Crystal structure and thermal stability of new iron phosphates KMFe(PO 4 ) 2 (M = Ni, Mg, and Co)

Three isostructural iron monophosphates KNiFe(PO 4 ) 2 (KNi), KMgFe(PO 4 ) 2 (KMg-LT, where LT means “low-temperature stable phase”), and KCoFe(PO 4 ) 2 (KCo-LT) are synthesized and structurally characterized from X-ray diffraction data. They crystallize in the monoclinic system with the space group P 2 1 / c . Their structures have in common a three-dimensional framework, built up by infinite zigzag chains of edge-sharing MO 6 (M = Ni, Mg or Co) octahedra, linked by FeO 5 and PO 4 polyhedra via vertices and edges to form a rigid skeleton. The K + ions are located in formed tunnels. DTA showed that KNi has a congruent melting at 941°C, whereas KMg-LT and KCo-LT undergo irreversible phase transitions from P 2 1 / c to different high-temperature structures with the C 2 / c symmetry. IR absorption bands are assigned to different vibrations of the PO 4 tetrahedron.