Vacancy ordering phase transition in ZrBe₂(H/D)_x: NMR and electronic structure study

The layered intermetallic compound ZrBe₂(H/D)_x (x≃1.5) was studied using deuterium and beryllium NMR. A temperature-induced structural transition in the deuteride reported earlier is further investigated here using ⁹Be NMR, which reveals the transition in both the hydride and deuteride at 200 and 235 K, respectively. Above the transition temperature a single pair of quadrupolar satellites is present. Each quadrupolar satellite splits into two resonances below the transition temperature, implying the existence of two different classes of Be sites. The temperature dependence of the satellite frequencies and the appearance of two-phase coexistence spectra show the transition to be first order. Be-D spin-echo double resonance (SEDOR) experiments were performed to identify the two sites; SEDOR demonstrates that the D environments of the two classes of Be sites are similar. Electronic structure calculations show the binding energy of H decreases gradually for x greater than 1.38. The calculations allow a vacancy ordered structure to be proposed.