The series Cs 3+2 n [ M O 4 ]O n ( M = Nb, Ta; n = 1, 2) with isolated tetrahedral oxidometalate anions [ M O 4 ] 3−

Abstract Four new compounds, Cs 5 [ M O 4 ]O and Cs 7 [ M O 4 ]O 2 with M = Nb, Ta were synthesised as serendipitous products from the reaction of Cs 3 O with HgO in Nb or Ta crucibles. They contain tetrahedral [ M O 4 ] 3 − ${\\left[\\mathit{M}{\\text{O}}_{4}\\right]}^{3-}$ oxidometalate anions and oxide anions [O] 2− . Cs 5 [NbO 4 ]O and Cs 5 [TaO 4 ]O both crystallise in the Na 5 AsO 5 structure type (orthorhombic, space group Pbcm , for the Nb compound: a = 7.5462(7), b = 20.169(2), c = 7.8697(8) Å, Z = 4) Cs 7 [NbO 4 ]O 2 and Cs 7 [TaO 4 ]O 2 adopt the Sr 7 [GeN 4 ]N 2 structure type as its first oxidic representatives (orthorhombic, space group Pbcn , for the Nb compound: a = 13.2911(3), b = 7.5411(5), c = 15.4100(6) Å, Z = 4). Together with the recently published compounds Cs 3 M O 4 ( M = Nb, Ta), representing the member of the series with n = 0, the new compounds form a series of double salt crystal structures that can be written as Cs 3+2 n [ M O 4 ]O n ( M = Nb, Ta; n = 0, 1, 2), and their members can be compared within the ionic concepts of the second Pauling rule on stability of ionic crystals. The bonding situation with predominating ionic interactions between the cations and the anionic units together with strong covalent contributions within the oxidometalate anions is evaluated with solid state DFT calculations and are corroborated by Raman spectroscopic measurements.