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Cite This: Inorg. Chem. 2017, 56, 14251-14259
Semiconducting Ba3Sn3Sb4 and Metallic Ba7−xSn11Sb15−y (x = 0.4, y = 0.6) Zintl Phases Haijie Chen,†,‡ Awadhesh Narayan,§,∥ Constantinos C. Stoumpos,† Jing Zhao,† Fei Han,‡ Duck Young Chung,‡ Lucas K. Wagner,§ Wai-Kwong Kwok,‡ and Mercouri G. Kanatzidis*,†,‡ †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Physics, University of Illinois at Urbana−Champaign, Champaign, Illinois 61801, United States Downloaded via UNIV OF TEXAS MEDICAL BRANCH on June 26, 2018 at 21:28:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We report the discovery of two ternary Zintl phases Ba3Sn3Sb4 and Ba7−xSn11Sb15−y (x = 0.4, y = 0.6). Ba3Sn3Sb4 adopts the monoclinic space group P21/c with a = 14.669(3) Å, b = 6.9649(14) Å, c = 13.629(3) Å, and β = 104.98(3)°. It features a unique corrugated two-dimensional (2D) structure consisting of [Sn3Sb4]6− layers extending along the ab-plane with Ba2+ atoms sandwiched between them. The nonstoichiometric Ba6.6Sn11Sb14.4 has a complex one-dimensional (1D) structure adopting the orthorhombic space group Pnma, with unit cell parameters a = 37.964(8) Å, b = 4.4090(9) Å, and c = 24.682(5) Å. It consists of large double Sn−Sb ribbons separated by Ba2+ atoms. Ba3Sn3Sb4 is an n-type semiconductor which has a narrow energy gap of ∼0.18 eV and a room temperature carrier concentration of ∼4.2 × 1018 cm−3. Ba6.6Sn11Sb14.4 is determined to be a metal with electrons being the dominant carriers.
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INTRODUCTION
The tin-containing ternary systems A/Sn/Pn (A = alkali or alkali earth metal; Pn = Sb, As) are a distinct series of Zintl phases. KSnSb has been explored for thermoelectric applications,27 while Ba2Sn3Sb6 and SrSn3Sb4 were reported to be new superconductors.28−30 Because of their weak bonding interactions between layers, NaSn2As2 and EuSn2As2 were explored as potential exfoliatable 2D compounds.31,32 In this work, we carried out a detailed investigation of the Ba/Sn/Sb system and discovered two new Ba/Sn/Sb compounds, Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4. On the basis of X-ray diffraction analysis, Ba3Sn3Sb4 features a corrugated 2D structure with alternating Ba2+ atoms and [Sn3Sb4]6− network. Ba6.6Sn11Sb14.4 adopts a complex 1D-like structure consisting of double infinite Sn−Sb ribbons that are separated by Ba2+ atoms. Charge transport measurements show that Ba3Sn3Sb4 is a narrow gap semiconductor (Eg ∼ 0.18 eV), while Ba6.6Sn11Sb14.4 is a normal metal.
Zintl phases are formed when a nearly complete electron transfer from an electropositive atom (generally, groups 1 and 2) to a relatively electronegative main group atom occurs. The structures and compositions of these phases are understood in terms of simple chemical principles defined by the so-called octet rule.1,2 This class of materials provides a useful framework for understanding the structures and bonding in many intermetallic solids.3 Zintl phases are determined to be formal semiconductors as a result of the expected energy gap between occupied (bonding and nonbonding) and unoccupied (antibonding) states in the electronic structure of closed-shell systems.4 With narrow band gaps and complex anionic frameworks stabilized by ionically bonded electropositive cations, many Zintl phases such as Ba4In8Sb16, BaGa2Sb2, Ba6Ge25−x, CaZn2Sb2, etc. have low lattice thermal conductivity and have been widely explored for use in thermoelectric applications.5−11 Zintl phases, however, can also be metallic because of conduction/valence band overlap or nonstoichiometry and furthermore can give rise to interesting physical phenomena, such as superconductivity as reported for BaSn3, Ca11Bi10−x, and Ba8Si46 to name a few.12−14 Inclusion of transition metals and rare earth elements can extend the diversity of Zintl compounds, endowing them with unusual electrical and magnetic properties, such as in Yb9Zn4Bi9,15 Sr14 MnSb11,16 Yb14 MnSb11,17 Eu 10 Mn6 Sb 13 ,18 EuIn2P 2 ,19 etc.20−26 © 2017 American Chemical Society
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EXPERIMENTAL SECTION
