First-principles investigations into the thermodynamics of cation disorder and its impact on electronic structure and magnetic properties of spinel Co(Cr_1–xMn_x)₂O₄

Abstract

Cation disorder over different crystallographic sites in spinel oxides is known to affect their properties. Recent experiments on Mn doped multiferroic CoCr2O4 indicate that a possible distribution of Mn atoms among tetrahedrally and octahedrally coordinated sites in the spinel lattice give rise to different variations in the structural parameters and saturation magnetisations in different concentration regimes of Mn atoms substituting the Cr. A composition dependent magnetic compensation behaviour points to the role conversions of the magnetic constituents. In this work, we have investigated the thermodynamics of cation disorder in Co (Cr1−xM nx) 2 O4 system and it's consequences on the structural, electronic and magnetic properties, using results from first-principles electronic structure calculations. We have computed the variations in the cation-disorder as a function of Mn concentration and the temperature and found that at the annealing temperature of the experiment many of the systems exhibit cation disorder. Our results support the interpretations of the experimental results regarding the qualitative variations in the sub-lattice occupancies and the associated magnetisation behaviour, with composition. We have analysed the variations in structural, magnetic and electronic properties of this system with variations in the compositions and the degree of cation disorder from the variations in their electronic structures and by using the ideas from crystal field theory. Our study provides a complete microscopic picture of the effects that are responsible for composition dependent behavioural differences of the properties of this system. This work lays down a general framework, based upon results from first-principles calculations, to understand and analyse the substitutional magnetic spinel oxides A (B1−xCx) 2 O4 in presence of cation disorder.

Role of Point Defects in Spinel Mg Chalcogenide Conductors

“…, we have demonstrated that the dominant defects in the chalcogenide spinels are anti-sites, signifying that p-type Mg In and n-type In • Mg are the primary intrinsic defects in MgIn 2 S 4 , while Sc • Mg or Mg Sc are the main defects in MgSc 2 S 4 (Se 4 ).These findings are similar to previous computational[75][76][77][78] and experimental 72 studies on ternary oxide spinels, with anti-sites dominating over other intrinsic defects, i.e. vacancies and interstitials.…”

Site Occupancies and Their Effects on the Physical Properties of Spinel : An Ab Initio Study

“…The details of calculations of the thermodynamic free energy, and the procedure of estimating the equilibrium y = 0 for a given temperature T and Fe concentration x have also been discussed in ref. . As done in ref.…”

“…We have modeled the system for various values of x and various degrees of cation disorder y according to the scheme presented in ref. . The details of calculations of the thermodynamic free energy, and the procedure of estimating the equilibrium y = 0 for a given temperature T and Fe concentration x have also been discussed in ref.…”

“…As done in ref. the magnetic configuration in all cases has been taken to be Néel configuration, with spins of A and B sub‐lattices anti‐aligned. Before fixing Néel configuration as the magnetic configuration, we have done several calculations with different spin configurations at different sub‐lattices.…”

“…Structural and magnetic measurements for Mn substituted CoCr 2 O 4 (Co) led the experimentalists to conclude that depending on the concentration x of the substituting Mn, the occupancies at the A and B sites change considerably, leading to phenomena such as magnetic compensation . The experimental observations and the inferences were complemented by the density functional theory based calculations, in conjunction with a thermodynamic model . In this first‐principles calculation, the site occupancies at A and B were obtained from a thermodynamic analysis, and the observed anomalous behavior of the total magnetization was analyzed based upon that.…”