Reply to the Comment on “Phase transitions, screening and dielectric response of CsPbBr 3 ” by Š. Svirskas, S. Balčiūnas, M. Šimėnas, G. Usevičius, M. Kinka, M. Velička, D. Kubicki, M. E. Castillo, A. Karabanov, V. V. Shvartsman, M. R. Soares, V. Šablinskas, A. N. Salak, D. C. Lupascu and J. Banys, J. Mater. Chem. A , 2020, 8 , 14015

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In this contribution, more evidence is provided to support the presence of low-temperature anomaly in CsPbBr 3 single crystals.

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Improving the phase transition temperature and enriching optoelectronics properties have become the top priorities. Herein, we have successfully synthesized two bilayer (2D/3D) perovskite materials based on a single layer perovskite.
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We present a mechanochemical procedure, with solvent-free, green-chemistry credentials, to grow all-inorganic CsPbBr3 perovskite. The crystal structure of this perovskite and its correlations with the physicochemical properties have been studied. Synchrotron X-ray diffraction (SXRD) and neutron powder diffraction (NPD) allowed us to follow the crystallographic behavior from 4 to 773 K. Unreported features like the observed negative thermal expansion of the b unit-cell parameter stem from octahedral distortions in the 4–100 K temperature range. The mechanochemical synthesis was designed to reduce the impact energy during the milling process, leading to a defect-free, well-crystallized sample characterized by a minimum unit-cell volume and octahedral tilting angles in the low-temperature orthorhombic perovskite framework, defined in the Pbnm space group. The UV–vis diffuse reflectance spectrum shows a reduced band gap of 2.22(3) eV, and the photocurrent characterization in a photodetector reveals excellent properties with potential applications of this material in optoelectronic devices.
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Lead halide perovskites belong to a broad class of compounds with appealing optoelectronic and photovoltaic properties. Doping with transition metal ions such as Mn²⁺ and Co²⁺ has recently been reported to substantially enhance luminescence and stability of these materials. However, so far atomic-level evidence for incorporation of the dopants into perovskite phases has been missing. Here, we introduce a general and straightforward method for confirming the substitutional doping of bulk perovskite phases with paramagnetic dopants. Using ¹³³Cs and ¹H solid-state MAS NMR relaxation measurements we provide for the first time direct evidence that, consistent with current understanding, Mn²⁺ is incorporated into the perovskite lattice of CsPbCl3 and CsPbBr3 and does not form clusters. We also show that, contrary to current conviction, Co²⁺ is not incorporated into the perovskite lattice of MAPbI3.
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Cesium lead bromide (CsPbBr3) was recently introduced as a potentially high performance thin-film halide perovskite (HaP) material for optoelectronics, including photovoltaics, significantly more stable than MAPbBr3 (MA = CH3NH3⁺). Because of the importance of single crystals to study relevant material properties per se, crystals grown under conditions comparable to those used for preparing thin films, i.e., low-temperature solution-based growth, are needed. We show here two simple ways, antisolvent-vapor saturation or heating a solution containing retrograde soluble CsPbBr3, to grow single crystals of CsPbBr3 from a precursor solution, treated with acetonitrile (MeCN) or methanol (MeOH). The precursor solutions are stable for at least several months. Millimeter-sized crystals are grown without crystal-seeding and can provide a 100% yield of CsPbBr3 perovskite crystals, avoiding a CsBr-rich (or PbBr2-rich) composition, which is often present alongside the perovskite phase. Further growth is demonstrated to be possible with crystal seeding. The crystals are characterized in several ways, including first results of charge carrier lifetime (30 ns) and an upper-limit of the Urbach energy (19 meV). As the crystals are grown from a polar aprotic solvent (DMSO), which is similar to those used to grow hybrid organic-inorganic HaP crystals, this may allow growing mixed (organic and inorganic) monovalent cation HaP crystals.
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Structural phase transitions ('type 0') in which there is no change of space group or of the occupied Wyckoff sites contrast with others in which diffusionless transformation can occur in a single step between higher- and lower-symmetry space groups (type I), through a low-symmetry transition state between relatively higher-symmetry initial and final structures (type II), and those where the mechanism is necessarily more complex (type III). A phenomenological model shows that type 0 transitions are necessarily first order, and may terminate at a critical point. The corresponding supercritical behaviour is a 'crossover' or 'diffuse transition' in which there is no discontinuity in any free-energy derivative. However, the location of the crossover is precisely defined at a minimum in the second derivative of the free energy with respect to a suitable order parameter. Isosymmetric transitions and/or crossovers occur in important mineralogical systems (pyroxenes, feldspars and carbonates) and non-linear optic materials (KTiOPO4). Non-monotonic variation of free-energy derivatives around the crossover can have a serious effect on the locations and slopes of phase equilibria in pressure-temperature space. Interaction between non-symmetry-breaking and symmetry-breaking order parameters appears to play a major role in stabilizing low-symmetry clinopyroxene and anorthite feldspar phases.
