Tuning the Mn valence state in new Ca0.66Mn2−xAlxO4 (x ≤ 0.4) oxides: impact on magnetic and redox properties

Abstract

New Ca_0.66Mn_2−xAl_xO₄ (x ≤ 0.4) solid solutions crystallizing with the CaFe₂O₄-type structure (SG: Pnma) were synthesized for the first time by the glycine-nitrate process. The structures were determined on the basis of XRD Rietveld analysis and electron diffraction investigation. While the CaMn₃O₆ (‘Ca_0.66Mn₂O₄’) oxide adopts a monoclinic unit cell, the Al substitution for Mn (x = 0.2, 0.4) leads to an orthorhombic cell with only two Mn atomic positions, with different valence states, and 33% of Ca sites empty. The Ca molar concentration decreases down to 0.6 in order to increase the Mn valence leading to a phase mixture, whereas a slight Ca content increase up to 0.7 leads to a decrease of Mn valence in the pure phase. The Al³⁺ ions are located at a specific Mn site because their ionic radii are close to that of Mn⁴⁺ and a more isotropic environment. The unit cell parameters and volume strongly decrease for a low Al content and tend to an asymptotic value of x = 0.33–0.4, around the limit of solubility. As the Al content increases, the Mn valence state in the same slightly distorted octahedral site increases up to 4+ whereas the other octahedral site is highly elongated and corresponds mainly to Jahn–Teller Mn³⁺. At x = 0.33, these two Mn sites correspond to Mn⁴⁺ and Mn³⁺ respectively. Moreover, the aluminium content increase induces a weakening of the global antiferromagnetic long range interactions between the ferromagnetic chains. The Al substitution leads to the change of the Mn valence distribution as well as the unit cell symmetry of the CaMn₃O₆ phase. These 1D tunnel networks stabilizing the Mn³⁺/Mn⁴⁺ valence states can be reduced under Ar/5%H₂ between T = 300 °C and T = 600 °C (heating rate = 2 °C min⁻¹) into pure Mn²⁺ rocksalt solid solution despite the large difference in ionic radii. The re-oxidation leads to the same CaFe₂O₄-type structure and several redox cycles can be operated. The relationship between the two double chains of the edge-sharing octahedral sites and the rocksalt-type framework is clear and should appear as the driving force for the structural transformation during the reduction/oxidation processes. Finally, Al substitution allows an increasing of the Mn–O bond covalence and consequently the reduction in temperature.