Interaction in the molten system Rb2O‐P2O5‐TiO2‐NiO. Crystal structure of the langbeinite‐related Rb2Ni0.5Ti1.5(PO4)3

The interaction in the molten system Rb₂O‐P₂O₅‐TiO₂‐NiO was investigated at different molar ratios Rb/P = 0.5‐1.3, fixed Ti/P = 0.15, Ti/Ni = 1.0 at temperature range 1073–953 K. The conditions of formation of complex phosphates RbTi₂(PO₄)₃, Rb₂Ni_0.5Ti_1.5(PO₄)₃ and RbNiPO₄ have been determined. The new phosphate Rb₂Ni_0.5Ti_1.5(PO₄)₃ (space group P2₁3, a = 9.9386(2) Å) has been obtained and investigated by the single crystal X‐ray diffraction and FTIR‐spectroscopy. It has langbeinite‐like structure, that is built up from mixed (Ni/Ti)O₆‐octahedra and РО₄‐tetrahedra. Rubidium atoms are located in closed cavities of 3D‐framework.