Synthesis. All elements were used as purchased, and the handling of chemicals was conducted in a nitrogen filled glovebox. Both Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4 were initially discovered during the exploration of Ba doping in the LaCrSb3 system by Sn flux.33 Subsequently, the reaction conditions were optimized to prepare the single phase product. A 1 mmol Ba chunk (0.137 g, 99.9%, SigmaAldrich), 2 mmol of Sb lumps (0.122 g, 99.9%, Sigma-Aldrich), and 10 Received: September 13, 2017 Published: November 8, 2017 14251
DOI: 10.1021/acs.inorgchem.7b02352 Inorg. Chem. 2017, 56, 14251−14259
Article
Inorganic Chemistry mmol of Sn lumps (1.187 g, 99.9%, American Elements) were added into an alumina crucible. Some amount of silica wool was placed atop the crucible to serve as a block for possible vapors, with a small length of alumina tube above to serve as a counterweight. All these were placed into a fused silica tube, evacuated to ∼10−4 Torr, and flamesealed. The tubes were placed inside a programmable furnace and heated to 1000 °C over 5 h, held at this temperature for 12 h, and cooled to 300 °C with the cooling rate of −5 °C/h. After soaking at 300 °C for a few hours, the tubes were quickly removed from the furnace and centrifuged to separate excess liquid Sn from the products. Two kinds of crystals, with different shapes, were found both loosely adhered to the crucible walls and captured on the silica wool. One is needle-like (∼75% yield), and the other is plate-like (∼25% yield). Single-Crystal X-ray Diffraction. Single crystals with well-defined facets were selected for X-ray diffraction. Data collections were carried out at room temperature (293 K) by using X-Area software with a STOE IPDS 2T single-crystal diffractometer at 50 kV and 40 mA with Mo Kα radiation (λ = 0.71073 Å).34 The crystal structure was solved via direct method and refined with the SHELX package.35 A summary of single-crystal data is given in Table 1. Atomic coordinates and bond lengths and angles are presented in Tables 2 and 3 for Ba3Sn3Sb4 and
for Ba6.6Sn11Sb14.4, respectively. More results of the single-crystal structural refinements are listed in Tables S1−S4. Powder X-ray Diffraction (PXRD). PXRD was measured to compare with the simulated patterns based on refined structures (Figure S1). The obtained single crystals were pulverized with an agate mortar. The PXRD patterns were collected on a Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. Scanning Electron Microscopy−Energy Dispersive X-ray Spectroscopy (SEM−EDS). Scanning electron microscope (SEM) images and element compositions of the synthesized crystals were obtained by a Hitachi S3400N-II scanning electron microscope equipped with an Oxford Instruments INCAx-act SDD EDS detector. Unpolished crystals mounted with carbon tape on an aluminum stub were imaged with an accelerating voltage of 20 kV and a probe current of 70 mA. The spectra were collected from numerous clean crystal surfaces using 120 s acquisition times. Transport Measurements. Resistivity and Hall effect as a function of temperature were measured on a Quantum Design PPMS. Contacts were made with gold wires attached to the sample surface using Dupont 4929N silver paste, and sample dimensions were determined using SEM images. Following our previous methods to maintain accuracy, the resistivity and Hall effect were measured on the same sample to exclude sample differences.36 The Hall resistivity, Rxy = [R(+H) − R(−H)]/2, was obtained by switching the magnetic field at each point to reduce the effect of Hall electrode misalignment. Absorption Spectra. Diffuse-reflectance IR absorption spectra were collected at room temperature, using a Nicolet 6700 FT-IR (Fourier transform infrared spectrometry) spectrometer. The spectra were collected on powder samples obtained by crushing a few large single crystals. The reflectance was converted to absorption using the Kubelka−Munk function: α/S = (1 − R)2/2R, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.37 The fundamental absorption edge was determined by linearly fitting the absorbance of the converted data against the energy axis. Electronic Structure. First-principles density functional theory calculations were carried out using the Quantum Espresso package.38 The Perdew−Burke−Ernzerhof form of the exchange correlation functional was used.39 A plane wave cutoff of 40 Ry was employed, and the Brillouin zone was sampled using a 8 × 4 × 8 k-point mesh. An experimentally determined crystal structure was used.
