An X‐ray crystallographic study of V 2 D

Both superlattice and fundamental reflections have been used in an X‐ray determination of the vanadium‐atom displacements induced by selected ordering of interstitial deuterium in a multi‐domain crystal of V 2 D. The magnitudes of these displacements along X and Z directions of the pseudo‐tetragonal (b.c.t.) unit cell are designated DAX and DAZ respectively, where Z is along the tetragonal c axis. Measurements of the lattice parameters yield DAX ≃0.03 and DAZ ≃0.17 Å. From intensity measurements, the best values for the displacements are DAX = 0.03±0.01 and DAZ = 0.15±0.02 Å, and the purely thermal Debye–Waller factor was found to be 2 B = 0.025 Å 2 . The temperature dependences of both the long‐range‐order parameter, S , and the lattice parameters of the b.c.t. unit cell were also investigated. Both verify that the phase transition from β (monoclinic) to α (b.c.c.) is first order in character with no intervening ɛ (tetragonal) phase as in V 2 H. There is additional evidence that the disordering process includes random occupancy of both Z octahedral and random tetrahedral sites.