Partial Substitution of Potassium with Sodium in the K2Ti2(PO4)3 Langbeinite-Type Framework: Synthesis and Crystalline Structure of K1.75Na0.25Ti2(PO4)3.

The interaction of TiN with Na2O-K2O-P2O5 melts was investigated at (Na+K)/P molar ratios of 0.9, 1.0, and 1.2 and at Na/K molar ratios of 1.0 and 2.0. Interactions in the system led to the loss of nitrogen and the partial loss of phosphorus and resulted in the formation of KTiP2O7 and langbeinite-type K2-x Na x Ti2(PO4)3 (x=0.22-0.26) solid solutions over the temperature range of 1173 to 1053 K. The phase compositions of the obtained samples were determined by using X-ray diffraction (including Rietveld refinement), scanning electron microscopy (using energy-dispersive X-ray spectroscopy and element mapping), FTIR spectroscopy, and thermogravimetric analysis/differential thermal analysis. K1.75Na0.25Ti2(PO4)3 was characterized by single-crystal X-ray diffraction [P213 space group, a=9.851(5) Å]. The 3D framework is built up by TiO6 octahedra and PO4 tetrahedra sharing all the oxygen vertices with the formation of cavities occupied by K(Na) cations. Only one of the two crystallographically inequivalent potassium sites is partially substituted by sodium, and this was confirmed by calculating the bond-valence sum. The thermodynamic stability of K1.75Na0.25Ti2(PO4)3 crystals and the preferable occupation sites of NaK cationic substitutions were investigated by DFT-based electronic structure calculations performed by the plane-wave pseudopotential method.