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

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• 作者：J. Antonio Barreda-Argüeso ; Fernando Aguado ; Jesús González ; Rafael Valiente ; Lucie Nataf ; Marta N. Sanz-Ortiz ; Fernando Rodríguez
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：August 25, 2016
• 年：2016
• 卷：120
• 期：33
• 页码：18788-18793
• 全文大小：414K
• 年卷期：0
• ISSN：1932-7455

文摘

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(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 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 O_h coordination is maintained in the 0–80 GPa pressure range.