Synthesis, crystal structure and $^{19}$F NMR parameters modelling of Catif$_6$(H$_2$o)$_2$ yielding to a revision of the bond-valence parameters for the Ti$^{4+}$/F$^-$ ion pair

International audienceCatif$_6$(H$_2$o)$_2$ was synthetized by a solvothermal method and was found to be isostructural to SrTiF$_6$(2H$_2$O). The structure, refined from Powder X-Ray Diffraction (PXRD) data, is built from the connectivity of dimers [Ca$_2$F$_{10}$(H$_2$O)$_4$] of square antiprisms (SAP) [CaF$_5$(H$_2$O)$_3$] and TiF$_6$ octahedra. To assign the six $^{19}$F resonances to the six fluorine crystallographic sites of same multiplicity, $^{19}$ F magnetic shielding tensors have been calculated using the GIPAW method. Unusually, the plot of the experimental 19F isotropic chemical shift (δ$_{iso}$) values, as a function of the calculated $^{19}$F isotropic shielding (ssiso) values, shows a much better alignment for the experimental than for the DFT-optimized structure. Whereas, the dynamics of the structural water molecules under experimental conditions are not captured by the optimized structure, the experimental averaged structure provides a good account of the experimental data and a reliable assignment of the $^{19}$F resonances to the F sites. Finally, the previously established bond-valence (BV) parameter Rij for the Ti$^{4+}$/F$^-$ ion pair (1.76 Å) leads to overestimated BV sum values for Ti. This trend has been observed for almost all other fluorotitanates. We then undertook to refine its value, using the Ti-F bond lengths from 70 carefully selected structures containing 134 Ti sites forming TiF$_6$ octahedra, and leading to R$_{ij}$. = 1.706 Å