High-pressure crystal-field spectra of single-crystal clinoferrosilite

Abstract

Electronic absorption spectra of a single crystal of clinoferrosilite, FeSiO 3 , have been collected between 4,000 and 15,000 cm (super -1) from room pressure to 4.8 GPa. Four bands are observed in all of the spectra: two peaks near 11,000 cm (super -1) and 5,100 cm (super -1) are attributed to the 5 A 1 --> 5 A 1 and 5 A 1 --> 5 B 1 transitions of Fe (super 2+) on the M2 site, and two peaks near 10,100 cm (super -1) and 8,000 cm (super -1) are assigned to the 5 B 2g --> 5 A 1g and 5 B 2g --> 5 B 1g transitions of Fe (super 2+) on the M1 site. At ambient conditions, crystal-field splitting (Delta 0 ) and crystal-field stabilization energy (CFSE) of M1 are estimated to be 8,681 cm (super -1) and 3,972 cm (super -1) , respectively, and Delta 0 and CFSE of M2 are estimated to be 7,262 cm (super -1) and 3,705 cm (super -1) , respectively. Between room pressure and 1.40 GPa, the crystal-field bands all shift to higher energies at rates between 118 and 287 cm-1 GPa (super -1) . At approximately 1.65 GPa, discontinuities in the peak positions between 270 to 600 cm (super -1) are observed that coincide with the transition from the P2 1 /c structure to the C2/c structure. Above the transition, the bands shift to higher energies at rates between 150 and 270 cm (super -1) GPa (super -1) . The C2/c polymorph of FeSiO 3 is predicted to gain additional stabilization energy relative to the P2 1 /c polymorph at high pressure and room temperature by increased crystal-field effects of Fe (super 2+) in the octahedral sites. The best estimates for CFSE of Fe (super 2+) on the M1 and M2 sites of the C2/c structure at 1.65 GPa are approximately 4,200 cm (super -1) and approximately 4,000 cm (super -1) , respectively, compared with approximately 4,000 and approximately 3,770 cm (super -1) for the M1 and M2 sites of the P2 1 /c structure at 1.4 GPa. The lowering of the transition pressure from MgSiO 3 to FeSiO 3 can be explained by increased crystal-field stabilization energy of Fe (super 2+) in the octahedral sites of the high-density phase, combined with a smaller contribution from the increase in cation size of Fe (super 2+) substituting for Mg (super 2+) .