Synthesis, crystal structures and properties of the new compounds KAg(XO) (X = Mo, W).

Two new isostructural compounds, namely heptapotassium silver tetrakis(tetraoxomolybdate), KAg(MoO) (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), KAg(WO) (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P6mc (Z = 2) with the unit-cell dimensions a = 12.4188 (2) and c = 7.4338 (2) Å for KAg(MoO) (single-crystal data), and a = 12.4912 (5) and c = 7.4526 (3) Å for KAg(WO) (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1(XO)] `pinwheels' of K1O octahedra and six XO tetrahedra (X = Mo, W) connected by common opposite faces into columns along the c axes. The octahedral columns are linked to each other through Ag1O tetrahedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (...Ag1O-X1O-empty octahedron-Ag1O...). Ag1 is located almost at the centre of the largest face of its coordination tetrahedron and seems to have some mobility. The new structure type is related to the BaNdAlO and CaBaSiO types, and to other structures of the α-KSO-glaserite family. The differential scanning calorimetry (DSC) and second harmonic generation (SHG) results show that both compounds undergo first-order phase transformations to high-temperature centrosymmetric phases.