Open Access

The first alkaline earth sulfatotungstate Ca[WO(SO4)3] is also the first one featuring a condensed anionic host structure. A solvothermal approach by using tungstic acid, CaSO4 and oleum led to Ca[WO(SO4)3] comprising corrugated two-dimensional (2D) layers in the a-b plane. It crystallises in a new structure type in space group Pnma (no. 62). Blue α-K2[MoO2(SO4)2] marks the first highly condensed structure for sulfatomolybdates, synthesized via melting MoO3 and K2S2O7. It crystallises in a new structure type in the non-centrosymmetric space group Cc (no. 9), yielding a doubled SHG response compared with potassium dihydrogen phosphate. Electron spin resonance spectroscopy proves the presence of Mo5+ ions causing the blue color. High condensation of the sulfate, tungstate and molybdate moieties seems to be remarkable considering the highly charged centres S6+, W6+, and Mo6+. Moreover, the electrostatic, optical, and thermal properties of the title compounds are elucidated. UV–vis measurements show a pronounced ligand-to-metal-charge–transfer of the tungstate and molybdate moieties dominating the absorption edge. Thermal gravimetric analysis reveals a rather high thermal stability for both compounds.