Enhanced n.m.r. of Cs2NaPrCl6, Cs2NaTbCl6 and Cs2NaTmCl6

Elpasolite compounds with the formula Cs2NaLnCl6, where Ln is a lantha­nide ion, have the cubic structure Fm3m (O5h) at room temperature, but some compounds (La, Ce, Pr, Nd) are known to have phase transitions at temperatures between 100 and 160 K to a lower symmetry (Anistratov et al. 1978, Nevald et al. 1979). On the other hand, Cs2NaHoCl6 remains cubic at least down to 0.6 K (Bleaney et al. 1981a), although its ground state is a Γ3 doublet. The compounds of the non-Kramers ions Pr3+, Tb3+, Tm3+ have singlet states Γ1 and we have investigated the enhanced n. m. r. spectra at low temperatures of 141Pr (I = 5/2), 159Tb (I = 3/2) and 169Tm (I = 1/2). The results are interpreted in terms of a predominantly cubic field with a small tetragonal distortion which splits the first excited state, the triplet Γ4. The magnitude of this splitting and its sign are consistent with a small elongation along the tetragonal axis for Cs2NaPrCl6. A novel feature is that the sign of the nuclear electric quadrupole interaction for Pr can be deduced from variations in the line width, ascribed to small, linked variations in the values of P and γ.