Synthesis,Structure And Physical Properties Of Low Dimensional V,Nb,Cr-Based Transition Metal Chalcogenide Single Crystals

Low-dimensional transition-metal chalcogenides have attracted a great attention in the fields of quantum materials and energy conversion due to their various structural characteristics and novel physical properties.However,an efficient strategy for exploring and synthesising new low dimensional transition metal chalcogenides is still lack of research,which is mainly resulted from the non-integer oxidation state of the metal atoms.Besides,growth of high quality single crystal is still challenging due to the high melting point of transition metals.Moreover,complex structural distortions occur in this system.These strongly hinder the design,discovery and development of low-dimensional transition metal chalcogenides.In this paper,we present the design and synthesis of low-dimensional transition metal chalcogenides in single crystal form in V,Nb and Ni systems and study the relationship between composition,crystal structures,band structures and physical properties.The main results are concluded as bellow:（1）We propose a eutectic polytelluride flux strategy as an excellent solution to address phase discovery and crystal growth in low dimensional transition metal tellurides.Utilizing a eutectic "K₃Te₄" polytellurdie flux,new phases are easily discovered and selectively synthesized by adjusting the ratio between Nb metal and flux in starting materials（K:Nb:Te₌3:x:4）.With high content of Nb in solvent（x=2 and 1）,the obtained crystals are KNb₃Te₃O_0.38 and K_0.9Nb₃Te₄,which are both subtellurides with diverse Nb clusters including face-sharing and edge-sharing Nb₆ octahedral columns and zig-zag Nb chains.Reducing the Nb content x to 0.33 leads to products as layered compound K_1.06NbTe₂ composed of[NbTe₆]trigonal prism with K intercalated between the layers.Both K_0.9Nb₃Te₄ and K_1.06NbTe₂ show a semimetallic ground state.Conversely,KNb₃Te₃O_0.38 exhibited semiconductor-like behavior below 29 K.single crystals of the known binary Nb tellurides can also be grown,and the obtained NbTe₂ shows a new polymor-phism with an extra trimerization along the b axis in Nb double zig-zag clustering.The precise control of structural dimensionality and oxidation state,combined with the facile crystal growth process make our synthetic strategy as an efficient route to access the new potential quantum materials in transition metal chalcogenide system.（2）We synthesized needle-like crystals of the V-cluster-based quasi-onedimensional（Q1D）materials AV₃Q₃O_δ（A=K,Rb,Cs,Q=Se,Te_）.We examine how changes in their average and local structure control their electronic properties.All compounds crystallize in the TlFe3Te₃-type structure（P6₃/m space group）with infinite（V₃Q₃）-double walled columnar chains separated by A+cations.Our single-crystal and synchrotron powder diffraction studies indicate oxygen atoms partially occupy the center site of the V₆ octahedral metal cluster cages in KV₃Te₃O_0.33,RbV₃Te₃O_0.32 and CsV₃Te₃O_0.35.whereas KV₃Se₃ is structurally oxygen-free.Our synchrotron X-ray pair distribution function（PDF）analyses indicate that the oxygen-free[V₆]cluster octahedra in KV₃Se₃ are highly distorted perpendicular to the chain direction.In contrast,the oxygen-centered V cluster in KV₃Te₃O_0.33 exhibits a V₃-triangle-trimerization along the chain direction.Resistivity measurements indicate that KV₃Se₃ exhibits metallic behavior,whereas a dramatic metal-semiconductor-metal transition emerges in KV₃Te₃O_0.33,RbV₃Te₃O_0.32,and CsV₃Te₃O_0.35 because of oxygen disorder and changes in local structure captured from our electronic structure analyses of the Fermi surface.Our investigation of the AV₃Q₃O_δ family demonstrates the importance of understanding local changes in structure driven by electronic instabilities,which can guide the search for new quantum materials in other lowdimensional cluster-compound materials.（3）We report a new heterolayered compound CrNiTe₄,which is constructed by CrTe₂ and NiTe₂ layers.The Ni-Te bond lengths in[NiTe₆]octahedron of is 2.5811（12）?,and the in-plane Te-Ni-Te bond angle is 83.05（3）°;The Cr-Te bond lengths（2.7019（13）?）and in-plane Te-Cr-Te bond angles（88.68（3）°）are comparable to the binary phases.The title compound is stable in atmospheric environment below 200℃.Magnetic measurements show that the Curie temperature of CrNiTe₄ is close to room temperature with T_c=290 K.A crossover from negative to positive magnetoresistance was also observed between 200 and 100 K,which is caused by the change of carrier type.The nanoflakes of CrNiTe₄ can be obtained by mechanical exfoliation.Our work report the first heterolayered crystal composed by alternately stacking two TMDCs layers.Given the stability and high Curie temperature,we suggest that CrNiTe₄ is promising for application in electronic and spintronic devices.