Hindered rotation and phase transition in sodium gallohydride according to the NMR data

Polycrystalline sodium gallohydrides with different hydrogen isotope compositions, NaGaH₄, NaGaH_2.56D_1.44, and NaGaD₄, were studied over the temperature range 100–375 K by the ²H, ²³Na, and ^69,71Ga NMR methods in magnetic fields of 1.4, 1.88, and 7.04 T. The data on spin-lattice deuteron relaxation T ₁(²H) were used to determine the activation energy (～35 kJ/mol) of anion rotational motions and quadrupole coupling constant (QCC) χ _Q ⁰ (²H) = 90 kHz for the rigid lattice. To within measurement errors, the activation energy and ²H QCC were independent of the isotope composition. The low-temperature ²H NMR line was a doublet with ²H QCC χ(²H) = 70 kHz. The difference between χ ₀ and χ(T) was interpreted in terms of the librational averaging of the electric field gradient (EFG) according to the Bayer equation. The frequency of librations at 113 K was estimated (31 cm^?1). The shape of the ^69,71Ga NMR line was indicative of second-order quadrupole interaction with a nonzero asymmetry parameter η of the EFG tensor. Temperature variations in the QCC and η parameter at gallium were considered using the distorted tetrahedron model. The η value changed from 0.65 to zero (axial EFG tensor symmetry characteristic of tetragonal lattices) over the temperature range 140–295 K and again increased to 0.3 at 300–365 K, which was evidence of a change in the orientation of the distorted tetrahedral anion in the lattice. Changes in the asymmetry parameter were in agreement with the phase transition in sodium gallohydride recorded as a heat capacity jump.