Abstract

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.