Low temperature susceptibility and phase diagrams of the Nb-H and Ta-H systems

Abstract

The low temperature Nb-H phase diagram was evaluated by measuring the static magnetic susceptibility χ ( T, x) in the temperature range 4.2 K ⩽ T ⩽ 270 K and the hydrogen concentration range 0 < x = [ H]/[ Nb] < 1.1. χ( T, x) depends weakly on the temperature T but decreases rapidly with the hydrogen concentration x. Phase transitions manifest themselves as susceptibility anomalies, determined either at constant concentration or at constant temperature, and appear as discontinuities, peaks or kinks. These features aid in the construction of the phase boundaries in the x- T plane. The low temperature phase diagram determined by the susceptibility method is divided into two parts: for x < 0.7 the phase boundaries are nearly horizontal, whereas for x > 0.7 they are vertical. Low temperature phase transitions, which manifest themselves in the χ ( T = 0 K, x) dependence, have been established for x values of 0.46, 0.73, 0.78, 0.83, 0.85, about 0.9 and about 1.0. Intermediate temperature phases occur in the range 0.15 < x < 0.65 and at x = 0.80 and x = 0.82. Examination of the Ta-H phase diagram suggested that the susceptibility measurements made previously in this system should be reevaluated. This led to a modification of the phase boundaries of the ζ phases at x ≈ 0.59 and x = 0.72. The revised diagram was confirmed by a new set of measurements.