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A Conceptional Approach to Materials for Resistivity Switching and Thermoelectrics
Abstract
Recently silver(I)-(poly)chalcogenides have received special interest in energy conversion materials because of their thermoelectric and resistivity switching properties. In order to understand these features, a new topological principle has been developed, which is an useful tool to plan new synthesis strategies and to vary functional properties. It has been shown that anion substitution is powerful tool to tune the physical properties in terms of a thermoelectric performance optimization. Herein we report on the recently introduced substance class of silver(I)-(poly)chalcogenidehalides and the well-established class of silver(I)-chalcogenidehalides. We focus our report on the electrochemical behaviour of all known silver-(poly)telluridehalides Ag10Te4Br3, Ag23Te12Br, Ag20Te10BrI,Ag5Te2Cl and selected substituted phases.
... The change in the magnitude of Seebeck during p-n-p type conduction switching in Ag 10 Te 4 Br 3 , AgBiSe 2 , AgCuS, and Ag 10 Te 4 Br 2.6 Cl 0.4 are 1400, 730, 1760 and 1800 mV K À1 , respectively. 2,6,17,23 Temperatures of the p-n-p type transition in several compounds are given in Table 1. ...
... 43,44 In order to shift the temperature driven p-n-p type conduction switching to an applicable temperature, various cationic/anioinic substitutions in Ag 10 Te 4 Br 3 were investigated. 23 Although little shift in transition temperature was observed with 2 mol% iodine doping (Ag 10 Te 4 Br 2.8 I 0.2 ), but the jump in the Seebeck coefficient was much smaller compared to that of pristine Ag 10 Te 4 Br 3 (Fig. 5). 23 Interestingly, the solid solution Ag 10 Te 4 Br 2.6 Cl 0.4 shows a significant jump in the Seebeck coefficients with a DS B 1800 mV K À1 (Fig. 1 and 5). ...
... 23 Although little shift in transition temperature was observed with 2 mol% iodine doping (Ag 10 Te 4 Br 2.8 I 0.2 ), but the jump in the Seebeck coefficient was much smaller compared to that of pristine Ag 10 Te 4 Br 3 (Fig. 5). 23 Interestingly, the solid solution Ag 10 Te 4 Br 2.6 Cl 0.4 shows a significant jump in the Seebeck coefficients with a DS B 1800 mV K À1 (Fig. 1 and 5). 23 Sulphur and selenium substitution on the Te sublattice resulted in the complete disappearance of the p-n-p conduction switching (Fig. 5), 23 because the Peierls distortion within the covalently bonded Te-substructure was disturbed effectively by a lightered homologue, sulphur or selenium. ...
Modern technological inventions have been going through a “renaissance” period. Development of new materials and understanding of fundamental structure-property correlations are the important steps to move further for advanced technologies. In modern technologies, inorganic semiconductors are the leading materials which are extensively used for different applications. In the current perspective, we present discussion on an important class of material that show fascinating p-n-p type conduction switching, which can have potential application in diode or transistor devices that operate reversibly on temperature or voltage change. We highlight the key concepts, present the current fundamental understanding and show the latest developments in the field of p-n-p type conduction switching. Finally, we point out the major challenges and opportunities in this field.
... 12,13 Conductivity type can also be inverted upon structural phase transitions, for example, under variation either in pressure 5,7,14 or in temperature. [15][16][17][18][19][20] Note that doping and implantation result in irreversible changes in electronic transport properties, whereas variations linked to pressure-or temperature-driven structural phase transitions are often reversible. However, in the latter case, a multiple cycling across phase transitions can severely damage specimens. ...
Under applied high pressure, the electronic, optical, structural, and other properties of narrow-bandgap telluride semiconductors are subjected to dramatic changes. They can include, for instance, structural and electronic topological transitions. In this work, we investigated the electronic properties of single crystals of three families of tellurides, namely, HgTe, PbTe, and Bi2Te3 by measurements of the thermoelectric power (the Seebeck coefficient) and electrical resistance under high pressure up to 10 GPa. The applied pressure led to spectacular variations in the electronic transport of all three tellurides. We addressed these effects to electronic topological transitions that could be driven by significant narrowing of the bandgaps in the normal-pressure phases of these compounds. In particular, at about 1 GPa, we observed an n-p switching in the conductivity of HgTe, which was well reproducible under multiple pressure cycling. In contrast, in PbTe, we found that an electronic topological transition irreversibly turns the conductivity from p- to n-type. An electronic topological Lifshitz transition in p-type Bi2Te3 crystals with a low carrier concentration enhanced the n-type conductivity in a narrow pressure region about 2–3 GPa and resulted in a double p–n–p conductivity inversion. An irreversible p–n conductivity switching in p-type Bi2Te3 happened already on decompression from a high-pressure phase from about 8 GPa. The stress-controlled p–n inversions of the electrical conductivity in these industrially important telluride materials can potentially find emergent applications in micro- and nanoelectronics.
... Since semiconductor elements are the basis of modern electronics, both novel materials and alternative methods, which could effectively tune their optoelectronic properties, are always in much demand. 62 Variation in the temperature can induce drastic changes in electronic transport properties (e.g., p-n switching), for example, in Ag 10 Te 4 Br 3 , 63 AgBiSe 2 , 64 AgCuS, 65 Ag 10 Te 4 Br 2.6 Cl 0.4 , 66 and in La x Sr 2−x TiFeO 6 perovskites. 67 However, this approach can only be used to a limited extent in practical micro-or nanoelectronic devices. ...
