Static and dynamic crystal-field effects in ferrous fluosilicate

Abstract

This paper examines the crystal fields acting on Fe2+ ions in ferrous fluosilicate, and the way that they influence the magnetic and spectroscopic properties of the compound. The crystal structures of ferrous fluosilicate and of the structurally similar magnesium, manganese, cobalt, nickel, and zinc compounds are described to establish the point symmetries of the metal-ion sites. In the low-temperature monoclinic (P21/c) phase of the Fe2+, Mg2+, Co2+, and Mn2+ compounds, although the metal-ion site is required crystallographically to have only inversion symmetry, all of the available low-temperature data are consistent with a crystal field of C2h symmetry. In the high-temperature phases of these salts, and for nickel and zinc fluosilicates at all temperatures, the site symmetries are either exactly or approximately C3i. The dominant effect of the phase transition on the Fe2+ crystal field in FeSiF6∙6H2O is to remove the nonaxial terms [Formula: see text] and [Formula: see text] that are present in the low-temperature phase. The other terms appear to be relatively unaffected.Both the time-dependent (spin-lattice relaxation) and time-independent (vibronic coupling) manifestations of the dynamic crystal field at the Fe2+ ions are qualitatively examined. A new interpretation of the 57Fe Mössbauer spectra of FeSiF6∙6H2O in the low-temperature phase is described, from which relaxation rates comparable to those deduced from other measurements are obtained. Evidence that the measured 57Fe Mössbauer quadrupole splittings of Fe2+ ions in FeSiF6∙6H2O and in ZnSiF6∙6H2O show significant effects of vibronic coupling is examined.