Periodic DFT Study of the Structural and Electronic Properties of Bulk CoAl₂O₄ Spinel

Abstract

In this study, structural and electronic properties of CoAl2O4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co2+ and Al3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al3+ at tetrahedral sites (and Co2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl2O4 can be interpreted as dominated by the local behavior of Co2+ at octahedral sites.

Optical characterization of fourfold (Td)- and sixfold (Oh)-transition-metal species in MgAl2O4:Co2+ by time-resolved spectroscopy

“…The present finding illustrates the complexity of investigating PL properties in the spinel doped with transition-metal ions, due to firstly the possibility of multiple sites for a given impurity [6][7][8]11] and the presence of unintentional impurities, which are even present in nominally pure spinels [9,12,13]. Both effects make the study of transition-metal PL in MgAl 2 O 4 subtle and complicated as it was pointed out in previous investigations on this crystal [7][8][9][10][11][12][13] and some other stoichiometric n.Al 2 O 3 :m.MgO doped with Co 2+ and Cr 3+ [11,13,14] report different PL bands as well formation of distinct Co 2+ and Cr 3+ sites with sixfold and fourfold coordination. The origin of some PL bands and the structure of the formed Co 2+ and Cr 3+ species are still controversial and deserve clarification.…”

Zircon to monazite phase transition in CeVO4: X-ray diffraction and Raman-scattering measurements

“…The thermodynamic phase transition between two structures occurs when the Gibbs free energy (G) is the same for both phases. In order to obtain the Gibbs free energy we use a quasi-harmonic Debye model that allows obtaining G at room temperature from calculations performed for T= 0 K [34]. Figure 6(b) shows the pressure dependence of the Gibbs free energy difference at T = 300 K for the scheelite and the monazite phases with respect to the zircon phase, which is taken as reference.…”

Theoretical investigation of the inversion parameter in Co3−sAlsO4 (s=0–3) spinel structures

“…All of these unit cell models incorporate the possibility of a choice of the particular coordinates of some of the cations among the different positions available in the tetrahedral and/or octahedral sites. As in our previous work [14], we have explored several configurations within the same value of the inversion parameter. Differences of energies are within the uncertainties of the total energy of the crystal (see also Ref.…”

“…For the Brillouin zone the number of k-points has been adjusted to the size of the supercell considered in order to keep the same density of k-points in the reciprocal space, within the Monkhorst-Pack scheme (6× 6 × 6 for the primitive cell and 6 × 6 × 3 for the double unit cell). The convergence of the energy with respect to the k-points was carefully checked [14].…”