The nature of deuterium arrangements in YD3 and other rare-earth trideuterides

Abstract

Abstract The efficacy of different structural models for describing the observed neutron-powder-diffraction (NPD) measurements of bulk polycrystalline YD3 as well as other hexagonal rare-earth (i.e., Nd, Tb, Dy, Ho, Er, and Tm) trideuteride powders has been investigated via Rietveld refinement. Between the two possible structural configurations, centrosymmetric P-3c1 and noncentrosymmetric P63 cm, the latter can be excluded due to very high correlations found between the positions of the D sites. Hence, the true “diffraction-average” structure for YD3 and all other rare-earth deuterides studied is centrosymmetric (P-3c1). This seems to contrast with the prior evidence from first-principles calculations and various spectroscopic probes suggesting that the true local symmetry is not P-3c1, but rather, noncentrosymmetric. A possible way to reconcile the apparently conflicting conclusions from NPD and spectroscopic measurements is by assuming that the real structure is a twinned arrangement of nanosized, noncentrosymmetric configurations. For example, we demonstrate that the diffraction-average centrosymmetric P-3c1 structure can result from a superposition of individual, noncentrosymmetric P3c1 twins. A comparison of neutron vibrational spectra for YH3 and YD3 confirms that both compounds share similar structural arrangements.