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 abstractThe superstructure of SrGe(PO4)2 at room temperature (α) results from the off-centering of cation SrII.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► 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.