Search

High pressure can provoke spin transitions in transition metal-bearing compounds. These transitions are of high interest not only for fundamental physics and chemistry, but also may have important implications for geochemistry and geophysics of the Earth and planetary interiors. Here we have carried out a comparative study of the pressure-induced spin transition in compounds with trivalent iron, octahedrally coordinated by oxygen. High-pressure single-crystal M\"{o}ssbauer spectroscopy data for FeBO$_3$, Fe$_2$O$_3$ and Fe$_3$(Fe$_{1.766(2)}$Si$_{0.234(2)}$)(SiO$_4$)$_3$ are presented together with detailed analysis of hyperfine parameter behavior. We argue that $\zeta$-Fe$_2$O$_3$ is an intermediate phase in the reconstructive phase transition between $\iota$-Fe$_2$O$_3$ and $\theta$-Fe$_2$O$_3$ and question the proposed perovskite-type structure for $\zeta$-Fe$_2$O$_3$.The structural data show that the spin transition is closely related to the volume of the iron octahedron. The transition starts when volumes reach 8.9-9.3 \AA$^3$, which corresponds to pressures of 45-60 GPa, depending on the compound. Based on phenomenological arguments we conclude that the spin transition can proceed only as a first-order phase transition in magnetically-ordered compounds. An empirical rule for prediction of cooperative behavior at the spin transition is proposed. The instability of iron octahedra, together with strong interactions between them in the vicinity of the critical volume, may trigger a phase transition in the metastable phase. We find that the isomer shift of high spin iron ions depends linearly on the octahedron volume with approximately the same coefficient, independent of the particular compounds and/or oxidation state. For eight-fold coordinated Fe$^{2+}$ we observe a significantly weaker nonlinear volume dependence.

Magnesite (MgCO3), calcite (CaCO3), dolomite [(Ca, Mg)CO3], and siderite (FeCO3) are among the best-studied carbonate minerals at high pressures and temperatures. Although they all exhibit the calcite-type structure ( $${\text{R}}\bar{3}{\text{c}}$$ ) at ambient conditions, they display very different behavior at mantle pressures. To broaden the knowledge of the high-pressure crystal chemistry of carbonates, we studied spherocobaltite (CoCO3), which contains Co2+ with cation radius in between those of Ca2+ and Mg2+ in calcite and magnesite, respectively. We synthesized single crystals of pure spherocobaltite and studied them using Raman spectroscopy and X-ray diffraction in diamond anvil cells at pressures to over 55 GPa. Based on single crystal diffraction data, we found that the bulk modulus of spherocobaltite is 128 (2) GPa and K′ = 4.28 (17). CoCO3 is stable in the calcite-type structure up to at least 56 GPa and 1200 K. At 57 GPa and after laser heating above 2000 K, CoCO3 partially decomposes and forms CoO. In comparison to previously studied carbonates, our results suggest that at lower mantle conditions carbonates can be stable in the calcite-type structure if the radius of the incorporated cation(s) is equal or smaller than that of Co2+ (i.e., 0.745 A).

A study of Fe,Al-bearing bridgmanite in Earth‘s mantle and synthesis of pure Fe-bridgmanite with anomalously low compressibility., The physical and chemical properties of Earth’s mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa. The compressibility of ferric iron–bearing bridgmanite is significantly different from any known bridgmanite, which has direct implications for the interpretation of seismic tomography data.