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As represented by hydrogen storage in metal hydride, research on the diffusion processes of hydrogen in metals are achieving remarkable development. We have studied the diffusion rate of hydrogen in V-a – merals with relatively low concentration region [1-4]. In order to clarify the diffusion process in phase transition, calorimetric studies are indispensable for thermo-dynamical analysis. Our main concerns are the basic thermodynamic properties of the Nb-H system in the range of high concentration of hydrogen. T. Plackowski, et.al. showed that NbHx with high concentration (x ~ 0.8) has an interesting specific heat behavior during ’phase transition [5]. Fig. 1 shows the phase diagram of the Nb-H system [6]. Several structural phases exist at different temperatures in the concentration region. The high-temperature ’ phase, existing above T ~ 420 K for x > 0.3, is an disorder state in which hydrogen atoms are randomly distributed over tetrahedral interstitial sites (TIS) of the metal lattice. The lattice parameter a of the ’ phase was found to vary linearly with H concentration. The  phase, existing for hydrogen concentrations 0.7 < x < 1.1, below T ~ 420 K, is an ordered interstitial solid solution of the face centered orthorhombic (FCO). The hydrogen atoms are located here at the specified TIS in the (110) planes of the basic metal lattice The ’  transition is accompanied by a doubling of the unit cell, which results in the orthorhombic lattice parameters being equal to a ≈ b ≈ 2 × a0 and c

Heat capacity measurements of NbH alloys and an analysis of phase transition process