Synthesis and Characterization of MnCrO₄, a New Mixed-Valence Antiferromagnet

A new orthorhombic phase, MnCrO₄, isostructural with MCrO₄ (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH₄)₂Cr₂O₇ + 2 Mn­(NO₃)₂, followed by calcination at 400 °C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO₄ phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn²⁺Cr⁶⁺O₄. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at <i>T</i>_N ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above <i>T</i>_N. MnCrO₄ is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 °C to form first Cr₂O₃ plus Mn₃O₄ and then Mn_1.5Cr_1.5O₄ spinel

Synthesis and Characterization of MnCrO₄, a New Mixed-Valence Antiferromagnet

A new orthorhombic phase, MnCrO₄, isostructural with MCrO₄ (M = Mg, Co, Ni, Cu, Cd) was prepared by evaporation of an aqueous solution, (NH₄)₂Cr₂O₇ + 2 Mn­(NO₃)₂, followed by calcination at 400 °C. It is characterized by redox titration, Rietveld analysis of the X-ray diffraction pattern, Cr K edge and Mn K edge XANES, ESR, magnetic susceptibility, specific heat and resistivity measurements. In contrast to the high-pressure MnCrO₄ phase where both cations are octahedral, the new phase contains Cr in a tetrahedral environment suggesting the charge balance Mn²⁺Cr⁶⁺O₄. However, the positions of both X-ray absorption K edges, the bond lengths and the ESR data suggest the occurrence of some mixed-valence character in which the mean oxidation state of Mn is higher than 2 and that of Cr is lower than 6. Both the magnetic susceptibility and the specific heat data indicate an onset of a three-dimensional antiferromagnetic order at <i>T</i>_N ≈ 42 K, which was confirmed also by calculating the spin exchange interactions on the basis of first principles density functional calculations. Dynamic magnetic studies (ESR) corroborate this scenario and indicate appreciable short-range correlations at temperatures far above <i>T</i>_N. MnCrO₄ is a semiconductor with activation energy of 0.27 eV; it loses oxygen on heating above 400 °C to form first Cr₂O₃ plus Mn₃O₄ and then Mn_1.5Cr_1.5O₄ spinel