Preparation, phase equilibria, and crystal chemistry of lanthanum, praseodymium, and neodymium hydroxide chlorides

Abstract

The preparation of hydroxide chlorides of lanthanum, praseodymium, and neodymium has been achieved by hydrothermal methods at 550[deg]C and 1530 atm. Three phases (Ln(OH)3, Ln(OH)2.55Cl0.45, and Ln(OH)2Cl) have been characterized by analytical and X-ray methods. The observed compositions are closely defined by the x = 0, 0.5, and 1 members of the homologous anion substitution series, Ln(OH)3-xClx. The previously unreported Ln(OH)2.55Cl0.45 phases are clearly substoichiometric in chloride and are described by the Ln7(OH)18Cl3 composition. X-ray diffraction data for the phases at x = 0.45 show a pronounced UCl3-type substructure and complex superstructure reflections that have been indexed on a hexagonal cell. For the lanthanum phase, a = 17.662 (6) and c = 3.914(1) A. Efforts to obtain single crystals have been unsuccessful. Thermal decomposition processes of the hydroxide chlorides have been investigated, and the characterization of LaO(OH)0.55Cl0.45, a monoclinic YOOH-type intermediate phase, is reported. Structural features and phase equilibria of the hydroxide chlorides are discussed and analogies with the hydroxide nitrate systems are drawn.