The Exploration Of Synthesis And Physical Properties Of Layered Transition Metal Compounds

The layered transition metal compounds is always the hot topic among the research of condensed matter and materials science,since its great potential both in fundamental research and application.Due to the flexibility of structures and chemical compositions,abundant physical phenomenon have been discovered in these materials,such as super-conductivity,charge density waves,magnetic order states,etc.Moreover,the interlayer interaction in layered transition metal compounds is van der Waals gap.This makes it possible to modulate the structure and physical properties of layered transition metal compounds via intercalating various guest species into the van der Waals gap.In this dissertation,we synthesized a series of layered transition metal compounds and investigated their physical properties.We synthesized the ternary thorium nitride ThNF and ThNCl.Although ThNF and ThNCl share the similar structure with MNX(M=Ti,Zr,Hf)compounds,the interaction between adjacent ThNF and ThNCl lay-ers is not van der Waals gap.Besides,we synthesized a series of misfit compounds(PbSe)1.16(TiSe2)m(m=1,2)and systematically studied their structure and physical prop-erties.(PbSe)1.16(TiSe2)2 with two-layer TiSe2 shows superconductivity at 2.3 K while(PbSe)1.16(TiSe2)with one-layer TiSe2 is non-superconducting.The absence of su-perconductivity in(PbSe)1.16(TiSe2)could be attributed to the much higher doping level compared with(PbSe)1.16(TiSe2)2.Moreover,via electrochemical intercalation method,we fabricated a series of intercalated TaS2(CTA+)x.The T,of intercalated TaS2(CTA+)x is considerably enhanced compared with original TaS2,which proves that electrochemical intercalation method is effective in tuning the materials proper-ties.Finally,we successfully intercalated organic ion tetrabutyl ammonium(TBA+)into the van der Waals gap of Cr2Ge2Te6 with electrochemical intercalation method.Surprisingly,the intercalated(TBA)Cr2Ge2Te6 shows metallic behavio'r and the Curie temperature is highly increased as high as 208 K from 67 K for pristine Cr2Ge2Te6.The dissertation could be divided into five chapters as follows:1.IntroductionIn this chapter,we briefly review the history of the development of superconduc-tors.Besides,the crystal structure and physical properties of ternary transition metal nitride and transition-metal-dichalcogenides is briefly introduced.Moreover,we re-view the research progress of magnetic van der Waals crystals.Some typical synthesis methods are also introduced.2.The synthesis and physical properties of the ternary thorium nitride ThNF and ThNClIn this chapter,we report the synthesis and physical properties of ternary thorium nitride ThNF and ThNCl.Via the refinement of their crystal structure,although ThNF and ThNCl share the similar structure with MNX(M=Ti,Zr,Hf,X=F,Cl,Br)com-pounds,we concluded that the interaction between adj acent ThNF and ThNCl layers is not van der Waals gap.Through the combination of X-ray photoelectron spectroscopy and band calculation,the Th-N bond in both ThNF and ThNCl is covalent bond,in contrast to the ionic bond in Zr(Hf)-N in ZrNCl and HfNCl.Besides,the Th-F bond in ThNF is ionic bond while the Th-Cl bond in ThNCl is covalent bond.For ThNF,the strong electronegativity of F ions leads to the bonding of Th to the F both in the nearest neighbor layer and next nearest neighbor layer,which results in the absence of van der Waals gap between ThNF layers.For ThNCl,the covalent bond between Th-Cl reduces the repulsion between adjacent Cl layers,which results in the absence of van der Waals gap between ThNF layers.Thus,it is the absence of van der Waals gap that leads to the failure of intercalating alkali-metals or organic molecules in ThNF and ThNCl,thereby the unsuccessful attempt to realize the high-T,superconductivity in the 5 f-electron thorium-based MNX compounds.3.The structure and physical properties of misfit compounds(PbSe)1.16(TiSe2)m(m=1,2)In this chapter,we have successfully synthesized the bulk single crystalline mis-fit compound(PbSe)1.16(TiSe2),which bears similar structure to the superconductor(PbSe)1.16(TiSe2)2.The latter shows superconductivity at 2.3 K while the former is non-superconducting.The misfit compound(PbSe)1.16(TiSe2)is constituted by dou-ble rock-salt PbSe layers and one layer TiSe2 stacking in c-direction,in contrast to the two layers TiSe2 in(PbSe)1.16(TiSe2)2.Anisotropic resistivity measurements indi-cate much stronger anisotropy of(PbSe)1.16(TiSe2)2 than that of(PbSe)1.16(TiSe2),sug-gesting much weaker interlayer coupling and lower dimensionality in the former.The angle-resolved photoemission spectroscopy(ARPES)indicates the amount of charge transfer increases unexpectedly by more than 250%from(PbSe)1.16(TiSe2)2 to the(PbSe)1.16(TiSe2).The absence of superconductivity in(PbSe)1.16(TiSe2)could be at-tributed to the much higher doping level compared with(PbSe)1.16(TiSe2)2.4.Tunable superconductivity by electrochemical intercalation in TaS2In this chapter,we use the electrochemical intercalation method to intercalate CTA+cations into 2H-TaS2.The single layer TaS2 is intercalated with CTA+ cations and form this hybrid material,which is determined by X-ray diffraction and high resolution transmission electron microscope.The direction of CTA+ is approximately perpendic-ular to the lamellar structure of TaS2.The superconducting transition temperature is enhanced from 0.8 K(pristine 2H-TaS2)to a maximum of 3.7 K.The phase diagram of TaS2(CTA+)x system is mapped out and shows a dome-like behavior.Our stud-ies demonstrate that the electrochemical intercalation method is a facile and effective way to controllably manipulate the electronic properties of different two-dimensional materials.5.The modulation of magnetic properties in Cr2Ge2Te6 via organic ion intercala-tionIn this chapter,we successfully realized the intercalation of organic ion,tetra-butyl ammonium(TBA+),into the Van der Waals gap of Cr2Ge2Te6.The single layer Cr2Ge2Te6 is intercalated with TBA+cations and form the hybrid superlattice.Surpris-ingly,the intercalated(TBA)Cr2Ge2Te6 shows metallic behavior and the Curie temper-ature is highly increased as high as 208 K from 67 K for pristine Cr2Ge2Te6.Moreover,the magnetic easy axis of(TBA)Cr2Ge2Te6 is reoriented to ab-plane from the c-axis for pristine Cr2Ge2Te6.The calculation results indicate that the intercalated TBA+ ions transfer electron to Cr2Ge2Te6 and alter the ground state of Cr2Ge2Te6 from semicon-ductor to metal.Thereby,the origin of the ferromagnetism changed from weak super-exchange interaction to strong double exchange interaction,which results in the huge increase of Curie temperature.