Speaker
Mr
Ivan Korchemkin
(Lobachevsky State University of Nizhni Novgorod)
Description
The family of phosphates with a general formula A(I)B(II)PO4 (A(I) − monovalent metal; B(II) − divalent metal with a tetrahedral coordination) includes the groups of compounds with different structure types: β-tridymite, arcanite, glaserite, maricite, olivine. The compounds of this family have been studied because of their ferroelectric, electrochemical and luminescent properties, catalytic activity. Interest in β-tridymite structure phosphates, containing cesium in a high concentration and possessing thermal, chemical and radiation stability, are conditioned by an opportunity to use them as ceramic for 137Cs γ-radiation sources for commercial and medical applications. Data on thermodynamic properties of β-tridymite type phosphates are limited by compounds CsZnPO4, CsMgPO4. It is known, that some species of liquid radioactive wastes contain high concentrations of stable isotopes (sodium, nickel, iron, chromium) and radionuclide cesium 137Cs. Investigations of hydrolytic stability of single-phase CsMgPO4 ceramic, which was obtained by “hot” and “cold” pressure and spark plasma sintering method, were shown that these objects can be used as matrix for storage of cesium containing radioactive nuclear wastes.
In this work we have studied phase formation regularities and thermophysical properties of phosphates system CsMPO4 (M = Co, Mn), Cs1-xNaxNiPO4 (0 ≤ x ≤ 1).
The samples were synthesized by the co-precipitation method. The aqueous solution of phosphoric acid taken in accordance with the stoichiometry sample was added to stoichiometric mixture of metal nitrate or chloride solutions. The reaction mixture was dried at 353 K and thermally treated at 873 and 973 K. All the thermal treatment stages were alternated with grinding.
X-ray powder diffraction measurements indicated that limited solid solutions in Cs1-xNaxNiPO4 system of the tridymite and maricite structural types were obtained. The samples of CsMnPO4 and CsCoPO4 were crystallized at ambient temperature in tridymite structural type.
The samples were investigated by DTA method over the temperature range 298−1273 K. Thermal decomposition of NaNiPO4 (x = 1) was researched.
Crystal structure of CsNiPO4 and NaNiPO4 were refined by Rietveld method at 298 K. CsNiPO4 crystal structure is characterized by three-dimensional framework, which is formed six-membered rings of tetrahedra NiO4 and PO4, linked by common vertices, and by large cavities that are occupied by Cs cations. NaNiPO4 crystal structure consists of edge-sharing chains of NiO6 octahedra running parallel to the b axis, which are crossconnected by the PO4 tetrahedra, giving rise to large ten-coordinate cavities in which the Na ions are located.
The heat capacity measurements of CsMnPO4 and CsCoPO4 crystalline phosphates were performed over the temperature range 5−650 K. The temperature dependence of the heat capacity of crystalline CsMnPO4 showed that for the phosphate two phase transitions were found at 6.86 К and 126.99 K. The temperature dependence of the heat capacity of crystalline CsCoPO4 showed that for the phosphate three phase transitions were found at 311, 481 and 512 K. For the studied compounds the characteristics of phase transitions and the thermodynamic functions over the temperature range from T (0 to 650) K and standard entropy of formation at 298.15 K were calculated.
The thermal expansion behavior of CsMPO4 (M = Ni, Co, Mn) and NaNiPO4 was studied by high-temperature X-ray powder diffraction at the temperature range 298−973 K. The samples expand anisotropically and belong to high-thermal expansion materials. The prepared ceramics showed a volume thermal expansion coefficient of (3÷7) • 10−5 K−1.
The high specific content of the cesium in β-tridymite structure some phosphates in combination with thermal and hydrolytic stability account for the prospects for their using as safe ceramic materials for cesium γ-radiation sources.
This work was made using equipment Shimadzu Corp.
Primary author
Mr
Ivan Korchemkin
(Lobachevsky State University of Nizhni Novgorod)
Co-author
Dr
Vladimir Pet'kov
(Lobachevsky State University of Nizhni Novgorod)
