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\ifmmode^\circ\else\textdegree\fi{}K, the best-fit parameters to the spin Hamiltonian are found to be ${g}_{\mathrm{II}}={g}_{\ensuremath{\perp}}=2.0030$, ${{B}_{2}}^{0}=343.23$ Oe, ${{B}_{4}}^{0}=\ensuremath{-}0.69102$ Oe, and $|{{B}_{4}}^{3}|=15.6$ Oe. The signs of the zero-field splitting parameters, determined by comparing relative line intensity ratios at 1.5 and at 77\ifmmode^\circ\else\textdegree\fi{}K, are such that the \ifmmode\pm\else\textpm\fi{}\textonehalf{} 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 $\ensuremath{\theta}$, it is concluded that the sites occupied by this ion have a mosaic structure whose width is given by $\ensuremath{\delta}\ensuremath{\theta}=0.04$ deg. An x-ray diffraction analysis of the calcite single crystal yields a mosaic angle of $\ensuremath{\delta}\ensuremath{\theta}=0.011$ deg.