The structure of the metastable K18Ta5Zr5F63 phase

Abstract

A metastable phase K₁₈Ta₅Zr₅F₆₃ was prepared by molten salt synthesis of K₂TaF₇ and K₂ZrF₆ in a sealed Pt crucible. This is the first example of a structure of a fluoride complex compound containing both tantalum and zirconium as central atoms. The asymmetric part of the unit cell contains two Ta/Zr sites and one pure Zr site. The Ta1/Zr1 ion is coordinated by seven F ions forming one capped trigonal prism. The Ta2/Zr2 ion is coordinated by six F ions forming a trigonal prism, and this polyhedron is fully ordered. The Zr3 ion is coordinated by six F ions, which are disordered over two positions. All (Zr/Ta)F_n (n = 6–8) polyhedra are isolated from each other, although the ZrF₆ units have shared faces, forming an infinite channel along the c-axis. ¹⁹F MAS NMR experiments agree with the proposed structural model, identifying all five central non-equivalent polyhedra. The K₁₈Ta₅Zr₅F₆₃ phase decomposes within several months to its initial components; this can be monitored by NMR, DSC and XRD experiments. Moreover, accelerated decomposition can be achieved by thermal treatment, resulting in the formation of a K₃ZrF₇ phase.