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Download fileStructural and Thermal Properties of BaTe2O6: Combined Variable-Temperature Synchrotron X‑ray Diffraction, Raman Spectroscopy, and ab Initio Calculations
journal contribution
posted on 2016-08-05, 19:52 authored by Karuna Kara Mishra, S. Nagabhusan Achary, Sharat Chandra, T. R. Ravindran, Anil K. Sinha, Manavendra
N. Singh, Avesh K. TyagiVariable-temperature Raman spectroscopic
and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to
understand structural stability and anharmonicity of phonons. The
structural and vibrational studies indicate no phase transition in
it over a wider range of temperature (20 to 853 K). The structure
shows anisotropic expansion with coefficients of thermal expansion
in the order αb ≫ αa >
αc, which was attributed to the anisotropy in bonding
and structure of BaTe2O6. Temperature evolution
of Raman modes of BaTe2O6 indicated a smooth
decreasing trend in mode frequencies with increasing temperature,
while the full width at half-maximum (fwhm) of all modes systematically
increases due to a rise in phonon scattering processes. With the use
of our earlier reported isothermal mode Grüneisen parameters,
thermal properties such as thermal expansion coefficient and molar
specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic
(implicit) contribution to the total anharmonicity is delineated and
compared. The temperature dependence of phonon mode frequencies and
their fwhm values are analyzed by anharmonicity models, and the dominating
anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external
modes of TeOn (n = 5,
6) are found to be strongly anharmonic. The ab initio electronic structure
calculations indicated BaTe2O6 is a direct band
gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals,
namely, O-2p states in the valence band maximum and the sp-hybridized
states in the conduction band minimum, are mainly involved in the
electronic transitions. In addition a number of electronic transitions
are predicted by the electronic structure calculations. Experimental
photoluminescence results are adequately explained by the ab initio
calculations. Further details of the structural and vibrational properties
are explained in the manuscript.
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Keywords
Experimental photoluminescence resultsanharmonicstructure calculationsmixed-valence tellurium compoundtransitionsynchrotron X-ray diffraction studiesab initio calculationsBaTe 2 O 6band gap semiconductorphonon mode frequenciesO -2p statesanharmonicityab Initio Calculations Variable-temperature Raman spectroscopic