Electron-Paramagnetic-Resonance Study of Cr Ions and Exchange-Coupled Cr Ion Pairs in the BiI3Structure

This paper reports the results of an EPR study of ${\mathrm{Cr}}^{3+}$ single ions in Bi${\mathrm{I}}_3$ and Sb${\mathrm{I}}_3$, and exchange-coupled pairs of ${\mathrm{Cr}}^{3+}$ ions in Bi${\mathrm{I}}_3$. The spectrum for ${\mathrm{Cr}}^{3+}$ single ions in Bi${\mathrm{I}}_3$ can be fitted by the spin Hamiltonian ${\mathcal{H}}_S=g\beta \mathbf{H}·\mathbf{S}+D{S_z}^2$, where $g=2.058\mathrm{}$ and $D=-0.5\mathrm{}$ kOe. The spin Hamiltonian for ${\mathrm{Cr}}^{3+}$ single ions in Sb${\mathrm{I}}_3$ is given by ${\mathcal{H}}_S=g_{\parallel }\beta H_z{S}_z+g_{\perp }\beta (H_x{S}_x+H_y{S}_y)+DS_z^{}{}_{}{}^2$, where $g_{\parallel }=2.078\mathrm{}$, $g_{\perp }=2.088\mathrm{}$, and $D=+3.85\mathrm{}$ kOe. The spectrum for exchange-coupled intralayer pairs of ${\mathrm{Cr}}^{3+}$ ions in Bi${\mathrm{I}}_3$ can be fitted by a spin Hamiltonian which has the terms $J{\mathbf{S}}_1·{\mathbf{S}}_2+\frac{3}{2}{J}_z{S}_{1z}{S}_{2z}+\frac{1}{4}K[{S_1}^{+}{S_2}^{+}+{S_1}^{-}{S_2}^{-}]$ in addition to the single-ion terms for each ${\mathrm{Cr}}^{3+}$ ion. In the above expression $J$ is negative, $J_z+34\mathrm{}$ kOe, and $K=2.0\mathrm{}$ kOe. Two of the results are rather unusual: a $g$ value greater than 2, and an anisotropic exchange parameter about $\frac{1}{5}$ the isotropic exchange parameter. Explanations which involve the quite high covalent bonding and the extremely large spin-orbit constant on the halogens are given for both unusual parameters. The relevance of the EPR results to the magnetic anisotropy in the chromium trihalides is discussed. Also observed and included in this paper is the ${\mathrm{Cr}}^{2+}$ single-ion spectrum of Bi${\mathrm{I}}_3$: Cr. The spin Hamiltonian for the ${\mathrm{Cr}}^{2+}$ transitions is given by ${\mathcal{H}}_S=g\beta \mathbf{H}·\mathbf{S}+D{S_z}^2+E({S_x}^2-{S_y}^2)$, where $g=1.98\mathrm{}$, $D=6.9\mathrm{}$ kOe, and $E=1.6\mathrm{}$ kOe.