NMR Studies of Phase Transitions. I. NaSH

Abstract

NaSH is one of a group of compounds M+(QR)— whose high-temperature crystalline form might be called pseudo-NaCl cubic, but which crystallize in less symmetrical lattices at low temperatures. Two models have been proposed to account for the obviously linear ion QR— replacing a spherical ion in the cubic phase. Pauling suggested that the QR— ion tumbles rapidly and randomly enough to have an effective spherical form to the surrounding ions. Frenkel suggested that the QR— ions are static and point along one of the eight cube body diagonals (〈111〉 directions) at random so as to give long-range cubic symmetry. Previous attempts, based on x-ray diffraction experiments, to decide between these models have been unsuccessful. We have studied the H1 and Na23 nuclear magnetic resonances in polycrystalline samples of NaSH at temperatures between —65° and 200°C. Linewidth changes at 70°C and above reveal a 10° region of transition from the rhombohedral to the cubic phase. In this transition region, diffusion of the Na+ ions causes major narrowing of the Na23 resonance and partial narrowing of the H1. The narrowing of the proton resonance is continued at temperatures above 110°C by diffusion of the SH— ions. The second moment of the proton absorption observed at ∼90°C, when compared with values calculated for several models, is consistent with rapid, random reorientations of the SH— ion among the eight 〈111〉 directions in the cubic phase. Similar calculations and comparisons provide evidence that the SH— ion reorients also in the rhombehodral phase. Here, however, the stable orientations appear to be in the six pseudo-〈110〉 directions perpendicular to the trigonal axis. Thus, the phase transition is a cooperative process resulting from the increasing population, at higher temperatures, of the pseudo-〈111〉 directions in the rhombohedral phase. The Na23 resonance is observed in the rhombohedral phase, even though the Na23 quadrupole splitting is generally large enough in noncubic crystals to obliterate the NMR absorption in powders. Electric-field gradient calculations, assuming point charges, indicate that the Na23 quadrupole splitting can be negligible in the dynamic structure we propose.