Khang Hoang

Publications (31)47.43 Total impact

• Article: Origin of the Structural Phase Transition in Li_{7}La_{3}Zr_{2}O_{12}

  N Bernstein, M D Johannes, Khang Hoang
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  ABSTRACT: Garnet-type Li_{7}La_{3}Zr_{2}O_{12} is a solid electrolyte material for Li-ion battery applications with a low-conductivity tetragonal and a high-conductivity cubic phase. Using density-functional theory and variable cell shape molecular dynamics simulations, we show that the tetragonal phase stability is dependent on a simultaneous ordering of the Li ions on the Li sublattice and a volume-preserving tetragonal distortion that relieves internal structural strain. Supervalent doping introduces vacancies into the Li sublattice, increasing the overall entropy and reducing the free energy gain from ordering, eventually stabilizing the cubic phase. We show that the critical temperature for cubic phase stability is lowered as Li vacancy concentration (dopant level) is raised and that an activated hop of Li ions from one crystallographic site to another always accompanies the transition. By identifying the relevant mechanism and critical concentrations for achieving the high conductivity phase, this work shows how targeted synthesis could be used to improve electrolytic performance.
  Physical Review Letters 11/2012; 109(20):205702. · 7.37 Impact Factor
• Article: Origin of the structural phase transition in Li7La3Zr2O12

  N. Bernstein, M. D. Johannes, Khang Hoang
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  ABSTRACT: Garnet-type Li7La3Zr2O12 (LLZO) is a solid electrolyte material with a low-conductivity tetragonal and a high-conductivity cubic phase. Using density-functional theory and variable cell shape molecular dynamics simulations, we show that the tetragonal phase stability is dependent on a simultaneous ordering of the Li ions on the Li sublattice and a volume-preserving tetragonal distortion that relieves internal structural strain. Supervalent doping introduces vacancies into the Li sublattice, increasing the overall entropy and reducing the free energy gain from ordering, eventually stabilizing the cubic phase. We show that the critical temperature for cubic phase stability is lowered as Li vacancy concentration (dopant level) is raised and that an activated hop of Li ions from one crystallographic site to another always accompanies the transition. By identifying the relevant mechanism and critical concentrations for achieving the high conductivity phase, this work shows how targeted synthesis could be used to improve electrolytic performance.
  09/2012;
• Article: Decomposition mechanism and the effects of metal additives on the kinetics of lithium alanate.

  Khang Hoang, Anderson Janotti, Chris G Van de Walle
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  ABSTRACT: First-principles density functional theory studies have been carried out for native defects and transition-metal (Ti and Ni) impurities in lithium alanate (LiAlH(4)), a potential material for hydrogen storage. On the basis of our detailed analysis of the structure, energetics, and migration of lithium-, aluminium-, and hydrogen-related defects, we propose a specific atomistic mechanism for the decomposition and dehydrogenation of LiAlH(4) that involves mass transport mediated by native point defects. We also discuss how Ti and Ni impurities alter the Fermi-level position with respect to that in the undoped material, thus changing the concentration of charged defects that are responsible for mass transport. This mechanism provides an explanation for the experimentally observed lowering of the temperature for the onset of decomposition and of the activation energy for hydrogen desorption from LiAlH(4).
  Physical Chemistry Chemical Physics 02/2012; 14(8):2840-8. · 3.57 Impact Factor
• Source

  Available from: ArXiv

  Article: First-principles studies of the effects of impurities on the ionic and electronic conduction in LiFePO$_{4}$

  Khang Hoang, Michelle D. Johannes
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  ABSTRACT: Olivine-type LiFePO$_{4}$ is widely considered as a candidate for Li-ion battery electrodes, yet its applicability in the pristine state is limited due to poor ionic and electronic conduction. Doping can be employed to enhance the material's electrical conductivity. However, this should be understood as incorporating electrically active impurities to manipulate the concentration of native point defects such as lithium vacancies and small hole polarons which are responsible for ionic and electronic conduction, respectively, and {\it not} as generating band-like carriers. Possible effects of monovalent (Na, K, Cu, and Ag), divalent (Mg and Zn), trivalent (Al), tetravalent (Zr, C, and Si), and pentavalent (V and Nb) impurities on the ionic and electronic conductivities of LiFePO$_{4}$ are analyzed based on results from first-principles density-functional theory calculations. We identify impurities that are effective (or ineffective) at enhancing the concentration of lithium vacancies or small hole polarons. Based on our studies, we discuss specific strategies for enhancing the electrical conductivity in LiFePO$_{4}$ and provide suggestions for further experimental studies.
  01/2012;
• Article: Formation of small hole polarons in olivine phosphate cathode materials

