Diffuse neutron scattering from yttrium hydride

The yttrium hydride phase diagram exhibits a vertical phase boundary down to low temperatures. This is seen as a result of extensive ordering. The hydrogen atoms occupy tetrahedral sites almost exclusively. The ordering first manifests itself as pairs of hydrogen atoms above and below the yttrium atom on next nearest neighbour tetrahedral sites. Plots of resistivity versus temperature show an anomalous behaviour in the 170 K region indicating a liquid-gas type transition. The neutron scattering from the system shows a weak ridge perpendicular to the c* at Qz = 1.4. This feature sharpens as the temperature is reduced. The feature is also seen in to have a periodic modulation along to c*. Models by McKergow et al. with chains of the pairs along c but uncorrelated along the basal directions can be seen to match the data along c*. Experiments by Blaschko et al. on αphase LuD and ourselves on αphase YD also indicate modulation of the ridge perpendicular to c*. This implies some correlation along the basal directions although this correlation is much weaker. Our data from SXD at ISIS shows ridges at Qz=4/3, 8/3 corresponding to the pairs. The other features have yet to be successfully explained. To this end we have undertaken Reverse Monte Carlo analysis to model the data. Here an initial configuration of atoms is assumed, a lattice of yttrium atoms with pairs of hydrogen randomly arranged, the scattering intensity is calculated, an atom is then moved and the change in the scattering intensity is calculated. The program accepts the move if it improves the fit. Comparisons between experiment and model will be presented. (orig.)