Table 1. Crystal Data and Structure Refinements for Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4 at 293(2) Ka empirical formula fw wavelength cryst syst space group unit cell dimensions
V Z density abs coeff F(000) cryst size θ range for data collection index ranges reflns collected indep reflns completeness to θ = 25.00° refinement method
Ba3Sn3Sb4 1255.09 0.71073 Å monoclinic P21/c a = 14.669(3) Å, α = 90.00° b = 6.9649(14) Å, β = 104.98(3)° c = 13.629(3) Å, γ = 90.00° 1345.1(5) Å3 4 6.197 g/cm3 21.934 mm−1 2088 0.7637 × 0.2568 × 0.0338 mm3 3.26 to 25.00°
Ba6.6Sn11Sb14.4 3967.53 0.71073 Å orthorhombic Pnma a = 37.964(8) Å, α = 90° b = 4.4090(9) Å, β = 90° c = 24.682(5) Å, γ = 90° 4131.4(14) Å3 4 6.379 g/cm3 21.922 mm−1 6620 0.4104 × 0.0612 × 0.0500 mm3 2.698 to 26.826°
−17 ≤ h ≤ 17, −8 ≤ k ≤ 7, −16 ≤ l ≤ 16 8161 2366 [Rint = 0.0853] 99.7%
−47 ≤ h ≤ 47, −5 ≤ k ≤ 5, −31 ≤ l ≤ 31 30595 4965 [Rint = 0.0899] 99.6%
full-matrix least-squares on F2 2366/0/92
full-matrix least-squares on F2 4965/6/210
data/restraints/ params GOF 1.227 Robs = 0.0337, wRobs = final R indices 0.0876 [>2σ(I)]a R indices [all data]a Rall = 0.0349, wRall = 0.0884 extinction coeff 0.00120(7) largest diff peak and 1.888 and −1.893 hole (e Å−3)
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RESULTS AND DISCUSSIONS Synthesis. We investigated several different reaction ratios of Ba/Sb/Sn and found that Ba3Sn3Sb4 single crystals form from 1:1:10 and 1:2:10, and Ba6.6Sn11Sb14.4 single crystals are obtained from 1:2:10 and 1:3:10 ratios. The known Ba2Sn3Sb6 was observed from a 1:4:10 ratio.28 Full exploration of other combinations of starting materials (3:1:10 and 2:1:10) resulted in binary BaSn and SnSb compounds. As shown in Figure 1a, the Ba3Sn3Sb4 crystals are chunk-shaped with the longest axis up to ∼400 μm. The EDS result indicates a ratio of 3:3:4 for Ba:Sn:Sb. Ba3Sn3Sb4 is moderately stable and turns black on the surface upon standing in air after a week. The needle-like Ba6.6Sn11Sb14.4 crystals grow up to 500 μm long, Figure 1b. The EDS result indicates a ratio of 4:7:9 for Ba:Sn:Sb. Unlike Ba3Sn3Sb4, Ba6.6Sn11Sb14.4 is stable in air. Direct solid state syntheses using a stoichiometric combination of elements were attempted to obtain either Ba3Sn3Sb4 or Ba6.6Sn11Sb14.4 but were unsuccessful, which was confirmed by PXRD (Figure S2). Structure of Ba3Sn3Sb4. Ba3Sn3Sb4 features anionic layers of [Sn3Sb4]6− which are corrugated and separated by Ba2+ ions along the b-axis, Figure 2. As shown in Figure 3, the [Sn3Sb4]6− network consists of [SnSb3] trigonal pyramidal building blocks sharing corners as well as between [SnSb2] units. Selected interatomic distance and angles are listed in Table 2. For the Sn
1.041 Robs = 0.0383, wRobs = 0.0631 Rall = 0.0569, wRall = 0.0678 0.000183(7) 2.955 and −2.507
R = ∑||Fo| − |Fc||/∑|Fo|, wR = {∑[w(|Fo|2 − |Fc|2)2]/∑[w(|Fo|4)]}1/2, and calcd w = 1/[σ2(Fo2) + (0.0253P)2 + 19.1514P], where P = (Fo2 + 2Fc2)/3 for Ba3Sn3Sb4 and w = 1/[σ2(Fo2) + (0.0232P)2] for Ba6.6Sn11Sb14.4. a
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DOI: 10.1021/acs.inorgchem.7b02352 Inorg. Chem. 2017, 56, 14251−14259
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Inorganic Chemistry
