Structure of K₂TaF₇ at 720^oC – a combined use of synchrotron powder data and solid state DFT calculations

Ľubomír Smrčok1, Michela Brunelli², Miroslav Boča¹ and Marian Kucharík¹

1Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9,SK-845 36 Bratislava, Slovak Republic, ²European Synchrotron Radiation Facility,B.P. 220, F-38043 Grenoble CEDEX, France

uachsmrk@savba.sk

The structure of the title compound was optimized by energy minimization in the solid state using a plane waves DFT computation for which the lattice parameters were obtained by the LeBail technique from synchrotron X-ray powder diffraction data collected at 720^oC. Owing to the sample’s corrosiveness, it had to be loaded in a thin-walled Pt capillary. It was found that the structure corresponds to that of the β-K₂TaF₇ phase. The Ta atoms in the TaF₇^- polyhedra are seven-fold coordinated by fluorine atoms positioned within 1.977 to 2.007 Å. The K atoms are surrounded by eleven (K1) and eight (K2) fluorine atoms. Every F atoms in the structure is surrounded by three K atoms. The F-K contact distances vary from 2.57 to 3.32 Å. It was shown that solid state DFT methods could be an accurate alternative to Rietveld refinement, providing a remedy to the chronic difficulty of standard powder refinements, which is the lack of information extractable from a powder pattern [1]. The size of problems tractable by solid state DFT methods running on a laboratory computer nowadays reaches ~500-1000 atoms per unit cell, depending on the level of approximation used by the computational method employed. These numbers well exceeds the widely accepted limits for unrestrained powder refinements, which frequently fail in providing accurate results even for the structures with much smaller numbers of atoms. Moreover, since theoretical calculations are frequently done in the P1 space group, simultaneous optimization of geometries of possibly symmetrically equivalent units within a unit cell provides a good measure of internal consistency of structure optimization and/or solution [2]. On the other hand, in practice some problems are encountered when treating structures with variable occupancies of the atomic sites, because the quantum physics/chemistry methods do not have any analogue to occupancy parameters routinely used in crystallography. Attempt to preserve occupational variability leads to computational supercells, which can easily cease to be tractable by the standard computational resources.

This work was partially supported by Slovak Grant Agency VEGA under the contracts 2/6179/26. We thank the European Synchrotron Radiation Facility ESRF, Grenoble, France, for provision of beam time on the high-resolution powder diffraction beam line ID31.

[1] Ľ.Smrčok , V.Jorík, E. Scholtzová, V.Milata, Acta Cryst., B63 (2007) 477-484

[2] ] Ľ.Smrčok, M.Brunelli, M.Boča, M.Kucharík, J.Appl.Cryst. 41 (2008) 634-636