Neutron scattering studies on the vibrational excitations and the structure of ordered niobium hydrides:the λ phases

Abstract

Neutron vibrational spectroscopy and neutron diffraction have been used to study theoptical hydrogen vibrations in and the structure of ordered niobium hydridesNbHx and deuteridesNbDx in theconcentration range 0.73<x<1.0 and at temperatures between 13 and 250 K. The measurements performedat temperatures above 200 K confirm the established model of theβ phase. Neutron diffraction data collected in the regime of theλ phases(T<200 K,0.73<x<1.0) show that the structurefor x<0.85 is made up ofmicrodomains with β phase structure. Neutron vibrational spectra display up to three families of peaks that canbe assigned to three different hydrogen sites: an undisturbed tetrahedral site within themicrodomains and two strongly disturbed tetrahedral sites within two different kinds ofmicrodomain boundaries. Incommensurately modulated phases probably existin this concentration range. For hydrogen concentrations around there is evidence for a change of symmetry of the basic cell as well as achange of the modulation at even higher concentrations. In the range of theγ phase at , strong isotopic effects occur. The results give new insights into the phase diagram.