Long-range order inV2H near the β-ε phase transition

The integrated intensity I of h/2 0 h/2¯ superstructure reflections has been measured with x rays for a single crystal of ${\mathrm{V}}_2$H. Just below the critical temperature $T_c$ of the β-ε phase transition, I varies as (1-T/$T_c$ ${)}^{2\beta }$ with β=0.153±0.007, increasing to β=0.17 and 0.20 when referred to a rigid volume and c lattice parameter, respectively, through a lattice-expansion correction. By comparing heating and cooling runs the second-order character of this phase transition has been demonstrated. A shift of $T_c$, however, was observed when data were taken without first thermally cycling the crystal close to $T_c$.