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We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior of the multiferroic hexagonal polymorph of YMnO3 between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric-ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at 1258±14 K corresponding to a unit-cell tripling and a change in space group from centrosymmetric P63/mmc to polar P63cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition (analyzed in terms of symmetry-adapted displacement modes) clearly support previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K3 mode. We therefore verify previous suggestions that YMnO3 is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group P63/mcm does not occur. However, we do find evidence for an isosymmetric phase transition (i.e., P63cm to P63cm) at ≈920 K, which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization.
Cesium–lead–bromide (CsPbBr3) is the simplest all inorganic halide perovskite. It serves as a reference material for understanding the exceptional solar cell properties of the organic–inorganic hybrid halide perovskites and is itself discussed as an alternative absorber material. Broadband dielectric spectroscopy has proven to yield an in depth understanding of charge screening mechanisms in the halide solar cell absorbers based on methylammonium and modifications hereof. For a deeper understanding of charge carrier screening, we have investigated CsPbBr3 across wide temperature (120 K–450 K) and frequency ranges. Besides the two known phase transitions at 403 K and 361 K, the dielectric data show another anomaly around 220 K, which can be interpreted as another phase transition. XRD and EPR studies confirm the presence of this anomaly, but Raman scattering spectra do not show any lattice anomalies in the vicinity of 220 K. This additional anomaly is of first order character (different transition temperatures upon cooling and heating) but hardly influences the lattice dynamics. Our broadband dielectric investigations of CsPbBr3 display the same microwave limit permittivity as for MAPbX3 (εr ≈ 30, X = Cl, Br, I, MA = CH3NH3⁺) but do not afford a second permittivity relaxation up to this frequency. Our prior assignment of the second contribution in the methylammonium compounds being due to the relaxation dynamics of the methylammonium ion as a dipole is herewith proven. Nevertheless, CsPbBr3 shows large charge carrier screening up to very high frequencies which can still play a vital role in charge carrier dynamics and exciton behaviour in this material as well.
Thermal degradation becomes the main obstacle for industrial applications of all-inorganic cesium lead halide (CsPbX3, X=Cl, Br, I) perovskite optoelectronic devices. A complete understanding of thermal degradation of CsPbX3 perovskites is required but greatly challenging for achieving optoelectronic devices with long-term stability particularly under extreme settings. Herein, we present an in situ spectroscopic study of thermal stability of CsPbX3 nanocrystals between the cryogenic temperature and high temperature. The low-frequency Raman signatures of CsPbX3 nanocrystals dramatically evolve but differentiate from the halogen atoms at elevated temperatures, acting as potent indicators of their crystalline structures and phase transitions. The merging of doublet Raman bands of CsPbX3 nanocrystals indicates their high-temperature phase transitions. CsPbX3 (X=Br, I) nanocrystals undergo a state of high-degree of disorder with featureless Raman spectra before thermally decomposed. Such understanding is of particular importance for future design and optimization of high-performance CsPbX3 perovskite devices with long-term stability under extreme settings.
Perovskite-like metal-organic frameworks (MOFs) are hybrid materials of high interest for their potential in information storage technology, as Pb-free substitutes for the widely used lead zirconate titanate (PZT) family of multiferroics. We report here a new, microwave-assisted method of synthesis for perovskite-like MOFs, which exploits the advantages of rapid and volumetric heating by microwaves in order to achieve synthesis within minutes, compared to days required by previously reported methods. The preliminary results demonstrate a broad control over the size and morphology of the products, by minor changes in the reaction conditions. An investigation of the effects of size and morphology on the magnetic and dielectric properties is presented here. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
Structural phase transitions in perovskite-type CsPbBr3 have been investigated by neutron diffraction method. Phase transitions occur at 88°C and 130°C, which are respectively second and first order. The phase transition at 130°C is caused by condensation of the M3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensation of the doubly degenerate R25-like mode (Z9 mode) at the Z point of the tetragonal Brillouin zone. Group theoretical considerations based on these results reveal that the crystal trans-forms from cubic perovskite structure (Oh1-Pm3m) to tetragonal D4h5-P4/mbm at 130°C and further to orthorhombic D2h16-Pmbn at 88°C. Possible atomic displacements induced at the phase transitions are obtained from the eigenvectors of the condensing modes.
