Polymorphism of new rubidium magnesium monophosphate

Novel phase RbMgPO4, synthesized by solid state reaction, sustains phase transitions at 169 and 184 °C. The medium (β)- and high- (γ) temperature forms (orthorhombic, respectively Pna21 and Pnma, Z=4) are typical stuffed tridymites but the ambient form (α) exhibits an unusual three-fold Pna21 superstructure that results from the change of coordination of one third of the Mg atoms. Cell parameters are as follows: for α: a=26.535(1) Å, b=9.2926(3) Å, c=5.3368(2) Å; for β: a=8.7938(3) Å, b=9.3698(3) Å, c=5.3956(1) Å; for γ: a=8.7907(3) Å, b=5.4059(1) Å, c=9.3949(3) Å.