Ethane-like Units Research Articles

Low thermal conductivity in a new mixed metal telluride Mn1.8(1)In0.8(1)Si2Te6.

The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn1.8(1)In0.8(1)Si2Te6. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group: P3̄1m) structure of the title phase has cell constants of a = b = 7.0483(7) Å and c = 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si2Te6 units. The optical absorption study of a polycrystalline Mn1.8In0.8Si2Te6 sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the p-type semiconducting nature of the sample with high values of S (301 μV K-1 to 444 μV K-1). The total thermal conductivity (ktot) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m-1 K-1 at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.

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, 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 deff = 3.4 pm V-1 in the static limit.

Structure and ionic conductivity of new Ga2S3-Sb2S3-NaI chalcogenide glass system

A new glass system of (100-x) (0.2Ga2S3-0.8Sb2S3)-xNaI (x = 5–30 mol%) was investigated through a melt-quenching method for solid-state electrolyte application. Na+ was added into the glass matrix as a conductive ion to study its effect on the properties of glass samples. XRD result indicated that the glasses retained their amorphous state until the NaI content exceeded 30 mol%. With increased NaI component, the density and hardness of glasses slightly decreased. DSC measurement showed that the glass-transition temperature of samples decreased, whereas the glass thermal stability improved. We speculated that the incorporation of NaI may have caused the destruction of [S3Ga–GaS3] ethane-like units and formation of [GaS4−xIx] structure, thereby decreasing the connectivity of the glass network. The conductivity of glass samples was further determined using the AC impedance method.

Eu11Zn4Sn2As12: A Ferromagnetic Zintl Semiconductor with a Layered Structure Featuring Extended Zn4As6 Sheets and Ethane-like Sn2As6 Units

We report the synthesis, structure, and magnetic properties of a new Zintl phase and structure type, Eu11Zn4Sn2As12. The structure and composition of this phase have been established by single-crystal X-ray diffraction and electron microprobe analysis. Eu11Zn4Sn2As12 crystallizes in monoclinic space group C2/c (No. 15) with the following lattice parameters: a = 7.5679(4) A, b = 13.0883(6) A, c = 31.305(2) A, and β = 94.8444(7)° [R1 = 0.0398; wR2 = 0.0633 (all data)]. The anisotropic structural features staggered ethane-like [Sn2As6]12– units and infinite ∞2[Zn2As3]5– sheets extended in the a–b plane. Eu cations fill the space between these anionic motifs. Temperature-dependent magnetic properties and magnetoresistance of this Zintl phase have been studied, and the electronic structure and chemical bonding were elucidated using first-principles quantum chemical calculations (TB-LMTO-ASA). Quantum chemical calculations show that the ethane-like units can be considered as consisting of covalent single bonds;...

Study of effects of Te addition and oxygen contamination on the surface short range order structure of Ge[sbnd]Bi[sbnd]Se glass by XPS

The short range order structure of Bi13Ge20Se67-xTex glasses (x = 0, 5, 10, 15, 20 at.%) is explored by using high-resolution X-ray photoelectron spectroscopy. Results show that the incorporated oxygen from surface containment of specimens and Te substitution for Se exert important impacts on the network structure of glasses. Oxygen breaks the GeGe homopolar bonds in the GeSeBi glass, leading to a significant reduction of the over-constrained GeGe(Se)3 ethane-like units. Te addtion promotes the formation of mixed BiSe(Te)3/2 nanolayers as crystal precursors, thus initiating crystallinzation in competition to a topological self-organization process. The present work will be helpful to deepen the understanding of ChG glasses for applications in fields of optoelectronics and photonics.

Thermal, structural, and defect studies on Pb modified Ge[sbnd]Se glasses

We report the detailed thermal, structural, microstructural and electrical transport studies on PbxGe42−xSe58 with (0≤x≤20) glasses. The influence of Pb content on thermal and local atomic structure and its qualitative consequences on the electronic properties has been elucidated. The composition dependent variation of glass transition temperature (Tg), change in specific heat capacity at the glass transition (ΔCp) and non-reversing enthalpy change (ΔHNR) complement with each other for enabling the role of Pb content on structural relaxation behavior of these glasses. The Raman spectroscopic studies reveal the presence of different structural species and its variation with Pb content, i.e. pronounced decrease in the amount of ethane-like structural units with the incorporation of Pb content. The addition of Pb content into Ge42Se58 glass, the network connectivity becomes loosened due to the depolymerization. The observed Raman band position of ethane-like structural units show noticeable blue shift with an increase of Pb content, which dictates the development of stress in the network structure caused by the predominant ionic character. The microstructural studies by positron annihilation life-time spectroscopy reveal the nature of defect states and its variation with Pb content. The composition dependent dc conductivity and its activation energy shows marked change across the p-n transition threshold due to the change in the charged defect concentration. The influence of local atomic structure and nature of defect states responsible for the carrier type reversal (CTR) has been discussed.

