Heat capacity measurement of VO 1± x and niobium doped VO 1+ x

Abstract

The heat capacities of VO 1± x , (O/V = 0.883 and 1.063 (δ-phase) and 1.267 (ϵ-phase) were measured in the temperature range of 324–920 K with an adiabatic scanning calorimeter. A steep decrease in the heat capacity near room temperature and a subsequent anomalous peak were observed for each sample in the low temperature range below 600 K. At high temperature, around 900 K, an anomalous peak was seen in V0 0.883 and VO 1.063. The high temperature heat capacity peak was thought to be due to the order-disorder rearrangement of clusters of oxygen and vanadium vacancies and/or clusters of interstitial vanadium and vanadium vacancies. The low temperature heat capacity anomaly was interpreted by atomic migration to induce an increase in the degree of order of the “frozen in” state at some higher temperature. The heat capacities of VO 1+ x doped with 1.6 and 30 at% of Nb were also measured in the same temperature range, and the results supported the presence of the similar order-disorder transition to undoped VO 1± x at high temperature. The enthalpy and entropy changes at the high temperature transition were calculated from the excess heat capacity. The entropy change at the transition obtained experimentally was compared with that calculated from the configurational change of the two types of defect clusters mentioned above. The electrical conductivities of VO 1.063 and VO 1.267 were measured in the temperature range 320–1200 K. The δ-VO 1± x phase (VO 1.063) was observed to be semiconducting and the slope in the conductivity curve changed around the peak temperatures of low and high heat capacity anomalies. The ϵ-VO 1+ x phase (VO 1.267) was a metallic conductor and the slope in the conductivity of VO 1.267 changed slightly near the peak temperature of the low temperature heat capacity anomaly.