Thermophysical properties for compounds with Kosnarite-type structure

V. Pet'kov1, E. Asabina1 and I. Schelokov1

1Lobachevsky State University of Nizhni Novgorod, Russia

Keywords: calorimetry, phosphate, kosnarite-type structure
property: heat capacity, thermal conductivity, thermodynamic functions
material: ceramic, thermo-insulating material

Data on the thermophysical properties for the AnMe2.25-0.25n(PO4)3 (A = Na, Cs; Me = Ti, Zr, Hf; n = 0, 1 and 5) and B0.5Zr2(PO4)3 (B = Ca, Sr, Ba) compounds of the kosnarite-type structure are reported. The phosphates belong to kosnarite KZr2(PO4)3 type structure, which is known as thermally stable and thermo-insulating materials, which can accommodate a large fraction of cation substitutions. The studied compounds were synthesized by solid-state and precipitating method starting from the following reactants (chemically pure grade): NaNO3, CsCl, CaCO3, SrCO3, BaCO3, TiO2, ZrOCl2·8H2O, HfO(NO3)2·2H2O, NH4H2PO4. The obtained samples were characterized by X-ray powder diffraction, electron microscopy and microprobe analysis, chemical elemental analysis.

The heat capacities of the phosphates were measured between (6 and 650) K. Isostructural solid-to-solid phase transitions of Na5Me(PO4)3 (Me = Zr, Hf) have been found, and their nature is connected with the centering of off-centered Me atoms in octahedral sites and an occupation transfer between sodium sites in the structure. The experimental data were used to calculate the characteristics of the phase transitions and the standard (p = 0.1 MPa) thermodynamic functions: there are the transition temperatures T, enthalpy of transition ΔtrsH0, entropy of transition ΔtrsS0, molar heat capacities , enthalpy [H0(T) – H0(0)], entropy S0(T) and Gibbs energy [G0(T) – H0(0)] reported over the range from T → (0 to 650) K. Standard entropies of formation at T = 298.15 K were estimated for the phosphates.

From hydrofluoric acid solution microcalorimetry, the enthalpies of solution of Zr3(PO4)4, NaZr2(PO4)3, CsZr2(PO4)3 and Na5Zr(PO4)3 have been determined and the enthalpies of formation have been derived. By combining the data obtained by the two techniques, their Gibbs energies of formation at 298.15 K have been obtained. Thermodynamic functions of reactions of solid-state synthesis were calculated and the usability of ceramic technology for the compounds of kosnarite-type structure was proved.

Thermal conductivity of representatives of phosphates was studied in the temperature interval from (298 to 650) K. The obtained data showed that the kosnarite-type materials have lower thermal conductivity than zirconia.

This work was supported by the Russian Foundation for Basic Research (project no. 11-03-00032).

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