  M D Johannes, Khang Hoang, J L Allen, K Gaskell
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  ABSTRACT: We investigate small hole polaron formation in olivine phosphate materials, LiMnPO4, LiFePO4, LiCoPO4 and LiNiPO4, using a combination of first principles calcns. and exptl. XPS spectra. We det. that details of the electronic structure near the Fermi energy, relevant to localization of a hole polaron, are not accurately captured using the std. GGA + U methodol. and require a hybrid functional approach. We show that polaron formation is possible in all but the Ni-based compd. and discuss the possibility of an electronic instability caused by electron withdrawal during delithiation caused by oxygen evolution. [on SciFinder(R)]
  ECS Trans. 01/2012; 41(29, Intercalation Compounds for Rechargeable Batteries):35-42.
• Source

  Available from: Karen Gaskell

  Article: Hole polaron formation and migration in olivine phosphate materials

  M. D. Johannes, Khang Hoang, J. L. Allen, K. Gaskell
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  ABSTRACT: By combining first principles calculations and experimental XPS measurements, we investigate the electronic structure of potential Li-ion battery cathode materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that determine small hole polaron formation and migration. We show that small hole polaron formation depends on features in the electronic structure near the valence-band maximum and that, calculationally, these features depend on the methodology chosen for dealing with the correlated nature of the transition-metal d-derived states in these systems. Comparison with experiment reveals that a hybrid functional approach is superior to GGA+U in correctly reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot support small hole polarons, but that the other three compounds can. The migration barrier is determined mainly by the strong or weak bonding nature of the states at the top of the valence band, resulting in a substantially higher barrier for LiMnPO4 than for LiCoPO4 or LiFePO4.
  11/2011;
• Article: The particle-size dependence of the activation energy for decomposition of lithium amide.

  Khang Hoang, Anderson Janotti, Chris G Van de Walle
  Angewandte Chemie International Edition 07/2011; 50(43):10170-3. · 13.45 Impact Factor
• Source

  Available from: ArXiv

  Article: Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations

  Khang Hoang, Michelle Johannes
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  ABSTRACT: We report first-principles density-functional theory studies of native point defects and defect complexes in olivine-type LiFePO4, a promising candidate for rechargeable Li-ion battery electrodes. The defects are characterized by their formation energies which are calculated within the GGA+U framework. We find that native point defects are charged, and each defect is stable in one charge state only. Removing electrons from the stable defects always generates defect complexes containing small hole polarons. Defect formation energies, hence concentrations, and defect energy landscapes are all sensitive to the choice of atomic chemical potentials which represent experimental conditions. One can, therefore, suppress or enhance certain native defects in LiFePO4 via tuning the synthesis conditions. Based on our results, we provide insights on how to obtain samples in experiments with tailored defect concentrations for targeted applications. We also discuss the mechanisms for ionic and electronic conduction in LiFePO4 and suggest strategies for enhancing the electrical conductivity.
  05/2011;
• Source

  Available from: Anderson Janotti

  Article: Formation and migration of native defects in NaAlH$_4$

  Gareth B. Wilson-Short, Anderson Janotti, Khang Hoang, Amra Peles, Chris G. Van de Walle
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  ABSTRACT: We present a first-principles study of native defects in NaAlH$_4$. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include $V_{\rm{AlH_4}}^+$, $V_{\rm{Na}}^-$, $V_{\rm{H}}^+$, H$_i^-$, and (H$_2$)$_i$. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for diffusion of defects and defect pairs. We suggest that $V_{\rm{AlH_4}}^+$ and H$_i^-$, or $V_{\rm{Na}}^-$ and $V_{\rm{H}}^+$, may be responsible for diffusion necessary for desorption. We discuss the possible role of $V_{\rm{H}}^+$-H$_i^-$ complex formation. The values we find are in the range of activation energies reported for catalyzed desorption. Comment: 26 pages, 8 figures, 1 table; to appear in Phys. Rev. B 80 (2009)
  11/2009;
• Source

  Available from: Subhendra D Mahanti

  Article: Impurity clustering and impurity-induced bands in PbTe-, SnTe-, and GeTe-based bulk thermoelectrics

