Crystal Structures And Dielectric Properties Of B-site Defected Hexagonal Perovskites La₂Ba₂MW₂O₁₂?M=Mn,Fe,Co?

B-defected hexagonal layered perovskite oxide A₄B₃O₁₂ has attracted much attention due to its diverse coordination stacking structure and rich physical and chemical properties.It is found that the ferroelectric relaxation behavior can be precisely controlled by introducing different types of transition metal ions at B site,which provides a useful reference for the design and development of high-frequency communication functional materials in the future.In this paper,three target compounds La₂Ba₂MW₂O₁₂?M=Mn,Fe,Co?were synthesized by high temperature solid-state reaction.Among them,a novel La₂Ba₂Fe W₂O₁₂compound was synthesized for the first time by the improved precursor two-step solid state method.The space group of the compounds is R-3?No.148?,and the stacking sequence of AO₃ is?hhcc?₃.In addition,the crystal structure analysis confirmed that La³⁺and Ba²⁺ions of A-site are arranged orderly along the c axis,and W⁶⁺ion towards to the A-site empty layer,which leads to the out-of-center distortion of WO₆ octahedron,thus stabilizing the coordination environment of O ion.The valence of B-site transition metal ions were determined by X-ray photoelectron spectroscopy combined with analysis of chemical bond valence sum.The ⁵⁷Fe M?ssbauer spectroscopy of La₂Ba₂Fe W₂O₁₂further confirmed that Fe ion exhibits+2 valence and mainly shows high-spin state?t2g⁴eg²?.By analyzing the M/W-O chemical bond parameters and surface electron energy band structure,it can be concluded that the different to out-of-center distortion of WO₆ and band gap are closely related to the electronic configuration of M²⁺ions.It is found that the high-spin electron configuration of Co²⁺ions can inhibit the second-JT distortion of WO₆octahedron,while the high-spin electron configuration of Mn²⁺and Fe²⁺ions can induce the out-of-center distortion of WO₆ octahedron,thus leading to the coordinated lattice distortion of B-site and significantly enhancing the covalent hybridization intensity of d(M²⁺)-p(O^2-).The dielectric properties of Fe and Co systems reveal significant low-temperature dielectric relaxation behavior.The dipole activation energy data indicate that the 3d electron short-range transition between M²⁺ions dominates the relaxation polarization of the compounds.It is speculated that the difference of relaxation degree is determined by the energy levels of different d(M²⁺)-p(O^2-)hybrid orbitals,which further affects the thermally activated electron hopping.In conclusion,the intrinsic relationship between the B-site ion electronic configuration,BO₆ octahedral lattice distortion and dielectric properties of the B-defected hexagonal perovskite oxide La₂Ba₂MW₂O₁₂ were revealed.It is proved that the dielectric relaxation behavior of the compounds in the low temperature can be attributed to the relaxation polarization mechanism of the electron short-range hopping of the transition metal ions at the B-site.The above work provides valuable experimental basis for revealing the intrinsic relationship between the crystal structure and physicochemical properties of this kind of perovskite.