Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
BaCuSiTe3: A Noncentrosymmetric Semiconductor with CuTe4 Tetrahedra and Ethane-like Si2Te6 Units Parisa Jafarzadeh,† Luke T. Menezes,† Mengyang Cui,† Abdeljalil Assoud,† Weiguo Zhang,‡ P. Shiv Halasyamani,‡ and Holger Kleinke*,† †
Department of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, ON, Canada N2L 3G1 Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
‡
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S Supporting Information *
ABSTRACT: BaCuSiTe3 was prepared from the elements in a solidstate 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.
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to exhibit good NLO properties.20,21 Here we report a new noncentrosymmetric telluride discovered by our group, BaCuSiTe3, including a first assessment of its NLO and transport properties.
INTRODUCTION Nonlinear optical (NLO) materials have been used for decades to modulate the frequencies of lasers in order to increase their range of applications. One differentiates between frequency upconversion, for example from the standard Nd:YAG laser wavelength of 1064 nm (∼1 μm) to shorter wavelengths (higher frequencies) of, e.g., 532 or 355 nm or lower (UV/ DUV range), and downconversion, i.e., to higher wavelengths in the mid/far-IR range of 3−20 μm.1−6 For the former, established materials include several oxides, most notably LiNbO37 and several borates such as β-BaB2O4.8 IR NLO materials, on the other hand, are typically chalcogenides, including the commercialized AgGaS2,9 AgGaSe2,10 and ZnGeP211 and newer materials such as BaGa4Se7.12,13 The optimization of IR NLO materials is complex, for in addition to a large second-order harmonic generation response (SHG, dij), one also needs a high laser damage threshold (LDT), a wide IR transmission range, a wide band gap, and phase-matching behavior.14 Specifically, a wider band gap generally results in a higher LDT but smaller dij values. On top of that, a necessary criterion for the existence of NLO behavior is a noncentrosymmetric space group. Typically that is achieved by incorporating noncentrosymmetric building blocks like tetrahedra or distorted polyhedra based on cations with lone electron pairs or second-order Jahn−Teller effects.2 Our exploratory research into ternary and higher chalcogenides has revealed several hitherto-undiscovered noncentrosymmetric materials with such motifs, including SrSn2Se4 (space group Fdd2),15 Ba2SnSe5 (P212121),16 BaAg2SnSe4 (I222) and BaCu 2 SnSe 4 (Ama2), 1 7 BaMnSnSe 4 (Fdd2), 1 8 and Sr19−xPbxGe11Se44 (P63),19 some of which were later shown © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Syntheses and Analyses. BaCuSiTe3 was prepared from the elements in the stoichiometric ratio (Ba pieces, 99.7%, Strem Chemicals; Cu powder, 99.5%, Alfa Aesar; Si powder, 99.9%, Alfa Aesar; Te broken ingots, 99.99%, Strem Chemicals). The elements were added to carbon-coated silica tubes in an argon-filled glovebox, and the tubes were evacuated to ∼10−3 mbar and sealed. In a programmable furnace, the sealed tubes were heated to 973 K within 10 h, kept at this temperature for 2 h, and finally slowly cooled to room temperature. After grinding, the samples were reheated at 773 K to enhance the homogeneity and yield. Attempts to prepare analogous compounds with Ag instead of Cu and with Ge instead of Si were not successful. Se atoms could be introduced into this material, partially replacing Te, but attempts to substitute more than 0.5 Te atoms per formula unit failed. Phase identification was performed with an Inel powder X-ray diffractometer equipped with a position-sensitive detector and Cu Kα1 radiation. The obtained X-ray diffraction pattern exhibited no peaks characteristic of any known material (Figure S1). Preliminary differential scanning calorimetry measurements under argon using a Netzsch Luxx thermal analyzer revealed an incongruent melting point at around 760 K. Scanning electron microscopy/energy-dispersive X-ray analysis (SEM/EDX) was performed using a QuantaFeg 250/Oxford Instrument x-act system with an applied acceleration voltage of 25 Received: May 31, 2019
