Heat Capacity of La_1-XSr_XCoO_3-δ

Abstract

  Isobaric heat capacities, C_p, of La_0.207Co_0.207O_0.586, La_0.179Sr_0.032Co_0.210O_0.579 and La_0.109Sr_0.109Co_0.218O_0.564 from near absolute zero Kelvin (2 K) to high temperature (1340 K) were measured by a relaxation method and differential scanning calorimetry. In the C_p values for La_0.207Co_0.207O_0.586, two peaks at 628 K and 788 K were observed, consistent with the phase transition of from semi-conductor to metal and the phase transition at which magnetic moment increased as a function of temperature, respectively. Also, in the C_p values for La_0.179Sr_0.032Co_0.210O_0.579, such two-kinds of the phase transitions, that is the phase transition of semi-conductor to metal and the phase transition increasing the magnetic moment, were observed in the lower temperature ranges than in La_0.207Co_0.207O_0.586. The phase transition from ferromagnetic- to paramagnetic-metal was caused at 239 K in La_0.109Sr_0.109Co_0.218O_0.564, while such two kinds of phase transitions observed in La_0.207Co_0.207O_0.586 and La_0.179Sr_0.032Co_0.210O_0.579 were disappeared. The Debye temperature, Θ_D, of La_0.207Co_0.207O_0.586 was determined from the measured C_p values below 10 K. Its isochoric heat capacity, C_V, was calculated by inserting Θ_D into the Debye function, and the isochoric entropies of vibration, Δ₀^TS_V, was determined, followed by calculating the entropy of vibration of oxygen vacancy, Δ_fS_T^vib._V(Vö), by differentiation of Δ₀^TS_V with the Vö content. The Δ_fS_T^vib._V(Vö) obtained at 298 K and 800 K were 42.86 J·K^-1·mol^-1 and 44.00 J·K^-1·mol^-1, respectively. Such a large Δ_fS_T^vib._V(Vö) appears to be one of important reason for foming Vö.