First-principle calculations of the thermal properties of SrTiO₃ and SrO(SrTiO₃)_n (n=1,2)

The thermal properties of SrTiO₃ and SrO(SrTiO₃)_n (n=1,2) with layered perovskite structure are analyzed using the Debye-Grüneisen model combined with ab initio calculations. The thermal expansion coefficient, specific heat at constant pressure C_P and specific heat at constant volume C_V, adiabatic bulk modulus B_S and isothermal bulk modulus B_T, entropy, and Debye temperature are investigated. At temperatures higher than 550°C, the thermal expansion coefficient and the discrepancies between C_P and C_V, as well as that between B_S and B_T, of Sr₃Ti₂O₇ increase the fastest as the temperature rises, followed by those of Sr₂TiO₄, and those of SrTiO₃ increase the slowest. The bulk module and Debye temperature of Sr₂TiO₄, Sr₃Ti₂O₇, and SrTiO₃ increase with decreasing SrO/SrTiO₃ ratio at 0 K. With increasing temperature, however, the bulk modulus and Debye temperature of Sr₃Ti₂O₇ both rapidly decrease and even fall below those of Sr₂TiO₄ when the temperature is higher than specific values. We also analyzed the thermal properties of these three compounds in the pressure range from 0 GPa to 16 GPa at 300 K.