MIIGe(PO4)2 (M=Ca, Sr, Ba): Crystal structure, phase transitions and thermal expansion

Three earth alkali-germanium monophosphates MIIGe(PO4)2 (M=Ca, Sr, Ba) were prepared by solid state reaction and their structures, previously unknown, studied by Rietveld analysis. BaGe(PO4)2 and high-temperature β-SrGe(PO4)2 (space group C2/m, Z=2) are fully isotypic with yavapaiite, whereas CaGe(PO4)2 and low-temperature α-SrGe(PO4)2 (C2/c, Z=4) are distorted derivatives. The phase transition between the two forms is observed for the first time. The thermal expansion, resulting from several structural mechanisms, is very anisotropic. - Graphical abstract: The superstructure of SrGe(PO4)2 at room temperature (α) results from the off-centering of cation SrII. Highlights: → Crystal structures of yavapaiite-type MIIGe(PO4)2 (M=Ca, Sr, Ba) have been solved. → Distortion and superstructure result from Ca and Sr off-centering. → A global scheme for the polymorphism of yavapaiites is proposed.