NaGe6As6: Insertion of sodium into the layered semiconductor germanium arsenide GeAs

Abstract NaGe6As6 is a ternary arsenide prepared by reaction of the elements at 650 °C. It crystallizes in a new monoclinic structure type [space group C2/m, Z = 2, a = 22.063(2), b = 3.8032(4), c = 7.2020(8) Å, β = 92.7437(15)°] that can be considered to be derived by inserting guest Na atoms between [Ge6As6] layers identical to those found in the layered binary arsenide GeAs. An unusual feature in both structures is the presence of ethane-like Ge2As6 units in staggered conformation, with Ge–Ge dumbbells oriented either parallel or perpendicular to the layers. Electronic band structure calculations have shown that the electron excess in NaGe6As6 is accommodated by raising the Fermi level across a 0.6 eV band gap in semiconducting GeAs so that it cuts the bottom of the conduction band, resulting in an n-doped semiconductor.

Mechanisms of structural accommodation of Se deficiency in binary Ga2Se3–GeSe2 glasses: Results from 77Se MATPASS/CPMG NMR spectroscopy

The nature of the selenium coordination environments in the structure of binary Ga2Se3–GeSe2 chalcogenide glasses is investigated using two-dimensional 77Se magic angle turning-phase adjusted spinning sidebands/Carr–Purcell–Meiboom–Gill (MATPASS/CPMG) nuclear magnetic resonance (NMR) spectroscopy. The structural network of these glasses consists of corner and edge-sharing (Ga/Ge)Se4 tetrahedra in which the coordination numbers of Ga, Ge, and Se atoms are primarily 4, 4, and 2, respectively. Consequently, the addition of Ga2Se3 to GeSe2 results in the formation of Ge–Ge homopolar bonds to satisfy the coordination constraints of the constituent atoms. These Ge–Ge bonds are randomly distributed throughout the network such that any clustering of (Se3/2)Ge–Ge(Se3/2) ethane-like units is avoided to the maximum extent. However, for glasses containing ≥30mol% Ga2Se3, the appearance of three-coordinated Se atoms provides an alternate mechanism to accommodate for the Se deficiency in the limit where the Ge–Ge bonds are nearly saturated. These three-coordinated Se atoms are formed by corner-sharing between three GaSe4 tetrahedra that is reminiscent of oxygen triclusters shared by three AlO4 tetrahedra reported in peraluminous oxide glasses and liquids.

Quaternary arsenides ACdGeAs2 (A = K, Rb) built of ethane-like Ge2As6 units.

Reactions of the elements at high temperature resulted in the quaternary arsenides KCdGeAs2 (650 °C) and RbCdGeAs2 (600 °C). Single-crystal X-ray diffraction analysis reveals that they adopt a new triclinic structure type (space group P1̅, Pearson symbol aP20, Z = 4; a = 8.0040(18) Å, b = 8.4023(19) Å, c = 8.703(2) Å, α = 71.019(3)°, β = 75.257(3)°, γ = 73.746(3)° for KCdGeAs2; a = 8.2692(13) Å, b = 8.4519(13) Å, c = 8.7349(13) Å, α = 71.163(2)°, β = 75.601(2)°, γ = 73.673(2)° for RbCdGeAs2). Two-dimensional anionic layers [CdGeAs2](-) are separated by A(+) cations and are built from ethane-like Ge2As6 units forming infinite chains connected via three- and four-coordinated Cd atoms. Being Zintl phases, these compounds satisfy charge balance and are expected to be semiconducting, as confirmed by band structure calculations on KCdGeAs2, which reveal a band gap of 0.8 eV. KCdGeAs2 is diamagnetic.

