Orientational Freezing in KCN-KBr Studied by Magnetic Resonance

The NMR line shape and spin-echo behavior of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ NMR in powder samples of K${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_{0.5}$ ${\mathrm{Br}}_{0.5}$ are reported. The line shape and spin-echo data are best explained by the molecules' developing increasingly preferred orientations as the temperature is decreased. Models in which the molecular reorientations simply slow down with decreasing temperature do not fit our data. The quadrupolar freezing is nearly complete at temperatures well above the dielectric freezing temperature. The NMR results from KCN-KBr are compared to results from the orientational glasses $o-{\mathrm{H}}_2-p-{\mathrm{H}}_2$ and ${\mathrm{N}}_2$-Ar.