Table 2. Selected Interatomic Distance (Å) and Angles (deg) in Ba3Sn3Sb4 at 293(2) K with Estimated Standard Deviations in Parentheses Sn(1)−Sb(3) Sn(1)−Sb(4) Sn(1)−Sb(4) Sn(2)−Sb(2) Sn(2)−Sb(3) Sn(3)−Sb(1) Sn(3)−Sb(1) Sn(3)−Sb(2) Ba(1)−Sn(1) Ba(1)−Sn(1) Ba(1)−Sb(2) Ba(1)−Sb(3) Ba(1)−Sb(3) Ba(1)−Sb(3) Ba(1)−Sb(4) Ba(1)−Sb(4) Ba(1)−Sb(4) Ba(2)−Sn(1) Ba(2)−Sn(2) Sb(4)−Sn(1)−Sb(4) Sb(4)−Sn(1)−Sb(3) Sb(4)−Sn(1)−Sb(3) Sb(2)−Sn(2)−Sb(3) Sb(2)−Sn(3)−Sb(1) Sb(2)−Sn(3)−Sb(1)
2.934(1) 2.889(7) 2.908(5) 2.806(1) 2.944(7) 2.892(3) 2.949(7) 2.827(4) 3.642(1) 3.709(1) 3.389(1) 3.475(4) 3.625(1) 3.749(4) 3.572(2) 3.686(9) 3.732(9) 3.696(4) 3.545(7) 101.98(3) 90.50(3) 112.65(4) 104.09(4) 108.93(4) 100.39(3)
Ba(2)−Sn(2) Ba(2)−Sn(3) Ba(2)−Sb(1) Ba(2)−Sb(1) Ba(2)−Sb(2) Ba(2)−Sb(2) Ba(2)−Sb(3) Ba(2)−Sb(4) Ba(3)−Sn(1) Ba(3)−Sn(2) Ba(3)−Sn(2) Ba(3)−Sn(3) Ba(3)−Sb(1) Ba(3)−Sb(1) Ba(3)−Sb(2) Ba(3)−Sb(2) Ba(3)−Sb(3) Ba(3)−Sb(4)
3.725(1) 3.636(5) 3.545(1) 3.700(2) 3.622(4) 3.759(8) 3.610(1) 3.600(4) 3.639(6) 3.657(4) 3.678(1) 3.459(5) 3.514(5) 3.831(7) 3.589(9) 3.690(1) 3.422(9) 3.777(9)
Sb(1)−Sn(3)−Sb(1) Sn(3)−Sb(1)−Sn(3) Sn(2)−Sb(2)−Sn(3) Sn(1)−Sb(3)−Sn(2) Sn(1)−Sb(4)−Sn(1)
95.91(3) 106.23(3) 91.39(3) 76.10(3) 102.05(3)
along the b-axis, formed by Sb9 and Sb10 atoms. The second type, infinite ribbon-II, runs along the a-axis, see Figure 4c ([Sn5Sb7]), and has the width of five [SnSb3]-based units. It has an Sb−Sb zigzag chain in the edge, composed of Sb1 and Sb2 atoms. Ribbon-I and ribbon-II stack to form a pair which we refer to here as a double ribbon. For comparison, the corresponding ordered ribbon-I is plotted in Figure 5c. The distances of Sn7−Sb9 and Sn6−Sb10 are, respectively, ∼3.55 and ∼3.33 Å, which suggest weak interactions. As shown in Table S4, the U22 isotropic displacement parameters for Ba7, Sb9, and Sb10 atoms are 682, 234, and 255 × 103 Å2, respectively. The observed disorder along the b-axis should be due to geometrical requirements.40 Because a high density of Ba cations cannot be stabilized by the ideal structure, the defects are generated at the Sb positions mainly to compensate for the missing electrons that should be donated by the active metal Ba. This leads to the disordering of the atom sites along the b-axis. If we ignore the partial atom occupancies in Ba6.6Sn11Sb14.4, the ideal formula can be written as Ba7Sn11Sb15. Assuming complete electron transfer, the resulting 14 electrons from seven Ba atoms are distributed over the [Sn11Sb15]14− ribbons. All Sn atoms have the same [SnSb3] coordination, bonding to three Sb atoms. Concerning the Sb atoms, they adopt three different coordination environments: Sb7, Sb8, and Sb15 are two-coordinated with Sn atoms, Sb1 and Sb2 form Sb−Sb bonding, and the remaining Sb atoms are three-coordinated. According to the Zintl concept, two- and three-bonded Sb atoms carry charges of −1 and −3, respectively. Similar to Ba2Sn3Sb6, the formal oxidation states of all Sn atoms in the trigonal pyramidal coordination should be +2. If we assume the Sb1 atoms in Sb−Sb bonds as zero valence, the ideal formula can be rewritten as (Ba2+)7[(Sn2+)11(Sb−)3(Sb0)(Sb3−)11]. Ribbon-I is [Sn6Sb8]8−, and ribbon-II is [Sn5Sb7]6−. The exact
