Temperature-tuned bandgap characteristics of Bi₁₂TiO₂₀ sillenite single crystals

Abstract

Bi12MO20 (M: Si, Ge, Ti, etc.) compounds are known as sillenites having fascinating photorefractive characteristics. The present paper reports the structural and optical characteristics of one of the members of this family, Bi12TiO20 single crystals, grown by Czochralski method. X-ray diffraction pattern of the crystal presented sharp and intensive peaks associated with planes of cubic crystalline structure with lattice constant of a = 1.0142 nm. The optical properties were studied by means of room temperature Raman and temperature-dependent transmission experiments at various temperatures between 10 and 300 K. Raman spectrum indicated peaks around 127, 162, 191, 219, 261, 289, 321, 497 and 537 cm(-1). The analyses of transmittance spectra indicated the increase of direct bandgap energy from 2.30 to 2.56 eV as temperature was decreased from room temperature to 10 K. The temperature-dependent bandgap characteristics of Bi12TiO20 were analyzed by means of Varshni and O'Donnell-Chen models. The analyses under the light of these models resulted in absolute zero bandgap energy of E-g(0) = 2.56(4) eV, rate of change of bandgap energy of gamma = - 1.11 x 10(-3) eV/K and average phonon energy of < E-ph & rang; = 8.6 meV.