Synthesis and characterization of the hydrated rare-earth acid diphosphates LnHP2O7·3.5H2O (Ln = rare-earth elements)

Hydrated lanthanum acid diphosphates, of the formula LnHP2O7·3.5H2O, have been prepared from acidic LaCl3 and Na4P2O7 solutions (pH = 1), and characterized by XRD, IR and NMR spectroscopic techniques. In the series Ln = La–Sm, these phosphates crystallize in the orthorhombic system (Type I) but in the series Sm–Yb, they crystallize in the triclinic system (Type II). In the case of Sm diphosphates both structures can be obtained depending on the synthesis conditions. The diphosphate character of the compounds was assessed by IR and NMR and two environments, namely PO43− and HPO42−, were identified by 31P MAS spectroscopy. Finally, a different distribution of OH and water was deduced from NMR and IR spectroscopic analysis of the two series of diphosphates.