Measurement of the molar heat capacities of MoO 2 and MoO 3 from 350 to 950 K

Abstract

Molar heat capacities of MoO 2 and MoO 3 were measured in the range between 350 and 950 K by means of adiabatic scanning calorimetry. For MoO 2, 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 MoO 3, 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 MoO 2 and MoO 3 are estimated as C l,m(MoO 2) = D(469 K T ) + E(578 K T ) + E(876 K T ) and C l,m(MoO 3) = 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 MoO 2 is estimated as (6.0 ± 0.5) mJ · K −2 · mol −1. The excess heat capacity in MoO 3 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 MoO 6 octahedra in the MoO 2 and MoO 3 structures.