Том 56, № 4 ЖУРНАЛ СТРУКТУРНОЙ ХИМИИ Июль UDC 548.73:546.72:546.18:546.21 CRYSTAL STRUCTURE AND THERMAL STABILITY OF NEW IRON PHOSPHATES KMFe(PO4)2 (M = Ni, Mg, and Co) A. Badri1, M. Hidouri1, M.L. Lуpez2, M.L. Veiga2, C. Pico2, J. Darriet3, M. Ben Amara1 1UR Matйriaux Inorganiques, Facultй des Sciences, Universitй de Monastir, Monastir, Tunisie E-mail: mongi.benamara@fsm.rnu.tn 2Departamento de Quнmica Inorgбnica I, Facultad de Ciencias Quнmicas, Universidad Complutense, Madrid, Spain Received November, 4, 2013 Three isostructural iron monophosphates KNiFe(PO4)2 (KNi), KMgFe(PO4)2 (KMg-LT, where LT means ″low-temperature stable phase″), and KCoFe(PO4)2 (KCo-LT) are synthesized and structurally characterized from X-ray diffraction data. <...> They crystallize in the monoclinic system with the space group P21/c. <...> Their structures have in common a three-dimensional framework, built up by infinite zigzag chains of edge-sharing MO6 (M = Ni, Mg or Co) octahedra, linked by FeO5 and PO4 polyhedra via vertices and edges to form a rigid skeleton. <...> DTA showed that KNi has a congruent melting at 941 °C, whereas KMg-LT and KCo-LT undergo irreversible phase transitions from P21/c to different high-temperature structures with the C2/c symmetry. <...> IR absorption bands are assigned to different vibrations of the PO4 tetrahedron. <...> INTRODUCTION Iron phosphate materials have received much attention over the past three decades because they have shown to be of a great interest both for their applications in many fields such as catalysis [ 1, 2 ], ionic conductivity [ 3 ], and as electrode materials for rechargeable batteries [ 4 ] and rich structural chemistry [ 5, 6 ]. <...> As a part of our investigation on the A2O—MO—Fe2O3—P2O5 systems, where A and M are alkali and divalent cations respectively, we reported the crystal structures and physical characterization of phosphates KMgFe(PO4)2 [ 7 ] and KCuFe(PO4)2 [ 8 ]. <...> These two phases have similar stoichiometries, but very different structures. <...> Indeed, magnesium phosphate displays a bi-dimensional structure, built up from layers of corner-sharing (Fe,Mg)O4 and PO4 tetrahedra, the K+ <...>