Synthesis And Physical Properties Of Several Group VB Metal Layered Compounds

In this dissertation,we focus on two aspects of group VB metal layered compounds.One is the quantum phenomena such as charge density wave,superconductivity,and quantum linear magnetoresistance in van der Waals layered compounds.The other is searching for new MAX phases with non-van der Waals layered structure and comprehensively characterizing their physical properties.The main results are listed as followed.First of all,anisotropic magneto-transport and magnetic properties of two VB group metal ditellereides,NbTe2 and TaTe2,are systematically studied to reveal their Fermi surface morphology.Anomalous linear magnetoresistance is observed in both 1T'-NbTe2 and LT-TaTe2.The linear magnetoresistance can be well explained by the quantum linear magnetoresistance model,which indicates the existence of a large normal Fermi surface and small Fermi pockets with linear band dispersions.The small Fermi pockets causes the linear magnetoresistance in 1T-NbTe2 and LT-TaTe2.What's more,a complicated anisotropy is observed in LT-TaTe2 under high fields and low temperatures but is absent in 1T'-NbTe2.The magnetoresistance anisotropy indicates the anisotropy of the small Fermi pockets in LT-TaTe2.Additionally,de Hass-van Alphen oscillations were observed when B||?001?or B??001?,the periods of oscillations and the area of Fermi surface cross sections are given.Our results provide helpful information to understand the Fermi surface morphology of 1T' phase and LT phase.Secondly,the charge density wave in Ta2S2C and Ta2Se2C are studied.Different from conventional charge density wave with single metal layer,we firstly found charge density waves with double metal layers in Ta2S2C and Ta2Se2C.Superlattices with 3^1/2×3^1/2 and 7^1/2 ×7^1/2 were observed in Ta2S2C and Ta2Se2C,respectively.We proposed four configurations of the commensurate charge density waves,and point out the phase shift between double metal layers will significantly affect the final configuration.Thirdly,the doping effects on charge density wave and superconductivity are presented here,and a detailed electronic phase diagram of Ta2S2-xSexC is given.We found no superconductivity above 0.3 K in Ta2S2C,and confirmed that Ta2Se2C is a superconductor with Tc=1.07 K.The commensurate charge density wave is gradually suppressed by the increase of Se content x,and superconductivity will emerge.When the Se content x is increased to?0.8,the commensurate charge density wave is almost suppressed,the superconductivity will reach to the optimization Tc?4 K.In our case,the superconductivity not only competes with commensurate charge density wave but also can coexist with it.The results of heat capacity and critically fields of superconducting transition prove that Ta2S2-xSexC is a type-? weak coupling superconductor.Finally,four new MAX phases,Nb3As2C,Nb3P2C,Ta3P2C,and V3As2C,are found by us.The new MAX phases are expressed by M3A2X,which is different from conventional MAX phases as expressed by Mn+iAXn.The discovery of 321 phases breaks the conventional knowledge about MAX phases.Experimental results combine with the first principle calculations proved the excellent mechanical and electrical properties in 321 phases.The measured bulk modulus of Nb3As2C is up to 225?5?GPa,the value is one of the highest among MAX phases.The electronic structures of 321 phases are quite different from the ones in Mn+iAXn.No charge compensation exists in 321 phases,which is confirmed by the Hall effect measurements and the calculated Fermi surface morphology.The Fermi surface of 321 phases is mainly composed of two large Fermi surfaces with open orbits and a small hole pocket near K point.