Crystal-field dependence with Co$^{2+}$-F$^{-}$ distance in the perovskite KCoF$_{3}$

Perovskite crystals are attractive for structural studies at high pressure in a wide multidisciplinary science. Perovskite or distorted-perovskite oxides are relevant as solid state devices as many of them exhibit properties such as colossal magnetoresistance, exotic magnetism, or even high T$_{\mathrm{C}}$ superconductivity. In geoscience, the understanding of high-pressure posperovskite phase is noteworthy. In this sense, it must be noted that NaCoF$_{3}$ transforms to the postperovskite phase at a moderate pressure ($P =$ 15 GPa) in comparison to oxides [1]. However KCoF$_{3}$ seems to lack this transition as it remains in the perovskite structure up to higher pressure [2]. This work reports a structural study in KCoF$_{3}$ as a function of pressure in the 0-60 GPa range. Its large pressure perovskite stability is noteworthy. Therefore, this is a unique system to establish correlations between crystal and electronic structures in a high-symmetry phase, where Co$^{2+}$ ions are located in a perfect octahedral environment. Here we present an x-ray diffraction study in correlation with single-crystal optical absorption spectroscopy. The variation of the crystal-field strength and Racah parameters of Co$^{2+}$ with the crystal volume (and Co-F distance) up to 60 GPa are presented, discussed and compared with available data in other structures involving oxides and chlorides.