Synthesis and characterization of the phosphates Na1+xMg1+xCr2−x(PO4)3 (x = 0; 0.2) and NaZnCr2(PO4)3 with the α-CrPO4 structure

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Three isostructural chromium phosphates, Na1+xMg1+xCr2−x(PO4)3 with x=0 (I) and 0.2 (II), and NaZnCr2(PO4)3 (III) have been synthesized for the first time. Their analysis by single and powder X-ray diffraction techniques showed that they crystallize in the orthorhombic system with space group Imma. The structure is closely related to that of the α-CrPO4 type with one main feature being a disorder, in the studied compounds, between the Cr3+ and M2+ (M=Mg or Zn) cations. The 3D framework is built up by PO4 tetrahedra, M(1)O6 octahedra and M(2)2O10 bioctahedral units of edge sharing octahedra, linked through common corners and edges. This framework delimits interesting tunnels in which the Na+ cations are located.The observed bands in the FT-IR spectra for I and III were assigned to corresponding vibrations of the PO4 tetrahedra. The EPR spectra exhibit two different resonance lines S1 and S2, characteristic of octahedral Cr3+ ions. The thermal evolution of EPR integral intensities of these signals were assigned to antiferromagnetic (AFM) coupling between nearest neighbors Cr3+ ions. The AFM ordering within these compounds at low temperature was also revealed from the thermal dependence of the molar magnetic susceptibility. At high temperature, a deviation from the Curie–Weiss law is observed and it was attributed to a magnetic frustration phenomenon.