Synthesis, crystal structure and 19F NMR parameters modelling of CaTiF6(H2O)2 yielding to a revision of the bond-valence parameters for the Ti4+/F- ion pair

CaTiF6(H2O)2 was synthetized by a solvothermal method and was found to be isostructural to SrTiF6.2H2O. The structure, refined from Powder X-Ray Diffraction (PXRD) data, is built from the connectivity of dimers [Ca2F10(H2O)4] of square antiprisms (SAP) [CaF5(H2O)3] and TiF6 octahedra. To assign the six 19F NMR reso-nances to the six fluorine crystallographic sites of same multiplicity, 19F magnetic shielding tensors have been calculated using the GIPAW method. Unusually, the plot of the experimental 19F isotropic chemical shift (delta iso) values, as a function of the calculated 19F isotropic shielding (sigma iso) values, shows a much better alignment for the experimental structure than for the DFT-optimized one. 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 19F resonances to the F sites. Finally, the previously established bond-valence (BV) parameter Rij for the Ti4+/F- ion pair (1.76 angstrom) 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 TiF6 octahedra, and leading to Rij = 1.706 angstrom.

Automated summary

CaTiF6(H2O)2 is isostructural to SrTiF6.2H2O and its structure is composed of dimers [Ca2F10(H2O)4], square antiprisms [CaF5(H2O)3] and TiF6 octahedra. 19F NMR resonances were assigned to fluorine crystallographic sites through the calculation of magnetic shielding tensors. The experimental structure shows better agreement with the experimental data compared to the DFT-optimized structure. The refinement of the Ti-F bond lengths leads to a revised bond-valence parameter Rij of 1.706 angstrom for the Ti4+/F- ion pair.