Partial Substitution of Potassium with Sodium in the K(2)Ti(2)(PO(4))(3) Langbeinite‐Type Framework: Synthesis and Crystalline Structure of K(1.75)Na(0.25)Ti(2)(PO(4))(3)

The interaction of TiN with Na(2)O–K(2)O–P(2)O(5) 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 KTiP(2)O(7) and langbeinite‐type K(2−x)Na(x)Ti(2)(PO(4))(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. K(1.75)Na(0.25)Ti(2)(PO(4))(3) was characterized by single‐crystal X‐ray diffraction [P2(1)3 space group, a=9.851(5) Å]. The 3D framework is built up by TiO(6) octahedra and PO(4) 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 K(1.75)Na(0.25)Ti(2)(PO(4))(3) crystals and the preferable occupation sites of Na(K) cationic substitutions were investigated by DFT‐based electronic structure calculations performed by the plane‐wave pseudopotential method.