Quadrupole interactions in lithium borodeuteride

摘要

Polycrystalline samples of lithium borohydride and borodeuteride, LiBH _4 and LiBD_4, are studied by ~2H, ~7Li, and ~(10,11)B NMR in 7.04 T and 9.35 T magnetic fields in the temperature range 116-580 K. The ~(10,11)B NMR line shape of the orthorhombic phase of LiBH_4 and LiBD_4 suggests that first-order quadrupole interaction takes place. The quadrupole coupling constant (QCC) χ _q and asymmetry parameter η of the electric field gradient tensor for ~(11)B are described by linear temperature dependences: χ _q (~(11)B) = 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 LiBH_4 and LiBD_4 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 BD _4 ~- anion about their proper symmetry axes C_2 and C_3.