Study On The Structure And Magnetoelectric Properties Properties Of Transition Metal Compounds Containing Pb²⁺ And Bi（3+） Ions

Due to the spatial effects of 6s2 lone pair electrons,the compounds containing Pb²⁺ and Bi³⁺ ions show unique physical properties,such as ferroelectricity and nonlinear optical effect,and thus have attracted much attention in the field of materials science.This study focuses on two compounds with 6s2 lone pair electrons Pb₃Mn₇O₁₅ and MnBi2S4.We start rational design of new compounds and solid solutions based on these compounds according to its structure feature and the chemical similarity of transition metal elements.Then we use diffraction techniques（including Powder X-ray Diffraction and Neutron Diffraction）,Electron Microscopy techniques（including SEM and EDX）combined with other physical property measurements to determine their crystal structures,phase transitions,electrical and magnetic properties.Based on these results we discussed the relationship between the crystal structures and physical properties.The main results are summarized in the following:1.We optimized the synthesis condition for Pb₃Mn₇O₁₅ from MnO2 and PbO raw materials.The optimized temperature is 850 ?C with 5 wt% overdose of PbO.Pb₃Mn₇O₁₅ is orthorhombic（Pnma）crystal at room temperature,and changes to hexagonal（P63/mcm）at 142 ℃,and changes to another hexagonal structure at 300 ℃.Both phase transitions are revisable.Pb₃Mn₇O₁₅ starts to decompose to PbO and Mn3O4 spinel at 950 1 000 ℃.Pb4Mn9O20 decompose and transform into Pb₃Mn₇O₁₅ at 750 800 ℃ irrevisably.2.We identified two new solid solutions Pb3NixMn7-xO15-δ（0 ≤ x ≤ 0.6）and Pb3 Cox Mn7-xO15-δ（0 ≤ x ≤ 0.5）.Both solid solution has different crystal structures depending on the doping levels.Pb3NixMn7-xO15-δ is orthorhombic（Pnma）at 0 ≤ x ≤ 0.2 and hexagonal（P63/mcm）at 0.2 ≤ x ≤ 0.6.Pb3CoxMn7-xO15-δ is orthorhombic（Pnma）at 0 ≤ x ≤ 0.25 and hexagonal（Pnma）at 0.25 ≤ x ≤ 0.5..The valence state of Ni and Co is +3.The doping ions tend to occupy the position for Mn ion in the Kagome layers preferably,which frustrates the magnetic exchange interaction between the manganese ions in the layers and results in the shifting of magnetic transition to lower temperatures.Both solid solutions are semi conductors.As the doping amount increases,the resistivity decreased gradually.3.We identified a new solid solution Mn1–xCuxBi2S4（0.1 ≤ x ≤ 0.4）.The valence state of Cu,Mn and Bi is +1,+2 and +3 in the solid solution respectively.The substitution of Mn2+ by Cu+ is accompanied by the introduction of vacancies of positive charges.The substitution process of Cu+ in solid solution Mn1-x CuxBi2S4 can be described by the following formula:Mn²⁺→Cu⁺ + hole⁺The positive charge results in the presence of the nominal Bi⁵⁺ and Mn³⁺ ions,and also make the abnormal volume expansion with the doping amounts.The doping of Cu+ ions dilutes the interactions between the magnetic ions,and thus the antiferromagnetic order disappears in the solid solution with 0.2 ≤ x ≤ 0.4...