Structural transitions at low temperatures in vanadium deuterides

X-ray and neutron-diffraction techniques have been used to ascertain the structures of vanadium deuterides with compositions between V₂D and V₄D₃ over a temperature range 5–425 °K. The β phase, V₂D, has a monoclinic crystal structure at room temperature with a≃c = 4.46; b = 3.00 Å, β = 95.5°. The deuterium atoms prefer octahedral interstitial sites. Near 425 °K this phase transforms to a b.c.c. phase, and the deuterium atoms prefer tetrahedral sites. The b.c.c. phase of V₄D₃ has been designated α′ and transforms to α′′ at about 220 °K. This transition is an ordering of deuterium as shown by additional neutron-diffraction peaks. This α′′ structure can be indexed with a≃c≃4.46; b≃3.00 Å; β = 90°. Below 160 °K an additional neutron peak appears at 2θ = 12° which indicates a doubling of the b lattice parameter. This transformation to α′′′ phase involves further ordering. The metal sublattice in α′ and α′′′ is very near b.c.c. Therefore, the neutron patterns do not allow a determination of octahedral or tetrahedral site occupancy. The exact crystal structure cannot be determined. The V–D system is not completely analogous to the V–H system and some of the differences are discussed.