Paramagnetic Resonance Absorption Spectrum of Trivalent Iron in Single-Crystal Calcite

Abstract

The electron paramagnetic resonance absorption spectrum of trace amounts of trivalent iron in single crystals of natural calcite has been observed at 9.6 kMc/sec. The spectrum is symmetric about the crystal [111] direction which, together with the doublet character of each spectral component, suggests that the trivalent iron ions occupy sites closely approximating those of the calcium ions. At 77°K, the best-fit parameters to the spin Hamiltonian are found to be $g_{\mathrm{II}}=g_{\perp }=2.0030\mathrm{}$, ${B_2}^0=343.23\mathrm{}$ Oe, ${B_4}^0=-0.69102\mathrm{}$ Oe, and $\left|{B_4}^3\right|=15.6\mathrm{}$ Oe. The signs of the zero-field splitting parameters, determined by comparing relative line intensity ratios at 1.5 and at 77°K, are such that the ±½ doublet spin state lies lowest. From this, it is concluded that the sign of the cubic-field splitting parameter is positive. The spectrum is found to exhibit a strong linewidth anisotropy. From the dependence of this broadening on the polar angle $\theta$, it is concluded that the sites occupied by this ion have a mosaic structure whose width is given by $\delta \theta =0.04\mathrm{}$ deg. An x-ray diffraction analysis of the calcite single crystal yields a mosaic angle of $\delta \theta =0.011\mathrm{}$ deg.