Volume and bond length dependences of the electronic structure of 6-fold and 8-fold coordinated Co2+ in pressure transformed CoF2

Abstract

This work reports an optical absorption study on CoF2 under high-pressure conditions in the 0-80 GPa range. A recent structural study1 performed on the pressure-induced phase-transition sequence of CoF2 reveals that this compound undergoes a structural transformation associated with a change of Co2+ coordination from 6 (rutile type) to 8 (fluorite type) at 15 GPa, what implies a profound change in the Co2+ electronic ground state associated with the inversion of d orbitals from (t2g5eg2) to (eg4t2g3). This allows us to explore the Co2+ electronic structure, and hence 10Dq, as a function of the density and the Co-F bond length in both CoF6 (rutile phase) and CoF8 (fluorite phase) coordinations. The results are compared with those obtained in KCoF3 (perovskite) in a wide range of Co-F distances (from 2.04 to 1.83 Å).