Molecular Germanium Selenophosphate Salts: Phase-Change Properties and Strong Second Harmonic Generation

A new series of germanium chalcophosphates with the formula A(4)GeP(4)Q(12) (A = K, Rb, Cs; Q = S, Se) have been synthesized. The selenium compounds are isostructural and crystallize in the polar orthorhombic space group Pca2(1). The sulfur analogues are isostructural to one another but crystallize in the centrosymmetric monoclinic space group C2/c. All structures contain the new molecular anion [GeP(4)Q(12)](4-); however, the difference between the sulfides and selenides arises from the change in crystal packing. Each discrete molecule is comprised of two ethane-like P(2)Q(6) units that chelate to a central tetrahedral Ge(4+) ion in a bidentate fashion. The selenides were synthesized pure by stoichiometric reaction of the starting materials, whereas the sulfides contained second phases. The band gaps of the molecular salts are independent of the alkali metal counterions and have a value of 2.0 eV for the selenides and 3.0-3.1 eV for the sulfides. All A(4)GeP(4)Se(12) compounds melt congruently, and the potassium analogue can be quenched to give a glassy phase that retains its short-range order as shown by Raman spectroscopy and powder X-ray diffraction. Interestingly, K(4)GeP(4)Se(12) is a phase-change material that reversibly converts between glassy and crystalline states and passes through a metastable crystalline state upon heating just before crystallizing into its slow-cooled form. Initial second harmonic generation (SHG) experiments showed crystalline K(4)GeP(4)Se(12) outperforms the other alkali metal analogues and exhibits the strongest second harmonic generation response among reported quaternary chalcophosphates, ~30 times that of AgGaSe(2) at 730 nm. A more thorough investigation of the nonlinear optical (NLO) properties was performed across a range of wavelengths that is almost triple that of previous reports (λ = 1200-2700 nm) and highlights the importance of broadband measurements. Glassy K(4)GeP(4)Se(12) also exhibits a measurable SHG response with no poling.

Flux growth and structure of two compounds with the EuIn2P2structure type,AIn2P2(A= Ca and Sr), and a new structure type, BaIn2P2

Single crystals of the new Zintl phases AIn2P2 [A = Ca (calcium indium phosphide), Sr (strontium indium phosphide) and Ba (barium indium phosphide)] have been synthesized from a reactive indium flux. CaIn2P2 and SrIn2P2 are isostructural with EuIn2P2 and crystallize in the space group P63/mmc. The alkaline earth cations A are located at a site with 3m symmetry; In and P are located at sites with 3m symmetry. The structure type consists of layers of A2+ cations separated by [In2P2]2- anions that contain [In2P6] eclipsed ethane-like units that are further connected by shared P atoms. This yields a double layer of six-membered rings in which the In-In bonds are parallel to the c axis and to one another. BaIn2P2 crystallizes in a new structure type in the space group P2(1)/m with Z = 4, with all atoms residing on sites of mirror symmetry. The structure contains layers of Ba2+ cations separated by [In2P2]2- layers of staggered [In2P6] units that form a mixture of four-, five- and six-membered rings. As a consequence of this more complicated layered structure, both the steric and electronic requirements of the large Ba2+ cation are met.

Host dependence of spectroscopic properties of Dy^3+- doped and Dy^3+, Tm^3+-codped Ge-Ga-S-CdI_2 chalcohalide glasses

Two serial Dy(3+)-doped and Dy(3+), Tm(3+)-codoped (100-x)(0.8GeS(2).0.2Ga(2)S(3)).xCdI(2) (0<or=x<or=20) chalcohalide glasses were prepared and characterized. By analyzing the absorption and emission measurements of the two serial chalcohalide glasses with the Judd-Ofelt analysis, we were able to calculate its Judd-Ofelt strength parameters Omega t (t = 2, 4, 6), transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections. In the Dy(3+)-doped glasses, increase of CdI(2) had positive effect up to 1330 nm fluorescence and the emission cross section (sigma emi) was estimated to be 4.19 x 10(-20)cm(2) for the 0.2 wt% Dy(3+)-doped 64 GeS(2).16 Ga(2)S(3).20 CdI(2) glass. In the Dy(3+), Tm(3+)-codoped glasses, increase of CdI(2) diminished the amount of ethane-like units [S(3)(Ga)Ge-Ge(Ga)S(3)], improved the Tm(3+): 3F 4-->Dy(3+): 6H 11/2 energy transfer efficiency and intensified the mid-infrared emissions. The emission cross sections (sigma emi) of the 2900 and 4300 nm fluorescences were estimated to be 1.68 x 10(-20) and 1.20 x 10(-20)cm(2) respectively for the 0.2 wt% Dy(3+) and 0.5 wt% Tm(3+) codoped 64 GeS(2).16 Ga(2)S(3).20 CdI(2) glass. These novel chalcohalide glasses are promising candidate materials for fiber-amplifiers and mid-infrared laser devices.

