INCOHERENT PHASE-TRANSITIONS OF HYDROGEN AND DEUTERIUM IN NIOBIUM

The incoherent phase transitions of H and D in Nb have been studied by X-ray scattering. For all investigations Nb was loaded with hydrogen in situ in order to prevent precipitation of the new phase, before starting the experiment, which irreversibly changes the crystal lattice and gives misleading results. The incoherent phase boundary was determined from the variation of the lattice parameter with hydrogen concentration. The lattice parameter varies linearly with concentration at least up to 0.34 H/Nb. The thermal expansion coefficient increases with hydrogen concentration and shows an anomalous behaviour just above the alpha - alpha ' phase transition. For the alpha - alpha ' transition of H and D in Nb the critical temperature is Tc=171+or-1 degrees C and the critical concentration cc=0.31+or-0.01 H/Nb. The triple point temperature of the alpha + beta to alpha + alpha ' transition is 88 degrees C for H and 99 degrees C for D in Nb.