Synthesis,Structures And Properties Of Novel Pnictides And Chalcogenides

Pnictides and Chalcogenides have exhibited diverse structure units and featured rich information on crystallographic chemistry. Rich structures are often related to interesting physical properties, which have drawn intensive attention recently, especially for the applications involving semiconducting, superconducting, thermoelectricity, materials for Li ion batteries, hydrogen storage, colossal magnetoresistance, nonlinear optics (NLO), ect.. The discoveries of novel multinary compounds with complex structures bring very interesting properties. Based on these discussions, we have synthesized a series of pnictides and chalcogenides. The details are described as follows:A series of new multinary phosphide Zintl compounds, Ba5Ga2GeSnP8, Ba5Al2GeSnP8, Ba5Ga6GeP12 and Ba5Ga6SnP12, have been synthesized through the high temperature Pb or Sn flux reactions. These two new structures feature different [Tr2P4] and [Tr5P12] polyanion chains, which can be closely related via some well-know Zintl analogues such as Ba3Al2P4 and Sr3In2P4. The thermal stabilities and the optical absorption properties of Ba5Ga6GeP12 and Ba5Ga6SnP12 are studied, both compounds are stable till 1000K.The optical band gaps are 1.39 and 1.21 eV for Ba5Ga6GeP12 and Ba5Ga6SnP12, respectively, and these were consistent with theory calculations. Besides, electronic structures of the tetrel atoms in these two compounds are obviously different, which support the differences on the crystal structures.A new quaternary antimonide-based Zintl compounds Ba23M2oSb25 (M=Ga, Ge) have been synthesized through the high temperature Pb flux reactions. The compound features a very complex 0-dimensional structure with a huge unit cell. Forty M and thirty Sb atoms connect to form a big flower-like cluster, Ba2+ cations fill in the space of the clusters. There are also isolated Sb2 dimers and Sb3 trimers disperse between these clusters. A single crystal of about 0.5cm×0.5cm×0.5cm was obtained through optimized conditions. The thermol conductivity of the compound was measured to be 0.189W/m-K at room tempreture, and this was even lower than the glass-limit. The temperature dependent electrical resistivity was measured to calculate the electron and lattice thermol conductivity, and we found that the κ1 was extremely low especially at high tempreture. This can be attributed to the complex structure, huge unit cell, so many disorders and heavy atoms.Two new quaternary pnictide-based Zintl phases Ba4AgGa5P8 and Ba4AgGa5As8 have been synthesized by Pb-flux reactions. The two compounds are isostructural and adopt the noncentrosymmetric (NCS) group Iba2 (No. 45). The GaPn slabs extend on (010) planes and (Ag/Ga)2Pn4 chains stretch along [001] direction give rise to a large polarization along c axis which may bring strong second-harmonic generation (SHG) responses. Theoretical calculations indicate that both compounds feature large SHG responses, approximately two and three times larger than the d36 of AgGaSe2, and the calculated cutoff-energy-depending static SHG coefficients show that the large NLO coefficients of Ba4AgGa5Pn8 mainly arise from the distorted GaPn4 tetrahedra. The optical band gaps of Ba4AgGa5P8 and Ba4AgGa5As8 are measured to be 1.38 and 0.8 eV, supported by the theoretical calculations. Besides, both compounds have moderate birefringence (△n),0.157 and 0.206 for Ba4AgGa5P8 and Ba4AgGa5As8, respectively, suggesting that these crystals may be suitable for the phase-matching condition in the SHG process.Two new barium chalcogenides, Ba3GeS5 and Ba3InS4Cl, were synthesized by using high temperature BaCl2-flux reactions and their structure feature discrete [InS4] and [GeS4] tetrahedral anions. Despite the similar chemical formula, the structures of Ba2GeS5 and Ba3InS4Cl are subtly different due to the size effects originated from the tetrahedral anions. The optical band gaps were measured to be 3.0 and 3.42 eV for Ba3GeS5 and Ba3InS4Cl, respectively. The absorption coefficient of Ba3GeS5 increases slowly with the energy increased, indicating the existence of an indirect band gap. However, for Ba3InS3Cl the curve reaches the peak quickly, typically observed for the absorption of the direct band gap semiconductor. These results are also consistent with the theoretical predictions. Besides, the analysis on the projected density of orbitals suggests that the p-orbital of the isolated S3 atom in Ba3GeS5 has significant contribution to the states right below the Fermi level. This indicate that the isolated S anions should play a very important role in affecting the physical properties of this compound. When this isolated S2- is substituted by the Cl-, the occupied p-orbitals of Cl atoms are located at much lower energy and have very little contribution to the states near the Fermi level. In addition, the population of In atom around the Fermi level become obviously innegligible. As a result, the band structure of Ba3InS4Cl is significantly different from that...