Structure and properties of GeS 2–Ga 2S 3–CdI 2 chalcohalide glasses

Chalcohalide glasses in the GeS 2–Ga 2S 3–CdI 2 pseudo-ternary system were prepared by 3–5N pure raw materials. Structures of these glasses were studied with Raman spectroscopy. Several properties, namely, glass transition temperature, optical transmission, density and microhardness have also been measured. Based on the Raman spectra, it can be speculated that the glass network is mainly constituted by [GeS 4], [GaS 4] tetrahedra with some mixed-anion tetrahedra [S 3GeI], [S 2GeI 2] and [S 3GaI], which are interconnected by bridging sulfurs and/or short S–S chains. In the glasses with little CdI 2, some part of Ge(Ga) exists in the forms of the ethane-like units [S 3(Ga)Ge–Ge(Ga)S 3] because of the lack of sulfur, but the amount of these units will decrease with the addition of CdI 2. Additionally, in the glasses with high content of CdI 2, some [CdI n ] structural units (s.u.) will be formed and dispersed homogenously in glass network. These novel glasses have relatively high glass transition temperatures ( T g ranges from 512 to 670 K), good thermal stabilities (the maximum of difference between T x and T g is 185 K) and UV–vis optical transmission, large densities ( d ranges from 3.162 to 3.863 g/cm 3) and microhardness (large than 150 kg/mm 2 generally). All properties evolutions follow the structural variations.

Large and Ultrafast Third-Order Nonlinear Optical Propertiesof Ge-S Based Chalcogenide Glasses

We report ultrafast third-order nonlinear optical (NLO) properties ofseveral chalcogenide glasses GeSx (x = 1.8, 2.0, 2.5)measured by femtosecond time-resolved optical Kerr gate technique at820 nm. The third-order nonlinear susceptibility of GeS1.8 glassis determined to be as large as 1.41×10−12 esu, which is themaximum value of the third order nonlinear susceptibility χ(3)for the three compositions investigated. The symmetric Gauss profiles ofoptical Kerr signals reveal the nature of ultrafast nonlinear responseof these samples, which are originated from the ultrafast polarizationof the electron clouds. By detailed microstructural analysis of theseglasses based on the chain-crossing model (CCM) and therandom-covalent-network model (RCNM), it can be concluded thatχ(3) value of GeSx glasses can be enhanced greatly by S–Scovalent bonds or S3Ge–GeS3 ethane-like units.

Study of thermal and optical properties of GeS 2–Ga 2S 3–Ag 2S chalcogenide glasses

Chalcogenide glasses in the GeS 2–Ga 2S 3–Ag 2S pseudo-ternary system were prepared by melt-quenching technique. The structural evolvement of the glasses was studied by Raman spectroscopy. The Raman results show that the addition of Ag 2S involves the breaking of [S 3Ge–GeS 3] ethane-like units and the formation of Ag–S ionic bonds. The results of differential scanning calorimetry (DSC) show that the glasses have relatively high-glass transition temperatures and good thermal stabilities. These GeS 2–Ga 2S 3–Ag 2S glasses have a wide range of transmission approximately from 0.50 to 12.5 μm. In addition, with the method of Maker fringe, SHG was observed in the 0.9GeS 2–0.05Ga 2S 3–0.05Ag 2S glass irradiated by an electron beam. The value of second-order nonlinear optical susceptibility d is as large as 6.6 pm/V, and the poling mechanism of electron beam irradiation was also discussed in this work.

Study on the structure dependent ultrafast third-order optical nonlinearity of GeS2–In2S3 chalcogenide glasses

The results of the femtosecond time-resolved optical Kerr at 820nm in GeS2–In2S3 chalcogenide glasses indicate that the response time in GeS2–In2S3 glasses is subpicosecond, which is predominantly due to the distortion of the electron cloud. The value of χ(3) in 0.95GeS2–0.05In2S3 glass is also as large as 2.7×10−13esu, and it reduces with the addition of In2S3, which may be ascribed to the microstructure evolution of GeS2–In2S3 glasses. It is deduced that the intrinsic [Ge(In)S4] tetrahedral structure units that possess the high hyperpolarizability may do great contribution to the enhancement of third-order optical nonlinearity while [S3Ge–GeS3] ethane-like molecular units make no considerable contribution to that in femtosecond time scale. These GeS2–In2S3 and GeS2–In2S3-based chalcogenide glasses would be expected to be the promising materials for all-optical switching devices.

