Reverse Monte Carlo investigation of the short-range order in α-VD0.8

The short-range deuterium ordering in bcc α-VD0.77 has been investigated by Reverse Monte Carlo modelling based on a total neutron scattering measurement. Structure modelling was performed in two different modes. Firstly, the deuterium atoms were confined to the ideal tetrahedral and octahedral positions. Secondly, they were allowed to move freely (under certain reasonable constraints) within the V matrix. In the first mode, only an insignificant fraction of the deuterium atoms are separated by less than 2 A, in good agreement with earlier findings. However, when allowed to displace from the ideal sites, about 12% of the D atoms become engaged in D-D separations of only 1.56 A. The significance of this result is discussed. Comparisons show that some of the structural features of the long-range deuterium ordering in the low-temperature phase δ-VD0.75 are maintained up to a length scale of 3 A in the short-range order α-phase