Synthetic Exploration And Properties Of New Pnictide Zintl Phases

Owing to the complex structures as well as the rich combination of various compositions,pnictide Zintl phases have exhibited potential applications in many fields,such as lithium battery,superconductivity,giant magnetoresistance and thermoelectrics.In this work,a series of new pnictide Zintl phases were synthesized through the solid-state syntheses or metal-flux reactions.Their crystal structures,magnetic and thermoelectric properties were studied.The main results are listed as below:(1)The structures and physical properties of Sr14MgSb11 and Eu14MgSb11 have been systematically studied.They were synthesized from high-temperature solid-state reactions in Ta tubes at 1323 K.Their structures crystallize in the tetragonal space group 141/acd(No.142,Z= 8)with the cell parameters of a=17.5691(14)/17.3442(11)A and c=23.399(4)/22.981(3)A for the Sr-and Eu-containing compounds,respectively.Temperature-dependent resistivity data indicate that Sr14MgSb11 is an intrinsic semiconductor,while Eu14MgSb11 has a poor metallic behavior.The corresponding thermoelectric properties were probed as well,which demonstrated high potential as new thermoelectric materials because of their very low thermal conductivity and moderate Seebeck coefficient.Magnetic susceptibility measurements confirmed that Eu14MgSb11 was antiferromagnetic.Electronic structure calculations suggest a band gap of Sr14MgSb11 as 1.18 eV.(2)Single crystals of new three bismuth compounds,A14MgBi11(A = Ca,Sr,Eu),were synthesized through solid-state reactions in welded-Ta tubes at high temperature(950-1100?).Their structures are similar to Ca14AlSb11,which all crystallize in the tetragonal space group I41/acd with unit cell parameters a/c = 17.0470(17)/22.665(5)A,17.854(2)/23.580(6)A and 17.6660(7)/23.2446(18)A for Ca14MgBi11 Sr14MgBi11 and Eu14MgBi11 respectively.Magnetic susceptibility was measured under a field of 1000 Oe for Eu14MgBi11,which is suggested antiferromagnetic with ?=-11.75 K.The calculated DOS by Wien2K indicated that Ca14MgBi11,was a p-type semiconductor with Eg?1.12 eV.Thermoelectric figure of merit for A14MgBi11(A = Ca,Sr,Eu,Yb)polycrystal samples approached maximum values of 0.21,0.71,0.46 and 0.55 at 1073 K,owing to the intrinsic low thermal conductivity dependent on their complex structure and large molecular mass.The discovery of this new series of materials is important,for which the idea of expanding the 14-1-11 thermoelectrics from antimonide-to bismide-based materials will be very promising.(3)Sr14MgAs11 and Eu14MgAs11 were synthesized by reacting elements at high temperature.They all crystallize in the tetragonal space group 141/acd(No.142,Z = 8)and single-crystal X-ray diffraction date(a = 16.581 A,c=22.052 A for Sr-and a ?16.3046 A,c = 21.857 A for Eu-)were refined.Occupancy of As4 in Sr14MgAs11 is 16.3%and reduction of As content results in the formula of Sr14MgAs10.33 after structure refinements.TG-DSC measurement suggests that Eu14MgAs11 is stable within the range from 300 K to 970 K.Temperature-dependent magnetic susceptibility data indicated that Eu14MgAs11 had an antiferromagnetic transformation temperature at 7.2 K.Optical absorption measurements showed that Sr14MgAs11 and Eu14MgAs11 were indirect semiconductor,with band gaps of 2.05 eV and 2.05 eV,respectively.According to their high thermal stability as well as complex crystal structures,these compounds may be considered as potential thermoelectrics.(4)New Zintl phase Sr9Mg4.56Sb9 was discovered from metal flux reactions,which is the first Mg-containing compound with 9-4-9 proportion.It belongs to orthogonal space group Pnma(No.62,Z=4)with unit cell parameters a=13.0962(11)A,6=4.8193(4)A,c=46.259(4)A.The anionic framework of this compound is composed of one-dimension chains built of[MgSb4]tetrahedrons and[MgSb3]plane triangular units by edge-or corner-sharing.Extensive disorder exists at the position of Mg and Sb.Low thermal conductivity and good thermoelectric performance are possible for this compound according to its high-density point defect and complex structure.(5)A Zintl phase compound Sr11Mg4As10,is prepared using the Sn-flux reactions.It crystallizes with a new structure type in the orthorhombic space group Pnnm(No.58)with cell parameters a = 14.542(6)A,b=22.018(10)A,c = 4.4981(19)A.The anionic structure is composed of[MgAs4]tetrahedrons with Sr2+ cations filled among the chains.Electronic structure calculation suggested that Sr11Mg4As10 might be a semiconductor.(6)Polycrystalline samples of Ca2Si were prepared using electromagnetic induction melting furnace.Its electrical and therrmal properties were studied and reported for the first time.The electric resistivity increased slowly with the increasing temperature,which indicated a metallic behavior.Thermal conductivity of this material is very low,only about 1 W/m K.Since the Seebeck coefficient is relatively low,the maximum ZT is around 0.1 at 873 K.The Ca2Si1-xSnx(0<x<1)series were prepared through alloying to further tune the carrier concentrations in order to improve the thermoelectric performance.