Synthesis And Physical Properties Study Of Li_x（NH₃）_yBi₂Se₃ And Ba₆Nb₁₁Se₂₈ Layered Compounds

Layered compounds have rich physical properties and potential applications due to their unique structural properties.The interlayer action of this material is generally weak van der Waals force,and the dissociation energy is low.Some guest groups（atoms,molecules,and ionic groups,etc.）can be easily inserted into the interlayers of layered materials.This process is called intercalation reaction,which can adjust the crystal structure,chemical valence state,and electronic state of the parent material.Moreover,this control method is controllable,and can start from multiple dimensions and control the relevant physical parameters to customize the material according to the needs.In this dissertation,two common layered materials,Nb Se₂and Bi₂Se₃,were intercalated by flux method and liquid ammonia method,respectively.Their crystal structures were regulated,and their low-temperature transport properties were systematically characterized.The following innovative results were obtained:1.The bulk superlattice van der Waals material Ba₆Nb₁₁Se₂₈ was successfully synthesized by the cosolvent method.This is the second case of layer-cake superconductor after Ba₆Nb₁₁S₂₈ layer-cake superconductor.It is proved that the research idea of realizing two-dimensional superconductivity in bulk materials by building a layer-cake superlattice structure is feasible.A superconducting phase with a superconducting transition temperature of 2.2 K was observed in Ba₆Nb₁₁Se₂₈,which was more than two times higher than that of Ba₆Nb₁₁S₂₈.The anisotropy in the superconducting upper critical field is three times higher than that of the TMDs parent compound（Nb Se₂）,with the characteristic of a Pauli breakthrough ratio exceeding 2.0.The angle dependence of its upper critical field also fits well with the 2D Tinkham model.2.In Ba₆Nb₁₁Se₂₈,the Nb Se₂ monolayer is encapsulated in the corresponding Ba₃Nb Se₅layer,but this material has less anisotropy in the normal state and theβvalue is very small,not higher than 20.At 5 K,the measured carrier mobility in Ba₆Nb₁₁Se₂₈ is only 1.24 cm V^-1s^-1,a value comparable to that reported in monolayer Nb Se₂.The ratio of the mean free path to the superconducting coherence length（?）/ξ₀ is 0.018,indicating that the Ba₆Nb₁₁Se₂₈ sample used in this experiment is within the dirty limit.3.We successfully synthesized Li_0.48（NH₃）_yBi_2.07Se₃ crystal by self-flux method and liquid ammonia method.We found that the resistivity of Li_0.48（NH₃）_yBi_2.07Se₃ is not much different from that of the parent Bi₂Se₃,both of which are metallic behaviors.The intercalation of Li and ammonia complex groups between Bi₂Se₃ layers has little effect on the resistivity.4.Although Li_0.48（NH₃）_yBi_2.07Se₃ did not exhibit the superconducting behavior as we expected,we made Bi₂Se₃ two-dimensional through intercalation,and realized the weak antilocalization（WAL）effect that only appeared in few-layer Bi₂Se₃ in bulk materials.The feasibility of realizing the idea of two-dimensional material through intercalation reaction is verified,which can avoid the complicated process of preparing dissociated layered materials and avoid the disadvantage that thin film samples are easily degraded by air oxidation.It provides a new idea and method for the study of layered two-dimensional materials.