Synthesis and Structural, Electrical, and Magnetic Properties of New Iron–Aluminum Alluaudite Phases β-Na2Ni2M(PO4)3 (M = Fe and Al)

Herein we report the studies of different physical properties (structural, magnetic, thermal, morphologic, electrical, and electrochemical) of two new allotropic β-Na2Ni2M(PO4)3 (NNMP) phosphates, with M = Fe and Al. Pure orthorhombic single-phase powders were prepared under air, using an autocombustion synthesis method. They crystallize in the orthorhombic Imma space group with similar unit cell parameters [a = 10.1592(2), b = 13.0321(3), c = 6.4864(2) A] and [a = 10.3993(1), b = 13.1966(1), c = 6.4955(1) A] for β-Na2Ni2M(PO4)3 (NNAP) and β-Na2Ni2Fe(PO4)3 (NNFP), respectively. Crystal structures of both compounds were determined using X-ray powder diffraction and Rietveld method refinements, which indicate the occurrence of Ni2+ in the 8g site, and of M3+ in the 4a site of the structure. The structure consists of a three-dimensional anionic framework obtained by the association on MO6, NiO6, and PO4 polyhedra, sharing edges and corners. The resulting three-dimensional structure creates monodimensional ch...