Cs2Hg3S4: A Low-Dimensional Direct Bandgap Semiconductor

Dec 17, 2014 - Yihui He , Oleg Y. Kontsevoi , Constantinos C. Stoumpos , Giancarlo G. Trimarchi , Saiful M. Islam , Zhifu Liu , Svetlana S. Kostina , ...
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Cs2Hg3S4: A Low-Dimensional Direct Bandgap Semiconductor Saiful M. Islam,† S. Vanishri,† Hao Li,# Constantinos C. Stoumpos,† John. A. Peters,‡ Maria Sebastian,‡ Zhifu Liu,‡ Shichao Wang,† Alyssa S. Haynes,† Jino Im,§ Arthur J. Freeman,§ Bruce Wessels,‡ and Mercouri G. Kanatzidis*,†,# †

Department of Chemistry, ‡Department of Materials Science, Engineering, §Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, United States # Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Cs2Hg3S4 was synthesized by slowly cooling a melted stoichiometric mixture of Hg and Cs2S4. Cs2Hg3S4 crystallizes in the Ibam spacegroup with a = 6.278(1) Å, b = 11.601(2) Å, and c = 14.431(3)Å; dcalc = 6.29 g/cm3. Its crystal structure consists of straight chains of [Hg3S4]n2n− that engage in side-by-side weak bonding interactions forming layers and are charge balanced by Cs+ cations. The thermal stability of this compound was investigated with differential thermal analysis and temperature dependent in situ synchrotron powder diffraction. The thermal expansion coefficients of the a, b, and c axes were assessed at 1.56 × 10−5, 2.79 × 10−5, and 3.04 × 10−5 K−1, respectively. Large single-crystals up to ∼5 cm in length and ∼1 cm in diameter were grown using a vertical Bridgman method. Electrical conductivity and photoconductivity measurements on naturally cleaved crystals of Cs2Hg3S4 gave resistivity ρ of ≥108 Ω·cm and carrier mobility-lifetime (μτ) products of 4.2 × 10−4 and 5.82 × 10−5 cm2 V−1 for electrons and holes, respectively. Cs2Hg3S4 is a semiconductor with a bandgap Eg ∼ 2.8 eV and exhibits photoluminescence (PL) at low temperature. Electronic band structure calculations within the density functional theory (DFT) framework employing the nonlocal hybrid functional within Heyd−Scuseria−Ernzerhof (HSE) formalism indicate a direct bandgap of 2.81 eV at Γ. The theoretical calculations show that the conduction band minimum has a highly dispersive and relatively isotropic mercury-based s-orbital-like character while the valence band maximum features a much less dispersive and more anisotropic sulfur orbital-based band.

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INTRODUCTION Metal chalcogenide is an important class of inorganic materials that exhibits numerous attractive properties such as catalysis,1 electronics and optoelectronics,2 solar energy conversion,3 superconductivity,4 nonlinear optics,5 thermoelectric energy conversion,6 phase change data storage,7 and hard radiation detection.8 We have been interested in new X-ray and γ-ray detector materials operating at room temperature because of the need to develop advanced but low cost capabilities for nuclear materials detection, nuclear medical imaging, environmental radioactivity monitoring, and related applications.9 To date, only a few semiconductors have been proposed and utilized as detector materials such as Si,8e high purity Ge,10 CdTe,11 TlBr,8e CdxZn1−xTe (CZT),11 and HgI2.11 The top room temperature γ-ray detector material is CZT, but it has limited use because of its high cost deriving from the difficulty in obtaining detector grade single-crystal in high yield.12 We are interested in new materials with bandgap range 1.5 ≤ Eg ≤ 3 eV, resistivity ρ ≥ 108 Ω·cm, high density d ≥ 6 g·cm−3, and high atomic numbers (Z ≥ 40). All these properties should coexist to obtain high mobility lifetime products μτ, called the figure of merit of the X- and γ-ray detector. To meet this © XXXX American Chemical Society