Silicon, germanium and their alloys are classical semiconductors that play an important role in fundamental sciences and are the basis for modern microelectronics, optoelectronics, energy conversion, and other applications. The thermoelectric power (Seebeck coefficient) of p- and n-type materials in combination with the electrical and thermal conductivities characterizes the efficiency of thermal-to-electric energy conversion. In this work we experimentally show how one can effectively control the thermoelectric power of silicon-germanium alloys using an applied high pressure. We measured the Seebeck coefficient and the electrical resistance for several Si1−xGex crystals with different compositions under applied high pressure in (i) their original semiconductor cubic-diamond structure, (ii) across a phase transition to a metal phase at about 9–13 GPa, and (iii) across phase transformations to different metastable phases on pressure release. These studies were carried out for several pressurization and decompression cycles. The Si–Ge samples were examined by X-ray diffraction and Raman spectroscopy. After the high-pressure cycling the Si–Ge samples transformed into two metastable phases, namely, a cubic bc8 phase (Si-III) with a p-type electrical conductivity in the Si-dominant samples, and a tetragonal st12 phase (Ge-III) in the Ge-dominant alloys, whose conductivity type depended on the Si content. The dramatic pressure-driven changes in the thermoelectric power of Si–Ge crystals we found suggest that these semiconductors are promising for use in various stress-controlled electronic junctions, such as switches, p–n diode elements, n–p–n (p–n–p) transistors, and multi-layer heterostructures with alternating types of electrical conductivity.
... Although these Ag/Cu based chalcogenide exhibit interesting p-n-p switching but their conduction is dominated by mobile cations(Ag þ /Cu þ ) in the superionic phase therefore their stability with temperature is an issue of concern. [7] Hence an electronic semiconductor exhibiting conduction switching can be a good choice for application. Chalcogenide such as bismuth telluride based alloys are electronic semiconductor and they are extensively used as conventional thermoelectric material for converting waste heat into electrical energy in the temperature range of 300 C. [8][9] Bismuth telluride is a V-VI chalcogenide compounds with narrow band gap semiconductor of 0.16-0.3 ...
Bismuth telluride based alloys are electronic semiconductors, which exhibit n‐ or p‐type conduction due to the formation of Te vacancies or antisite defects, i.e., substitution of Bi on Te site or vice versa. Here, it is demonstrated that the temperature dependent Seebeck coefficient of Bi2Te2.7Se0.3 exhibits a reversible change in conduction from p‐ to n‐type at temperatures >487 K without exhibiting any structural transformation. The detailed characterization revealed that conversion of BiTe/Se antisite defects into Te vacancies is responsible for the p–n transition. The observed p–n transition makes Bi2Te2.7Se0.3 an ideal candidate for temperature controlled electronic switches. Bismuth telluride based alloys usually exhibit n‐ or p‐type conduction due to Te vacancies or due to the formation of Bi vacancies or anitisite defects, i.e., Bi substitution on Te sites or vice versa. The temperature dependent Seebeck coefficient of Bi2Te2.7Se0.3 exhibits a reversible change in conduction from p‐ to n‐type at temperatures >487 K without exhibiting any structural transformation. The conversion of BiTe/Se antisite defects into Te vacancies is responsible for p to n transition of Bi2Te2.7Se0.3.
... Previously such p-n type conduction switching assis- ted large change in thermopower was reported in the literature for few chalcogenide-based compounds, only in the last five years. But for the very first time it was observed in perovskite-based oxide materials and also the change in thermopower was larger than the previously reported results, as shown in the comparison 20,21,[98][99][100][101][102] bar chart in Fig. 12. Such p-n switching coupled with a large change in thermopower in a single material as a result of only temperature change, opens up a new avenue for developing novel high-temperature diodes, thyristors, sensors, switches, thermistors, etc. ...
Recently, double perovskite-based oxide materials have been proposed for thermoelectric (TE) applications due to their environment-friendly nature, high-temperature stability, better oxidation resistance, and lower processing cost compared to conventional chalcogenides and intermetallics. In this review article, we have comprehensively summarized our recent research studies on Sr2B′B″O6-based double perovskites for high-temperature TE power generation. We have shown that decoupling of phonon-glass and electron-crystal behavior is possible in oxides by reducing thermal conductivity due to induced dipolar glassy state as a result of relaxor ferroelectricity. We have also introduced metal-like electrical conductivity (-1/410⁵ S/m) in these ceramics that are inherently insulator in nature. Moreover, we have observed interesting behavior of temperature-driven p-n type conduction switching assisted colossal change in thermopower in some of these oxides, hitherto, obtained only in chalcogenides. The charge transport mechanism in these complex oxides has been analyzed by small polaron hopping conduction model in conjugation with defect chemistry.
... In Ag 10 Te 4 Br 3 , a similar p−n−p switching behavior has been attributed to a charge density wave (CDW) linked to a Peierls transition 15 that influences the electronic density of states close to the Fermi level, and it has been suggested that this thermopower modulation could be used to develop improved TE materials. 16 A strong anomaly was also observed in the temperature dependence of the thermal diffusivity ( Figure S7). ...
