Structure And Property Study Of Several Kinds Of Strongly Correlated Electron Materials

In this dissertation,we mainly studied the structures and physical properties of three kinds of compounds.The first is the cobalt doped bismuth layer magnetoelectric materials.The XRD,SAED,ZFC-FC,M-H and GSAS methods were used to study the dependence of crystal structure and magnetic property on the doping content.Moreover,a double Cs-corrected TEM was used to investigate the relationship between the crystal structures and magnetic properties.The second is the double perovskite Y₂CrMnO₆.By using atomic resolution HAADF and ABF images,SAED patterns,PPA software,first principles calculations,and image simulations on HAADF,ABF and SAED,two new kinds of layer structures of Y₂CrMnO₆ were disclosed and possible properties were predicted.The third is the studies on the crystal structural evolution of Co₂P₂O₇ under pressure by using in situ high pressure angle dispersive x-ray diffraction with synchrotron radiation.Furthermore,the first principles calculations and XRD patter simulations were also carried out to reveal the intrinsic mechanisms of Co₂P₂O₇ phase transformation.The main points are summarized as follows:One of the core issues for the A/B site doping in the bismuth layer magnetoelectric materials is to find out the evolution of the crystal structure and elemental distribution,the magnetic structure,and the coupling effects between spin and lattice with the increase of ion substitution.Here,we have conducted systematic structural and physical property studies on the series samples of Bi₅Ti₃Fe_1-xCo_xO₁₅.This work represents that Bi₅Ti₃Fe_1-xCox O₁₅ forms a single four layer perovskite-like structure for 0?x<0.67,while begins to arise three layer perovskite-like structure block for x?0.67.With different cobalt contents,the compounds show antiferromagnetism,spin state determined magnetism,or magnetic anisotropy determined magnetism.The weak feromagnetism at room temperature is considered to be induced by the larger displacement of Co³⁺ions from the center of octahedra and the change of the spin state of Co³⁺ions.It was also observed that Fe and Co elements are homogeneously substituted in the three layer structure block accompanied with the rotation?and/or distortion?of BO₆ octahedra.Double perovskites are generally with small structure distortion induced by the expansion/contraction and tilt of the BO₆ octahedron.It is difficult to precisely determine their structures,especially the phase with notable pseudo-symmetry structure and two or more phases with slightly different structures.In this dissertation,the double perovskite Y₂CrMnO₆ synthesized under high pressure was studied by scanning transmission electron microscopy and first-principles calculations.The new finding is that a rock-salt ordered structure and a layer ordered structure are coexistent in Y₂CrMnO₆.Besides the Cu²⁺-based compound,this Mn³⁺-based compound is another new system with the layer ordered structure.Unlike the structures of YCrO₃and YMnO₃,the ordered structures of Y₂CrMnO₆ have half-metal characteristics.The crystal structural evolution of Co₂P₂O₇ was studied by using in situ high pressure angle dispersive x-ray diffraction with synchrotron radiation.The results demonstrate that?phase of Co₂P₂O₇ goes through a partially irreversible structural transformation to?phase under pressure.The pressure is conductive to reduce the longest Co-O bond length of?phase and then more uniform Co-O bonds and regular hexagonal arrangement of CoO₆ octahedra of?phase are favored.According to the Birch-Murnaghan equation,the fitted bulk moduli B₀ are 158.1?±5.6?GPa for?phase and 276.5?±6.5?GPa for?phase,respectively.Furthermore,the first principles calculations show that these two phases of Co₂P₂O₇ have almost equal total-energies,and also have similar band structures and spin-polarized density of states at their ground states.This may be the reason that these two phases of Co₂P₂O₇ can coexist in the pressure released state.It is found that the band gap energies decrease with increasing pressure for both phases.