Synthesis, Crystal Structures And Properties Of Pnictide Zintl Phases

The discoveries of pnictides based Zintl compounds always catch more and more attention from solid state chemist and material researcher. The research of new pnictides based Zintl compounds is very important in many fields:1. in solid state chemistry field, exploring new Zintl compounds will draw deep knowledge in crystal structure and solid state chemistry. Pnictides based Zintl compounds exhibit interesting structures:3D-networks,2D-layers,1D-chains and OD-clusters.2. Exploring new materials. Many pnictides based Zintl compounds exhibit really good performance in thermoelectric materials, high temperature superconductivity and anode materials for lithium batteries. Going through pnictides based Zintl compounds may discover many useful materials, which is also helpful in understanding the relationship between structure and properties. Our interests are focused on exploring P、As and Sb based new Zintl compounds, we synthesized four systems in P and As containing compounds to research their semiconductivity properties and magnetism and so on. Two new potential thermoelectric systems were also discovered in our experiments, we also carry research of chemical doping in these Sb based systems.The details are described as fellows:1. A series of ternary Zintl phases, Ca2CdP2, Ca2CdAs2, Sr2CdAs2, Ba2CdAs2, and Eu2CdAs2, have been synthesized through high temperature metal flux re-actions, and their structures have been characterized by single-crystal X-ray diffraction. They belong to theYb2CdSb2structure type and crystallize in the orthor-hombic space group Cmc21. Among these, Ca2CdAs2was found to form with another closely related structure, depending on the experimental conditions (monoclinic space group Cm). UV/vis optical absorption spectra for both forms of Ca2CdAs2show band gaps on the order of1.0eV, suggesting semiconducting prop-erties, which have also been confirmed through electronic band structure calculations based on the density-functional theory. Results from differential scanning calorimet-ry measurements probing the thermalstability and phase transitions in the two polymorphs are discussed. Magnetic susceptibility measurements for Eu2CdAs2, indicating divalent Eu2+cations, are presented as well.2. Two new ternary Zintl phases, Sr5Sn2As6and Eu5Sn2As6, have been synthesized, and their structures have been accurately determined through single-crystal X-ray diffraction.Both compounds crystallize in orthorhombic space group Pbam (No.55, Z=2). Their structure belongs to the Sr5Sn2P6type, which can be closely related to the Ca5Ga2As6type. Electronic band structure calculations based on the density functional theory reveal an interesting electronic effect in the structure formation of these two types of Zintl phases, which substantially affect their corresponding electronic band structure. Related studies on the thermal stability, magnetism, and thermoelectric properties of EusSn2As6are presented as well.3. Through high temperature Pb-flux reactions, two new arsenide Zintl compounds, Ba3Zn2As4and Ba3Cd2As4, were successfully obtained and their structures were accurately determined with Single-Crystal X-ray Diffraction. Both compounds are isotypic to Ba3Cd2Sb4and crystallize in the monoclinic space group C2/m.Electrical resistivity measurement on Ba3Cd2As4reveals semiconducting behavior between10and100K, which results in a very small band gap of0.01eV. According to TG/DSC analyses, Ba3Cd2As4exhibits good thermal stability and does not decompose below950K.4. Two new ternary Pnictides compounds:Eu2Zn2P3and Eu2Cd2As3were synthesized by metal flux method. Structure was accurately determined by single crystal X-ray diffraction. Eu2Zn2P3and Eu2Cd2As3crystallize in C2/m space group (No=12) with the cell parameter:a=15.653(5)/16.402(1) A; b=4.127(1)/4.445(4) A; c=11.552(4)/12.311(1) A; β=126.647(4)/126.515(7)°for Eu2Zn2P3and Eu2Cd2As3respectively.Temperature-dependent DC magnetization measurements of Eu2Cd2As3indicate Curie-Weiss paramagnetism from10K to300K. The measured effective moment is consistent with Eu2+ground states. Temperature-dependent electrical resistivity measurements show semiconducting behavior up to300K and reveal band gaps of0.059eV for Eu2Cd2As3. According to TG/DSC analyses,Eu2Cd2As3decomposes at about1000K. 5. Two new Zintl compounds A10LaCdSb9, namely Ca9.81(1)La0.97(1)Cd1.23(1)Sb9and Yb9.78(1)La0.97(1)Cd1.24(1)Sb9, were designed and synthesized by applying the Zintl concept. Though both compounds are isoelectronic with their Ca11InSb9and Yb11InSb9analogues, they crystallize in a new structure type with the orthorhombic space group Ibam (No.72) and feature very complex anion structures, which are composed of unique [Cd2Sb6]12-clusters, dumbbell-shaped [Sb2]4-dimers and isolated [Sb3-] anions. For Yb9.78(1)La0.97(1)Cd1.24(1)Sb9, an extremely low lattice thermal conductivity of0.29W.m-1.K-1was observed at875K, which almost approaches the lower limit of nonglassy or nonionically conducting bulk materials ever known. According to Thermogravimetirc (TG) and Differential Scanning Calorimetry (DSC) analyses, both compounds show very good thermal stability and no melting or phase transition processes were found below1173K. Though related thermoelectric property studies on Yb9.78(1)La0.97(1)Cd1.24(1)Sb9only present a maximum zT of0.11at920K, due to its low Seebeck coefficients, these materials are still very promising for its high temperature stability and low thermal conductivity. Furthermore, as there exist mixed cations with different charges, it makes this system very flexible in tuning the related electrical properties.6. For high temperature thermoelectric materials there are still very few choices when concerning practical applications. In this work, we demonstrate the design and synthesis of a new class of complex Zintl compounds Ca1-xRExAg1-ySb (RE=La, Ce, Pr, Nd, Sm)(P63mc, No.186, LiGaGe structure type), which exhibit excellent high temperature thermoelectric performance. The structure of such new phases is derived from a family-rich Zintl compound CaAgSb (Pnma, No.62, TiNiSi structure type) and by doping trivalent rare-earth cations into the calcium sites, the crystal structure of CaAgSb is delicately modified with the crystal system changed from orthorhombic to hexagonal. More interestingly, the interlayer Ag-Sb interactions are substantially weakened, which results in the formation of an almost coplanar [AgSb] layer. Such a structure change also leads to a better energy separation between the conduction bands and the valence bands as well as an increased density of p-states right below the Fermi level, suggested by the theoretical calculations and explaining well the significant thermoelectric performance enhancement of these new materials. For a p-type composition-optimized material Ca0.84Ce0.16Ag0.87Sb, an efficiency of zT～0.7can be achieved at1079K. Combined with their excellent thermal stability and high electrical conductivity, these materials can be very promising candidates for substituting p-type Si-Ge alloys in high temperature power generation.