Temperature Variation ofLa139Nuclear Quadrupole Resonance in LaF3

The temperature dependence of the pure quadrupole transitions of ${\mathrm{La}}^{139}$ in La${\mathrm{F}}_3$ has been studied from 88 to 447°K. ${\mathrm{La}}^{139}$ has a nuclear spin of $\frac{7}{2}$, so three $\left|\Delta m\right|=1\mathrm{}$ transitions were observed. A spectrometer of crossed-coil geometry was used. The Bayer torsional-motion mechanism and an Einstein phonon spectrum are used to interpret the measurements. The characteristic (or Einstein) temperatures are 465 and 641°K. Values for the quadrupole coupling constant, asymmetry parameter, and electric-field-gradient components at the La sites are tabulated. The observed temperature variation is approximately two orders of magnitude larger than predicted by the simple theory employed. Mechanisms are discussed to account for this discrepancy.