Pressure-Induced Distortive Phase Transition in Chromite-Spinel at 29 GPa
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Pressure-Induced Distortive Phase Transition in Chromite-Spinel at 29 GPa Jinfu Shu1, Wendy L. Mao2, Russell J. Hemley1, and Hokwang Mao1 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC, 20015 2 LANSCE, Los Alamos National Laboratory, Los Almos, NM, 87545 ABSTRACT A natural chromite single crystal was compressed in a helium pressure medium to just above 35 GPa at ambient temperature and studied with energy-dispersive x-ray diffraction. The volume compression of the cubic, spinel-type structure was fit to a third-order Birch-Murnaghan equation of state with parameters a0= 8.338(4) Å, V0 = 579.6(9) Å3, K0 =179(10) GPa, and K’ = 3.9(9) up to 29 GPa. A distortive phase transition was discovered at higher pressures to a CaAl2O4-type orthorhombic structure, with two cubic unit-cell axes increasing and the third decreasing with increasing pressure. The transition can be fit to a Landau-type strain-order formulism with approximately 5% volume decrease from the cubic phase at 35 GPa. The transition may be triggered by electronic or magnetic transitions in the 3d elements Fe and Cr cations. INTRODUCTION Chromite (FeCr2O4), an end-member mineral in the spinel group, is ubiquitously present as a accessory mineral in the Earth’s crust and mantle over a wide range of pressures (P) and temperatures (T). The face-centered cubic (fcc) unit cell of spinel contains 32 oxygen atoms that accommodate 16 cations in octahedral coordination sites and 8 cations in tetrahedral coordination sites (Fig. 1), forming a great variety of spinels with divalent (e.g., Mg, Mn, Fe), trivalent (e.g., Al, V, Ti, Cr, Fe), or tetravalent (e.g., Si, Ti) cations in these sites. Since the beginning of high P-T experimental explorations of the deep mantle, the spinel to post-spinel structural transitions have long been a central subject in deep Earth studies; many different types of transitions have been discovered (see for example, 1-6). Transformations of chromite-spinel to CaFe2O4- and CaTi2O4- structured polymorphs were recently discovered in a shockmetamorphosed meteorite 7, 8. Corresponding high P-T experiments in laser-heated diamondanvil cells (DAC) observed these phase transitions at 12.5 and 20 GPa, respectively, thus demonstrating the use of chromite as a geobarometer. At room temperature with pressure alone, these ceramic spinels usually cannot overcome the kinetic barriers associated with the first-order phase transition to form the stable high-pressure phases. Transition element bearing spinels are also particularly interesting due to their electronic and magnetic ordering and materials applications. The drastic pressure effect often leads to distortive or isostructural electronic or magnetic tranistions that can proceed readily at room temperature. Chromite has two 3d elements, Fe2+ and Cr3+, and both have potentially intriguing electronic or magnetic transitions. In ferrites, a Verwey distortive transition was reported in magnetite (Fe3O4) at 6.5 GPa and 82.6 K 9, and spin-p
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