THE STRUCTURE AND PHYSICOCHEMICAL CHARACTERISTICS OF A SYNTHETIC PHASE COMPOSITIONALLY INTERMEDIATE BETWEEN LIEBIGITE AND ANDERSONITE

During the synthesis of liebigite and andersonite, starting from tetra sodium uranylcarbonate and calcium ions, an unknown stable and crystal line intermediate phase is formed at a Ca2+/Na+ ratio lying between th ose necessary to form the above-cited compounds. Chemical analysis of this yellow-green intermediate phase results in the formula: Ca1.54Na0 .63[UO2(CO3)(3)].xH(2)O (x = 5.38). The scanning electron micrographs show foliated lathlike crystals. From single-crystal data, the crystal s are found to be orthorhombic, a 18.150(3), b 16.866(6), c 18.436(3) Angstrom, space group Pnnm. The basic structural units consist of thre e UO2(CO3)(3) units. Each U atom is [8]-coordinated, resulting in a di storted hexagonal-bipyramidal polyhedron. A chemical analysis indicate s that the structure is short of positive charges, which could be bala nced by the presence of oxonium ions. Results of the crystal-structure analysis support this proposal, which ensures a tight fit of the wate r molecule that is not attached to either Na+ or Ca2+ ions. The powder -diffraction spectrum calculated from the single-crystal structure cor responds very well with the measured one. The fluorescence and phospho rescence spectra are given, as well as information on the thermal deco mposition, as revealed by thermogravimetric and differential scanning calorimetric analyses.