atoms (Sn1 and Sn3) that form trigonal pyramids, the Sn−Sb distances in Ba3Sn3Sb4 range from 2.806(1) to 2.949(7) Å. The two-coordinated Sn2 possesses Sn−Sb distances of 2.806(1) and 2.944(7) Å, respectively. Similar Sb−Sn distances were also found in Ba2Sn3Sb6 and SrSn3Sb4.28,29 The Sb−Sn−Sb angles of the trigonal [SnSb3] pyramid are in the range 90.50− 112.65°, similar to that for SrSn3Sb4.29 All Sb atoms are twocoordinated with Sn atoms. According to the Zintl concept, the formal charge of Sb can be assigned as −3. The Sn atoms in the trigonal pyramid have a valence of +2, resulting in a net charge of −6 per formula unit of [Sn3Sb4]. Thus, the charge-balanced formula of Ba3Sn3Sb4 can be written as (Ba2+)3(Sn2+)3(Sb3−)4. The Ba2+ atoms are located between the 2D [Sn3Sb4]6− layers and surrounded by Sb and Sn atoms in distorted bicapped square antiprismatic geometries as shown in Figure S3. The Ba1 atom is coordinated with three Sn atoms and seven Sb atoms; Ba2 is bonded to five Sn atoms and six Sb atoms, and Ba3 to four atoms and six Sb atoms. The Ba−Sn distances range from 3.459(5) to 3.725(1) Å and the Ba−Sb distances from 3.389(1) to 3.831(7) Å, which are comparable to those found in related systems, such as Ba4In8Sb16.5 Structure of Ba 6. 6 Sn 11 Sb 14.4 . The structure of Ba6.6Sn11Sb14.4 along the b-axis is shown in Figure 4. It features two different kinds of infinite parallel Sn−Sb ribbons which are one atom thick and are stacked along the a-axis direction while running parallel to the b-axis. Two of these ribbons (one from each kind) first stack pairwise to form double ribbons, see Figure 5. These ribbons that are almost as wide as the length of the c-axis are separated by Ba2+ atoms (Figure 5a). Both ribbons contain corner shared [SnSb3] trigonal pyramidal structures as building blocks. The first type, infinite ribbon-I, runs along the a-axis ([Sn6Sb8]), as shown in Figure 5b, is six [SnSb3]-pyramids wide along the b-axis. It has a disordered Sb−Sb straight chain 14253
DOI: 10.1021/acs.inorgchem.7b02352 Inorg. Chem. 2017, 56, 14251−14259
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Inorganic Chemistry
Table 3. Selected Interatomic Distance (Å) and Angles (deg) in Ba6.6Sn11Sb14.4 at 293(2) K with Estimated Standard Deviations in Parentheses Sn(1)−Sb(2) Sn(1)−Sb(3) Sn(2)−Sb(3) Sn(2)−Sb(4) Sn(3)−Sb(4) Sn(3)−Sb(5) Sn(4)−Sb(5) Sn(4)−Sb(6) Sn(5)−Sb(6) Sn(5)−Sb(7) Sn(6)−Sb(8) Sn(6)−Sb(9) Sn(7)−Sb(10) Sn(7)−Sb(11) Sn(8)−Sb(11) Sn(8)−Sb(12) Sn(9)−Sb(12) Sn(9)−Sb(13) Sn(10)−Sb(13) Sn(10)−Sb(14) Sn(11)−Sb(14) Sn(11)−Sb(15) Sb(1)−Sb(2) Ba(1)−Sn(6) Ba(1)−Sn(7) (×2) Ba(1)−Sb(2) (×2) Ba(1)−Sb(7) Ba(1)−Sb(8) (×2) Ba(1)−Sb(11) Ba(2)−Sn(5) Ba(2)−Sb(1) (×2) Sb(3)−Sn(1)−Sb(2) Sb(4)−Sn(2)−Sb(3) Sb(5)−Sn(3)−Sb(4) Sb(5)−Sn(4)−Sb(6) Sb(6)−Sn(5)−Sb(7) Sb(9)−Sn(6)−Sb(8) Sb(10)−Sn(7)−Sb(11) Sb(11)−Sn(8)−Sb(12) Sb(13)−Sn(9)−Sb(12) Sb(13)−Sn(10)−Sb(14)
2.990(3) 2.924(7) 2.948(7) 2.934(8) 2.994(1) 2.950(1) 2.827(2) 3.069(9) 2.899(5) 3.004(1) 3.000(1) 2.862(5) 2.913(7) 3.031(9) 2.868(5) 2.938(5) 2.998(9) 2.935(1) 2.918(9) 2.927(3) 2.968(1) 2.936(9) 2.927(3) 3.493(5) 3.576(1) 3.555(3) 3.638(7) 3.533(2) 3.570(4) 3.637(3) 3.580(6) 99.62(3) 94.60(3) 100.50(3) 98.51(3) 110.49(3) 111.10(4) 144.60(4) 96.40(3) 95.48(3) 95.82(3)
Ba(2)−Sb(2) Ba(2)−Sb(5) Ba(2)−Sb(6) (×2) Ba(2)−Sb(7) (×2) Ba(3)−Sn(6) (×2) Ba(3)−Sn(8) Ba(3)−Sn(9) (×2) Ba(3)−Sb(9) Ba(3)−Sb(10) Ba(3)−Sb(15) (×2) Ba(4)−Sn(4) Ba(4)−Sn(6) Ba(4)−Sn(11) (×2) Ba(4)−Sb(6) (×2) Ba(4)−Sb(7) Ba(4)−Sb(8) (×2) Ba(5)−Sn(7) (×2) Ba(5)−Sn(8) Ba(5)−Sn(10) (×2) Ba(5)−Sn(11) Ba(5)−Sb(9) Ba(5)−Sb(10) Ba(5)−Sb(14) Ba(5)−Sb(15) (×2) Ba(6)−Sb(1) (×2) Ba(6)−Sb(2) Ba(6)−Sb(3) (×2) Ba(6)−Sb(4) (×2) Ba(6)−Sb(5) Ba(7)−Sb(3) (×2) Ba(7)−Sb(4) (×2) Sb(15)−Sn(11)−Sb(14) Sb(1)−Sb(2)−Sn(1) Sn(1)−Sb(3)−Sn(2) Sn(2)−Sb(4)−Sn(3) Sn(4)−Sb(5)−Sn(3) Sn(5)−Sb(6)−Sn(4) Sn(8) -Sb(11)−Sn(7) Sn(8)−Sb(12)−Sn(9) Sn(10) -Sb(13)−Sn(9) Sn(10)−Sb(14)−Sn(11)
