Synthesis, crystal structures and properties of the new compounds K 7– x 7– x x Ag 1+ x 1+ x x ( X X O 4 4 ) 4 4 ( X X = Mo, W)

摘要

Two new isostructural compounds, namely heptapotassium silver tetrakis(tetraoxomolybdate), K 7– x Ag 1+ x (MoO 4 ) 4 (0 ≤ x ≤ 0.4), and heptapotassium silver tetrakis(tetraoxotungstate), K 7– x Ag 1+ x (WO 4 ) 4 (0 ≤ x ≤ 0.4), have been synthesized and found to crystallize in the polar space group P 6 3 mc ( Z = 2) with the unit‐cell dimensions a = 12.4188?(2) and c = 7.4338?(2)?? for K 6.68 Ag 1.32 (MoO 4 ) 4 (single‐crystal data), and a = 12.4912?(5) and c = 7.4526?(3)?? for K 7 Ag(WO 4 ) 4 (Rietveld analysis data). Both structures represent a new structure type, with characteristic [K1( X O 4 ) 6 ] `pinwheels' of K1O 6 octahedra and six X O 4 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 4 tetrahedra along with the K2 and K3/Ag2 polyhedra, forming the polar rods (…Ag1O 4 – X 1O 4 –empty octahedron–Ag1O 4 …). 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 Ba 6 Nd 2 Al 4 O 15 and CaBaSiO 4 types, and to other structures of the α‐K 2 SO 4 –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.