Phase transformations in the solid solutions ZrV2Hx at high hydrogen concentrations (4

Abstract

We have studied, by means of neutron powder diffraction, the phase transformations in the hydrogen solid solutions ZrV2Hx in the temperature range of 1.4–310K and at the hydrogen concentration of 4<x<5. In the concentration range of 4<x<4.4, near 290K, we have found an order–disorder phase transition resulting in the formation of a hydrogen-ordered phase with the propagation vector k=0. This phase is a derivative of the known superstructure ZrV2H4. When the hydrogen concentration increases, x>4, the superstructure ZrV2H4 transforms in such a way, that hydrogen atoms become more closely packed. As a result, in ZrV2Hx>4 the hydrogen atoms are ordered in the interstices of two types, 2Zr+2V and 1Zr+3V, whereas in ZrV2H4 they are ordered only in the interstices 2Zr+2V. When the hydrogen concentration reaches x=5 the solid solution ZrV2Hx separates, below 210K, into the ordered phase with k=0, ZrV2Hx≈4.4, and a disordered phase ZrV2Hx≈5.1. In the phase ZrV2H5.1 the hydrogen atoms occupy statistically the interstices 2Zr+2V (H3.1) and 1Zr+3V (H2.0), the hydrogen distribution not changing down to the lowest temperature, 1.4K.