CALORIMETRIC STUDY OF PHASE-TRANSFORMATION IN TANTALUM HYDRIDES

Interstitial order-disorder transformations in tantalum hydrides up to the composition TaH//0//. //7//5 were studied by heat capacity measurements between minus 100 and plus 100 degree C. It is shown that the beta //1 phase with an ordered structure in the range TaH//0//. //4//5 to TaH//0//. //7//5 transforms to the disordered alpha phase stepwise through the beta //2 phase near Ta//2H and through the alpha plus delta two-phase region in hydrogen-rich compositions, and directly in intermediate compositions. The phase structures are given except for beta //2. On the basis of the results, a revised phase diagram is presented.