Modulation On The Defect Structures And Thermoelectric Properties Of Novel Transition Metal Zintl Phases

Intermellatic compounds are interesting for their diverse crystal structures and closely related physical properties.As a specific class of polar intermetallic compound,Zintl phase has drawn increasing attention in the field of thermoelectric materials due to the high electrical conductivity and low thermal conductivity.Herein,a series of new polar intermetallic compounds were synthesized by solid-state and metal flux reactions,and their defect structures as well as related magnetic and thermoelectric properties were studied.The details are described as fellows:Two new cage-like copper rich ternary antimonides,Sr4Cu25.37(18)Sb12 and Eu4Cu26.45(9)Sb12 were synthesized by Pb-flux method.Sr4Cu25.37Sb12 and Eu4Cu26.45Sbi2 are isostructural and crystallize in the tetragonal space group Cmcm(no.63)with unit cell parameters:a = 4.3200(7)A,b = 14.528(2)A,c = 12.2594(19)A for Sr4Cu25.37Sb,2,and a = 4.2874(7)A,b=14.611(2)A,c = 12.1946(19)A for Eu4Cu26.45Sb12,respectively.They adopt a new structure with a very complex three-dimensional framework,which composed of polyhedral[CuPnn]through sharing corner or edge with cations filled into the cavities.Metallic conducting properties were suggested by the measured electrical transport properties as well as the extensive Cu vacancies in the structure.Magnetic susceptibility measurements on Eu4Cu26.06(i3)Sbi2 indicated divalent Eu2+ ions coupled by the weak ferromagnetic interactions.A complex Zintl series,AM1-xAg1-ySb(A = Ca,Yb;M = Mn,Zn;0<x<1;0<y<1),were discovered and identified,which crystallize in the LiGaGe structure type with the hexagonal space group P63mc(No.186).Bearing unique anionic layers and extensive defects in the structure,these novel Zintl phases have very low intrinsic thermal conductivity and good electrical conductivity.For example,the thermal conductivities of AMn(1-x)/2AgxSb(A = Ca,Yb;x = 0.1,0.2)are less than 0.55 W·m-1·K-1 at 323K.For a specific composition CaZn0.4Ag0.18Sb,decent figure of merit can be achieved and in addition,a broad high-zT plateau(zT>1.0)was observed from 800～1100 K.The discovery of these new Zintl phases is very significant.The excellent thermoelectric performance accompanied implies that the LiGaGe-type intermetallics should deserved more focused interest,which may be a valuable resource for the exploration and design of high-zT thermoelectrics in practical applications.A series of new copper pnictides intermetallics,A2Mg3Cu9Pn7(A = Sr,Eu;Pn =P,As)and Eu5Mg2.39Cu16.61As12,were synthesized through the high temperature flux reactions.By the single-crystal X-ray diffraction method,their structures were determined to be Zr2Fe12P7-type(S.G.P-6,No.174,cell parameters:A2Mg3Cu9P7(A=Sr,Eu),a=9.7736(19)A,9.7417(7)A,c = 3.9280(16)A,3.9008(5)A;A2Mg3Cu9As7(A=Sr,Eu),a = 10.0550(10)A,10.0410(8)A,c = 4.0544(9)A,4.0364(6)A)for compounds A2Mg3Cu9Pn7(A = Sr,Eu;Pn = P,As)and Zr5Co19P12-type(S.G.P-62m,No.189,cell parameters:a=13.2521(6)A,c =4.0910(3)A)for Eu5Mg2,39Cu16.61As12,respectively.These compounds can be viewed as an extensive branch of the ternary RE-M-Pn system(RE = rare-earth elements;M=transition metals;Pn = pnictogen)with the metal-to-nonmetal ratio is close or equal to 2:1.Interestingly,with the divalent alkaline-earth cations introduced these metal-rich phases exhibit electron deficiency according to the electronic band structure.Eu2Mg3Cu9As7 was metallic based on the resistivity measurements,consistent with theoretical prediction.The magnetic susceptibility of Eu2Mg3Cu9As7 suggested divalent oxidation state and weak ferromagnetic coupling for the europium cations in the structure.A series of new Zintl phases,Mg1-xMn2+xAs2(x = 0.17,0.49,0.69),were synthesized,which adopt the CaAl2Si2 structure type with the trigonal space group P-3m1(No.164).These compounds all possess ferromagnetism over room temperature.The Curie temperature increases with increasing Mn content.We also verified the presence of ferromagnetic(FM)/antiferromagnetic(AFM)orders variation at low temperature,which is consistent with an AFM low-field state.Electrical resistivity measurement based on Mg0.31Mn2.69As2 in a zero-magnetic field indicates that is a typical doped semiconductor.A new layered intermetallic compound,CaZnGe,was synthesized by the Pb-flux method.It crystallizes in the hexagonal crystal system with space group P-6m2(NO.187)and unit cell parameters:a = 4.236(3)A,b = 12.756(19)A,V= 198.3(4)A3.Its two-dimensional framework built by flat and puckered ZnGe-layers adopting ABBA-type stacked along the direction of the hexagonal c-axis.On the basis of electronic structure calculations,CaZnGe is a metallic,which is in good agreement with the measurements.Thermoelectric properties of this compound were measured from 323 to 973 K on a pellet sample prepared by spark plasma sintering.The resistivity increases first and then decreases with the increasing temperature,with which a maximum resistivity of 1.84 mΩ·cm was approched at 473 K.The Seebeck coefficient ranges from 64 μV·K-1 at 323 K to a maximum of 77μV·K-1 at 473 K.The thermal conductivity of this material,by contrast,is relatively low at room temperature.