  Khang Hoang, S D Mahanti, Mercouri G Kanatzidis
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  ABSTRACT: Complex multicomponent systems based on PbTe, SnTe, and GeTe are of great interest for infrared devices and high-temperature thermoelectric applications. A deeper understanding of the atomic and electronic structure of these materials is crucial for explaining, predicting, and optimizing their properties, and to suggest new materials for better performance. In this work, we present our first-principles studies of the energy bands associated with various monovalent (Na, K, and Ag) and trivalent (Sb and Bi) impurities and impurity clusters in PbTe, SnTe, and GeTe using supercell models. We find that monovalent and trivalent impurity atoms tend to come close to one another and form impurity-rich clusters, and the electronic structure of the host materials is strongly perturbed by the impurities. There are impurity-induced bands associated with the trivalent impurities that split off from the conduction-band bottom with large shifts towards the valence-band top. This is due to the interaction between the $p$ states of the trivalent impurity cation and the divalent anion which tends to drive the systems towards metallicity. The introduction of monovalent impurities (in the presence of trivalent impurities) significantly reduces (in PbTe and GeTe) or slightly enhances (in SnTe) the effect of the trivalent impurities. One, therefore, can tailor the band gap and band structure near the band gap (hence transport properties) by choosing the type of impurity and its concentration or tuning the monovalent/trivalent ratio. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the whole class of PbTe-, SnTe-, and GeTe-based bulk thermoelectrics. Comment: 37 pages, 13 figures, 2 tables; added new figures and paragraphs; to appear in Phys. Rev. B
  11/2009;
• Article: First-principles study of the formation and migration of native defects in NaAlH4

  Gareth B. Wilson-Short, Anderson Janotti, Khang Hoang, Amra Peles, Chris G. Van de Walle
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  ABSTRACT: We present a first-principles study of native defects in NaAlH4. Our analysis indicates that the structure and energetics of these defects can be interpreted in terms of elementary building blocks, which include VAlH4+, VNa−, VH+, Hi−, and (H2)i. We also calculate migration barriers for several key defects, in order to compare enthalpies of diffusion to experimentally measured activation energies of desorption. From this, we estimate activation energies for the diffusion of defects and defect pairs. We suggest that VAlH4+ and Hi−, or VNa− and VH+, may be responsible for diffusion necessary for desorption. We discuss the possible role of VH+-Hi− complex formation. The values we find are in the range of activation energies reported for catalyzed desorption.
  Physical Review B 11/2009; 80(22):224102. · 3.69 Impact Factor
• Article: Role of K/Bi disorder in the electronic structure of β-K2Bi8Se13

  Khang Hoang, Aleksandra Tomic, S. D. Mahanti, Theodora Kyratsi, Duck-Young Chung, S. H. Tessmer, Mercouri G. Kanatzidis
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  ABSTRACT: We have carried out tunneling spectroscopy and first-principles studies for β-K2Bi8Se13, a promising thermoelec. material with partially disordered mixed K/Bi sites. The tunneling data, obtained with a scanning tunneling microscope (STM), show that the system is a semiconductor with a band gap of ∼0.4 eV and band-tail states near the valence-band top and the conduction-band bottom. First-principles calcns., on the other hand, show that β-K2Bi8Se13 can be semimetallic or semiconducting depending on the arrangements of the K and Bi atoms in the mixed sites. The electronic structure of β-K2Bi8Se13 near the band-gap region is largely detd. by unbonded Se p states and states assocd. with strained bonds which are present due to K/Bi disorder and by the Bi p-Se p hybridization which tends to drive the system toward metallicity. Among the different K/Bi arrangements investigated, we have identified a structural model (quasidisordered structure) that is able to satisfactorily reproduce the at. and electronic structures of β-K2Bi8Se13; i.e., the local compn. in the mixed channels as obsd. exptl. and the band gap and tails as seen in the STM measurements. Transport properties of β-K2Bi8Se13 can be qual. understood in terms of the electronic structure obtained in calcns. using the above structural model. [on SciFinder(R)]
  Phys. Rev. B: Condens. Matter Mater. Phys. 01/2009; 80(Copyright (C) 2013 American Chemical Society (ACS). All Rights Reserved.):125112/1-125112/8.
• Article: First-principles study of the electronic, optical, and lattice vibrational properties of AgSbTe_ {2}

