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 V2D. 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 2B = 0.025 Å2. 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 V2H. There is additional evidence that the disordering process includes random occupancy of both Z octahedral and random tetrahedral sites.
A large vanadium single crystal is loaded with deuterium (D) to V2D and cooled to form a multidomain ordered crystal. The ordered state and ordering transition are studied with X‐ray scattering revealing a D‐induced modulation of the vanadium lattice which produces X‐ray peaks at the D superlattice positions. The disordering is monitored using this modulation peak together with a careful measurement of the temperature variation of the tetragonal lattice parameters. These values of c and a are fit with a phenomenological expression incorporating the order parameter, the thermal expansion, and the volume dilat iondue to tetrahedral and octahedral D occupancy in V. The transition is shown unambiguously to be first order with some hysteresis.
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