We report on the thermoelectric performances of intrinsic n-type AgBiSe2, a lead-free material with more earth-abundant and cheaper elements than intrinsic p-type homologous AgSbTe2. Pb doping yields n-type AgBiSe2 to p-type but leads to poor electrical transport properties. Nb doping enhances the thermoelectric properties of n-type AgBiSe2 by increasing the carriers concentration. Featured with intrinsically low thermal conductivity (0.7 Wm-1K-1), low electrical resistivity (5.2 mΩ×cm) and high absolute Seebeck coefficient (-218 µV/K), the thermoelectric figure of merit ZT at 773K is significantly in-creased from 0.5 for solid-state synthesized pristine AgBiSe2 to 1 for Ag0.96Nb0.04BiSe2, which makes it a promising n-type candidate for medium tempera-ture thermoelectric applications.
A new state-of-the-art thermoelectric material, Tl2Ag12Te7+δ, which possesses an extremely low thermal conductivity of about 0.25 W m–1K–1 and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its sub cell is a variant of the Zr2Fe12P7 type, but ultimately its structure was refined as a composite structure of a Tl2Ag12Te6 framework and a linear Te atom chain running along the c axis. The super space group of the framework was determined to be P63(00γ)s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain sub structure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ)s. The modulation leads to the formation of Te2 and Te3 fragments in this chain, and a refined formula of Tl2Ag11.5Te7.4. The structure consists of a complex network of three-dimensionally connected AgTe4 tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl2Ag12Te7, Tl2Ag12Te7.33, and 2x Tl2Ag12Te7.5, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.
Understanding the complex phenomenon behind the structural transformations is a key requisite to developing important solid-state materials with better efficacy such as transistors, resistive switches, thermoelectrics, etc. AgCuS, a superionic semiconductor, exhibits temperature-dependent p-n-p-type conduction switching and a colossal jump in thermopower during an orthorhombic to hexagonal superionic transition. Tuning of p-n-p-type conduction switching in superionic compounds is fundamentally important to realize the correlation between electronic/phonon dispersion modulation with changes in the crystal structure and bonding, which might contribute to the design of better thermoelectric materials. Herein, we have created extrinsic Ag/Cu nonstoichiometry in AgCuS, which resulted in the vanishing of p-n-p-type conduction switching and improved its thermoelectric properties. We have performed the selective removal of cations and measured their temperature-dependent thermopower and Hall coefficient, which demonstrates only p-type conduction in the Ag1- xCuS and AgCu1- xS samples. The removal of Cu is much more efficient in arresting conduction switching, whereas in the case of Ag vacancy, p-n-p-type conduction switching vanishes at higher vacant concentrations. Positron annihilation spectroscopy measurements have been done to shed further light on the mechanisms behind this structural transition-dependent conduction switching. Cation (Ag+/Cu+) nonstoichiometry in AgCuS significantly increases the vacancy concentration, hence, the p-type carriers, which is confirmed by positron annihilation spectroscopy and Hall measurement. The Ag1- xCuS and AgCu1- xS samples exhibit ultralow thermal conductivity (∼0.3-0.5 W/m·K) in the 290-623 K temperature range because of the low-energy cationic sublattice vibration that arises as a result of the movement of loosely bound Ag/Cu within the stiff S sublattice.
Double perovskite materials have been studied in detail by many researchers, as their magnetic and electronic properties can be controlled by the substitution of alkaline earth metals or lanthanides in the A site and transition metals in the B site. Here we report the temperature-driven, p-n-type conduction switching assisted, large change in thermopower in La³⁺-doped Sr2TiFeO6-based double perovskites. Stoichiometric compositions of LaxSr2-xTiFeO6 (LSTF) with 0 x 0.25 were synthesized by the solid-state reaction method. Rietveld refinement of room-temperature XRD data confirmed a single-phase solid solution with cubic crystal structure and space group. From temperature-dependent electrical conductivity and Seebeck coefficient (S) studies it is evident that all the compositions underwent an intermediate semiconductor-to-metal transition before the semiconductor phase reappeared at higher temperature. In the process of semiconductor-metal-semiconductor transition, LSTF compositions demonstrated temperature-driven p-n-type conduction switching behavior. The electronic restructuring which occurs due to the intermediate metallic phase between semiconductor phases leads to the colossal change in S for LSTF oxides. The maximum drop in thermopower (ΔS ∼ 2516 V K⁻¹) was observed for LSTF with x = 0.1 composition. Owing to their enormous change in thermopower of the order of millivolts per kelvin, integrated with p-n-type resistance switching, these double perovskites can be used for various high-temperature multifunctional device applications such as diodes, sensors, switches, thermistors, thyristors, thermal runaway monitors etc. Furthermore, the conduction mechanisms of these oxides were explained by the small polaron hopping model.