  Lin-Hui Ye, Khang Hoang, A. J. Freeman, S. D. Mahanti, Jian He, Terry M. Tritt, M. G. Kanatzidis
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  ABSTRACT: First-principles calculations of the electronic, optical, and lattice vibrational properties of AgSbTe2 were performed with the generalized gradient approximation (GGA) and the screened-exchange local-density approximation (sx-LDA) method, which successfully corrects the band-gap problem found with GGA. We find a vanishing density of states at the Fermi level, which is consistent with the semiconducting behavior of AgSbTe2. Various optical properties, including the dielectric function, absorption coefficient, and refractive index, as functions of the photon energy are also calculated with the sx-LDA and are found to be in good agreement with experiments. Phonon spectra obtained by the cumulant force constant method show that the optic modes of AgSbTe2 are very low in frequency and should scatter strongly with acoustic modes during heat transport. The calculated specific-heat curve is in general agreement with experiment. Ag/Sb disorder is expected to have a small effect on the electrical transport but may introduce strong phonon scattering due to the large force constant disorder. The scattering of acoustic phonons by optic modes and the possible Ag/Sb disorder may explain the extremely low lattice thermal conductivity of AgSbTe2.
  Phys. Rev. B. 06/2008; 77(24).
• Source

  Available from: Subhendra D Mahanti

  Article: Substitution of Bi for Sb and its Role in the Thermoelectric Properties and Nanostructuring in Ag1−xPb18MTe20 (M = Bi, Sb) (x = 0, 0.14, 0.3)

  Mi-Kyung Han, Khang Hoang, Huijun Kong, Robert Pcionek, Ctirad Uher, Konstantinos M. Paraskevopoulos, S. D. Mahanti, Mercouri G. Kanatzidis
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  ABSTRACT: We have performed a comparative investigation of the Ag1−xPb18MTe20 (M = Bi, Sb) (x = 0, 0.14, 0.3) system to assess the roles of Sb and Bi on the thermoelectric properties. Detailed charge transport data including electrical conductivity, the Seebeck coefficient, the Hall coefficient, and thermal conductivity are presented. Optical reflectivity data support the conclusions of the transport studies. For comparable nominal compositions, the carrier concentrations are lower in the Sb analogs and the mobilities are higher. The Seebeck coefficient decreases dramatically in going from Sb to Bi. High resolution transmission electron microscopy (TEM) images of both samples reveal that all systems contain compositional fluctuations at the nanoscopic level and are nanostructured. Compared to PbTe, the lattice thermal conductivity of AgPb18BiTe20 is substantially reduced. The lattice thermal conductivity of the Bi analog is, however, higher than the Sb analog, and this correlates with the decrease in the degree of mass fluctuation between the nanostructures and the matrix (for the Bi analog). As a result the dimensionless figure of merit ZT of Ag1−xPb18BiTe20 is found to be substantially smaller than that of Ag1−xPb18SbTe20. Electronic structure calculations performed within the density functional theory and generalized gradient approximation show marked differences in the band structure near the Fermi level between the two analogs providing useful insights on the carrier transport in these systems.
  04/2008;
• Article: Atomic and electronic structures of thallium-based III-V-VI2 ternary chalcogenides: Ab initio calculations

  Khang Hoang, S. D. Mahanti
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  ABSTRACT: The atomic and electronic structures of III-V-VI2 ternary chalcogenides ( III=Tl , V=Sb and Bi, and VI=Te , Se, and S) have been studied using ab initio electronic structure calculations. Most of these compounds are found to take rhombohedral structures as their lowest energy structures (except for TlSbS2 , which takes a triclinic structure), in agreement with experiments. There is a disagreement between theory and experiment in the case of TlSbSe2 , wherein our calculations identify a rhombohedral structure (as yet hypothetical), which has lower energy than a monoclinic one (given by experiment). Band-gap formation in these ternaries are controlled by a highly directional hybridization between the cation (Sb, Bi), anion (S, Se, Te), and Tlp states, and the electronic structure near the Fermi level is sensitive to the ordering on the cation sublattice.
  Physical Review B - PHYS REV B. 01/2008; 77(20).
• Source

  Available from: Subhendra D Mahanti

  Article: Atomic ordering and gap formation in Ag-Sb-based ternary chalcogenides.

  Khang Hoang, S D Mahanti, James R Salvador, Mercouri G Kanatzidis
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  ABSTRACT: Novel semiconductors with tailored properties can be designed theoretically based on our understanding of the interplay of atomic and electronic structures and the nature of the electronic states near the band-gap region. We discuss here the realization of this idea in Ag-Sb-based ternary chalcogenides, which are important optical phase change and thermoelectric materials. Based on our studies we propose new systems for high-performance thermoelectrics.
  Physical Review Letters 11/2007; 99(15):156403. · 7.37 Impact Factor
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  Available from: ucsb.edu

  Article: REAu2In4 (RE = La, Ce, Pr, Nd): polyindides from liquid indium.

