SPECIFIC-HEAT OF NBHX, WITH HIGH HYDROGEN CONCENTRATION

The specific heat of a NbHx solid interstitial solution was measured i n the temperature range 80-420 K for hydrogen concentrations 0.73 < x < 0.94. A series of first-order phase transitions was observed between alpha', beta and different types of low-temperature phases, and the p hase diagram was established. Jumps of the reduced entropy per hydroge n atom, Delta s/x, were calculated for each transition as a function o f the hydrogen concentration and analyzed in terms of configurational degrees of freedom. An abnormal lowering of the specific heat in the b eta phase with increasing hydrogen content was revealed with a small a nomaly at x approximate to 0.87.