## Abstract

The enhanced nuclear magnetic resonance spectrum of $^{165}$Ho (I = $\frac{7}{2}$) in the cubic compound Cs$_2$NaHoCl$_6$ has been measured from 20 K down to 0.6 K. Along a \[111\]-axis the value of ($\gamma$-$\gamma_I$)/2$\pi$ rises from 0.70 GHz T$^{-1}$ at T = 20 K to 1.98 (2) GHz T$^{-1}$ at 2 K. The temperature variation can be fitted accurately by using a cubic crystal field which gives a doublet $\Gamma_3$ as the ground state, with a triplet $\Gamma_4$ at 10.3 (2) cm$^{-1}$, followed by a singlet $\Gamma_1$ at 30 to 40 cm$^{-1}$. Below ca. 4 K the second-order Zeeman splitting of the doublet gives rise to a temperature-dependent anisotropy consistent with the cubic symmetry. This has been studied both through the enhanced $^{165}$Ho spectrum and through the paramagnetic shift in the $^{23}$Na (I = $\frac{3}{2})$ spectrum. At higher temperatures the paramagnetic shifts of both $^{23}$Na and $^{133}$Cs (I = $\frac{7}{2}$) are shown to be accurately proportional to the computed value of ($\gamma-\gamma_I$)/2$\pi$ for $^{165}$Ho. No departure from cubic symmetry is observed.