Effect of dilution on the spin pairing transition in rhombohedral carbonates

Abstract

The compressibility of an iron-bearing magnesite was determined by means of single crystal diffraction up to 64 GPa. Up to 49 GPa the pressure-evolution of the unit cell volume of the solid solution with 12% of Fe²⁺ can be described by a third-order Birch–Murnaghan equation of state with parameters V ₀=281.0(5) Å³, K ₀=102.8(3) GPa, K. The spin pairing of the Fe²⁺ d-electrons occurs between 49 and 52 GPa, as evidenced by a discontinuous volume change. The transition pressure is increased by about 5 GPa compared with the iron end-member; an effect consistent with a cooperative contribution of adjacent clusters to the spin transition. The trend is, however, opposite in the periclase–wüstite solid solution. Differences among the two structures, in particular in the Fe–Fe interactions, that might explain the different behavior are discussed.

Keywords:

• magnesite
• siderite
• spin transition
• compressibility
• single crystal diffraction

Acknowledgements

The UNLV High Pressure Science and Engineering Center is supported by DOE-NNSA Cooperative Agreement DE-FC52-06NA262740. Diffraction data were collected at HPCAT (Sector 16), APS, Argonne National Laboratory. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. We thank GSECARS and COMPRES for the use of the Gas Loading System. HPSynC is supported as part of the Efree, an Energy Frontier Research Center funded by DOE-BES under Award DE-SC0001057.