While continuing our investigations of thallium chalcogenides because of their outstanding thermoelectric properties, we discovered a new selenide with an interesting pnp switching behavior around 400 K. Tl2Ag12Se7 was prepared via high temperature reaction from the elements in the stoichiometric ratio. This selenide crystallizes in a new structure type, namely a √3 x √3 x 1 super cell of the Zr2Fe12P7 type, adopting space group P–3, a = b = 18.9153(18) Å, c = 4.3783(4) Å, V = 1356.6(2) ų (Z = 3). The structure consists of a complex network of three-dimensionally connected AgSe4 tetrahedra that include linear channels filled with thallium atoms. This material is a semiconductor with an experimentally derived activation gap of 0.8 eV, and extraordinarily low thermal conductivity of < 0.45 W m–1K–1. A reversible phase transition causes a pnp switch from p-type to n-type and back to p-type conduction beginning around 390 K, with the thermopower changing fast from +230 μV K–1 at 390 K to –230 μV K–1 at 410 K, and then back up to +75 μV K–1 at 420 K.
Nominal In4Se3-x with x = 0.65, derived from the prototype compound In4Se3, is a good thermoelectric material achieving a Z value of 1.48 at 705 K. This value is close to state of the art thermoelectric materials. In literature this special behavior is explained by a Peierls-distortion or Charge Density Wave within the compound caused by the selenium deficit. While extensive thermoelectric studies for different compositions are reported in literature, a solid proof of the existence of selenium deficit samples is still missing. In this work we report about the synthesis, phase analyses, and temperature dependent single crystal determinations of In4Se3-x samples in order to clarify these structural features.
Copper and silver based chalcogenides, chalco-halides, and halides form a unique class of semiconductors, as they display mixed ionic and electronic conduction in their superionic phase. These compounds are composed of softly coupled cationic and anionic substructures, and undergo a transition to a superionic phase displaying changes in the substructure of their mobile ions with varying temperature. Here, we demonstrate a facile, ambient and capping agent free solution based synthesis of AgCuS nanocrystals and their temperature dependent (300–550 K) thermoelectric properties. AgCuS is known to show fascinating p–n–p type conduction switching in its bulk polycrystalline form. Temperature dependent synchrotron powder X-ray diffraction, heat capacity and Raman spectroscopy measurements indicate the observation of two superionic phase transitions, from a room temperature ordered orthorhombic (β) to a partially disordered hexagonal (α) phase at ∼365 K and from the hexagonal (α) to a fully disordered cubic (δ) phase at ∼439 K, in nanocrystalline AgCuS. The size reduction to the nanoscale resulted in a large variation in the thermoelectric properties compared to its bulk counterpart. Temperature dependent Seebeck coefficient measurements indicate that the nanocrystalline AgCuS does not display the p–n–p type conduction switching property like its bulk form, but remains p-type throughout the measured temperature range due to the presence of excess localized Ag vacancies. Nanocrystalline AgCuS exhibits a wider electronic band gap (∼1.2 eV) compared to that of the bulk AgCuS (∼0.9 eV), which is not sufficient to close the band gap to form a semimetallic intermediate state during the orthorhombic to hexagonal superionic phase transition, thus AgCuS nanocrystals do not show conduction type switching properties like their bulk counterpart. The present study demonstrates that ambient solution phase synthesis and size reduction to the nanoscale can tailor the order–disorder phase transition, the band gap and the electronic conduction properties in superionic compounds, which will not only enrich solid-state inorganic chemistry but also open a new avenue to design multifunctional materials.
Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulphide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (ΔS ~1757 µVK-1) at the superionic phase transition (T ~364 K). In addition, its thermal conductivity is ultra-low in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy and positron annihilation spectroscopy measurements. Using first-principles calculations we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulphur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.
Ag10Te4Br3 is the first compound of this substance class, where covalent interactions in the tellurium substructure led to the formation of Peierls distorted linear chains, responsible for a strong modulation of the electronic structure. This compound represents the first example within a series of pnp switching compounds, where the interplay of mobile ions, in combination with the attractive interactions, created by partially covalent‐bonded anion substructures defines a new substance class of resistivity switching phases. Herein, we report on the partial substitution of mobile copper ions into the cation substructure of cation conducting Ag10Te4Br3. An X‐ray powder phase analysis substantiated the continuous intake of copper in the title compound. Thermoanalytic investigations were performed to analyse the influence of two mobile species on the structural phase transitions of the polymorphic coinage metal (poly)telluride bromide.
Nominal In4Se3x with x = 0.65, derived from the prototype compound In4Se3, is a good thermoelectric material achieving a Z value of 1.48 at 705 K. This value is close to state of the art thermoelectric materials. In literature this special behavior is explained by a Peierls-distortion or Charge Density Wave within the compound caused by the selenium deficit. While extensive thermoelectric studies for different compositions are reported in literature, a solid proof of the existence of selenium deficit samples is still missing. In this work we report about the synthesis, phase analyses, and temperature dependent single crystal determinations of In4Se3x samples in order to clarify these structural features.
Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag10Te4Br3 is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 V K-1, in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.
Coinage-metal(I) polychalcogenide halides represent a new class of materials featuring high ion dynamics of different substructures
capable of an effective phonon scattering process in the solid state. The interplay of mobile coinage-metal ions such as Ag+ or Cu+ on the one hand and the temperature-driven redox reaction of a linear, partially covalent-bonded Te chain on the other hand
is responsible for tremendous variations in the electronic structure of such materials. A huge drop of the Seebeck coefficient
within a very small temperature range and an extremely low thermal conductivity are the key properties of Ag10Te4Br3, the first representative of this substance class. Two different sets of compounds with the general formula (CM)10Q4X3 and (CM)23Q12X (CM=coinage metal Cu or Ag, Q=chalcogen, X=halogen) have been found, and recent experiments point toward the existence
of formerly unknown copper(I) (poly)chalcogenide halides and two more silver(I) (poly)chalcogenide halides in this field.
