Anisotropic thermal expansion and anharmonic phonon behavior of mullite-type Bi2Ga4O9

• Anisotropic lattice thermal expansion of Bi2Ga4O9 was modeled using extended Grüneisen first-order approximation.
• The model includes harmonic, quasi-harmonic and intrinsic anharmonic contributions to the internal energy.
• Temperature dependent Raman frequency shift and line-width was analyzed using symmetric phonon decay channel.

We report the lattice thermal expansion and the temperature-dependent phonon behavior of the mullite-type Bi2Ga4O9 complex oxide. The thermal expansion was studied using composite data collected from powder and single crystal X-ray diffraction between 100 K and 1273 K. The lattice expansion occurred in the order of a < c < b. The anisotropic expansion behavior was monitored with respect to thermal expansion coefficients and the anisotropy factor. The volume thermal expansion was expressed using an extended Grüneisen first-order approximation to the zero-pressure equation of state; the model includes harmonic, quasi-harmonic and intrinsic anharmonic contributions to the internal energy as a function of temperature. The temperature dependent Raman spectra were collected from a single crystal between 78 K and 1273 K. The shift of the frequencies and the broadening of the line-widths with increasing temperature helped to analyze the anharmonicity and the thermal behavior of some phonons.

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