challenge, we have been engaged in the synthesis and crystal growth of suitably selected ternary chalcogenides.8a−d,13 In this contribution, we describe the synthesis, structure, crystal growth, and thermal behavior of the new compound Cs2Hg3S4. This compound is a direct bandgap semiconductor with Eg ∼ 2.8 eV and exhibits photoluminescence (PL) at low temperature. Temperature dependent in situ synchrotron powder diffraction of Cs2Hg3S4 shows unexpected thermal behavior. We also show that Cs2Hg3S4 is a direct bandgap semiconductor and exhibits promising mobility lifetime products μτ for electrons and holes, which highlights the material’s potential for hard radiation detection.

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EXPERIMENTAL SECTION

Synthesis of Cs2Hg3S4. Cs2Hg3S4 was synthesized using elemental Hg (1812.0 mg, 9 mmol) of high purity (6N, Aldrich) and Cs2S4 (1188.0 mg, 3 mmol) in a stoichiometric ratio. Cs2S4 is prepared by combining stoichiometric amounts of high purity Cs (6.7452 g, 0.056 Received: November 5, 2014 Revised: December 14, 2014

A

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performed using the known thermal expansion of alumina (Al2O3) a = 4.759 09(1) Å and c = 12.991 78(3) Å unit cell parameters as a function of temperature. The diffractometer is controlled via EPICS.18 Data are collected while continually scanning the diffractometer 2θ arm. A mixture of NIST standard reference materials, Si (SRM 640c) and Al2O3 (SRM 676) is used to calibrate the instrument, where the Si lattice constant determines the wavelength for each detector. Corrections are applied for detector sensitivity, 2θ offset, small differences in wavelength between detectors, and the source intensity, as noted by the ion chamber before merging the data into a single set of intensities evenly spaced in 2θ. Scanning Electron Microscopy. Images and semiquantitative energy dispersive X-ray spectroscopy (EDS) analyses were obtained using a Hitachi S-3400 scanning electron microscope equipped with a PGT energy-dispersive X-ray analyzer. Spectra were collected using an accelerating voltage of 15 kV and a 90 s accumulation time. Single-Crystal X-ray Crystallography. Data collections were performed on a STOE IPDS II diffractometer using Mo Kα radiation (λ = 0.710 73 Å) operating at 50 kV and 40 mA at 100 K under nitrogen atmosphere. Integration and numerical absorption corrections were performed using X-AREA, X-RED, and X-SHAPE. All structures were solved using direct methods and refined by full-matrix least-squares on F2 using the SHELXTL program package.19 A complete list of crystallographic information, data collections, structure refinement, atomic coordinates, isotropic and anisotropic displacement parameters and selected inter atomic distances and angles are given in Tables 1, 2, 3, and 4. Differential Thermal Analysis. Thermal analysis was performed using a Shimadzu DTA-50 thermal analyzer. Ground crystalline samples (∼40 mg) were sealed under vacuum (∼10−4 mbar) in a fused silica ampule. During the sealing, Cg2Hg3S4 containing end of the silica tubes was dipped in liquid nitrogen in order to prevent its melting. A similar amount of Al2O3 was sealed in a separate ampule under vacuum and used as a reference. The samples were heated and cooled at a rate of 5 °C/min to a maximum temperature of 700 °C. Two cycles of heating and cooling were performed to check the reproducibility of the obtained values. UV−Vis Spectroscopy. Diffuse reflectance spectra of the selected crystals of Cs2Hg3S4 were collected in the range of 200−2500 nm using a Shimadzu UV-3101 PC double-beam, double-monochromator spectrophotometer. The procedures used to extract the bandgap have been described elsewhere.20 Transmission UV/vis spectra of single crystal samples of Cs2Hg3S4 were recorded with a PerkinElmer LAMBDA 1050 Spectrophotometer. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron studies were performed using a Thermo Scientific ESCALAB 250 Xi spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) and operated at 300 W. Samples were analyzed under vacuum (P < 10−9 mbar), whereas survey scans and high-resolution scans were collected using pass energy of 25 eV. Binding energies were referred to the C 1s binding energy at 284.6 eV. A low-energy electron flood gun was employed for charge neutralization. Prior to XPS measurements, a cleaved crystal was attached on cupper foil and mounted on stubs and successively put into the entry-load chamber to pump. Resistivity and Photoconductivity Measurements. The room temperature current−voltage characteristics were measured in 2-probe configurations using a Keithley 617 electrometer. Carbon paint was used as the contact material. Photoconductivity was measured in a homemade setup as described elsewhere.8b,21 Gold electrodes of ∼100 nm were evaporated on the front and back surfaces of the samples. A semiconductor diode laser (405 nm) chopped at 575 Hz was focused on the surface of the samples. Positive and negative bias voltages of up to 200 V were applied across the samples. The time-dependent photocurrent of the sample was measured as a function of the bias voltages using a lock-in amplifier. The resulting signal was subsequently recorded by a computer. Photoluminescence Measurements. The sample was cooled to 10 K using a closed-cycle He cryostat (APD DE-202) and excited with