A comparable linear Te chain is present in a number of different compounds such as some alkaline-earth polychalcogenides M5Te3, the mineral stuetzite Ag4.5Te3, and the closely related compounds Ag11AsTe7 and Ag12Te6S. In most cases the thermoelectric potential of these materials has not been verified. A topological approach for the description
of the structural features has been developed to understand the electronic properties of these complex materials in detail.
KeywordsRedox switches-chalcogen chains-ion dynamics-polychalcogenide halides-structure topology-effective phonon scattering
Mo <sub>3</sub> Sb <sub>7-x</sub> Te <sub>x</sub> is a high temperature thermoelectric material, reported to reach figure of merit ( ZT )=0.8 at 1023 K. Various p -type samples of Ni <sub>y</sub> Mo <sub>3</sub> Sb <sub>7-x</sub> Te <sub>x</sub> were prepared with y≤0.1 and 1.5≤x≤1.7 via high temperature reactions at 993 K. Adding transition metal atoms into the empty cube formed by Sb atoms significantly alters the band structure and thus the thermoelectric properties. Electronic band structure calculations indicate that adding Ni slightly increases the charge carrier concentration, while higher Te content causes a decrease. Thermoelectric properties were determined on pellets densified via hot pressing at 993 K. Seebeck as well as electrical and thermal conductivity measurements were performed up to 1023 K. The highest ZT value thus far was obtained from a sample of nominal composition Ni <sub>0.06</sub> Mo <sub>3</sub> Sb <sub>5.4</sub> Te <sub>1.6</sub> , which amounts to 0.93 at 1023 K.
Thermoelectric energy harvesting-the transformation of waste heat into useful electricity-is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500-900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In(4)Se(3-delta), which achieves the ZT value of 1.48 at 705 K-very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.
The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and
utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for
high-temperature applications. We show that the material system AgPbmSbTe2+m may be suitable for this purpose. With m = 10 and 18 and doped appropriately, n-type semiconductors can be produced that exhibit a high thermoelectric figure of merit material ZTmax of ∼2.2 at 800 kelvin. In the temperature range 600 to 900 kelvin, the AgPbmSbTe2+m material is expected to outperform all reported bulk thermoelectrics, thereby earmarking it as a material system for potential
use in efficient thermoelectric power generation from heat sources.
Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. Such a development is contingent on identifying materials with higher thermoelectric efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelectric materials is approaching. We review recent advances in the field, highlighting the strategies used to improve the thermopower and reduce the thermal conductivity.
X-ray powder and EDX phase analyses, thermal analyses, impedance spectroscopic measurements, and crystal structure determinations of selected phases are reported to highlight the structural and electric properties of the solid solutions Ag10Te4Br3-xClx and Ag10Te4Br3-yIy. Bromide can be partially substituted by chloride or iodide up to x = 1.6 and y = 0.2, respectively, without changes in the structural properties of the tetramorphic, ion-conducting Ag10Te4Br3. Results from X-ray powder phase analyses have been complemented by data of single crystal structure determinations of selected samples. At r. t. the fully ordered γ-Ag 10Te4Br3 structure type is realized in all chloride containing phases, but a partial iodide substitution leads to the stabilization of the disordered β-Ag10Te4Br 3 structure type. Impedance spectroscopic measurements reveal a significant increase by one order of magnitude of the r. t. silver ion conductivity of Ag10Te4Br2.8I0.2 as compared with Ag10Te4Br3.
The phase diagrams of Ag 2SAgI, Ag
2SeAgI, Ag 2TeAgI, Ag
2TeAgBr, and Ag 2TeAgCl were
investigated. The system Ag 2S-AgI shows two broad regions of
solid solution which are based on the structure of the high-temperature
phases of the constituent compounds. The high-temperature modification
of Ag 3SI is part of one of these regions. The system Ag
2SeAgI resembles the system Ag
2TeAgI; both contain limited regions of terminal
solid solutions. The AgI-based solid solutions decompose peritectically.
In the system Ag 2TeAgBr a compound Ag
3TeBr was found. Ag 3TeBr undergoes a phase
transition at 590 ± 20 K. The low-temperature form has hexagonal
symmetry with the lattice parameters a = 748.8(1) pm and c = 4357.6(6)
pm. The compound Ag 5Te 2Cl was found in the Ag
2TeAgCl system. In both systems a restricted terminal
solid solution, based on the high-temperature form of Ag 2Te,
was observed. Ag 5Te 2Cl has a reversible phase
transformation at 329 ± 3 K with ΔHtr = 9.82
± 0.4 kJ mole -1. β-Ag 5TeCl, the
low-temperature form probably has the space group P2 {1}/{n},
a = 1365.5(1), b = 1386.1(1), c = 764.23(2) , β = 90.201(1)°,
and Z = 4, α-Ag 5Te 2Cl has the space group
{I4}/{mcm} with a = 975.5(3), c = 783.0(1) pm, and Z = 4. The
anion sublattice is built of octahedra, which share all their vertices
with neighboring octahedra. The Ag + ions are distributed
over octahedral holes of this network. The phase is similar in behavior
to Ag 8GeTe 6 and may be a silver-ion conductor.
