Study on order-disorder transition of HfO alloys (O/Hf = 0.11 - 0.22) by heat capacity measurement

Abstract

The heat capacities of hafnium-oxygen solid solutions, HfO X ( X = 0.11, 0.19 and 0.22), were measured from 325 to 905 K by using an adiabatic scanning calorimeter. Entropy changes due to the order-disorder transition of the oxygen sublattice were estimated from the heat capacity data in this study and were recomputed for HfO X ( X = 0.14 and 0.17) whose heat capacities had been measured in our previous study. The experimental transition entropy change obtained for the composition of X = 0.17 (≈ 1/6) was in good agreement with the theoretical transition entropy change calculated on the basis of the crystal analyses using statistical thermodynamics, while the experimental values for the other compositions (X ≠ 0.17) were smaller than the theoretical values. The difference between the experimental and theoretical values was considered to be caused by the increasing degree of configurational freedom in the ordered structures due to the increments of oxygen vacancies for X < 1/6 or the increments of interstitial oxygen atoms for X > 1/6.