Mn3+ stabilization in complex phosphate–fluoride fluxes and its incorporation into langbeinite framework

Two new mixed valence manganese-containing orthophosphates with a langbeinite structure have been obtained from the complex fluoride–phosphate melts using the high-temperature flux technique. MIVF4 and K2MIVF6 (MIV—Zr, Hf) were used as fluoride and polyvalent metal precursors. Obtained langbeinites were investigated using a single-crystal X-ray diffraction, FTIR-, UV–VIS- and electron paramagnetic resonance (EPR)-spectroscopy. Both compounds crystallize in a cubic system (sp. gr. P213) with cell parameters a=10.2106(12) and 10.1896(9) Å for K1.96Mn0.57Zr1.43(PO4)3 and K1.93Mn0.53Hf1.47(PO4)3, respectively. The rigid langbeinite-like framework is built up from the isolated [MO6] octahedra and [PO4] tetrahedra interlinked via oxygen vertices. Potassium cations occupy positions in the large closed cavities of the framework. Statistical distribution of manganese and tetravalent metal over two crystallographic positions in octahedral oxygen environment can be observed. The presence of the mixed valence manganese in the reported phosphates was confirmed using UV–VIS and EPR-spectroscopy.