Heat capacities, third-law entropies and thermodynamic functions of the geometrically frustrated antiferromagnetic spinels GeCo2O4 and GeNi2O4 from T = (0 to 400) K

The molar heat capacities of GeCo2O4 and GeNi2O4, two geometrically frustrated spinels, have been measured in the temperature range from T = (0.5 to 400) K. Anomalies associated with magnetic ordering occur in the heat capacities of both compounds. The transition in GeCo2O4 occurs at T = 20.6 K while two peaks are found in the heat capacity of GeNi2O4, both within the narrow temperature range between 11.4 < (T/K) < 12.2. Thermodynamic functions have been generated from smoothed fits of the experimental results. At T = 298.15 K the standard molar heat capacities are (143.44 +/- 0.14) J (.) K-1 (.) mol(-1) for GeCo2O4 and (130.76 +/- 0.13) J (.) K-1 (.) mol(-1) for GeNi2O4. The standard molar entropies at T = 298.15 K for GeCO2O4 and GeNi2O4 are (149.20 +/- 0.60) J (.) K-1 (.) mol(-1) and (131.80 +/- 0.53) J (.) K-1 (.) mol(-1) respectively. Above 100 K, the heat capacity of the cobalt compound is significantly higher than that of the nickel compound. The excess heat capacity can be reasonably modeled by the assumption of a Schottky contribution arising from the thermal excitation of electronic states associated with the CO2+ ion in a cubic crystal field. The splittings obtained, 230 cm(-1) for the four-fold-degenerate first excited state and 610 cm(-1) for the six-fold degenerate second excited state, are significantly lower than those observed in pure CoO. (C) 2004 Elsevier Ltd. All rights reserved.