Implications of including a magnetic ion (Cr3+ and Fe3+) at the vanadium site in a geometrically frustrated spinel MgV2O4: magnetic and catalytic properties

Abstract

The vanadium sublattice in a geometrically frustrated MgV₂O₄ is substituted partially with Cr³⁺ and Fe³⁺. With a successful Rietveld refinement of the powder X-ray diffraction patterns of MgVCrO₄ and MgVFeO₄, Cr and Fe occupy the octahedral sites of a spinel structure. Extensive field-dependent and temperature magnetic measurements on these samples reveal exciting results. MgV₂O₄ remains paramagnetic till 3 K, with a small divergence at around 20 K between ZFC and FC data. MgVCrO₄ exhibits antiferromagnetic behavior with a Néel temperature of 13.6 K. MgVFeO₄ shows spin-glass behavior resulting from the frustration with a glass transition temperature of 194 K. This sample shows a typical ferromagnetic behavior, with a coercivity of 194.5 Oe within an applied field of ±2 kOe. All three systems have been found to have a frustration index in the range of 1–25, and the effective magnetic moment decreases in the order MgVFeO₄ > MgVCrO₄ > MgV₂O₄. While the inclusion of chromium does not alter the bandgap of MgV₂O₄, iron substitution increases the bandgap. The partial replacement of V³⁺ with Cr³⁺ and Fe³⁺ increases the catalytic ability of the system in terms of the oxidative degradation of methylene blue dye. The catalytic efficiency follows the order MgVFeO₄ > MgVCrO₄ > MgV₂O₄, matching well with the trend noticed in the porosity, surface area, and redox ability of Fe³⁺, Cr³⁺ and V³⁺ in these samples. The degradation pathway has been followed by analyzing the intermediates from these experiments by mass spectrometry, and a plausible mechanism is proposed.