CRYSTAL STRUCTURE AND THERMAL STABILITY OF NEW IRON PHOSPHATES KMFe(PO4)2 (M = Ni, Mg, and Co)
A. Badri1, M. Hidouri1, M.L. López2, M.L. Veiga2, C. Pico2, J. Darriet3, M.Ben Amara1
1Université de Monastir, Monastir, Tunisie
2Universidad Complutense, Madrid, Spain
3Institut de Chimie de la Matière Condensée de Bordeaux, CNRS, Pessac-Cedex, France
Keywords: iron phosphates, synthesis, crystal structure, phase transitions, IR
Abstract
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. 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 P21/c to different high-temperature structures with the C2/c symmetry. IR absorption bands are assigned to different vibrations of the PO4 tetrahedron.
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