Measurement of the molar heat capacities of MoO2 and MoO3 from 350 to 950 K

Molar heat capacities of MoO2 and MoO3 were measured in the range between 350 and 950 K by means of adiabatic scanning calorimetry. For MoO2, a sharp heat-capacity anomaly with a molar enthalpy change of (178 +- 24) J.mol-1 and a molar entropy change of (0.207 +- 0.028) J.K-1.mol-1 was observed at 865 K, which had not been detected by drop calorimetry. For MoO3, two heat-capacity anomalies with molar enthalpy changes of (88 +- 21) and (60 +- 36) J.mol-1 were found at 808 K and 857 K, respectively; neither anomaly had been detected by the drop method. The lattice molar heat capacities of MoO2 and MoO3 are estimated as Csub(l,m)(MoO2) = D(469 K/T) + E(578 K/T) + E(876 K/T) and Csub(l,m)(MoO3) = D(208 K/T) + 2E(488 K/T) + E(1170 K/T), where D(x) and E(x) are the Debye and Einstein functions, respectively. The temperature coefficient of the electronic molar heat capacity of MoO2 is estimated as (6.0 +- 0.5) mJ.K-2.mol-1. The excess heat capacity in MoO3 found at higher temperatures is interpreted as being due to vacancy formation with a molar activation energy of (98 +-5) kJ.mol-1. The origin of the heat-capacity anomalies is inferred as arising from the slight movement of distorted MoO6 octahedra in the MoO2 and MoO3 structures. (author)