Micro-structural study of the GeS 2–In 2S 3–KCl glassy system by Raman scattering

Room temperature Raman spectra of samples on three serials within the GeS 2–In 2S 3–KCl glassy system have been investigated systematically. According to XRD patterns and Raman spectra of several pseudo-binary systems, the Cl atoms, which was added into the GeS 2–In 2S 3 glasses through KCl, was considered to be leading to the breaking of In–In bonds among the S 3In–InS 3 ethane-like units and the forming of InS 4− x Cl x , InS 6− x Cl x mixed polyhedra. Considering the effect of K + ions upon mixed anion units (InS 4− x Cl x and InS 6− x Cl x ) and the corresponding micro-structural model, the Raman spectral evolution of the GeS 2–In 2S 3–KCl glasses can be elucidated successfully. The microstructure of the GeS 2–In 2S 3–KCl glasses was considered to be that the potassium atoms, which exist in the form of chlorine atoms as its nearest neighbor, are homogeneously dispersed in the glassy net formed by the micro-structural units such as InS 4, InS 6, InS 4− x Cl x , InS 6− x Cl x , GeS 4 polyhedra and S 3In(Ge)–In(Ge)S 3 ethane-like units.

Raman scattering studies of the Ge–In sulfide glasses

Room temperature Raman scattering measurements of samples of the (1− x)GeS 2– xIn 2S 3 ( x=0.00–0.35) system have been conducted together with the FTIR transmission spectra of partial samples. Based on the Raman scattering and FTIR transmission spectra of the prepared defect spinel polycrystalline In 2S 3, the additional peaks at 306 and 245 cm −1 were ascribed to the local symmetric stretching vibration of InS 4 tetrahedra and InS 6 octahedra, respectively. According to the Raman scattering spectral evolution of the Ge–In sulfide glasses, the microstructure of the studied glasses was considered to be that S 3Ge–GeS 3 and S 3In–InS 3 ethane-like units originated from the sulfur deficient with the addition of In 2S 3 are homogeneously dispersed in a disordered polymer network formed by GeS 4, InS 4 tetrahedra and a small quantity of InS 6 octahedra interconnected by sulfur bridges.

Properties and microstructure of GeS2−Ga2S3−CdS glasses

The samples of the GeS2−Ga2S3−CdS pseudo-ternary glassy system were prepared by conventional melt-quenching techniques. The microstructure of the GeS2−Ga2S3−CdS glasses was analyzed thoroughly using Raman spectra and the relationships among the composition, microstructure and properties (such as thermal properties, densities, optical properties) were probed. The experimental results indicate that the GeS2 acts as the network former, the Ga2S3 as the net intermediate, and the CdS as the net modifier. The GeS2 and Ga2S3 exist in the form of [GeS4/2], [GaS4/2] tetrahedra or S3 Ge(Ga)−(Ga)GeS3 ethane-like units within the glassy network, and the addition of CdS mainly breaks the Ge(Ga)−(Ga) Ge bonds among the ethane-like units, leading to the formation of [GeS4/2], [GaS4/2] tetrahedra. The Tg and Tx have tight relations on the congregated degree of glassy network, however, λris, n and d are hardly involved into the connectional dependence of the space arrangement.

Microstructural probing of (1−x) GeS2−x Ga2S3 system glasses by Raman scattering

Raman scattering measurement of (1−x) GeS2−x Ga2S3 system glasses was conducted in order to understand the microstructural change caused by the addition of Ga2S3. According to the change of Raman spectra with the addition of Ga2S3, two main structural transformations were deduced: the gradual enhancement of ethane-like structural units S3Ge−GeS3 (250 cm−1) and S3 Ga−GaS3 (270 cm−1) and the appearance of charge ambalanced units [Ga2S2(S1/2)4]2− and [Ga(S1/2)4]− And this change of structural aspect seems to give us a clue to understanding the cause of the increased rare-earth solubility.