Electron Spin Resonance Absorption Spectrum of Y3+-Stabilized CO33− Molecule–Ion in Single-Crystal Calcite

An electron spin resonance absorption spectrum of effective spin $\mathrm{S\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2}$ exhibiting two hyperfine structure doublet patterns has been observed in naturally occurring single crystals of calcite. The nuclei responsible for these doublets are identified from electron–nucleus double resonance and relative intensity measurements to be ⁸⁹Y and ¹³C. From the mean of the spectroscopic splitting tensor values as well as from the ¹³C hyperfine structure separations, it is deduced that this spectrum is due to the CO₃^3 − molecule–ion which is charge stabilized by a neighboring Y³⁺ ion. Experimental data suggest the existence of two sets of six equivalent centers each having an orthorhombic $g$ tensor which is diagonal in a coordinate system whose $z$ axis is canted with respect to the crystal [111] direction. The ⁸⁹Y and ¹³C hyperfine tensors exhibit orthorhombic and axial symmetries, respectively, with the $z$ axis of each being parallel to the crystal [111] direction. From an analysis of the ⁸⁹Y hyperfine structure spectral components, an estimate is obtained for the unpaired electron spin density transferred to the Y^3 + ion. This taken together with the ⁸⁹Y contact term provides an estimate to the hyperfine field of the Y²⁺ ${\text{(4d}}^1)$ ion. Over the temperature range 4.2°‐450°K, the ⁸⁹Y hyperfine splitting increases by about 5% and asymptotically reaches a maximum value at the upper temperature. Attempts to interpret this effect using existing theories on the variation of hyperfine structure with temperature have proven unsatisfactory.