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 CuO₂ 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 Rb_1-?V₂Te₂O???0.2?,Rb_1-x-x V₂Te₂O,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.Ba₂Ti₂Cr₂As₄O crystallizes in a tetragonal lattice?I4/mmm?,which can be regarded as an intergrowth of BaCr₂As₂ and BaTi₂As₂O along the c axes with mixed occupancies among Cr atoms and Ti atoms.The material is of metallic conduction accompanied by an anomaly at around350 K,probably related to AFM transition in the[Cr₂As₂]^2-layers.No obvious density wave instability is detected in the[Ti₂As₂O]^2-layers,while the superconductivity is still beyond reach above 2 K.2.Rb_1-?V₂Te₂O???0.2?:The crystal structure of Rb_1-?V₂Te₂O bears two-dimensional V₂O 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 d_yz/d_xzorbital polarization,and spin-density-wave or orbital order for the lower-temperature state.The solid solution of vanadium oxytelluride Rb_1-x-x V₂Te₂O?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 VTe₄O₂ 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 V₂Te₂O.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.AV₂Ch₂O?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,AV₂T e₂O?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.