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Electric-field gradients ͑EFG's͒ in yttrium trideuteride (YD 2.98 ) have been determined using deuteron magnetic resonance ͑DMR͒ and compared with values for the P3 c1 and P6 3 cm structures calculated from first principles. The experimental DMR spectra at low temperatures ͑180-290 K͒ could be decomposed into two components with intensity ratio 2:1 in accordance with the expected two groups of deuterium sites, i.e., tetrahedral and octahedral sites in the crystal lattice. Their characteristic quadrupole frequencies, q , and asymmetry parameters, , have been compared with the theoretical values obtained for the structures with space groups P3 c1 ͑no. 165͒ and P6 3 cm ͑no. 185͒. The agreement between the experimental and calculated results is much better for the latter structure. The spectra at higher temperatures ͑300-370 K͒ transform from a two-component configuration into three quadrupolar doublets plus a low-intensity narrow central component. One of the doublets keeps the same parameters as those observed at low temperatures. The two others indicate changes in the tetrahedral site environments. One possible explanation could be a thermally activated quasitwo-dimensional hopping of the D atoms to nearby sites within the ab plane.

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