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

Abstract We report the lattice thermal expansion and the temperature-dependent phonon behavior of the mullite-type Bi 2 Ga 4 O 9 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 Gruneisen 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.