Crystal structures and magnetic properties of iron (III)-based phosphates Na4NiFe(PO4)3 and Na2Ni2Fe(PO4)3

Crystal structures from two new phosphates Na4NiFe(PO4)(3) (I) and Na2Ni2Fe(PO4)(3) (II) have been determined by single crystal X-ray diffraction analysis Compound (I) crystallizes in a rhombohedral system (S G R-3c Z=6 a = 8 7350(9)angstrom c=21 643(4)angstrom R-1= 0 041 wR(2) = 0 120) Compound (II) crystallizes in a monoclinic system (S G C2/c Z = 4 a = 11 729(7)angstrom b = 12 433(5)angstrom c= 6 431(2)angstrom beta = 113 66(4) R-1= 0 043 wR(2) = 0 111) The three-dimensional structure of (I) is closely related to the Nasicon structural type consisting of corner sharing [(Ni/Fe)O-6] octahedra and [PO4] tetrahedra forming [NiFe(PO4)(3)](4+) units which align in chains along the c-axis The Na+ cations fill up trigonal antiprismatic sites within these chains The crystal structure of (II) belongs to the alluaudite type Its open framework results from [Ni2O10] units of edge-sharing [NiO6] octahedra which alternate with [FeO6] octahedra that form infinite chains Coordination of these chains yields two distinct tunnels in which site Na+ The magnetization data of compound (I) reveal antiferromagnetic (AFM) interactions by the onset of deviations from a Curie-Weiss behaviour at low temperature as confirmed by Mossbauer measurements performed at 4 2 K The corresponding temperature dependence of the reciprocal susceptibility x(-1) follows a typical Curie-Weiss behaviour for T> 105 K A canted AFM state is proposed for compound (II) below 46 K with a field-induced magnetic transition at H approximate to 19 kOe revealed in the hysteresis loop measured at 5 K This transition is most probably associated with a spin-flop transition (C) 2010 Elsevier B V All rights reserved