mol; 99.9%, Aldrich) and S (3.5693 g, 0.11 mol; 9.98% 5N Plus Inc.) in liquid ammonia. Because Cs2S4 is air sensitive, all the reactants were loaded into carbon coated fused silica tubes under nitrogen ambience in a glovebox. Because Hg is volatile, the lower part of the silica tubes were dipped in liquid nitrogen and flame-sealed under 10−4 mbar vacuum. The sealed tubes were then transferred to a furnace and heated to 670 °C in 24 h, held at that temperature for about 12 h and then cooled to 200 °C in 24 h, and finally fast cooled down to room temperature in about 3 h. The product thus obtained is yellow crystals with platy habit and having cleavage planes. These crystals were found to be air sensitive and hence they need to be stored in an inert atmosphere. Warning: Hg is highly toxic, and great care should be exerted with appropriate protective equipment in both synthesis and handling. Crystal growth of Cs2Hg3S4. To grow large single crystals, the Bridgman growth method was employed using a two zone vertically aligned furnace. A typical amount of 8−10 g of Cs2Hg3S4 was used for this purpose. The synthesized compound is loaded into the tapering end of a fused silica ampule and sealed under 10−4 mbar vacuum. The ampule was then placed in a two zone Bridgman furnace with top zone at 685 °C and lower zone at 550 °C. The material is heated to 685 °C in 12 h and then held at that temperature for about 6 h for homogenization. The ampule was then slowly lowered at a speed of 1.8 mm/h through the temperature gradient (∼4 °C/cm), and once the growth was over, the furnace was slowly cooled to room temperature. The purity of the crystals was confirmed by powder X-ray diffraction, scanning electron microscopy, energy dispersive analysis, and optical spectroscopy. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data were collected on ground crystalline samples of Cs2Hg3S4 with a flat sample geometry using a silicon-calibrated CPS 120 INEL powder Xray diffractometer (Cu Kα graphite-monochromatized radiation) operating at 40 kV and 20 mA equipped with a position-sensitive detector. Simulated patterns were generated using the CIF of each refined structure and the Visualizer program within FindIt (see the Supporting Information). For the variable temperature synchrotron PXRD sample preparation, finely powdered Cs2Hg3S4 was thoroughly mixed with amorphous SiO2 in a ∼1:4 mass ratio in order to reduce the X-ray absorption of Cs2Hg3S4. The homogeneous mixture was placed in a 0.3 mm ID quartz capillary and flame-sealed under a 10−4 mbar vacuum. Variable temperature PXRD patterns were collected using beamline 17-BM-B at the APS, running at 50 keV (λ = 0.727 Å) with a PerkinElmer a-Si C-window. The capillaries were placed in a flow cell apparatus and heated using an electrical resistance heater. During data collection, the sample was translated normal to the incident X-ray beam in order to average the diffraction of the whole capillary content. The detector distance was 600 mm and 65 s exposures were averaged together for each PXRD pattern with a dark before each frame. A LaB6 standard was used to refine the sample-to-detector distance and imaging plate tilt relative to the beam. Fit-2D was used to process the raw images.14 High-resolution, high-temperature synchrotron powder diffraction data were collected using beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory using an average wavelength of 0.413 87 Å. Discrete detectors covering an angular range from 0 to 4° 2θ are scanned over a 26° 2θ range, with data points collected every 0.0015° 2θ and scan speed of 0.1°/s. The 11-BM instrument uses X-ray optics with two platinum-striped mirrors and a double-crystal Si(111) monochromator, where the second crystal has an adjustable sagittal bend15 ion chambers monitor incident flux. A vertical Huber 480 goniometer, equipped with a Heidenhain encoder, positions an analyzer system composed of 12 perfect Si(111) analyzers and 12 Oxford-Danfysik LaCl3 scintillators, with a spacing of 2° along 2θ.16 Analyzer orientation can be adjusted individually on two axes. A three-axis translation stage holds the sample mounting and allows it to be spun, typically at ∼600 rpm (10 Hz). The sample temperature was adjusted by a homemade resistive heating furnace equipped with a SRS PTC10 programmable temperature controller.17 In situ calibration of the heater was B