  James R Salvador, Khang Hoang, S D Mahanti, Mercouri G Kanatzidis
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  ABSTRACT: The series of compounds REAu2In4 (RE = La, Ce, Pr, Nd) crystallize from excess In as rod-shaped single crystals. All members adopt the orthorhombic space group Pnma with a = 18.506(2) A, b = 4.6865(6) A, and c = 7.3414(9) A for LaAu2In4, a = 18.514(3) A, b = 4.6624(8) A, and c = 7.389(1) A for CeAu2In4, a = 18.420(4) A, b = 4.6202(9) A, and c = 7.376(2) A for the Pr analogue, and a = 18.406(2) A, b = 4.6114(5) A, and c = 7.4073(7) A for NdAu2In4. The REAu2In4 series can be regarded as polar intermetallic phases composed of a complex [Au2In4]3- polyanion network in which the rare-earth ions are embedded. The [Au2In4]3- network features In tetramer units, which defines the compounds as polyindides. Magnetic measurements found no magnetic ordering down to 2 K for any of the compounds. In addition, LaAu2In4 was found to be Pauli paramagnetic with a small susceptibility. Ab initio density functional methods were used to carry out electronic structure calculations to explore the bonding, the role of gold, and the contributions of different atoms to the density of states near the Fermi energy. We find that the density of states decreases slowly near Ef and reaches a minimum at about 0.5 eV above Ef.
  Inorganic Chemistry 09/2007; 46(17):6933-41. · 4.60 Impact Factor
• Article: REAu 2 In 4 (RE = La, Ce, Pr, Nd): Polyindides from Liquid Indium

  James R. Salvador, Khang Hoang, S. D. Mahanti, Mercouri G. Kanatzidis
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  ABSTRACT: 3- network features In tetramer units, which defines the compounds as polyindides. Magnetic measurements found no magnetic ordering down t o2Kf or any of the compounds. In addition, LaAu 2In4 was found to be Pauli paramagnetic with a small susceptibility. Ab initio density functional methods were used to carry out electronic structure calculations to explore the bonding, the role of gold, and the contributions of different atoms to the density of states near the Fermi energy. We find that the density of states decreases slowly near E f and reaches a minimum at about 0.5 eV above Ef.
  Inorganic Chemistry - INORG CHEM. 01/2007; 46(17):6933-6941.
• Article: Theoretical study of the deep defect states in PbTe thin films

  Khang Hoang, S. D. Mahanti, Puru Jena
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  ABSTRACT: The nature of deep defect states (DDS) in bulk PbTe has been studied recently using density functional theory and a supercell model [1]. It was found that substitution of Pb by the trivalent impurities Ga, In, and Tl gave rise to hyperdeep defect states (HDS) below the valence band (VB) and DDS near the band gap region. Here we discuss how these states are affected in a (100) PbTe film using a supercell slab model. The HDS and DDS are preserved in the film geometry. As one goes from the bulk-like layers to subsurface and surface layers, the HDS tends to move closer to the bottom of the VB and becomes narrower; the DDS also gets modified. We also find that the defect formation energy Ef as a function of the distance from the surface shows interesting features: all three impurities have lowest Ef in the first layer but Ef increases monotonically in the case of Ga, whereas there is a potential barrier in the second layer and a shallow potential ``valley'' between the second and the bulk-like layers in the case of In and Tl. This suggests that Ga impurities will be annealed out whereas the other two can be trapped in the subsurface region. [1] Salameh Ahmad, Khang Hoang, and S. D. Mahanti, Phys. Rev. Lett. 96, 056403 (2006).
  Physical Review B - PHYS REV B. 01/2007; 76(11).
• Article: Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems

  S. D. Mahanti, Khang Hoang, Salameh Ahmad
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  ABSTRACT: In recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κel,E) and for constant current (κel,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κel,J = LoσT, where Lo =2x10−8WΩ/K is the Lorentz number, overstimates κel,J by nearly 60% at 800K for carrier concentration n=5x1019/cm3. As a result, one tends to underestimate the lattice contribution κlatt = κexp - κel,J. We give theoretical values of effective Lorentz number L = κel.J/σT for different n and T.
  MRS Proceedings. 12/2006; 1044.