Preparation, phase equilibria, and crystal chemistry of lanthanum, praseodymium, and neodymium hydroxide chlorides
The preparation of hydroxide chlorides of lanthanum, praseodymium, and neodymium has been achieved by hydrothermal methods at 550°C and 1530 atm. Three phases ( Ln(OH) 3, Ln(OH) 2.55Cl 0.45, 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- xCl x. The previously unreported Ln(OH) 2.55Cl 0.45 phases are clearly substoichiometric in chloride and are described by the Ln7(OH) 18Cl 3 composition. X-ray diffraction data for the phases at x = 0.45 show a pronounced UCl 3-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) Å. Efforts to obtain single crystals have been unsuccessful. Thermal decomposition processes of the hydroxide chlorides have been investigated, and the characterization of LaO(OH) 0.55Cl 0.45, 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.