Investigation On Crystallographic Chemistry,Magnetic And Electrical Properties Of Double Perovskite Cuprates La₂CuMO₆(M=Ti,Mn,Ru)

Perovskite copper oxides have attracted extensive attention due to their unique structural characteristics and rich magnetic and electrical properties.For La₂Cu MO₆double perovskite system,the valence layer electron energy state and d-p orbital hybrid mode of M ions play a key role in the change of B site lattice chemical environment.Therefore,the valence of B site ion bond and the ionic electronegativity associated with(BVS)will determine the covalency of M-O chemical bond,which is closely related to the charge,orbit and spin order of the system.In this paper,a series of Cu based perovskite oxides La₂Cu MO₆(M=Ti,Mn,Ru)have been synthesized by high temperature solid-state method and sol-gel method.The results of full spectrum fitting and structure refinement of neutron powder diffraction and X-ray powder diffraction show that La₂Cu Ti O₆ is an orthorhombic Pnma(No.62)space group,and its strong anisotropic lattice strain and abnormal uplift at the back of diffraction spectrum can be attributed to the coordinated Jahn-teller lattice distortion induced by short-range order of Cu-Ti ions.High resolution transmission electron microscopy confirmed that the system has local ordered structure.The effective magnetic moments of Cu²⁺ions obtained by paramagnetic Curie Weiss fitting are close to the values of the spin only model of Cu²⁺ions,which further confirms the existence of Cu-Ti short-range ordered phase.The electrical properties of the system reveal that the resistivity of the system is very high from room temperature to 200°C,and the change of the resistivity in the high temperature region is consistent with the non adiabatic small polaron transition mechanism.La₂Cu Mn O₆ and La₂Cu Ru O₆ are monoclinic P2₁/n(No.14)space groups,in which M⁴⁺and Cu²⁺ions at B site are arranged in a long-range order of Na Cl type rock salt.Because Mn⁴⁺/Ru⁴⁺ions do not have the second-order Jahn teller activity of Ti⁴⁺ions,the B-site lattice distortion is strongly suppressed,which is conducive to d-p orbital hybridization.Magnetic characterization confirmed that the two systems have strong spin frustration,and the weak ferromagnetic behavior can be attributed to the spin tilt caused by the spin coupling competition of energy degeneracy.DC electrical properties is found that the two systems have excellent carrier transport ability in the temperature range from room temperature to 200°C,and the resistivity changes in the high temperature region also conform to the non adiabatic small polaron transition mechanism.In order to quantitatively describe the dependence of M-O bond covalent on the magnetic and electrical properties of the compounds,BVS modified ion electronegativity evaluation scale is proposed,and the covalent ratio of different M-O chemical bonds is calculated systematically.The results show that the degree of orbital hybridization of d(M)-p(O)-d(Cu)is closely related to the covalent ratio of M-O bond.d-electrons show strong off domain behavior in high covalent M-O bonds,and strong local behaviors in high ionic Cu-O bonds,which affect the phonon spin/electron coupling mode and intensity in cu-m ordered lattice.In conclusion,the BVS modified electronegativity of M ions in La₂Cu MO₆(M=Ti,Mn,Ru)and the influence mechanism of covalent ratio of M-O bond on the crystal structure,magnetic and electrical properties of the compounds are discussed in this paper,which provides a new idea for better understanding the structure property relationship of double perovskite copper oxides.