Preparation, phase equilibria, and crystal chemistry of La, Pr, Nd, Sm, Eu, and Gd hydroxide halides

The phase equilibria of the hydroxide halides of La, Pr, Nd, Sm, Eu, and Gd in hydrothermal media have been investigated. The hydroxide chlorides, bromides, and iodides of La, Pr, and Nd were prepared in the temperature and pressure range 5500 to 6000 and 1333 to 1530 atm. Equilibria of the hydroxide halides of Sm, Eu, and Gd show a temperature dependence; chlorides and bromides were prepared at 550 to 7500 and 1667 atm, and 350 to 5500 and 1333 to 1533 atm, respectively. The hydroxide iodides of Sm, Eu, and Gd were obtained at 5500 and 1400 to 1933 atm. The crystalline products, which have been characterized by analytical and X-ray methods, are unaffected by pressure. Several different phases have been identified in the lanthanide (Ln) systems. For Ln = La, Pr, and Nd, the products include Ln(OH)3, Ln7(OH)18X3, and Ln(OH)2Y (X = lattice l-, Br-, I-; Y = Cl-, Br-). For Ln = Sm, Eu, and Gd, a wider variety of phases are encountered, some of which are unidentified. The identified products include Ln(OH)3, Ln7(OH)18X3, Ln3O(OH)5Br2, Ln3(OH)8I (Ln = Sm, Eu, Gd), and Ln(OH)2Y (Ln = Eu, Gd; Y = Cl-, Br-). All observed Ln(OH)2Y phases have the Y(OH)2Cl-type structure. The Ln(OH)2Br phases have not been previously characterized. Diffraction data for the Ln7(OH)18X3 phases show a new hexagonal structure type with a pronounced UCl3-type substructure. The quaternary Ln3O(OH)5Br2 phases have an orthorhombic structure similar to that of Y3O(OH)5Cl2. Thermal decomposition processes of all the identified phases have been characterized. The structure of La7(OH)18I3 is a substitution derivative of the UCl3-type structure. The Gd3O(OH)5Br2 structure (Pmmn) is similar to that previously reported for Y3O(OH)5Cl2