Chlorine Nuclear Quadrupole Resonance in ErCl3 and Some Similar Chlorides

In ErCl₃ at 4.2°K the ³⁵Cl NQR frequencies are 4.4566 ± 0.0002 and 4.5157 ± 0.0002 MHz and the asymmetry parameters $\eta$ are 0.53 ± 0.02 and 0.52 ± 0.03 for the eightfold and fourfold sites, respectively. The electric field gradient $\mathrm{xx}$ principal axis at each site is within 4° of the bisector of the Er–Cl–Er angle. The above results are analyzed in a “bond‐switching” scheme. We report also the ³⁵Cl NQR frequencies in the trichlorides of Y, Dy, Ho, Tm, Yb, La, and U. The adequacy of the point‐change model as the primary source of the electric field gradient in these ionic crystals is tested.