First-principles Study On Electronic And Magnetic Properties Of Transition-metal Compounds With The Perovskite-type Structure

In this dissertation, by means of first-principles calculations within density functionaltheory, we investigate several transition-metal compounds with the perovskite and relatedstructures, concentrating on their structural, electronic, magnetic and ferroelectricproperties.The electronic and magnetic properties of BiCrO₃are investigated by using thegeneral gradient approximation （GGA） and GGA+U methods. It is found that the groundstate of BiCrO₃is a moderately correlated Mott-Hubbard insulator with the G-typeantiferromagnetic structure, which is in well agreement with the experiments. Moreover,the spin exchange constants between the Cr ions are estimated through the total energiescalculated for various magnetic orderings. The accuracy of these estimates is tested bycalculating the antiferromagnetic transition temperature TNfrom the molecular field theory.Additionally, the magnetic interaction in BiCrO₃is ascribed to the superexchangemechanism.The structural, electronic, and magnetic properties of a newly discoveredantiperovskite superconductor ZnNNi₃and related compounds ZnCNi₃and ZnNi₃areinvestigated based on full-potential linearized augmented plane wave method within theGGA scheme. It is found that the electronic structures of ZnNNi₃and ZnCNi₃are verysimilar. Contrarily, the band structure and Fermi surface in ZnNi3is changed considerably.Based on the free electron model, the Sommerfeld coefficients and the molar Pauliparamagnetic susceptibility for these compounds are evaluated. Furthermore, we alsoinvestigate the influence of the N/C-defect on the electronic and magnetic properties ofZnNNi₃and ZnCNi3. It is revealed that ZnCNi₃is more sensitive to the defect thanZnNNi₃, which may explain the fact why superconductivity has not yet been observed inZnCNi₃.The electronic structure and magnetism of layered oxyselenide La₂Mn₂Se₂O₃arestudied by using the GGA and GGA+U approaches. The G-type antiferromagnetic state isfound to be the most stable phase among the various magnetic configurations of interest, irrespective of the choice of the functional used, which is in well agreement with theexperiments. The GGA predicts the ground state as a semiconductor with an indirectbandgap of about0.52eV. This is related to a closed shell configuration and largeexchange splitting in the Mn3d states. Moreover, the magnetic properties are alsodiscussed in terms of the calculated Heisenberg spin exchange constants, suggesting astronger two-dimensional magnetically frustrated character for La₂Mn₂Se₂O₃comparedwith La₂Fe₂Se₂O₃and La₂Co₂Se₂O₃.By using the GGA and hybrid functional （HSE） methods, we study the structural,electronic and ferroelectric properties of the two recently synthesized high-pressureperovskite-type （orthorhombic, space group Pnma） and LiNbO3-type （rhombohedral,space group R3c） polymorphs of CdPbO₃. Besides providing structural and electronicresults in good agreement with available experiments, our results are able to correctlydescribe the pressure-induced Pnma to R3c structural phase transition, and mostimportantly predict the realization of proper ferroelectric behavior in LiNbO3-typeCdPbO₃with an electric polarization of52.3μC/cm². The proper covalent interactionmechanism driving the ferroelectric transition is discussed and explained in term of theanalysis of Born effective charges, potential-energy surfaces, charge density isosurfaces,and electric localization function.