Three Ways to (Pseudo)cubic Structure Models: Phase Transition and Pseudosymmetry in Orthorhombic Cs 3 MO 4 ( M = V, Nb, or Ta).

The new oxometalates Cs 3 NbO 4 and Cs 3 TaO 4 together with Cs 3 VO 4 crystallize with the K 3 NO 4 structure type [ Pnma , a = 12.495(2) Å, b = 9.0183(14) Å, and c = 6.6529(10) Å for the V compound; a = 12.928(2) Å, b = 9.177(3) Å, and c = 6.739(4) Å for the Nb; and a = 12.963(4) Å, b = 9.122(2) Å, and c = 6.774(1) Å for the Ta compound]. Their crystal structures were evaluated on the basis of single-crystal and powder X-ray diffractometry, assisted by vibrational spectroscopy, thermoanalysis, and DFT calculations. The crystal structures contain tetrahedral [ M 5+ O 4 ] 3- anions, representing the first occurrence of Nb and Ta in a tetrahedral oxidic environment. Many representatives of the orthorhombic K 3 NO 4 structure type have been described in the literature with a cubic structure model with disordered O atomic positions. Based on studies on Cs 3 M O 4 ( M = P, V, Nb, or Ta), we show here three different effects which can lead to (pseudo)cubic data sets. Two of them are problems of crystallographic nature (overlooked twinning or adverse atomic form factor ratios), but the third one, phase transformation into a plastic crystalline high-temperature modification, leads to a "truly" cubic structure with dynamically disordered (freely rotating) oxometalate anions. This might be of interest with respect to a large and growing number of sulfido- and selenidometalate materials which are today in discussion as solid-state electrolytes and to the mechanism of the unusually efficient ion transport therein.