The layered iron phosphate KMgFe(PO4)2: Crystal structure, Mössbauer spectroscopy and ionic conductivity

A new iron phosphate, KMgFe(PO4)2 has been synthesized and investigated by X-ray diffraction, Mössbauer spectroscopy and ionic conductivity. This compound crystallizes in the monoclinic space group C2/c with the parameters a = 18.529(7) Å, b = 5.402(3) Å, c = 9.374(9) Å, β = 120.64(5)° and Z = 4. Its original structure can be described as the stacking along the [101] direction of [MgFe(PO4)−2]∞ layers of corner-sharing MO4 (M = 0.5 Fe + 0.5 Mg) and PO4 tetrahedra. The K+ ions are occupying the inter-layer space. The Mössbauer spectroscopy confirms the occurrence of only tetrahedral Fe3+ ions and gives a definitive proof of the disordered character of their distribution. Ionic conductivity results obtained by the impedance spectroscopy technique show that this material is a two-dimensional ionic conductor, with low activation energy, 0.51 eV, that is interpreted on the basis of two-dimensional pathways.