Synthesis and Transport Properties of ZnSnP2-yAsy Chalcopyrite Solid Solutions.

This work focuses on the synthesis and properties of quaternary ZnSnP2-yAsy chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm-1 for ZnSnP2 to 10.3 S cm-1 for ZnSnAs2 at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 µV K-1 for ZnSnP2 to -100 µV K-1 for ZnSnAs2 at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m-1 K-1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10-3 and y = 1 reaching 2.1 × 10-3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10-3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs2, comparable with ZnSnP2.

Comparative Analysis of the Crystal Structures and Physical Properties of the Complex Group 14 Selenides Sr8Ge4Se17 and Ba8Sn4Se17.

During our group's continued exploration of group 14 chalcogenides, we discovered two new compounds, Sr8Ge4Se17 and Ba8Sn4Se17. Both compounds have an 8:4:17 stoichiometric ratio but adopt different centrosymmetric crystal structures. Sr8Ge4Se17 crystallizes in the triclinic P1̅ space group with a = 11.8429(18) Å, b = 12.172(3) Å, c = 13.624(3) Å, α = 114.472(5)0, ß = 97.396(5)0, γ = 107.040(5)0, and Z = 2. Ba8Sn4Se17 crystallizes in the monoclinic C2/c space group with a = 47.286(3) Å, b = 12.6294(5) Å, c = 25.7303(15) Å, ß = 104.585(5)0, and Z = 16. The unit cell of Ba8Sn4Se17 is approximately eight times larger than the unit cell of Sr8Ge4Se17, which is a consequence of the differently aligned tetrahedra, resulting in a quadrupled a axis, unchanged b axis, and doubled c axis. The lattice parameters and atomic coordinates were finalized via Rietveld refinements on data collected at a high-energy synchrotron beamline. Both compounds are semiconductors with band gaps in the visible region. Sr8Ge4Se17 and Ba8Sn4Se17 have optical band gaps of 1.88 and 1.93 eV, respectively. Both compounds have remarkably low thermal conductivities owing to their low symmetries and large unit cells. The minimum experimental thermal conductivity values of Sr8Ge4Se17 and Ba8Sn4Se17 are 0.45 W m-1 K-1 at 321 K and 0.31 W m-1 K-1 at 324 K, respectively.

Crystal structure, phase width, and physical properties of the barium tetrel selenides Ba6Si2-xGexSe12 (x = 0, 0.5, 1, and 1.5) with ultralow thermal conductivity.

The new compound Ba6Si2Se12 was synthesized, and the crystal structure and physical properties are reported here. Ba6Si2Se12 adopts a new structure type in the triclinic P1̄ space group with the lattice parameters a = 9.1822(7) Å, b = 12.2633(14) Å, c = 12.3636(18) Å, α = 109.277(3)°, ß = 104.734(2)°, and γ = 100.4067(16)°. Notably, the structure features disordered Se22- dumbbells that have also been observed in the germanium selenide with the analogous stoichiometry (Ba6Ge2Se12). Density functional theory calculations revealed that Ba6Si2Se12 is a semiconductor with a calculated band gap of 1.74 eV. UV/vis/NIR absorption spectra indicated that the experimental band gap of Ba6Si2Se12 is 1.89 eV. While exploring this compound's phase width, it was discovered that up to 75% of the Si could be substituted with Ge while retaining the structure type. Rietveld refinements were performed on the phase-pure samples of Ba6Si2-xGexSe12 (x = 0, 0.5, 1, and 1.5) using data collected at the Canadian Light Source's High Energy Wiggler Beamline. The cell parameters, Si/Ge occupancies, and disordered Se22- occupancies were studied. Raman spectra displayed the expected Si-Se and Ge-Se stretching modes from 215 cm-1 to 280 cm-1. The samples were also hot-pressed into pellets to determine their thermal conductivity values ranging from 0.5 to 0.4 W m-1 K-1 for the x = 0, 0.5, and 1.5 samples. The x = 1 sample stood out with a remarkably low thermal conductivity of 0.3 W m-1 K-1, consistent from room temperature up to 573 K.

Development of Nanostructured Bi2Te3 with High Thermoelectric Performance by Scalable Synthesis and Microstructure Manipulations.

