Mixed anionic double sodium-cobalt(II) diorthodiphosphate: structure and growing of single-crystals

Abstract

The solubility of cobalt oxide was determined in the melts of the system Na2O–P2O5–CoO–NaF in a wide range of molar ratios Na2O:P2O5 (0.5–2.0) and temperatures of 800–9000C at the sodium fluoride content of 10 wt.% and 20 wt.%. The region of existence and optimal conditions for growing monocrystals of complex phosphate Na4Со3(PO4)2P2O7 in the melts of the system Na2O–P2O5–CoO–NaF was found out. The synthesized compound was investigated using a number of physicochemical methods (IR spectroscopy, diffuse reflection spectroscopy, X-ray phase analysis, X-ray structure analysis and differential thermal analysis). The complete chemical analysis of the synthesized compound was performed and the melting temperature was determined 7650С, without decomposition). The X-ray diffraction analysis of Na4Со3(PO4)2P2O7 single crystals was carried out and the unit lattice parameters were determined. Phosphate crystals belong to the rhombic crystal system, sp. gr. Pna21; crystal lattice parameters are as follows: a=18.021(1) Å, b=10.389(2) Å, c=6.532(2) Å, V=1222.47(1) Å3, Z=4, сcalc=3.47 g cm–3. The following specific features of the structure of complex phosphate Na4Со3(PO4)2P2O7 have been established: the presence of conduction channels for the sodium ion along the oy axis and the presence of differently coordinated Na and Co polyhedra with an uncharacteristic coordination number. The optimal conditions for the growth of complex phosphate single crystals with a high weight yield were selected. The use of the synthesized compound Na4Со3(PO4)2P2O7 as a functional material with ion conductivity was suggested.