Synthesis,Crystal Structure And Physical Properties Of Layered Transition-metal Mixed-anion Compounds

Since the discovery of high-temperature superconductivity in cuprates,in which two-dimensional Cu02 planes serve as active conductive layer,exploring layered compounds has been an interest-ing route to search for superconductivity.The discovery of another member of high-temperature superconductor family,Iron based superconductors with planar Fe-square nets that are sandwiched by pnictogen or chalcogen monolayers,made the 3d-transition-metal systems become a flashpoint to search for potential superconductors.In these fascinating materials,superconductivity emerges as the competing orders,such as antiferromagnetism and charge-or spin-density wave,are suffi-ciently suppressed.In this dissertation,we focus on new layered 3d-transition-metal mixed anion compounds which are potential candidates for superconductivity.We synthesized several layered 3d-transition-metal mixed anion compounds,including Ba2Ti2Cr2As4O and Rb1-?V2Te2O(?? 0.2),Rb1-xV2Te2O,V2Te2O as well as AV2Ch2O(A?Cs,Ch =Te;A =Rb,Ch ?Se,S;A =K,Ch =Te,Se,S)with[V2O]sheets,via sold state and topochemical deintercalation,and investigated their crystal structures as well as physical properties.1.Ba2Ti2Cr2As40 crystallizes in a tetragonal lattice(I4/mmm),which can be regarded as an intergrowth of BaCr2As2 and BaTi2As20 along the c axes with mixed occupancies among Cr atoms and Ti atoms.The material is of metallic conduction accompanied by an anomaly at around 350 K,probably related to AFM transition in the[Cr2As2]2-layers.No obvious density wave instability is detected in the[Ti2As20]2-layers,while the superconductivity is still beyond reach above 2 K.2.Rb1-?V2TesO(?? 0.2):The crystal structure of Rb1-?V2Te2O bears two-dimensional V20 square nets sandwiched with tellurium layers.This compound exhibits metallic conductivity while a weak metal-to-metal transition(MMT)takes place at?100 K.Neither Bragg-peak splitting nor superlattice reflections can be detected within the resolution of conventional x-ray diffractions.Electronic structure calculations suggest dyz/dxz orbital polarization,and spin-density-wave or orbital order for the lower-temperature state.The solid solution of vanadium oxytelluride Rb1-xV2Te2O(0 ? x ? 0.35)has been syn-thesized by solid state reactions and characterized by x-ray diffraction(XRD),resistivity and mag-netic susceptibility measurements.The lattice parameters evolve systematically with increasing Rb vacancy,while the temperature of the metal-to-metal transition gradually reduces and eventually vanishes.Meanwhile,the Te-V-Te angles are transforming towards 90°,which reduces the struc-tural distortion of the VTe402 octahedra.However,superconductivity is still absent down to 0.3 K in this system..3.V2Te2O:we applied topochemical deintercalation methods and successfully removed all of the Rb,to gain a new phase V2Te20.The material possesses Te-terminated[V2OTe2]sandwich layers,stacking along the c axis with van der Waals interactions.The physical-property measure-ments and first-principles calculations indicate that it is a Pauli-paramagnetic metal with enhanced electron correlations.While the MMT is completely suppressed,superconductivity has not been observed yet above 0.03 K.4.AV2Ch20(A =Cs,Ch =Te;A =Rb,Ch =Se,S;A =K,Ch =Te,Se,S):We suc-cessfully synthesized several new layered transition-metal mixed anion compounds,denoted as AV2Ch2O,and investigated with XRD,resistivity and susceptibility measurements.Generally,AV2Te2O(A =Cs,Rb,K)are of metallic conduction,while AV2Ch2O(A =Rb,K Ch =Se,S)exhibit semiconductor-like behavior.No superconductivity has been observed above 2 K.The nine new layered 3d-transition-metal mixed anion compounds possess the structural sig-naures of cuprate and iron-based superconductors with similar anomalies.However,no supercon-ductivity is observed in these new compounds.