Influence of pairs of alkali metal cations on the formation of complex phosphates at the crystallization of multicomponent self-fluxes

Abstract

The regularities of phase formation in the systems (МІ1 + МІ2)2O—P2O5—TiO2—МІІO (МІ — Na, K, Rb; MII — Mg, Co, Ni) at the crystallization of multicomponent self-fluxes at the values of molar ratios: (МІ1 + МІ2)/Р = 1.0; Ti/Р = 0.25; MII/Ті = 1.0, and М І 1/М І 2 = 1.0 and 2.0 over the temperature interval of 1000-780 °C have been investigated. For mixed sodium-potassium-phosphate systems, regardless of the ratio of Na/K (1.0 or 2.0), the solidification without crystal formation was found. For Na/Rb-containing systems, the increasing of the so dium amount in the initial melt to the value of molar ratio Na/Rb = 2.0 promoted the crystalization of single crystals of NaTi2(PO4)3 doped by divalent metals ions. In the case of K-Rb-phosphate self-fluxes, it was found that the value of K/Rb = 2.0 is optimal for the growing of langbeinite-related single crystals (K/Rb)2MII0,5Ti1,5(PO4)3 (MII — Mg, Co, Ni) which belong to cubic system (space group P213). The calculated cell parameters for new phosphates (K/Rb)2MII 0.5Ti1.5(PO4)3 depend on the nature of MII: a = 9.851(6) Å for Mg, a = 9.853(9) Å for Co and a = 9.850 (1) - for Ni. In the FTIR spectra of phosphates (K/Rb)2MII0.5Ti1.5(PO4)3, the characteristic modes in the region of 520-650 сm–1 and 1000-1250 сm-1 which have been assigned to symmetric and asymmetric stretching vibrations (ν4, ν1 and ν3) of phosphate tetrahedron confirmed the presence of orthophosphate type anion in their composition. According to results of thermal analysis, the melting points of (K/Rb)2MII0.5Ti1.5(PO4)3 are at a temperatures of 1082 °С for MII — Ni, 1057 °С for MII — Co, and above 1100 °С for MII — Mg. The synthesized complex phosphates have been investigated using the powder X-Ray diffraction method, thermogravimetry, differential thermal analysis, and FTIR-spectroscopy.