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within the density functional theory formalism using the Projector Augmented Wave method22 implemented in Vienna Ab-initio Simulation Package.23 The energy cut off for plane wave basis was set to 350 eV and 5 × 5 × 5 k-point mesh was chosen for Brillouin zone sampling. To obtain more accurate bandgap which is usually underestimated with semilocal exchange-correlation functional like local density approximation (LDA) and generalized gradient approximation (GGA),24 we employed nonlocal hybrid functional within Heyd-Scuseria-Ernzerhof (HSE) formalism,25 which is known to predict more accurate bandgap in semiconductors and insulators.26 The experimentally observed crystal structure was used for band structure calculations.

Table 1. Details Concerning Data Collection and Structure Refinement of Cs2Hg3S4. crystal data

Cs2Hg3S4

crystal system space group a (Å) b (Å) c (Å) V (Å3) Z color MW (g·mol−1) μ (mm−1) ρX‑ray (g·cm−3) color crystal shape size/mm3 F(000) data collection Mo Kα radiation monochromator temperature (K) scan range θ (deg)

structure refinement software measured reflections independent reflections absorption correction parameters; GOf residuals Rint R1a R1alla wR2b extinction coefficient weighting scheme res. electron density Δρmax (Å−3) Δρmin (Å−3)

orthorhombic Ibam (72) 6.278(1) 11.601(2) 14.431(3) 1051.0(4) 4 yellow 995.83 51.26 6.29 yellow rectangular 0.09 × 0.08 × 0.08 1656

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RESULTS AND DISCUSSION Synthesis and Thermal Behavior. Cs 2 Hg 3 S 4 was synthesized as yellow plate-like crystals by slow cooling of a mixture of Hg and Cs2S4 in a stoichiometric ratio of 3:1 from 670 °C (Figure 1). The crystals exhibit high cleavage tendency. DTA shows that Cs2Hg3S4 exhibits a sharp endothermic peak upon heating (melting point) at ∼659 °C and a sharp exothermic peak upon cooling (crystallization point) at ∼613 °C, Figure 2. The sharp nature of the crystallization peak is indicative of the ease of crystallization of this material. No other thermal events were observed. X-ray powder diffraction after the DTA experiment showed no additional phases suggesting congruent melting behavior (Figure 3). Temperature dependent in situ synchrotron powder diffraction experiments are powerful in probing any phase transformations in ways other techniques cannot.27 These experiments provided further evidence of the phase purity of Cs2Hg3S4 and revealed additional detailed information about the thermal behavior (Figure 4). Based on the synchrotron study, Cs2Hg3S4 is stable up to ∼320 °C, at which point the known phase Cs2Hg6S78c surprisingly starts to crystallize (relevant 2θ at 4.16, 11.85, 11.98, 12.26°). The Cs2Hg6S7 is stable up to 470 °C before it disappears. Cs2Hg6S7 is the decomposition product of Cs2Hg3S4 in according to eq 1:

λ = 0.71073 Å graphite 100 K, in nitrogen atmosphere 2.82 ≤ θ ≤ 27.49 0≤h≤8 −15 ≤ k ≤ 15 −18 ≤ l ≤ 18 SHELX9719 2250 632 multiscan43 25; 1.06 0.079 0.040 0.051 0.089 0.00093(8) A = 0.0395B = 12.3925

2Cs2Hg 3S4 ⇆ Cs 2Hg6S7 + Cs 2S

Based on our experimental data, however, we did not observe the presence of Cs2S at any stage of the in situ experiment. We attribute the loss Cs2S to its facile volatility at higher temperature and/or its reactivity with SiO2, which is present in the experiment (see the Experimental Section), to form amorphous cesium silicate. As a result, Cs2S is removed from the above equation and therefore the equilibrium shifts toward Cs2Hg6S7. Cs2Hg3S4, on the other hand, melts at ∼580° (Figure 4). Upon cooling, Cs2Hg3S4 and Cs2Hg6S7 crystallize simultaneously at 470 and 410 °C, respectively. The small difference in melting and crystallization temperature between DTA and synchrotron experiments can be attributed to the different heating and cooling rates for the two experiments. Moreover, the absence of the melting and crystallization events of Cs2Hg6S7 in DTA can be rationalized by the different nature of the experiments (i.e., the absence of SiO2, different crystallization mechanism). Cs2Hg6S7 is not crystallized from its melt in a single abrupt step but rather nucleates and grows from the flux of Cs2Hg3S4. Another unusual feature is the temperature behavior of the Bragg peaks, 2θ at 13.9° (132) and ∼17.2° (134), (Figure 4). At room temperature these reflections are broad, indicating poor crystallinity or disorder within the structure. These peaks remain broad up to 470 °C above, at which they appear to

2.59 (close to Hg2) 2.09 (close to Hg2)

R1 = Σ||F0| − |Fc||/|F0|, F02 ≥ 2σ(F02). bwR2 = 1/[σ2(F02) + (A·P)2 + B·P]; P = (F02 + 2Fc2)/3.

a

Table 2. Atomic Coordinates and Isotropic Displacement Parameters for Cs2Hg3S4 atoms

Wyck.

x

y

z

Hg1 Hg2 Cs1 S1

4b 8g 8j 16k

1/2 0 0.75818(14) 0.2795(4)

0 0.15982(5) 0.12119(8) 0.1316(2)

1/4 1/4 0 0.1430(2)

a

(1)

Ueq

0.0249(3) 0.0259(3) 0.0239(3) 0.02077(5)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensors.

a 405 nm diode laser (50 mW) chopped at 577 Hz. The signal was detected using a photomultiplier tube (Hamamatsu R928) with an applied bias of 500 V coupled to a phase-sensitive lock-in amplifier. The PL signal was analyzed using a 3/4 m SPEX 1702 monochromator. Band Structure Calculations. To investigate the electronic structure of Cs2Hg3S4, first-principles calculations were carried out C

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Chemistry of Materials Table 3. Anisotropic Displacement Parameters for Cs2Hg3S4 atom

U11

U22

U33

U12

U13

U23

Hg1 Hg2 Cs S1

0.0233(5) 0.0192(4) 0.0312(5) 0.0194(10)

0.0239(4) 0.0345(4) 0.0212(5) 0.0222(11)

0.0246(4) 0.0211(3) 0.0255(4) 0.0208(9)

0.00000 0.00000 −0.0017(3) 0.0018(10)

0.00000 0.0019(2) 0.00000 0.0032(10)

0.00000 0.00000 0.00000 0.0015(9)