3.816(4) 3.687(2) 3.568(4) 3.626(4) 3.488(3) 3.475(9) 3.571(7) 3.767(5) 3.779(5) 3.584(2) 3.439(5) 3.582(3) 3.558(2) 3.667(5) 3.475(9) 3.527(8) 3.534(5) 3.564(6) 3.526(6) 3.524(5) 3.577(4) 3.606(5) 3.742(2) 3.513(8) 3.504(9) 3.546(8) 3.557(6) 3.520(2) 3.411(7) 3.136(5) 3.270(1) 109.47(3) 109.82(3) 94.71(3) 99.20(3) 93.92(3) 131.14(2) 104.06(3) 101.15(3) 95.03(3) 101.36(3)
Figure 1. (a) SEM image of a Ba3Sn3Sb4 single crystal. The EDS analysis gave a ratio of 3:3:4 for Ba:Sn:Sb. (b) SEM image of a needle-like Ba6.6Sn11Sb14.4 single crystal (along b-axis). The EDS analysis gave a ratio of 4:7:9 for Ba:Sn:Sb.
Selected interatomic distances and angles of Ba6.6Sn11Sb14.4 are listed in Table 3. The Sb(1)−Sb(2) bond is 2.990(3) Å. The Sn−Sb distances in Ba6.6Sn11Sb14.4 range from 2.827(2) to
partial occupancies for the disordered Ba7, Sb9, and Sb10 atoms are refined to be 0.63, 0.35, and 0.35, respectively (Table S2). This makes the compound highly nonstoichiometric. 14254
DOI: 10.1021/acs.inorgchem.7b02352 Inorg. Chem. 2017, 56, 14251−14259
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Inorganic Chemistry
Next, we performed first-principles calculations to gain insight into the electronic structure of Ba3Sn3Sb4. Because of the existence of partial site occupations and nonstoichiometric composition in Ba6.6Sn11Sb14.4, the calculations in this compound were not performed because they required prohibitively large cells. The calculated band structure from density functional theory is shown in Figure 7a. We find Ba3Sn3Sb4 to be a narrow gap semiconductor with an indirect band gap of nearly 0.15 eV, consistent with the absorption result. There are two conduction band minima, one lying along the Γ−Z direction and one at the D point of the Brillouin zone, while the valence band maximum is located along the Γ−Y direction. The presence of two conduction band minima (referred as band convergence) suggests Ba3Sn3Sb4 could be an interesting high performance n-type thermoelectric material and points to a future direction in the investigations of this material.41 The momentum-resolved atomic contributions to the band structure are shown in Figure S5. We also checked the influence of spin−orbit coupling on the band structure (Figure S6). We found that apart from lifting of certain band degeneracies the band structure remains rather similar to the case without inclusion of spin−orbit coupling. In particular, the band gap remains nearly identical with or without considering spin−orbit interaction. An inspection of the projected density of states (Figure 7b) reveals that the valence band manifold near the Fermi level primarily arises from Sb p states, while the conduction band manifold is a mixture of Sn and Sb states. Charge Transport Properties. The electrical resistivity (ρ) from 300 to 50 K of a single-crystal sample of Ba3Sn3Sb4 is shown in Figure 8. ρ is 0.23 Ω cm at 300 K and increases dramatically with decreasing temperature reaching 39 180 Ω cm at 50 K, which suggests a typical semiconducting behavior. This is consistent with the infrared absorption edge observed from diffuse-reflectance spectra. The activation energy was determined by fitting the temperature-dependent resistivity using the standard semiconductor equation:
Figure 2. Projected structure of Ba3Sn3Sb4 viewed along the b-axis. The blue, red, and green balls are Ba, Sb, and Sn atoms, respectively.
