NMR study of a temperature-induced structural transition in ZrBe ₂D _x

We have studied the motion of D atoms in ZrBe ₂D _1.56 and ZrBe ₂D _1.4 using deuterium NMR and observed a temperature-induced structural transition near 240 K. By comparing the behavior of hydride and deuteride samples with the same concentration (x=1.4), the absence of a transition in the hydride is shown to be an isotope effect. Above 240 K, the deuterium relaxation times T ₁ and T ₂ behave as expected from proton data in ZrBe ₂H _1.4. However, on decreasing the temperature below 240 K, T ₂ decreases rapidly and T ₁ increases rapidly, indicating that the D atom mobility is dramatically reduced. Above the transition temperature, the spectrum shows a quadrupolar doublet splitting of order 1 kHz, reflecting a small motionally averaged electric field-gradient (EFG) at the D atoms. Upon cooling below 240 K the line broadens rapidly, revealing a large distribution of quadrupolar splittings and EFGs at the individual sites. Thus substantial variations exist between the D-atom sites, despite their nominal equivalence in the neutron diffraction-determined structure. We suggest that the strong Be-H (Be-D) repulsion generates substantial Be displacements near vacant D-atom sites. The probable nature of the transition is discussed from the point of view of ordering of D-atom vacancies.