Synthesis, crystal structures and properties of the new compounds K7–xAg1+x(XO4)4 (X = Mo, W)

Two new isostructural compounds, namely hepta­potassium silver tetra­kis­(tetra­oxomolybdate), K7–xAg1+x(MoO4)4 (0 ≤ x ≤ 0.4), and hepta­potassium silver tetra­kis­(tetra­oxotungstate), K7–xAg1+x(WO4)4 (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P63mc (Z = 2) with the unit-cell dimensions a = 12.4188 (2) and c = 7.4338 (2) A for K6.68Ag1.32(MoO4)4 (single-crystal data), and a = 12.4912 (5) and c = 7.4526 (3) A for K7Ag(WO4)4 (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1(XO4)6] `pinwheels' of K1O6 octa­hedra and six XO4 tetra­hedra (X = Mo, W) connected by common opposite faces into columns along the c axes. The octa­hedral columns are linked to each other through Ag1O4 tetra­hedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (⋯Ag1O4–X1O4–empty octa­hedron–Ag1O4⋯). 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 Ba6Nd2Al4O15 and CaBaSiO4 types, and to other structures of the α-K2SO4–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.