EPR spectra of magnetically coupled pairs of G${\mathrm{d}}^{3+}$ ions in crystals of CsMg${\mathrm{Cl}}_3$, CsMg${\mathrm{Br}}_3$, and CsCd${\mathrm{Br}}_3$

The EPR spectra of crystals of CsMg${\mathrm{Cl}}_3$, CsMg${\mathrm{Br}}_3$, and CsCd${\mathrm{Br}}_3$ doped with ${\mathrm{Gd}}^{3+}$ are dominated by resonances arising from weakly coupled pairs. The host materials all adopt the linear-chain CsNi${\mathrm{Cl}}_3$ structure. The exchange interactions between the ${\mathrm{Gd}}^{3+}$ ions within the pairs are antiferromagnetic but extremely weak. At room temperature, the exchange parameter $J$ for the pair in CsMg${\mathrm{Cl}}_3$, CsMg${\mathrm{Br}}_3$, and CsCd${\mathrm{Br}}_3$ is equal to +0.00107, +0.00080, and +0.00068 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. The pairs consist of two ${\mathrm{Gd}}^{3+}$ ions associated with a ${\mathrm{Mg}}^{2+}$ or ${\mathrm{Cd}}^{2+}$ ion vacancy. The unusual stability of this type of pair apparently arises from the strict charge-compensation requirement of the linear-chain $\mathrm{Cs}M{X}_3$ lattice.