Deuterium superlattice formation in lanthanum trideuterides: The nuclear magnetic resonance evidence

Abstract

We report the results of the first deuterium magnetic resonance (DMR) investigation of lanthanum trideuterides LaD x (2.28 ⩽ x ⩽ 3.00). The DMR spectra distinguish the following three different crystalline electric field (CEF) environments of deuterium: (1) octahedral (O) sites which retain nearly cubic symmetry, (2) axially symmetric tetrahedral (T) and O sites and (3) strongly asymmetric T sites resulting from a pair of occupied neighboring O sites. The observed relative strengths and symmetries of these CEFs are consistent with the deuterium superlattice structure based on neutron diffraction measurements. Since no quadrupole splitting occurs at 300 K, rapid deuterium motion must occur among all T and O sites. The onset of quadrupole splitting for 2.28 ⩽ x ⩽ 2.62 occurs sharply in the range 240–250 K in which heat capacity and differential thermal analysis measurements have also revealed anomalies. Sites (1) and (2) predominate at x = 2.28, and sites (2) and (3) predominate at x = 2.48 and x = 2.62. At x = 2.78 rapid deuterium motion persists to still lower temperatures, the onset of quadrupole splitting (site (3)) being at 180–190 K. For 2.90 ⩽ x ⩽ 3.00, no quadrupole splitting is detected at temperatures as low as 120 K indicating that tetragonal (or other) distortion of the f.c.c. lanthanum lattice does not occur.