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DOI: 10.1021/acs.inorgchem.9b01608 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry kV. No heteroelements were detected, and the elemental percentages of the six measured crystals were obtained as Ba:Cu:Si:Te = 20(3):16.8(8):16.3(3):47(1), which compare reasonably well to the nominal percentages of 16.7:16.7:16.7:50. Single-Crystal Structure Studies. A dark-red block-like crystal was selected for data collection at room temperature on a Bruker Kappa Apex II CCD diffractometer utilizing Mo Kα radiation. With the search strategy part of the APEX II suite, the data were collected by scanning ω and ϕ in steps of 0.3° in different sets of frames to obtain full data coverage and low redundancy, with an exposure time of 30 s per frame. The data were corrected for Lorentz and polarization effects, and absorption corrections were applied to the data using the empirical multiscan method SADABS, part of the APEX II software,22 since the crystal faces could not be determined reliably for numerical absorption corrections. For solution and refinement, we utilized the SHELXTL package.23 On the basis of the systematic absences, the possible space groups were P21/c and Pc. Only in Pc was the structure solution employing the direct method successful. Two Ba, two Cu, two Si, and six Te sites were identified and subsequently refined. To test for Cu deficiencies, as often found in barium copper chalcogenides,24 we refined the occupancies of the two Cu sites, resulting in almost full occupancies of 98% and 97%, respectively. Therefore, the Cu sites were treated as fully occupied. Last, the tidy routine of the Platon program package25 was utilized to standardize the atomic positions. Its addsymm part was applied to identify potential additional symmetry elements; none were found. To verify that Se can indeed be incorporated, a red single crystal of a sample of nominal composition “BaCuSiSe0.5Te2.5” was also analyzed. This crystal exhibited a significantly smaller unit cell volume of 720.1 Å3 versus 746.5 Å3, as Se atoms are smaller than Te atoms. Refining all of the chalcogen sites Q as mixed-occupied by Se and Te resulted in more than 1% Se in the cases of the Q1, Q3, and Q6 sites. Subsequently Q2, Q4, and Q5 were treated as pure Te positions. The crystallographic data are summarized in Table 1. Table 2 lists the atomic positions, and selected bond distances are shown in Table 3. On the basis of these distances, it is not obvious why the Se atoms would prefer the Q1, Q3, and Q6 sites. The crystallographic
Table 2. Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters of BaCuSiTe3a Ba1 Ba2 Cu1 Cu2 Si1 Si2 Te1 Te2 Te3 Te4 Te5 Te6
y
z
Ueq/Å2
0.19906(4) 0.54975(4) 0.83808(9) −0.0000(1) 0.5375(2) 0.2936(2) 0.30310(4) 0.69025(4) 0.49856(4) 0.78718(4) 0.02482(4) 0.09644(4)
0.37286(3) 0.14529(3) 0.00228(7) 0.34223(8) 0.33357(15) 0.14402(14) 0.53934(4) 0.45838(4) 0.07479(3) 0.21618(4) 0.74989(3) 0.06670(4)
0.30771(2) 0.79552(3) 0.19445(6) −0.00014(7) 0.1106(1) 0.0018(1) 0.10428(3) 0.00119(3) 0.49840(3) 0.31061(2) 0.29909(3) 0.09948(3)
0.01879(7) 0.02081(7) 0.0250(1) 0.0286(2) 0.0128(2) 0.0130(2) 0.01515(7) 0.01702(7) 0.01638(7) 0.01708(7) 0.01643(7) 0.01991(7)
a
Se occupancies for BaCuSiSe0.41(2)Te2.59: 44(2)% on Q1; 23(2)% on Q3; 15.1(1.6)% on Q6.