Table 4. Selected Interatomic Distances (Å) and Angles (deg) in Cs2Hg3S4a d(Hg1−S1) × 4 d(Hg2−S1) × 2 d(Cs1−S1) × 2 d(Cs1−S1) × 2 d(Cs1−S1) × 2 d(Cs1−S1) × 2 d(Hg1−Cs1) × 4 a

2.575(2) 2.360(2) 3.536(3) 3.594(3) 3.647(2) 3.871(3) 4.1976(8)

d(Hg2−Cs1) × 4 d(Cs1−Cs1) ∠(S1−Hg1−S1) ∠(S1−Hg1−S1) ∠(S1−Hg1−S1) ∠(S1−Hg2−S1) ∠(Hg2−S1−Hg1)

3.9397(8) 4.138(2) 114.96(10) 106.32(10) 107.27(11)) 164.05(12)) 95.15(8)

Estimated standard deviation in parentheses.

Figure 3. Comparison of the powder diffraction patterns of Cs2Hg3S4 before (red) and after (blue) the DTA experiment. The amorphous background in the “after DTA” sample is from the sample holder.

Figure 1. Photographs of the crystals of Cs2Hg3S4. (A) Ingot obtained after Bridgman crystal growth, (B) crystal with naturally cleaved faces in the vertical direction, (C) crystals obtained by slowly cooling a mixture of Cs2S4 and Hg, and (D) crystal turns black after exposure in air for a few days. Figure 4. Temperature dependent in situ synchrotron powder diffraction (λ = 0.717 Å) of selected crystals of Cs2Hg3S4 (heating rate 10 °C/min). Green dashed lines are the simulation of powder diffraction, parameters obtained from X-ray single crystal refinement. At higher temperatures, peak positions shift to lower angle, which is in agreement to expansion of the cell with increasing temperature. Red rectangles highlight the region of the formation of Cs2Hg6S7 upon heating.

expansion of 7.61 × 10−5 K−1 within the 300−580 °C temperature region (Figure 5). These thermal expansions are consistent to those of Cs2Hg6S7 and HgS.8c,28 Crystal Structure. Cs2Hg3S4 crystallizes in an orthorhombic crystal system with the Ibam space group (Table 1). X-ray diffraction on polycrystalline compound (Figure 6) showed an excellent match with the theoretical pattern simulated from the single-crystal structure refinement. This indicates successful formation of a single phase of Cs2Hg3S4 and also attests to the accuracy of the proposed structural model. It is worth mentioning that A2M3Q4 stoichiometry adopts four different space groups, Pnma (e.g., A2Cd3Q4, (A = K, Rb; Q = S, Se, Te),29 Ibam (i.e., Cs2Mn3Q4; Q = S, Se),30 C2/c (i.e., Rb 2 Cd 3 Te 4 ),31 and Pbcn (i.e., K 2 Hg 3 Q 4 , Q = S, Se; Cs2Hg3Se4).32 The crystal structure of Cs2Hg3S4 (Figure 7) contains parallel infinite chains of [Hg3S4]n2n− similar to those found in the

Figure 2. Differential thermal analysis showing meting and crystallization temperature of Cs2Hg3S4. This compound melts at 659 and crystallizes at 613 °C (heating rate 5 °C/h), respectively.

sharpen suggesting improving crystallinity. Interestingly, the sharpening of the (13l) reflections and the crystallization of Cs2Hg3S4 coincides with the disappearance of the Cs2Hg6S7 phase. This behavior is reproducible for different data sets and different Cs2Hg3S4 batches. It is worth mentioning that at higher temperatures, the positions of the Bragg reflections shift to lower angles, in agreement with the expected thermal expansion with increasing temperature. The thermal expansion coefficients of the a, b, and c axes were assessed at 1.56 × 10−5, 2.79 × 10−5, and 3.04 × 10−5 K−1, respectively, giving a unit cell D

