Exploration On New Halide, Intercalated And Alloy Superconductors

Superconductivity is one of the major topics in condensed matter physics,and new superconductors have been emerging ever since the first discovery in 1911.During the exploration of new type of superconductors,researchers have noticed that some structural and physical properties are closely related to the occurance of superconductivity.For instance,superconductivity often arises on t he verge of a SDW/CDW ordering.Besides,some structural types,such as the A15-phase alloys,perovskite oxides or layered disulfides,etc.is apt to superconduct in many cases.In this thesis,researches are carried out in the light of those empirical rules.Layered-type bismuth sulfides,111-type TT'X superconductors,graphite fluoride intercalated compounds and FeSe intercalation with ammonia are studied in detail.In the history of superconductivity research,halide superconductors are rare,which is related to the strong electronegativity of halogen elements.However,through literature research we found that halogens can play an important role in conducting compounds.In addition to halogens as building blocks in some compounds,some superconductors,such as halogens in systems such as Y₂C₂I₂ and Mo₆S₆I₂,also participate in the construction of conduction bands in their three-dimensional structures.During the exploration of BiS₂ superconductors,we obtained a novel compound Bi₃O₂S₂Cl and discovered superconductivity in it.The superconducting transition temperature T_c is 2.6 K.Bi₃O₂S₂Cl polycrystalline samples were synthesized by solid-state reaction.On the surface of samples from some batch,we also collected single crystals of Bi₃O₂S₂Cl,and magnetic susceptibility measurement on the single crystals confirmed the bulk superconductivity.From Rietveld refinement of powder XRD results and analysis of HAADF-STEM images,we conclude that the sample contains a unique[BiS₂Cl]^2-layer,which distinguishes it from other BiS₂-based superconductors.Hall effect measurements indicate that the major carriers in the Bi₃O₂S₂Cl sample are electrons,and that their concentration does not change much in the measurement temperature range.At 275 K,the electron concentration n=9.34×10¹⁹ cm^-3.This concentration is close to those in the typical BiS₂-based superconductors,but significantly lower than the average metal,suggesting that it may be a heavily doped n-type semiconductor.By changing the ratio of the raw materials,we achieved the synthesis of the semiconducting parent phase in samples with higher S content.Alloy superconductors have always been an important aspect of superconductivity research.On the one hand,alloy superconductors mostly belong to the type of electron-phonon coupling BCS superconductors;on the other hand,alloys are easier to be fabricated to deliver applications.Among those alloy superconductors,the 111-type TT'X compounds?T,T'=transition metals;X=Al,Ga,Si,Ge,Sn,P,As,Sb,Bi,etc.?resemble a large amount of superconductors.We noticed that the TT'X-type superconductors containing Sc were rare,so we started to synthesize ScRuSi alloys and succeeded by discovering superconductivity in the orthohombic o-ScRuSi.The o-ScRuSi sample prepared by a rc-melting under an argon atmosphere showed a superconducting T_c of 3.1 K,H_c1=14 Oe,and H_c2=8700 Oe.Temperature depending magnetic susceptibility and specific heat measurements confirm the bulk superconductivity of the sample and indicate that it is likely to belong to a weakly coupled BCS superconductor.It is the first Co₂P-type alloy superconducting compound containing Sc.In addition,through high-temperature annealing,the superconducting o-ScRuSi phase completely transforms into the non-superconducting hexagonal phase h-ScRuSi,which is the first report of the phase transition in this compound.Among recently discovered superconducting compounds,a considerable part of them adopt intercalated structures,such as ThCr₂Si₂-type iron-based superconductors,alkali metal-ammonia intercalated FeSe,alkali metal intercalated graphite,and alkali metal intercalated aromatic compounds,etc..In fact,alkali metal intercalation can convert many semiconductors/insulators into superconductors.Based on this thought,we conducted an intercalation study of graphite fluoride.The guest compounds used for intercalation were:alkali metals?Li,Na,K?,organic amines?ethylenediamine,butylenediamine,1,8-octanediamine and dodecylamine?,pyridine and ferrocene.In some of the intercalation experiments,we obtained intercalated phases with larger layer distances and observed superconductivity in the intercalation products of Li and Na.The superconducting transition temperature of the intercalation product is 4.8 K or 8 K for Li,and is 4.8 K for Na.The newly discovered superconductors are stable in air and water,which is significantly different from known graphite intercalated superconductors,the stability makes CF intercalation superconductors more promising compared with other graphite-based or aromatic hydrocarbon superconductors.In the case of hydrothermal intercalation of FeSe,we discovered a new intercalated phase with no metallic ion involved.EDX measurement confirmed the absence of alkali metal elements,which is for the first time only ammonia/ammonium ion is intercalated between FeSe layers.XRD pattern reveals the layer distance to be 8?,comparable with those of the alkali metal intercalated compounds synthesized by liquid ammonia method.The resistivity of the intercalation compound suggests it to be a granular metal,while magnetic susceptibility measurement indicates low local magnetic moments and the absence of long-range magetic ordering in the compound.The ammonia/ammonium intercalated FeSe is likely to be a parent compound for the alkali metal intercalated FeSe superconductors.