Crystal-Field Theory Validity Through Local (and Bulk) Compressibilities in CoF2 and KCoF3

Crystal field theory (CFT) predicts that crystal field acting on an transition-metal (TM) ion complex of cubic symmetry varies as R-5, where R is the TM-ligand distance. Yet simple and old-fashioned, CFT is used extensively since it provides excellent results in most TM ion-bearing systems, although no direct and thorough validation has been provided so far. Here we investigate the evolution of the electronic and crystal structures of two archetypal Co2+ compounds by optical absorption and X-ray diffraction under high pressure. Both the electronic excited states and crystal-field splitting, Delta = 10Dq, between 3d(e(g) + t(2g)) orbitals of Co2+ as a function of volume, V, and Co-F bond length, R, in 6-fold octahedral (oct) and 8-fold hexahedral (cub) coordination in compressed CoF2 have been analyzed. We demonstrated that Delta scales with R in both coordinations as R-n, with n close to 5 in agreement with CFT predictions. The pressure-induced rutile to fluorite structural phase transition at 15 GPa in CoF2 is associated with an increase of R due to the 6 -> 8 coordination change. The experimental Delta(oct)/ Delta(cub) = 1.10 for the same R-values is close to 9/8, in agreement with CFT. A similar R-dependence is observed in KCoF3 in which the CoF6 O-h coordination is maintained in the 080 GPa pressure range.