Figure 3. Anionic [Sn3Sb4]6− framework of Ba3Sn3Sb4 with labeled Sn and Sb atoms. It shows periodic 1D chains composed of trigonal pyramidal [SnSb3] units along the b-axis which are bridged by Sn2 atoms, forming the corrugated 2D Sn−Sb framework along the abplane.
⎛ E ⎞ ρ = ρ0 exp⎜ a ⎟ ⎝ kBT ⎠
Here, ρ0, Ea, kB, and T denote a pre-exponential factor, the activation energy, the Boltzmann constant, and temperature, respectively.42 The power-law relation represents the impurity scattering process which becomes influential for heavily doped semiconductors. As shown in the inset in Figure 8, the fitted Ea is determined to be 0.1 eV in the temperature range 125−300 K. This is about half of the observed band gap (0.09 eV), suggesting that this activation energy is indeed associated with the band gap of the material. The hall resistivity (ρxy) of a Ba3Sn3Sb4 single crystal changes linearly with the applied magnetic field (Figure S7). The negative Hall resistivity at the positive magnetic field demonstrates that it is n-type with electrons as the dominant carriers. The corresponding temperature-dependent Hall coefficients (RH’s), slopes of the fitted lines, are shown in Figure 9a. These increase with decreasing temperature. Consistent with this behavior, the calculated carrier density (n = −1/(RHq)) decreases rapidly with falling temperature. The n = 4.4 × 1014 cm−3 at 75 K of is 4 orders of magnitude smaller than n = 4.2 × 1018 cm−3 at 300 K, Figure 9b. The carrier mobility (μ) of Ba3Sn3Sb4, evaluated by μ = 1/nqρ, is 6.48 cm2
Figure 4. Projected view of Ba6.6Sn11Sb14.4 structure along the b-axis. The disordered Ba and Sb atoms are plotted with yellow and pink colors, respectively. They (Ba7, Sb9, and Sb10) show high thermal parameters along the b-axis (Table S4).
3.069(9) Å. The Sb−Sn−Sb and Sn−Sb−Sn angles fall in the wide ranges, 94.60−144.60° and 93.92−131.14°, respectively. The coordination environments of Ba atoms are shown in Figure S4 with different polyhedral geometries. Ba−Sn and Ba− Sb distances range from 3.439(5) to 3.637(3) Å and 3.136(5) to 3.816(4) Å, respectively. Optical Absorption and First-Principles Calculations. Infrared absorption spectra were measured on polycrystalline samples of both compounds. As shown in Figure 6, an energy band gap was observed as a broad absorption around 0.18 eV for Ba3Sn3Sb4, suggesting the compound is a narrow gap semiconductor. No obvious band gap could be observed for Ba6.6Sn11Sb14.4 powders which is consistent with the metallic nature of this material. 14255
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Figure 5. (a) Detailed view of the anionic Sn−Sb network in Ba6.6Sn11Sb14.4 with labeled Sn and Sb atoms. It is composed of two different infinite ribbons along the b-axis, containing [SnSb3] trigonal pyramids extending along the bc-plane. Partially occupied Sb9 and Sb10 atoms are marked with pink color. (b) Detailed view of ribbon-I as well as the ordered ideal structure along a-axis ([Sn6Sb8]). The weak Sn7−Sb9 and Sn6−Sb10 bonds are plotted with pink dashed lines. (c) Detailed view of ribbon-II along the a-axis ([Sn5Sb7]).
Figure 6. Infrared absorption spectrum measured on Ba3Sn3Sb4 powders exhibits a band gap (Eg) ∼ 0.18 eV.