Nanostructuring of thermoelectric (TE) materials leads to improved energy conversion performance; however, it requires a perfect fit between the nanoprecipitates' chemistry and crystal structure and those of the matrix. We synthesize bulk Bi2Te3 from molecular precursors and characterize their structure and chemistry using electron microscopy and analyze their TE transport properties in the range of 300-500 K. We find that synthesis from Bi2O3 + Na2TeO3 precursors results in n-type Bi2Te3 containing a high number density (Nv ∼ 2.45 × 1023 m-3) of Te-nanoprecipitates decorating the Bi2Te3 grain boundaries (GBs), which yield enhanced TE performance with a power factor (PF) of ∼19 µW cm-1 K-2 at 300 K. First-principles calculations validate the role of Te/Bi2Te3 interfaces in increasing the charge carrier concentration, density of states, and electrical conductivity. These optimized TE coefficients yield a promising TE figure of merit (zT) peak value of 1.30 at 450 K and an average zT of 1.14 from 300 to 500 K. This is one of the cutting-edge zT values recorded for n-type Bi2Te3 produced by chemical routes. We believe that this chemical synthesis strategy will be beneficial for future development of scalable n-type Bi2Te3 based devices.

Ba6Ge2Se12 and Ba7Ge2Se17: Two Centrosymmetric Barium Seleno-Germanates with Polyatomic Anion Disorder.

Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba6Ge2Se12 and Ba7Ge2Se17, are presented. Ba6Ge2Se12 adopts the P21/c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and ß = 90.303(1)°, whereas Ba7Ge2Se17 crystallizes in the Pnma space group with a = 12.652(1) Å, b = 20.069(2) Å, c = 12.3067(9) Å. Both structures feature polyatomic anion disorder: [Se2]2- in the case of Ba6Ge2Se12 and [GeSe5]4- in the case of Ba7Ge2Se17. The anion disorder is verified by comparing pair distribution functions of ordered and disordered models of the structures. These anions are split unevenly across two possible sets of atomic coordinates. The optical band gaps obtained from the powdered samples are found to be 1.75 and 1.51 eV for Ba6Ge2Se12 and Ba7Ge2Se17, respectively. Differential scanning calorimetry experiments indicate that the compounds are stable under the exclusion of air up to at least 673 K. The thermal diffusivity measurements revealed thermal conductivities reaching values as low as 0.33 W m-1 K-1 in both compounds at 573 K.

Effect of Pb Substitution in Sr2-xPbxGeSe4 on Crystal Structures and Nonlinear Optical Properties Predicted by DFT Calculations.

We investigated the Sr2-xPbxGeSe4 series from 0 ≤ x ≤ 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds γ-Sr2GeSe4 and α-Pb2GeSe4 have already been reported previously, the substitution variants Sr1.31Pb0.69GeSe4 (space group Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr0.19Pb1.81GeSe4 (I4̅3d, a = 14.6177(3) Å) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from γ-Sr2GeSe4 to Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr2GeSe4 and Sr1.3Pb0.7GeSe4 (approximated with the supercell "Sr3PbGe2Se8") were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, deff, of γ-Sr2GeSe4 and "Sr3PbGe2Se8" at the static limit to be 10.8 and 8.8 pm V-1, respectively. The experimental deff values of γ-Sr2GeSe4, Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 were 2.6, 2.3, 0.68, and 0.79 pm V-1, respectively.

BaCuSiTe3: A Noncentrosymmetric Semiconductor with CuTe4 Tetrahedra and Ethane-like Si2Te6 Units.

BaCuSiTe3 was prepared from the elements in a solid-state reaction at 973 K, followed by slow cooling to room temperature. This telluride adopts a new, hitherto unknown structure type, crystallizing in the noncentrosymmetric space group Pc with a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, ß = 122.022(1)°, and V = 746.45(2) Å3 (Z = 4). The structure consists of a complex network of two-dimensionally connected CuTe4 tetrahedra and ethane-like Si2Te6 units with a Si-Si bond. This semiconducting material has an optical band gap of 1.65 eV and a low thermal conductivity of 0.50 W m-1 K-1 at 300 K. Calculations of its optical properties revealed a moderate birefringence of 0.23 and a second-order harmonic generation response of deff = 3.4 pm V-1 in the static limit.