Herein we report the structural, thermal and physical properties of Ag5Te2−ySeyCl (y=0–0.7). Polymorphic Ag5Te2Cl (y=0) is an ion conductor with two reversible phase transitions at 241 and 334 K. While the monoclinic room temperature β- and the low temperature γ-phase show conductivities below σ=10−3Ω−1cm−1, the tetragonal high temperature α-phase is a promising conductor for electronic devices with conductivities up to 10−1Ω−1cm−1 in addition to high moisture and photo stability. The α–β-phase transition temperature of ion conducting Ag5Te2Cl can be lowered by partial substitution of tellurium by selenium. From differential thermal analysis (DTA) and differential scanning calorimetry (DSC) a linear decrease of the α–β-phase transition temperature with increasing compositional parameter y to a minimum transition temperature of 239 K was observed for Ag5Te1.3Se0.7Cl. Phases up to a maximum selenium content of y=0.7 were stabilized by preservation of the β- and α-Ag5Te2Cl structure types. Ag5Te2−ySeyCl (y=0–0.9) phases were characterised by powder X-ray diffraction (XRD) at room temperature. The stabilization of the α-Ag5Te2Cl structure type in Ag5Te2−ySeyCl around room temperature results in an increase of conductivity of more than 2 orders of magnitude compared to the ternary phase. Conductivity of up to σ=5.1×10−2Ω−1cm−1 (314 K, α-Ag5Te1.6Se0.4Cl) was observed by impedance spectroscopy. In the range from room temperature down to 239 K any transition temperature can be chosen by simply varying the composition.
The distinction between valence electrons and essentially inactive core electrons is the basis of many classifying concepts in chemistry. However, it has recently been recognized that this is an oversimplification and should, at least in some areas of chemistry, be modified. Many cases are known where cations with closed d10 configurations are subject to homoatomic interactions that influence structure and properties. A characteristic and surprisingly uniform structural feature (e.g., of a number of compounds containing monovalent coinage metals) is a clusterlike assembly of d10 cations that corresponds in geometry and bond lengths to fragments of the metal structures themselves. Further evidence for a special type of bonding in such compounds is provided by their physical properties; for example, the absorption in the UV/VIS region shows a drastic redshift and the compounds are often conductors or semiconductors. The d electrons in such cases have obviously lost their pure “core” nature. All bonding models so far proposed for such systems involve mixing of higher orbitals.
Thermoelectric power generation is foreseen to play a much larger role in the near future, considering the need for alternative energies because of declining natural resources as well as the increasing efficiency of thermoelectric materials. The latter is a consequence of the discoveries of new materials as well as of improvements of established materials by, for example, nanostructuring or band structure engineering. Within this review, two major classes of high-temperature thermoelectrics are presented: clathrates formed by silicides and germanides, and complex antimonides including but not limited to the filled skutterudites. The clathrates and the skutterudites are cage compounds that exhibit low thermal conductivity, reportedly related to the rattling effect of the guest atoms, whereas the other antimonides achieve low thermal conductivity via defects or simply via the high complexity of their crystal structures.
Systematic examinations for new phases on the quasi-binary section AgX−Ag2Q (with X = halide and Q = chalcogenide) led to the exploration of the three new compounds Ag19Te6Br7, Ag19Te6Br5.4I1.6, and Ag19Te5SeBr7. All of them crystallize in new structure types with rigid anion and highly disordered silver substructures. A peritectic decomposition to the binary halides and chalcogenides and Ag23Te12Br was observed for each compound between 693 and 719 K. Polymorphism, a common feature for solid state ion conductors, was found for Ag19Te6Br7. Impedance spectroscopic investigations showed high conductivities (up to σ = 1.1 × 10 −2 Ω−1 cm−1, 323 K, Ag19Te6Br5.4I1.6) and low activation energies (Ea = 0.19 eV, α-Ag19Te6Br7) at room temperature. A topological description of the rigid and complex anion substructure was used to classify the three different structures. Linearly arranged nets of anions, separated from each other by the disordered silver ions, corrugate under partial substitution of either the halide or chalcogenide ions. The topological approach is a fundamental tool to understand the structure property relations in more detail.
Thermoelectric and phase analytical measurements were performed to investigate the physical properties of trimorphic Ag5Te2Cl. The material is a mixed electron/silver ion conductor featuring drastic property changes during a silver order/disorder phase transition at 334 K. The transition is characterized by a jump in the total electric conductivity by 2 orders of magnitude directly affecting the electric and thermoelectric properties. Silver ions are arranged in well-defined strands along the crystallographic c-axis characterized by a set of not fully occupied sites. Heat capacity measurements show a large effect, whereas the thermopower and thermal diffusivity drop significantly at the temperature of transition. Right after the transition, an attractive d10−d10 interaction within the disordered silver substructure occurs affecting the c-lattice parameter upon heating. Due to this interaction a modulation of the electronic structure and the thermoelectric properties can be observed which have been investigated in detail. While the thermopower stays low with increasing temperature the thermal diffusivity relaxes fast to values before the transition. At 355 K, the thermopower starts rising again, which is consistent with a small effect in the heat capacity and a reduction of the c-lattice parameter upon heating. Further heating leads to a reduction of the d10−d10 interactions and a drastic increase in the thermopower. The observed phenomenon must be regarded as a new example of a compound following the recently discovered concept of low-dimensional partially covalent-bonded structure units that can positively influence thermoelectric properties in bulk materials. Ag5Te2Cl is the first example where mobile d10 ions interact to create low-dimensional partially covalent-bonded subunits in a solid, which then leads to a switching of thermoelectric and electronic properties. The system shows very low thermal conductivities between 0.19 W m−1 K−1 and 0.60 W m−1 K−1 in the temperature range 298 to 500 K, reaching a maximal ZT value of 0.033 at high temperatures.
