EPR of Cr(III) in a Single Crystal of NaMgAl(C2O4)3·9H2O

The electron paramagnetic resonance of Cr(III) substituted for Al in a single crystal of NaMgAl(C₂O₄)₃·9H₂O was examined at two different microwave frequencies. The Cr(C₂O₄)₃³⁻ ion was found to be nonaxially symmetric in this host crystal and to occupy six nonequivalent positions in the unit cell. The distortion from axial symmetry is caused by the presence of one molecule of water of hydration per Cr(C₂O₄)₃³⁻. When gently heated the crystal loses one water molecule per Cr(C₂O₄)₃³⁻ ion, and the EPR spectrum becomes that of an axially symmetric species. Upon being exposed to water vapor the heated crystal slowly absorbs water, and the EPR spectrum of the original nonaxial species is regenerated. The spin‐Hamiltonian parameters for the two cases were determined. For the nonaxially symmetric, unheated crystal $g_{\mathrm{zz}}{\text{\hspace{0.17em}=\hspace{0.17em}1.9780, g}}_{\mathrm{xx}}{\text{\hspace{0.17em}=\hspace{0.17em}1.9822, g}}_{\mathrm{yy}}\text{\hspace{0.17em}=\hspace{0.17em}1.9812, D\hspace{0.17em}=\hspace{0.17em}0.7786}{\mathrm{cm}}^{\text{−1}}$ and $\text{E\hspace{0.17em}=\hspace{0.17em}0.0306}{\mathrm{cm}}^{\text{−1}}$ . For the axially symmetric, heated crystal, $g_{\Vert }\text{\hspace{0.17em}=\hspace{0.17em}1.978, g⊥\hspace{0.17em}=\hspace{0.17em}1.980}$ , and $\text{D\hspace{0.17em}=\hspace{0.17em}0.7783}{\mathrm{cm}}^{\text{−1}}$ . The ⁵³Cr hyperfine interaction for the heated crystal displayed a possible anisotropy with ${\text{|\hspace{0.17em}A\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}(15.7\hspace{0.17em}±\hspace{0.17em}0.2)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ and ${\text{|\hspace{0.17em}B\hspace{0.17em}|\hspace{0.17em}=\hspace{0.17em}(16.1\hspace{0.17em}±\hspace{0.17em}0.3)\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−4}}{\mathrm{cm}}^{\text{−1}}$ . The EPR spectrum of a Mn²⁺ impurity, presumably present as Mn(H₂O)₆²⁺, could be observed to occupy two different sites.