Bulk samples of hafnium (Hf) hydride and deuteride were prepared and the thermal properties, heat capacity (CP) and thermal conductivity (κ) were measured. In the CP-temperature curves for both samples, typical lambda-type peaks were observed at around 350 K, which was due to the second-order phase transition from the δ′-phase to the δ-phase. In Hf hydride, it is considered that the δ′-phase and the δ-phase consist of regularly arranged and randomly arranged hydrogen atoms, respectively. Therefore, it can be said that the δ′/δ phase transition observed in both Hf hydride and deuteride is an order-disorder phase transition. The values of κ as well as CP changed significantly at around the phase transition temperature.
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