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the crystal growth parameters. The loaded ampules were heated in a two-zone Bridgman furnace with the top zone at 685 °C and the lower zone at 550 °C. The material was melted at 685 °C and the melt was soaked for homogenization for 6 h and subsequently slowly lowered at a speed 1.8 mm/h through the temperature gradient to produce a yellow crystal ingot of ∼5 cm in length. The homogeneity of the grown crystal was confirmed by PXRD from different parts of the ingot. A naturally cleaved crystal of ∼3 × 2 × 1 mm was used for resistivity and photoconductivity measurements. The surface of the crystals slowly turns black as the material gradually decomposes upon exposure to air over a few days. A comparison of the X-ray powder diffraction pattern of the pristine ingot sample and air exposed Cs2Hg3S4 samples are presented in Figure 8. The PXRD of Cs2Hg3S4 upon a two day exposure to ambient air provide no evidence of the formation of other species, however, the PXRD after 16 days becomes complicated from the presence of several new weak diffraction peaks (Figure 8). Assignment of the diffraction pattern confirms the presence α-HgS, however, additional phases are observed as decomposition products, such as Cs2S, CsOH, Cs2SO4, and HgSO4. SEM images (Figure 9) of a naturally cleaved surface of fresh and exposed sample of Cs2Hg3S4 in air (24 h) are very much comparable and EDS showed essentially identical compositions with no evidence of other species. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy was performed on a freshly cleaved single crystal. XPS shows that the surface composition is of cesium, mercury, and sulfur, as expected (Figure 10). Peaks at 738.11 and 724.16 eV are the characteristics for 4f7/2 and 4f5/2 cesium cations35 (Figure 10a). Peaks at about 99.88 and 103.88 eV essentially provides confirmation of 4f5/2 and 4f7/2 of Hg2+ ions33,36 (Figure 10b). The X-ray photoelectron spectra of sulfur essentially show two bands at ∼162 and 168.48 eV (Figure 10c). These bands are in agreement with the energies of sulfur 2p orbitals.37 More precisely, the lower energy band corresponds to the S2− ion of the S2p energy. The band centered at 168.48 eV is the signature for sulfur 2p from SO42−.37a,b It is worth mentioning that the freshly cleaved crystal was unavoidably exposed to air for about 15 min during its transportation from the glovebox to the X-ray photoelectron spectrometer. Hence the presence of SO42− is a clear evidence of the high reactivity of the surface of Cs2Hg3S4. Optical Properties and Electronic Band Structure Calculations. Optical diffuse reflectance spectroscopy performed on ground powder of Cs2Hg3S4 indicate a clear bandgap of ∼2.80 eV (Figure 11). UV−visible transmission spectra obtained from cleaved single crystals (dimension ∼ 1 × 1 × 0.5 mm) show a similar bandgap of ∼2.73 eV (Figure 11). These values are consistent with each other and in agreement with the yellow color of the material. The sharp rise of the absorption edge is suggestive of direct electronic transitions and this is supported by the electronic band structure calculations discussed below. The calculated electronic band structure of Cs2Hg3S4 is shown in Figure 12a and predicts a direct bandgap of 2.81 eV at the Γ point of the Brillouin zone. This calculated value agrees well with the experimentally observed bandgap. Furthermore, the electronic band structure shows that the conduction band minimum (CBM) is defined by a highly dispersive and isotropic s-like band whereas the corresponding valence band maximum (VBM) is defined by a much less dispersive and anisotropic band.

Figure 5. Thermal expansion of Cs2Hg3S4 with temperature (a) expansion of a-axis, (b) expansion b-axis, (c) expansion c-axis and (d) expansion of the cell volume. Cell parameters were determined by Rietveld refinement.

Figure 6. Experimental (black) and simulated (red) powder diffraction pattern of Cs2Hg3S4. For the simulation parameters were obtained from the X-ray single crystal refinement. The slight mismatch in intensities between the experimental and simulated powder diffraction is due to the preferred orientation of the crystallites.

