K₂Fe^III _0.5Ti^III _0.5Ti^IV _1.0(PO₄)₃: Preparation and Characterization of a Langbeinite-related Phosphate Containing Iron(III) and Mixed-valent Titanium

Abstract

A potassium mixed iron(III)-titanium(III)-titanium(IV) phosphate K 2 Fe III 0.5 Ti III 0.5 Ti IV 1.0 (PO 4 ) 3 has been obtained using a two-step flux interaction in evacuated sealed silica tubes. It forms tetrahedrally-shaped dark violet crystals which belong to the cubic system (space group P2 1 3) with the cell parameter a = 9.8592(5) Å . The structure was refined from single-crystal X-ray diffraction data. [MO 6 ] octahedra and [PO 4 ] tetrahedra share their vertices forming a rigid 3D framework. The potassium cations are located in large closed cavities of the framework. A distribution of the 3d metals’ valence states in K 2 Fe III 0.5 Ti III 0.5 Ti IV 1.0 (PO 4 ) 3 has been proposed on the basis of magnetic measurements, structure investigations and bond-valence calculations as well as UV/vis and EPR spectroscopy.