Table 3. Selected Interatomic Distances (in Å) for BaCuSiTe3 Ba1−Te4 Ba1−Te1 Ba1−Te3 Ba1−Te2 Ba1−Te5 Ba1−Te1 Ba1−Te6 Ba1−Te2 Cu1−Te4 Cu1−Te3 Cu1−Te5 Cu1−Te6 Si1−Si2 Si1−Te4 Si1−Te1 Si1−Te2
Table 1. Crystallographic Details for BaCuSiTe3a formula formula weight crystal system space group a [Å] b [Å] c [Å] β [deg] V [Å3] Z density [g cm−3] absorption coefficient [mm−1] F(000) crystal size [mm] temperature [K] wavelength [Å] total data, unique data, R(int) observed data [I > 2σ(I)] no. of reflections, parameters R(Fo),b Rw(Fo2),c GOF (obs. data) min., max. residual electron density [e Å−3]
x
BaCuSiTe3 611.77 monoclinic Pc (no. 7) 7.5824(1) 8.8440(1) 13.1289(2) 122.022(1) 746.45(2) 4 5.444 19.62 1020 0.09 × 0.04 × 0.02 293(2) 0.71073 7711, 3596, 0.016 3570 3596, 111 0.015, 0.033, 1.10 −0.98, 0.77
3.4354(4) 3.4867(4) 3.5135(4) 3.5381(4) 3.5663(4) 3.6234(4) 3.6270(4) 3.8969(4) 2.5842(7) 2.5865(7) 2.6099(7) 2.8822(7) 2.339(2) 2.507(1) 2.514(1) 2.527(1)
Ba2−Te3 Ba2−Te6 Ba2−Te1 Ba2−Te2 Ba2−Te4 Ba2−Te5 Ba2−Te3 Ba2−Te6 Ba2−Te5 Cu2−Te2 Cu2−Te1 Cu2−Te6 Cu2−Te5 Si2−Si1 Si2−Te3 Si2−Te5 Si2−Te6
3.4829(4) 3.5457(4) 3.5498(4) 3.6115(4) 3.6224(4) 3.6957(4) 3.7617(4) 4.0059(4) 4.1113(5) 2.5719(7) 2.6191(7) 2.6775(8) 2.8590(8) 2.339(2) 2.497(1) 2.523(1) 2.527(1)
information file (CIF) is available from the joint CCDC/FIZ Karlsruhe deposition service under accession code CCDC 1919783. Physical Property Measurements. A ground BaCuSiTe3 sample was hot-pressed under a 5% hydrogen/95% argon atmosphere with an Oxy-Gon FR-210-30T-ASA-160-EVC hot-press furnace system using a hardened graphite die with an inner diameter of 12.7 mm. The densifying was performed at 583 K under a pressure of 47 MPa for 1 h, which resulted in a density equal to 99% of the theoretical density as determined via the Archimedes method. The resulting disk was polished to a height of 1.5 mm under argon, and its thermal diffusivity (D) was measured using a TA Instruments DLF 1200 system under argon. The thermal conductivity κ was determined via the formula κ = ρCpD, in which ρ is the pellet’s density and Cp is the specific heat derived from the Dulong−Petit law.26 The experimental error for κ was estimated to be ±5%. For the electrical properties, a rectangular-shaped pellet with dimensions of 10 mm × 2 mm × 1.5 mm was cut from the hotpressed disk. The electrical conductivity σ and the Seebeck coefficient α were measured on that rectangular-shaped pellet using a ULVACRIKO ZEM-3 system. The experimental errors were estimated to be ±3% for α and ±5% for σ. The diffuse-reflectance spectrum of BaCuSiTe3 was recorded from a cold pressed pellet on a PerkinElmer Lambda 1050 UV/vis/NIR spectrometer. The Kubelka−Munk function (α/S = (1 − R)2/(2R), where α is the absorbance, S is the scattering factor (a constant for
a
Lattice parameters for BaCuSiSe0.41(2)Te2.59: a = 7.491(2) Å, b = 8.710(2) Å, c = 13.019(3) Å, β = 122.016(1)°, V = 720.2(3) Å3. b R(Fo) = ∑||Fo| − |Fc||/∑|Fo|, where Fo and Fc are the observed and calculated structure factors, respectively. cRw(Fo2) = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. B
DOI: 10.1021/acs.inorgchem.9b01608 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry particles larger than ∼5 μm), and R is the reflectance) was used to convert the reflectance spectrum to the absorption spectrum. To measure the SHG of BaCuSiTe3, a sample was hot-pressed into a pellet to achieve high density. This pellet was broken up and sieved into six size ranges:
