Electrocrystallization of PrO2 and TbO2-x from alkali hydroxide melts and characterization of the fluorite-related TbO2-x

Single crystals of PrO2 and of the oxygen deficient fluorite related phase TbO2-x have been obtained by anodic electrocrystallization from alkaline hydroxide melts containing PrCl3 and TbCl3, respectively. Magnetic measurements, X-ray diffraction data and TEM investigations confirm the identity of the products. PrO2 crystallizes in a fluorite type of structure with a = 5.3945(3) Angstrom and shows paramagnetic behaviour with a magnetic moment as expected for Pr4+ (mu(eff) = 2.49 mu(B), Theta = -109 K, T-N = 10 K). According to precession photographs and an examination with an area sensitive X-ray detector, TbO2-x exhibits a superstructure of fluorite with a pseudocubic subcell with a(f) = 5.2810(1) Angstrom. This lattice constant is intermediate between those of TbO2 and Tb2O3 (C-type), the same is true for its paramagnetic behaviour (mu(eff) = 8.58 mu(B), Theta = - 22 K, T-N = 5 K). The supercell was found to be hexagonal with a = 25.836(1) Angstrom and c = 36.672(2) A, the symmetry of the intensity distribution being monoclinic. Electron diffraction revealed a cubic cell with a = 10.6 A, space group Ia-3, indicating reduction of the material in the electron beam. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.