Ag10Te4Br3 is the first representative of an M(group 11) (poly)chalcogenide halide with covalently bonded [Te2]2- units and isolated Te2- ions. It represents a new class of materials located between M(group 11) chalcogen halides, characterized by covalently bonded chalcogen structure motives, on the one hand and chalcogenide halides, consisting of isolated chalcogenide ions, on the other. A high silver mobility on distinct silver layers was observed for orthorhombic Ag10Te4Br3, resulting in a pronounced 1D silver mobility at room temperature. The covalent regions of the chalcogen substructure account for the lowest activation energies, determined after a detailed analysis of the silver joint probability density functions (jpdf). Impedance spectroscopic measurements were performed in order to substantiate the results from jpdf analyses. Ag10Te4Br3 is an excellent conductor, with conductivities on the same order of magnitude as β-Ag3SX (X = I, Br) or glassy silver chalcogenide halides. The occurrence of the [Te2]2- dumbbell was substantiated by Raman spectroscopy.
Single-crystal structure determinations at room temperature and elevated temperatures were carried out on the silver ion conductor Ag5Te2Cl, and the hitherto unknown structure of the room-temperature phase (β-Ag5Te2Cl) was determined. Ion pathways are discussed for the room- and high-temperature phase by analyzing the joint probability density functions. Main transport pathways at different temperatures were worked out to obtain detailed insight into the movement of the silver atoms in this material. Ag5Te2Cl is an excellent 1D ionic conductor having a pronounced silver mobility along complex silver columns. Conductivities are σ = 1.51 × 10-3 Ω-1 cm-1 at 323 K and σ = 4.30 × 10-1 Ω-1 cm-1 at 469 K. With use of a nonharmonic description of the silver distribution of the high-temperature phase, a promising structure model has been derived for the room-temperature phase. β-Ag5Te2Cl can be regarded as an intermediate phase between the disordered high- and the ordered low-temperature phase, showing an almost ordered silver distribution in addition to a slightly different arrangement of the anionic substructure.
X-ray powder diffraction, EDX and DSC measurements were performed in order to determine the structural and thermal properties of the solid solutions of Ag23Te12Cl1–xBrx, Ag23Te12Br1–yIy and Ag23Te12Cl1–zIz. A complete solid solution exists between the two ternary end members Ag23Te12Cl and Ag23Te12Br, whereas the substitution of bromine by iodine can only be observed up to y = 0.3 in Ag23Te12Br1–yIy under thermodynamically controlled synthesis conditions. Even chlorine can be partially substituted by small amounts of iodine up to a certain extend. Temperature dependent single crystal structure determinations of crystals with the nominal composition Ag23Te12Cl0.95I0.05 substantiated the significant influence of the iodine intake on the lattice parameters of the pseudo-tetragonal orthorhombic system, especially at low temperatures. A systematic deviation from the 1:1 ratio of the a and b lattice parameter (pseudo-tetragonal ratio) was observed upon cooling, which could not be detected in the case of the ternary compounds. X-ray powder diffraction experiments gave a first hint on the non-stoichiometry of this set of materials and all compounds show peritectic decomposition to binary phases at temperatures slightly above 680 K.
Recent progress in the field of silver(I)-chalcogenide halides and silver(I)-polychalcogenide halides led to the discovery of a reasonable number of new phases, solid solutions and even a new substance class of materials with favorable electronic and thermoelectric properties. Most of these new compounds are characterized by a pronounced ion mobility, polymorphism, and complex structures, which result in severe problems during the structure solution, the refinement, and even the description of the crystal structures. Herein we report on a very simple but effective building principle and a topological structure approach, which are both based on the separate description of anion substructures and the introduction of discrete building blocks. Such a topological principle helps to understand the complex structural relations and allows one to discuss all known chalcogenide and polychalcogenide halides in a continuous sequence of compounds. Following this principle it is possible to understand physical properties and to predict new phases in the system.
