EXAFS study of the local structure of zirconium tungstate and molybdate biomedical gel generators

Amorphous and crystalline zirconium tungstate and molybdate gel-generator systems for production of medical radionuclides Tc-99m and Re-188, have been investigated by means of X-ray powder diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The structures of the products of hydrothermal crystallisation of zirconium tungstate and molybdate gel-generators previously refluxed in water solution of concentrated hydrochloric acid are closely related to the structures of initial amorphous precursors. The coordination polyhedra of Zr atoms in both crystalline and amorphous zirconium tungstate and molybdate are pentagonal bipyramids built up from oxygen atoms surrounding the central atom in a regular manner. The distances and dihedral and torsion angles are close to the values for ideal polyhedra. The central Zr atoms are located in gravity center of oxygen polyhedra. There are no measurable differences between the geometry of Zr coordination polyhedra for crystalline Zr tungstate and molybdate and of their amorphous precursors (gel-generators). Moreover, the second shell atomic distributions in the structure of amorphous precursors show similarity to those observed for crystalline samples comprising two tungsten (molybdenum) atoms at 3.4 Angstrom and four tungsten (molybdenum) atoms at 3.8 Angstrom distances. Although present EXASF refinement of the atomic arrangement has resulted only in obtaining quantitative radial structure including interatomic distances, coordination, numbers and mean square displacements for corresponding absorber-scatterer pairs, the similarities in structure between the crystalline samples and the amorphous gel-generators are notable.