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

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journal contribution
posted on 01.08.2016, 00:00 authored by J. Antonio Barreda-Argüeso, Fernando Aguado, Jesús González, Rafael Valiente, Lucie Nataf, Marta N. Sanz-Ortiz, Fernando Rodríguez
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, Δ = 10Dq, between 3d(eg + t2g) 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 Δ 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 Δ­(oct)/ Δ­(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 Oh coordination is maintained in the 0–80 GPa pressure range.

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