The structures, thermal and physical properties of ion conducting polymorphic Ag5Te2Cl1−xBrx and Ag5Te2−ySyCl have been investigated. A maximum substitution degree of x = 0.65 and y = 0.3 was derived from X-ray powder diffraction. Mixtures of silver halides, silver chalcogenides and Ag3TeBr were observed for higher substitution degrees. Both silver chalcogenide halide systems show a Vegard type behaviour. Single crystal structure determinations of selected materials were performed at different temperatures to analyse the silver distribution in the tetragonal high temperature α- and the monoclinic room temperature β-phases. After non-harmonic refinement of the silver positions detailed joint probability density function analysis (jpdf) and determination of one particle potentials (opp) were carried out to investigate the diffusion pathways and bottlenecks of ion transport for those materials. A preferred anisotropic ion transport along the diffusion pathways for the α- and 1D zig-zag diffusion pathways for the β-phases were found. α–β and β-γ phase transitions were determined by DSC and DTA methods and conductivities were measured using temperature dependent impedance spectroscopy. The substitution of tellurium by sulphur lowered the α–β phase transition from 334 K (Ag5Te2Cl) to 270 K (Ag5Te1.8S0.2Cl) while the opposite trend was found for the Ag5Te2Cl1−xBrx phases. The α–β phase transition of Ag5Te2Cl0.35Br0.65 at 343 K represents the highest transition observed for the silver chalcogenide halides under discussion. Total conductivities of approx. 1 Ω−1 cm−1 (α-Ag5Te2Cl0.5Br0.5) and 0.24 Ω−1 cm−1 (α-Ag5Te1.8S0.2Cl) at 473 K were found being slightly higher (Br) and lower (S) than the conductivity observed for α-Ag5Te2Cl. A conductivity jump of more than two orders of magnitude, related to the α–β phase transitions, within the temperature range from 270 to 343 K is adjustable by simple variation of the composition and is therefore an extraordinary feature of these materials. The total conductivity is linearly correlated to the volume of the anion substructure and can be varied within more than half an order of magnitude.
A solid solution of the general formula Sn24P19.3ClyI8−y (y ≤ 0.8) with a crystal structure of the clathrate-I type (cubic, ) was prepared by a standard ampoule technique and found to be isostructural to the previously discovered Sn24P19.3BrxI8−x (x = 0–8). The unit cell parameter linearly decreases from 10.954(1) Å for y = 0 to 10.933(1) Å for y = 0.8. Sn24P19.3ClyI8−y (y ≤ 0.8) and Sn24P19.3BrxI8−x (x = 0–8) reveal a non-uniform communal distribution of the halogen atoms inside the cages of different size formed in the clathrate framework. The halogen atoms of smaller size (chlorine for Sn24P19.3ClyI8−y and bromine for Sn24P19.3BrxI8−x) preferentially occupy the smaller 20-vertex cages. Thereby the chlorine atoms do not show a complete segregation in the smaller cages, but mix with the iodine atoms in both types of cages. The magnetic and thermoelectric properties for Sn24P19.3ClyI8−y (y ≤ 0.8) as well as for Sn24P19.3BrxI8−x (x = 0–8) were investigated. Both solid solutions are diamagnetic semiconductors as expected for Zintl phases. The core diamagnetism of the guest atoms contributes primarily to the diamagnetic susceptibility of the compounds. The band gap in the case of Sn24P19.3BrxI8−x (x = 0–8) varies from 0.03 eV to 0.14 eV and appears to be a linear function of the guest halogen atom ratios. The lowest value of thermal conductivity, 0.5 W m−1 K−1 at room temperature, is observed for Sn24P19.3Br2I6 featuring the almost random distribution of the guest bromine and iodine atoms.Graphical abstract
Thermoelectric devices can convert heat into useful electricity with no moving parts. Considerable progress has been made in improving the efficiency of these devices over the past 15 years. The key ideas responsible for most of this progress will be examined using specific examples. Recent improvements in thermoelectric efficiency appear to be dominated by a reduction in the lattice thermal conductivity. This reduction is accomplished by the careful introduction of 0.1–5-nm sized “objects” that effectively scatter acoustic phonons without significantly affecting electronic transport. Future research directions will be discussed.
Controlling simultaneously the electric and thermal properties of materials can lead to very efficient thermoelectric devices. Advances following different routes were highlighted at a recent conference.
Zintl phases and related compounds are promising thermoelectric materials; for instance, high zT has been found in Yb_(14)MnSb_(11), clathrates, and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.
L'appareil décrit dans le texte est conçu pour mesurer le pouvoir thermoélectrique de monocristaux de forte résistivité (≃ 1010 Ω cm) entre 4 et 300 K. Il permet d'effectuer des mesures sur des échantillons de faibles dimensions (< 0,5 mm). Le dispositif a été testé sur un monocristal de magnétite de composition stœchiométrique. Bien que non construit pour, il est capable de mesurer le coefficient de Seebeck d'échantillons métalliques pour lesquels α < 10 μV/°C.
Two new silver (poly)chalcogenide halides, Ag23Te12Cl and Ag23Te12Br, were characterized by powder X-ray phase analysis, energy dispersive X-ray analysis, and crystal structure determinations at various temperatures. Thermal analyses of both compounds and electrochemical measurements for the bromide completed the investigation. The compounds Ag23Te12X (X = Cl, Br) are isostructural and crystallize orthorhombically (space group Pnnm, Z = 4) as systematic twins. The lattice parameter values derived from X-ray powder data were a = 21.214(2) A, b = 21.218(2) A, c = 7.7086(7) A, and V = 3469.8(6) A (3) for Ag23Te12Cl at 293 K and a = 21.170(1) A, b = 21.170(1) A, c = 7.7458(5) A, and V = 3471.4(4) A (3) for Ag23Te12Br at 298 K. An enhanced silver ion mobility was revealed by impedance spectroscopy investigations. No phase transitions were observed in the temperature range 100-750 K. These two silver(I) (poly)chalcogenide halides are the second set of representatives of a new class of coinage-metal (poly)chalcogenide halides in which both covalently bonded [Te2](2-) dumbbells and ionically bonded Te(2-) anions appear.