Some experimental investigations have been made in order to understand the nature of phase transitions in the perovskite-type crystal CsPbCl3. Measurements of the birefringence and conoscopic observations confirm three phase transitions occurring at 37°C, 42°C and 47°C, respectively. The crystal system in each phase and the superstructure below 37°C are determined from the results of these measurements and an X-ray work. A dielectric measurement shows that CsPbCl3 is neither ferroelectric nor antiferroelectric in any of the phases. The transition entropies estimated from the result of a specific heat measurement are considerably smaller than that reported by Møller. The elastic compliance measured by the method of composite oscillator shows anomaly at each transition point. Temperature-dependent Raman spectra demonstrate the existence of ``soft'' modes which are overdamped near the transition temperatures. The nature of these phase transitions is discussed on the basis of the experimental results.
The first- and second-order Raman phonon spectra of CsPbCl3 were measured in phase IV using Raman and resonance-Raman scattering. Twelve Raman active phonons were resolved at 32, 35.5, 42, 52, 72, 90, 110, 115, 121, 189, 200.5, and 375 cm−1 and correlated to the phonon spectrum of CsPbCl3 in phase I. The highest frequency longitudinal optical mode is 375 cm−1. © 1997 American Institute of Physics.
We report a Raman scattering investigation of CsPbCl3 as a function of temperature in four different scattering configurations allowing us to obtain the various irreducible representations of the appropriate symmetry groups describing the substance in its successive phase transitions: cubic, tetragonal, orthorhombic and monoclinic or orthorhombic. We identify the correct ‘parentage’ of the observed modes with respect to infrared reflectivity data reported in the literature in accord with the Lyddane–Sachs–Teller relation, which was not fulfilled in previous assignments. We demonstrate the presence of strong correlation chains, which maintain the compound in a pseudo-cubic state as far as x-ray, infrared and Raman spectroscopy are concerned. The intensity of the related central peak was analysed on the basis of a Debye-type relaxation as a function of temperature; the results show perfect correspondence of its variations and the temperatures of the phase transitions. In addition, an order–disorder phase transition is evidenced at low temperature, 193 K (−80 °C). Copyright © 2003 John Wiley & Sons, Ltd.
Measurements of dielectric properties, pyroelectricity, and the electron paramagnetic resonance spectrum of Gd<sup>3+</sup> as a function of temperature have been used to examine the phase transitions in CsPbCl 3 . The results indicate the presence of five phase transitions and the loss of a center of symmetry at 194 K. The results together with the apparent order of the transitions, and published data, enable the Landau criterion to be used so that the point group of each phase may be identified. A reasonable choice of space group is also made.
Studies of the EPR of dilute Mn<sup>2+</sup> in CsPbCl<sub>3</sub> have been carried out over the temperature range 77°–340°K. Crystalline phase changes as indicated by changes in the Pb site symmetry were evident at 320°K (46.5°C) and 185°K. The first of these has been reported previously while the latter is apparently a new observation. Between 185° and 320°K, the EPR spectra can be assigned to two and possibly four crystallographically equivalent sets of magnetically inequivalent sites, the spectra of the ith set derivable from the spin Hamiltonian <sub>i</sub>=gβ H · S +D<sub>i</sub>[S<sub>z</sub><sup arrange="stagger">2</sup>- 1 3 S(S+1)]+E<sub>i</sub>(S<sub>x</sub><sup arrange="stagger">2</sup>-S<sub>y</sub><sup arrange="stagger">2</sup>)+ 1 6 a<sub>i</sub>(S<sub>x</sub><sup arrange="stagger">4</sup>+S<sub>y</sub><sup arrange="stagger">4</sup>+S<sub>z</sub><sup arrange="stagger">4</sup>)+AS<sub>z</sub>I<sub>z</sub>+B(S<sub>x</sub>I<sub>x</sub>+S<sub>y</sub>I<sub>y</sub>) . Typical values of the crystal field parameters are: A = -87.5 G, B = -86.5 G, and a≈1 G; all sensibly temperature independent, while D and E are temperature dependent. For example at 195°K, D<sub>1,2,3,4</sub>=45 G and E<sub>1</sub>= -E<sub>2</sub>=7.2 G, E<sub>3</sub>=6.4 G, and E<sub>4</sub>= -5.0 G. Both D and E tend continuously to zero at 320°K. Thus, the individual site symmetries are evidently orthorhombic and the overall crystal symmetry is apparently orthorhombic or nearly so. Some discussion of possible crystal space groups is included.
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