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), K_7–x Ag_1+x (MoO₄)₄ (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), K_7–x Ag_1+x (WO₄)₄ (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P 6₃mc (Z = 2) with the unit‐cell dimensions a = 12.4188 (2) and c = 7.4338 (2) Å for K_6.68Ag_1.32(MoO₄)₄ (single‐crystal data), and a = 12.4912 (5) and c = 7.4526 (3) Å for K₇Ag(WO₄)₄ (Rietveld analysis data). Both structures represent a new structure type, with characteristic K1(X O₄)₆] `pinwheels' of K1O₆ octahedra and six X O₄ 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–X 1O₄–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.