The Development Of Theoretical Method And Theoretical Studies Of Ferroelectric Properties Of Transition Metal Oxides

Transition metal oxides are widely studied due to their abundant phase diagrams and physical properties.But just due to their complexities,there are still many problems to be solved in the study of transition metal oxides.We will perform theoretical and computational studies of the transition metal oxides from three aspects,i.e.development of the method,prediction of the properties,and discovery of new materials.Development of the method.Now the Density Functional Theory-based first principles calculations cannot describe the electronic structures of transition metal oxides such as zinc oxide?ZnO?precisely.We thus develop the hybrid functional pseudopoten-tial and perform first principles hybrid functional calculations with such pseudopotential.Employing this method,we for the first time reproduce the electronic structure of ZnO with just one tunable parameter in the framework of hybrid functional.We further generalize the method to a set of other binary semiconducting 3d transition metal oxides,finding that this method performs better than the traditional hybrid functional method for the electronic structure and have very good transferability.Prediction of the properties.Now studies about the atom-substituted relaxor ferroelectrics are mainly from experiments,and there are very few studies that provide a systematic prediction for the effect of substitution.In this work,we study systematically the transition metal atom substitution effect on the properties of a typical relaxor ferroelectric material PMN-PT.We find that different atom substitution has very different effects on the properties.For example,replacing Mg by Cu or Zn will enhance the structural distortion and polarization remarkably,which results in higher characteristic temperature T_m?However,replacing Ti by Mo or Tc will reduce greatly the structural distortion and polarization,and the T_mis decreased much,but the dispersion of T_mmay be increased.Such differences arise from the differences in the covalent bonds between the transition metal atoms and oxygen,instead of the differences in the cations'effective radii.Discovery of new material.Multiferroics,especially the two-dimensional multiferroics attract wide spread attention due to their rich properties and potential applications.In this article,we discover a new kind of two-dimensional multiferroic materials VOX₂?X=Cl,Br,I?with just one kind of cation.Our studies reveal that the coexistence of ferroelectricity and magnetism arises from the special electronic structures of the special crystal structures,that is,the d electron of the V⁴⁺is distributed in the plane perpendicular to the polarization which results in no screening of the ferroelectric distortion.The ferroelectricity of such systems is attributed to both the phonon mode softening by the V-O covalent bonding and the hybridization of the V d and X p.Our investigations indicate that no extra driving force is needed for the coexistence of both ferroic orders.That the V⁴⁺contributes to both the ferroelectricity and magnetism may indicate a degree of coupling between them.We also find that replacing V by Ti forms a new kind of stable layered ferroics.