Theoretical Study On Properties Of Magnesium Aluminates Spinel Materials

Spinel series materials, represented by magnesium aluminates spinel distribute widely in nature, their physical and chemical properties have been widely used in metallurgical, chemical, building materials, refractory and many other fields. Spinel series materials also show a good application prospects in other emerging areas such as electronics, information, communication, household appliances, aviation, aerospace, military, instrument, meter, automation and other areas. The theoretical and application research in Mg-Al spinel series materials has become one of the hot areas in material science. In this paper, the physical properties of Mg-Al spinel materials are studied using the plane-wave pseudopotential method (PWP) based on density functional theory (DFT) theory.First, the high-pressure phase transition path of magnesium aluminates spinel is modeled, phase transition pressure of three-phase magnesium aluminates spinel is calculated by state equation and thermodynamic formation enthalpy. It is found that the first method is suitable for calculating the phase transition under lower pressures, while at high pressures the error is large. The applicability of the second approach is stronger, the calculated results are rational. The research in the pressure reduced effect on the elastic constants and the bulk modulus for spinel phase and two high-pressure phases reveals that the variations in the three-phase elastic properties with pressures follow the linear Murnaghan relation. The research in mechanical stability reveals that the spinel phase is mechanically stable under the pressures of 0～30GPa, while the other two high-pressure phases are stable under the considered pressures of 0～50GPa.Secondly, three distorted lattice models are designed, according to the characteristics of spinel structure, by changing the location parameter of oxygen atoms around the ideal locations in the lattice. The impact of weak lattice distortion on the elastic properties of Fe-Cr spinel (FeCr₂O₄) is studied. The results show that the elastic properties are significantly dependent on the changes in lattice structure. The slight lattice changes make the bulk modulus of Fe-Cr spinel decrease. With increasing the location parameters of oxygen atoms, Young's modulus increase and the shear modulus decrease. Analyses of electronic properties of three slightly distorted structures show that the variations in the elastic properties are due to the electronic exchange between the Fe-Cr-atoms.Thirdly, the transition metal ion doping magnesium aluminates spinel A-cordinated and B-cordinated substitution effect and the effect of the manganese ion doping concentration on the electronic structure and optical properties of doped crystals are systematically studied. The microscopic reasons for the lattice distortion of the doped crystals are analyzed and corresponding microscopic models are established. The studies in the spin configuration of the Co³⁺ ion substitution in octahedron reveal that Co³⁺ ions in the octahedra take the forms of low-spin configuration, that is to say, the configurations have no spin structure, leading to the difficulty of achieving octahedral cobalt ions substitute. The electronic structure analysis of the doped crystals shows that the interaction between doping ions and oxygen ligands determines the two optical transition processes. The microscopic reasons for the fact that for all the doping systems absorption band edges occur red shift and apparent absorption takes place in the visible and near infrared region of doping system are interpreted according to the electronic structure of the doping system. The influences of the manganese ion concentration on the lattice constant, electronic structure and the optical absorption coefficient, taking tetrahedronly coordinated Mn-ions as an example, are studied. The microscopic mechanism of the doping ion concentration changes contributing to the variation in the lattice constant is illuminated. The effects of the manganese ion doping concentration on the electronic structure and the optical absorption coefficient are analyzed.In addition, the electrical and magnetic properties of CaFe₂O₄-XMn₂O₄ (X=Li,Na), which has the same structure of the spinel high-pressure phase CF are systematically studied. The investigation in the electromagnetic properties of CaFe₂O₄-LiMn₂O₄ show that the property of CaFe₂O₄-LiMn₂O₄ is consistent with that of same structral hypoxia Li_0.92Mn₂O₄, both of them are antiferromagnetic semiconductor mixed-valent compounds with the mechanical stability and disordered distribution of the charge. It is predicted that CaFe₂O₄-NaMn₂O₄ is an antiferromagnetic semiconductor mixed valence compound with charge, spin and orbital ordering. The specific physical phenomena of this compound distinguished from those of CaFe₂O₄-LiMn₂O₄ are explained. The microscopic reasons for the entirely different physical properties of the two manganese oxides with same structures are illuminated. It is predicted that CaFe₂O₄-NaMn₂O₄ is possible to become an applicable spin-electronic material.