Crystal-field analysis for the susceptibility of lanthanide compounds of the formCs2NaRCl6

We have used crystal-field theory to discuss susceptibility data for the compounds ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Cl}}_6$ where $R=\mathrm{C}\mathrm{e},\mathrm{N}\mathrm{d},\mathrm{T}\mathrm{b},\mathrm{D}\mathrm{y},\mathrm{H}\mathrm{o},\mathrm{T}\mathrm{m},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{Y}\mathrm{b}$. These materials have an octahedral coordination for the lanthanide ion which remains undistorted even at low temperatures. A combined analysis of all the susceptibility data provides the crystal-field interaction parameters $A_4〈r^4〉=254\mathrm{}〈r^4〉$ and $A_6〈r^6〉=4\mathrm{}〈r^6〉$ K with $〈r^4〉$ and $〈r^6〉$ in atomic units. The numerical values $A_4$ and $A_6$ are independent of the lanthanide ion within experimental error. Attempts to explain these results within a simple point-charge model were unsuccessful.