Effect of dilution on the spin pairing transition in rhombohedral carbonates

The compressibility of an iron-bearing magnesite [image omitted] was determined by means of single crystal diffraction up to 64GPa. 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 V0=281.0(5)angstrom 3, K0=102.8(3)GPa, K[image omitted]. 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 5GPa 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-wustite solid solution. Differences among the two structures, in particular in the Fe-Fe interactions, that might explain the different behavior are discussed.