Hydrothermal Synthesis And Magnetic Properties Of Transition Metal Fluoride Selenate

Nowadays,magnetic materials are widely used in electric motors,generators,loudspeakers,hard disks and magnetic refrigerators.Transition metal fluoroselenates have received great attentions owing to their remarkable physical and chemical characteristics,such as optical,electricitic,magnetic properties.The search for new materials with innovative architectures is the driving force of scientists,especially new materials with magnetic properties.The purpose of this thesis is to explore the 3d new transition metal fluoroselenates,and study its structure and magnetic properties.Two series of new transition metal fluoroselenates were synthesized by hydrothermal method.Structure determinations,and thorough characterization of new transition metal selenites were presented.The magnetic behavior of the compounds was studied by low temperature magnetic test and specific heat test.The details as follows:Two M₂（SeO₃）F₂fluoro-selenites(M=Mn²⁺,Ni²⁺)have been synthesized using optimized hydrothermal reactions.Their 3D framework consists of 1D-[MO₂F₂]^4-chains of edge-sharing octahedra with a rare topology of O-O and F-F alternatingμ₂bridges.The inter-chain corner sharing connections develops a complex set of M-X-M super-exchanges calculated by LDA+U.Their interplay induce prominent frustration in the Ni²⁺case,responsible for the cycloidal magnetic structure observed below T_N～21.5K.For comparison the Mn²⁺compound develops a nearly collinear spin（canted）ordering below T_N～26K with ferromagnetic chain-units.Three new alkali metal manganese fluoride selenate A₂Mn（SeO₄）F₃（A=K,Rb,Cs）have been synthesized through hydrothermal reactions using metal hydroxide as starting materials to balance hydrofluoric acid.All of the reported materials consist of one-dimensional_∞[Mn F₃O₂]^4-chains decorated with SeO₄^2-tetrahedral,where the A⁺cations are connected with each other via oxygen/fluorine atoms to form two-dimensional layers working as blocks.A₂Mn（SeO₄）F₃（A=Rb,Cs）are isotypic crystallizing in the monoclinic spacegroup P2₁/c,while K₂Mn（SeO₄）F₃adopted the orthorhombic space group Pbcn.A₂Mn（SeO₄）F₃（A=K,Rb,Cs）forms spin chains of Mn³⁺containing Mn-F-Mn bridges,showing canting AFM behaviors.