Synthesis,Structures And Properties Of Novel Arsenide Zintl Phases

Pnictide-based Zintl phases are of interest due to their diverse crystal structures and unusual electronic structures. In the past decade, abundant physical properties such as semiconducting, superconducting, colossal magnetoresistance, and thermoelectricity have been frequently reported for such polar intermetallic phases. We have synthesized a series of arsenide Zintl compounds by using metal flux method. Involves the main content includes:A new quaternary arsenide Zintl phase, Ba13Si6Sn8As22, has been synthesized from the Sn-flux reaction and its structure was determined by single-crystal X-ray diffraction methods. The compound crystallizes in the tetragonal noncentrosymmetric space group I-42m (No.121) with unit cell parameters of a=b=14.4857(3) A, c=13.5506(7) A, V =2843.40(17) A3. Its polyanion structure can be described as composed of [Si4AS10] adamantane-like clusters and SiAs4 tetrahedra, which are linked via the [Sn2As4] units constructed by two edge-sharing SnAs3 triangular pyramids. Differential thermal analysis and thermogravimetry measurements suggest that Ba13Si6Sn8As22 have good thermal stability and with temperature above 1045 K, it decomposes into Ba3Sn4As6 and some uncertain amorphous phases. For studies of the electronic band structure, density functional calculations were performed on Ba13Si6Sn8As22 and the results indicated the compound had a band gap of around 1.0 eV, which was confirmed by the optical diffuse reflectance spectrum.Two new Zint1 phases, Sr14Sn3As12 and Eu14Sn3As12, have been synthesized by flux reactions, and their structures were determined by single-crystal X-ray diffraction. Both compounds crystallize in the chiral trigonal space group R3 (No.146, Z=3) and their anionic structures all contain three separate anionic components:[Sn2As6]12-ethane-like octahedron, [SnAs3]7- triangular pyramid and three isolated As3" anions. Despite the similarity in their chemical formulas and anionic components, the signs of chirality in both compounds are oppisite. And the electronic structures indicate that the cations are not fully ionic and the covalent bonding interactions between cations and anions are non-negligible. The electronic structural difference induced by cations may be a primary reason for the opposite chiralities in both compounds.Two new Europium-containing Zintl phases, Eu3Si2As4 and Eu3Ge2As4, were discovered from metal flux reactions and the structures were identified by using the single-crystal X-ray diffraction method. Both compounds crystallize in monoclinic space group P21/c and are isotypic to Sr3Ge2As4. Magnetic property studies were performed and the results proved the long-range antiferromagnetic behavior for Eu3Ge2As4 but with a positive Weiss constant, which suggested the coexistence of both ferromagnetic and antiferromagnetic order of Eu-4f electrons in the structure. Density functional calculations were incorporated to understand the related magnetic properties as well.Two polymorphs of Sr3Sn2As4 have been synthesized from the Sn-flux reactions, and their structures were determined by single-crystal X-ray diffraction technique. α-Sr3Sn2As4 adopts the Sr3Sn2P4 structure type with the orthorhombic space group Cmca(a=25.798(2) A, b=12.8883(11) A, c=19.1244(16) A, V= 6358.8(9)A3, Z= 24), whereas β-Sr3Sn2As4 belongs to Ca3Si2As4 structure type and crystallizes in the monoclinic crystal system P21/c (a=7.7049(13), b=19.118(3), c=7.6877(13), β= 112.003(2)°, V=1049.9(3), Z=4). Despite the obvious structures differences, the polyanion units of both compounds feature similar [Sn2As6] octahedron. Differential thermal analysis and thermogravimetry measurements indicate that a-Sr3Sn2As4 has good thermal stability and melt at 1185K, while β-Sr3Sn2As4 starts to decompose above 800 K. Diffuse reflectance spectrum measurements proved both compounds had a band gap of about 0.9 eV, supported by the density functional calculations.New ternary antimonides Eu9Cd4+xSb9, Ca9Zn446(1)As9 and Ca9Mn4+xSb>9(x≈1/2) have been synthesized. Although these compounds have analogous chemical makeup and formulas, which may suggest isotypism, they actually belong to two different structure types. Eu9Cd4.45(1)Sbg is isostructural with the previously reported Ca9Zn4.5Sb9 (Pbam), and its structure has unit cell parameters a=12.9178(11) A, b=23.025(2) A, and c=4.7767(4) A. Ca9Zn4.46(1)As9 and Ca9Mn4.41(1)Sb9 constitute a new structure type, and they crystallize in the orthorhombic space group Pnma with unit cell dimensions a =11.855(2)/12.490(2) A, b=4.2747(8)/4.6292(8) A, and c=41.440(8)/44.197(8) A, respectively. The two structures are compared and contrasted, and the structural relationships are discussed. Differential thermal analysis and electrical resistivity measurements, performed on single crystals of Ca9Zn4+xAs9, indicate high thermal stability and semiconducting behavior. Magnetic susceptibility measurements on Eu9Cd4+xSb9 samples confirm the expected Eu2+([Xe]4/) ground state.