EPR spectrum of coupled pairs ofGd3+ions in single crystals of CsCdBr3

The EPR spectrum of crystals of CsCd${\mathrm{Br}}_3$ doped with small amounts of ${\mathrm{Gd}}^{3+}$ (≃ 0.1-at.% ${\mathrm{Gd}}^{3+}$) is dominated by resonances arising from weakly coupled pairs. The analysis of the spectrum shows the exchange interaction to be antiferromagnetic but extremely weak ($J=+0.00068\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$). The pair seems to consist of two ${\mathrm{Gd}}^{3+}$ ions associated with a ${\mathrm{Cd}}^{2+}$ ion vacancy. The unusual stability of this type of pair apparently arises from the charge balance requirements of the CsCd${\mathrm{Br}}_3$ lattice which has a linear-chain CsNi${\mathrm{Cl}}_3$-type structure. The formation of ${\mathrm{Gd}}^{3+}$-${\mathrm{Gd}}^{3+}$ pairs in the CsCd${\mathrm{Br}}_3$ crystals can be inhibited by the presence of other trivalent ions and small monovalent ions.