Heat capacity anomalies due to the cooperative Jahn-Teller effect in Cu 1− xNi xCr 2O 4

Abstract

Heat capacities of Cu 1− x Ni x Cr 2O 4 were measured for various x values, x = 0.0, 0.2, 0.5, 0.7, 0.85, and 1.0, in the temperature range 220 to 960 K. The heat capacity anomalies due to the cubic-tetragonal transition resulting from the cooperative Jahn-Teller effect were observed in all the samples and that due to the tetragonal-orthorhombic transition was observed in Cu 0.15Ni 0.85Cr 2O 4. The excess heat capacities due to the cubic-tetragonal transition were obtained from the cooperative part of the heat capacity curves, of which shapes were similar as those obtained from the theoretical calculation proposed by Kataoka and Kanamori. The enthalpy and entropy changes due to the cubic-tetragonal transition were obtained as 6.38, 5.47, 2.34, 1.21, and 0.89 kJ mol −1 and 8.69, 7.99, 4.29, 2.86, and 2.59 J K −1 mol −1 for Cu 1− x Ni x Cr 2O 4 with x values of 0.0, 0.2, 0.5, 0.7, and 0.85, respectively, which are roughly in agreement with the theoretical calculation by Kataoka and Kanamori.