Quadrupole interactions in lithium borodeuteride

Abstract

Polycrystalline samples of lithium borohydride and borodeuteride, LiBH4 and LiBD4, are studied by 2H, 7Li, and 10,11B NMR in 7.04 T and 9.35 T magnetic fields in the temperature range 116–580 K. The 10,11B NMR line shape of the orthorhombic phase of LiBH4 and LiBD4 suggests that first-order quadrupole interaction takes place. The quadrupole coupling constant (QCC) χq and asymmetry parameter η of the electric field gradient tensor for 11B are described by linear temperature dependences: χq(11B) = 177 − 0.24T and η = 0.043 + 0.0014T. The electric field gradient at the positions of boron nuclei is created by external charges, primarily lithium cations. In the range of 388–391 K, the 7Li NMR line shape reflects the coexistence of two phase modifications of LiBH4 and LiBD4 and the occurrence of a reversible first-order phase transition. In the temperature range of 390–530 K, the 7Li NMR line shape represents a first-order quadrupole perturbed spectrum with zero asymmetry parameter and a weakly temperature dependent 7Li QCC. The spin-lattice relaxation time and the NMR line shape of 2H are interpreted in terms of the reorientation of the BD4− anion about their proper symmetry axes C2 and C3.