Thermodynamic properties of compounds with kosnarite-type structure

Synthesis and thermodynamic properties of the crystalline kosnarite-type structure compounds, viz., A(x)Me(2.25-0.25x)(PO4)(3) (A = Na, Cs; Me = Ti, Zr, Hf; x = 0, 1 and 5) are reported. The heat capacities of the phosphates have been measured between 6 and 650 K. Investigations on the isostructural solid-to-solid phase transitions of Na5Zr(PO4)(3) and Na5Hf(PO4)(3) show centering of the off-centered Me atoms in octahedral sites and Na+ occupation transfer between sodium sites. The transition temperatures (T-trs(0)), enthalpy of transition (Delta H-trs(0)), entropy of transition (Delta S-trs(0)), molar heat capacities (C-p,m(0)), enthalpy (H-0(T) - H-0(0)), entropy (S-0(T)) and Gibbs) energy (G(0)(T) H-0(0)) are calculated from the experimental data. Standard enthalpies of formation at T = 298.15 K for the phosphates Zr-3(PO4)(4), NaZr2(PO4)(3), CsZr2(PO4)(3) and Na5Zr(PO4)(3) are estimated by solution reaction calorimetry. By combining the data obtained by the two techniques, their Gibbs energies of formation at 298.15 K have been obtained. Thermodynamic functions of the reactions for solid-state synthesis of the compounds of kosnarite-type structure are calculated.