Figure 8. Electrical resistivity, ρ, as a function of temperature for Ba3Sn3Sb4 from 300 to 50 K. The inset shows the fitted activation energy (Ea) of 0.1 eV in the temperature range 125−300 K.
V−1 s−1 at 300 K. μ increases slightly to 7.84 cm2 V−1 s−1 at 200 K, Figure 9c. The Ba6.6Sn11Sb14.4 compound is 3 orders of magnitude less resistive than Ba3Sn3Sb4 with a room temperature value of 0.90 mΩ cm, and the resistivity decreases almost linearly to 0.24 mΩ cm at 5 K, suggesting a typical metallic behavior, Figure 10. The low temperature data (5−50 K) show T2 behavior due to strong electron−electron scattering.43,44 Temperature-dependent in-plane Hall effect measurements conducted on a Ba6.6Sn11Sb14.4 single crystal are shown in Figure 11a. Similar to Ba3Sn3Sb4, ρxy changes linearly with the applied magnetic field (Figure S8) and exhibits n-type behavior with a negative Hall resistivity under positive magnetic field. As shown in Figure 11b, the calculated carrier concentration based on a single band approximation is n ∼ 2.95 × 1022 cm−3 near room temperature and decreases to 1.15 × 1022 cm2 at 5 K. The corresponding carrier mobility of Ba6.6Sn11Sb14.4 as a function of temperature is shown in Figure 11c. It increases with decreasing
Figure 7. (a) Electronic band structure and (b) density of states (DOS) of Ba3Sn3Sb4. It indicates a narrow indirect band gap.
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Figure 9. (a) In-plane Hall coefficients, −RH; (b) carrier density, n; and (c) mobility, μ, of Ba3Sn3Sb4 at different temperatures.
Figure 11. (a) In-plane Hall coefficients, RH; (b) carrier density, n; and (c) mobility, μ, of Ba6.6Sn11Sb14.4 at different temperatures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02352. Tables of atomic coordinates, displacement parameters, and anisotropic displacement parameters of Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4; powder X-ray diffraction patterns for Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4; coordination environments of Ba atoms; band structure with projected contributions from Sn and Sb atoms for Ba3Sn3Sb4; comparison of the band structures for Ba3Sn3Sb4 with and without spin−orbit coupling; and in-plane Hall resistivity (ρxy) at different temperatures (PDF)
Figure 10. Electrical resistivity, ρ, as a function of temperature for a single crystal of Ba6.6Sn11Sb14.4. It decreases with temperature, indicating metallic behavior. The resistivity at low temperature (5− 50 K) is well-fitted with the formula, 0.241 + 3.8 × 10−5T2 (inset).
temperature. μ is calculated to be 2.35 cm2 V−1 s−1 at 300 K and increases to a larger value of 22.52 cm2 V−1 s−1 at 5 K. These relatively low mobilities are consistent with the very large carrier concentration of the sample.
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Accession Codes
CCDC 1574148−1574149 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
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The discovery of two Zintl phases Ba 3 Sn 3 Sb 4 and Ba6.6Sn11Sb14.4 in the already crowded Ba/Sn/Sb system reveals its rich reaction chemistry and complex phase diagram. Therefore, we can anticipate more Zintl phases by such exploratory synthesis investigations in the related A/Sn/Pn (A = alkali or alkali earth metal; Pn = Sb, As) systems. Both Ba3Sn3Sb4 and Ba6.6Sn11Sb14.4 contain the same Sb-sharing trigonal pyramids of [SnSb3] but very different Sn/Sb structures. Stoichiometric Ba3Sn3Sb4 is a narrow band gap ntype semiconductor (∼0.18 eV) with converged conduction bands and could be a good thermoelectric material while the nonstoichiometric Ba6.6Sn11Sb14.4 is a normal metal with electrons as the dominant carriers.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Haijie Chen: 0000-0003-3567-1763 Constantinos C. Stoumpos: 0000-0001-8396-9578 Jing Zhao: 0000-0002-8000-5973 Mercouri G. Kanatzidis: 0000-0003-2037-4168 Present Address ∥
Materials Theory, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH 8093 Zurich, Switzerland. Notes
The authors declare no competing financial interest. 14257
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ACKNOWLEDGMENTS This work was supported primarily by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DEAC0298CH1088. Research at Argonne was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Computational resources were provided by the University of Illinois Campus Cluster. This work made use of the EPIC, Keck-II, and/or SPID facility(ies) of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR-1121262) at the Materials Research Center, the International Institute for Nanotechnology (IIN), and the Keck Foundation; and the State of Illinois, through the IIN.
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