Ternary U (IV) containing phosphates: Synthesis, structure and thermodynamic studies

Abstract

Five ternary compounds namely, SrLnU(PO4)3 (Ln = La (1), Nd (2), Sm (3), Eu (4) and Gd (5)) were synthesized by conventional solid state route. These compounds were characterized by powder X-ray diffraction (XRD) method. All the compounds crystallize with monazite type structure. Lattice parameter of these compounds shows the general trend of decreasing as ionic radii of Ln ions decreases. The structure refinement by Rietveld refinement shows that all the cations M (Sr2+, Ln3+ and U4+) occupy the same site in crystal lattice with random distribution, forming MO9 polyhedra. All the nine bond lengths (M–O) are unequal as general characteristic of Monazite compounds. The P–O bond lengths range from 1.44 Å to 1.60 Å. The FTIR (Fourier Transform Infrared) and Raman spectroscopy confirms the monazite type phase with characteristic phosphate symmetrical and asymmetrical bands. These compounds are stable with respect to phase up to 1573 K in air with partial oxidation of U (IV) to U (VI) as evidenced by Themogravimetric analysis (TGA) and chemical analysis. Thermal expansions of these compounds were measured by HT-XRD (High Temperature X-ray Diffraction). Volume thermal expansion coefficient of these compounds lies in range from 22.1 × 10−6 K−1 to 32.5 × 10−6 K−1. Heat capacities of the synthesized compounds were measured by heat flux Differential Scanning Calorimetry (DSC) using sapphire method in the temperature range of 303–858 K. Standard molar enthalpy of formation (ΔH°f) of each compound was derived from their respective enthalpy of dissolution (ΔHds) data measured by High Temperature Oxide Melt Calorimeter. Enthalpies of formation as a function of lanthanide substitution were analysed to gain insight into the thermodynamic stability of the synthesized compounds. Other important thermodynamic functions such as entropy, Som(T); Gibbs energy function, Фom(T){ Фom(T) = −[Gom(T) − Hom(298 K)]/T}; enthalpy, Hom(T); enthalpy of formation, ΔHof, m(T); Gibbs energy, Gom(T) and Gibbs energy of formation, ΔGof,m(T) were also derived for these compounds.