Sodium NMR in the trigonal phase of sodium hydrosulfide

Measurements of the sodium nuclear quadrupole coupling constant and spin–lattice relaxation time, while confined mainly to the trigonal phase, show the existence of the two known phase transitions at 358 and 113 K. The quadrupole coupling constant is 206 kHz at 358 K and decreases roughly linearly with decreasing temperature at a rate of about 0.66 kHz/K. The satellite peaks in the quadrupolar split powder pattern disappear above 358 K in the cubic phase. Below 130 K. where there is no known phase transition, the satellites broaden and again disappear but in this case it is because the fluctuation rate of the electric field gradient at the sodium site due to reorientation of the SH− ion is about equal to the quadrupolar splitting of the spectrum at this temperature. Only the resonance from the −1/2 ↔ 1/2 transition is observed down to 77 K.The spin–lattice relaxation of the sodium spins shows a very strong temperature dependence and a well-defined minimum similar to the previously observed results for the protons. The relaxation is caused by the nuclear quadrupole interaction made time dependent by the reorientation of the neighbouring SH− ions. A simple model calculation gives reasonable agreement between the calculated and experimental values of T1 at the minimum if the SH− ion is considered to move between two positions parallel and antiparallel to the trigonal axis. An extension of this model suggests that the lack of an enhanced relaxation rate at the 113 K phase transition may be a result of a parallel ordering of neighbouring SH− ions along the trigonal axis in the lowest temperature phase.