Thermodynamic and dielectric properties of hexagonal barium titanate near the phase transitions

The critical behavior of the thermodynamic quantities is studied for the two phase transitions near the transition temperatures (74 K and 222 K) in the hexagonal (h) BaTiO 3 . A linear relationship has been established between the variations of the frequency shifts and of the dielectric constant by using the literature data in h-BaTiO 3 . Temperature and pressure dependence of the mode Grüneisen parameters ( γ p and γ T ) are also described by a power-law formula as the other thermodynamic parameters near T pt in h-BaTiO 3 . Our results show that they explain the observed behavior adequately, in particular soft-mode frequency, dielectric constant, thermal expansion, the excess specific heat and the entropy near T pt in h-BaTiO 3 . Temperature dependence of the isobaric mode Grüneisen parameter ( γ p ), enthalpy and the Gibbs free energy, which we have evaluated, can be compared with experiments in h-BaTiO 3 .