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, β = 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 d eff = 3.4 pm V–1 in the static limit.
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 (N v ∼ 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.
We investigated the Sr2‑xPb x GeSe4 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, d eff, of γ-Sr2GeSe4 and “Sr3PbGe2Se8” at the static limit to be 10.8 and 8.8 pm V–1, respectively. The experimental d eff 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.
The strategy of partial isovalent anion substitution was used to discover the new noncentrosymmetric (NCS) quaternary oxyselenide Sr6Ge3OSe11. Sr6Ge3OSe11 crystallizes in the NCS trigonal space group P3m1 with lattice parameters of a = 10.268(5) Å, c = 6.363(3) Å, Z = 1. Spectroscopic studies revealed that Sr6Ge3OSe11 has a wide transparent range in the IR region and an optical band gap of 2.39 eV. The nonlinear optical (NLO) properties of this compound were calculated via DFT as well as measured experimentally using a KH2PO4 standard. The second-order nonlinear susceptibilities (dij ) of Sr6Ge3OSe11 were calculated to be d 15 = −12.9 pm V–1, d 22 = −15.4 pm V–1, d 33 = 15.0 pm V–1, and d eff = 17.0 pm V–1. Size-dependent second harmonic generation (SHG) intensity experiments revealed that Sr6Ge3OSe11 is phase-matchable at 1064 nm with an intensity equal to 0.62 × KH2PO4. Additionally, this material exhibits ultralow thermal conductivity despite its relatively low molar mass, which is of interest in the thermoelectric energy conversion.
Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 , are presented. Ba 6 Ge 2 Se 12 adopts the P2 1 /c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and β = 90.303(1)°, whereas Ba 7 Ge 2 Se 17 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: [Se 2 ] 2− in the case of Ba 6 Ge 2 Se 12 and [GeSe 5 ] 4− in the case of Ba 7 Ge 2 Se 17 . 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 Ba 6 Ge 2 Se 12 and Ba 7 Ge 2 Se 17 , 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.
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