Heat capacities, third-law entropies and thermodynamic functions of the negative thermal expansion materials, cubic α-ZrW2O8 and cubic ZrMo2O8, from T=(0 to 400) K

The molar heat capacities of crystalline cubic α-ZrW2O8 and cubic ZrMo2O8 have been measured at temperatures from (0.6 to 400) K. At T=298.15 K, the standard molar heat capacities are (207.01±0.21) J·K-1·mol-1 for the tungstate and (210.06±0.42) J·K-1·mol-1 for the molybdate. Thermodynamic functions have been generated from smoothed fits of the experimental results. The standard molar entropies for the tungstate and molybdate are (257.96±0.50) J·K-1·mol-1 and (254.3±1) J·K-1·mol-1, respectively. The uncertainty of the entropy of the cubic ZrMo2O8 is larger due to the presence of small chemical and phase impurities whose effects cannot be corrected for at this time. The heat capacities of the negative thermal expansion materials have been compared to the weighted sums of their constituent binary oxides. Both negative thermal expansion materials have heat capacities which are significantly greater than the sum of the binary oxides over the entire temperature region