A2M3Q4 stoichiometric chalcogenides structure type. The chains are assembled by distorted tetrahedral [HgS4]6− building blocks that are alternatively linked by two-coordinated Hg2+ ions. An alternative view to describe this structure is as a onedimensional spiro-polymer of eight-membered [Hg4S4] rings where, two- and four-coordinated Hg2+ ions alternatively linked via S2− ions. These negatively charged chains interact, albeit weakly, with one another as they arrange side by side to form slabs giving the material a two-dimensional character along the ab plane (Figure 7D). They are counter balanced by Cs+ cations that participate in ionic interactions with the S2− anions. The asymmetric unit of Cs2Hg3S4 possesses two crystallographically distinct Hg atoms, Hg1 and Hg2. The Hg1 atom attains a tetrahedral geometry with distances d(Hg1−S) = 2.575(2) Å (Table 4) and (S−Hg1−S) angles ranging from 106.32(2) to 114.96(7)° whereas the Hg2 atom has linear-like coordination geometry with d(Hg2−S1) = 2.360(2) Å and angles of (S1−Hg2−S1) 164.05(81)°. Such linear coordination geometry is found in Ba2HgS5 (d(Hg−S) ∼ 2.34 Å and angles of (S−Hg−S) ∼ 179.9°),33 K2Hg3S4 (d(Hg−S) ∼ 2.37 Å and angles of (S−Hg−S) ∼ 165.3°),32 and Cs2Hg3Sn2S8 (d(Hg−S) ∼ 2.345 Å and angles of (S−Hg−S) ∼ 180.0°).34 Atomic coordinates, thermal parameters and selected bond distances and angles for Cs2Hg3S4 are given in Tables 2 and 3. Bridgman Crystal Growth. The thermal behavior of Cs2Hg3S4, as revealed by DTA (Figure 2), was used to guide E

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Figure 7. Crystal structure of Cs2Hg3S4 represents (A, B) 1D infinite [Hg3S4]2− chain, (C) three-dimensional arrangement. (D) Side-by-side interactions using long Hg−S bonds giving 2D character. (E, F) Polyhedral representation with thermal ellipsoids at 50% probability limit.

Figure 8. Comparison of the X-ray powder diffraction of pristine Cs2Hg3S4, with 2 and 16 days air exposed sample, black, red and green, respectively. A “+” indicates forming impurity.

Figure 10. X-ray photoelectron spectra of (A) cesium, (B) mercury, and (c) sulfur in Cs2Hg3S4. The freshly cleaved crystals exposed in air for about 15 min. Red lines represent experimental and the others to simulated spectra, respectively. Figure 9. SEM images of the naturally cleaved surface of Cs2Hg3S4; crystal was exposed to air for about (A) 15 min and (B) 24 h. EDS shows no evidence of other species.

the two kinds of Hg atoms, which Hg(1) is tetrahedral and Hg(2) linear, the CBM mainly consists of Hg(1)-s and S-p orbitals while Hg(2)-s orbitals do not play a significant role in the CBM. Instead, the Hg(2)-s orbitals are hybridized with Hg(2)-d orbitals, via a so-called d-s hybridization,38 and interact with p-orbitals of neighboring S atoms. This interaction is

The PDOS calculations reveal that the highly dispersive and isotropic features of the CBM are attributed to the contribution of Hg-s orbitals to the CBM as shown in Figure 12b. Between F

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Figure 11. UV/vis optical absorption spectrum of polycrystalline powder of Cs2Hg3S4 showing bandgap 2.8 eV. Inset shows the transmission spectrum of a cleaved crystal of Cs2Hg3S4 that exhibits bandgap 2.73 eV.

Figure 13. PL of Cs2Hg3S4 at 10 K. (A) Two sharp peaks are seen at 2.99 and 2.98 eV; these are postulated to be related to the bandgap. (B) Scan of the region below the bandgap reveals three broad peaks at 1.68, 1.86, and 2.08 eV that might be attributed from impurities Cs2Hg6S7 (1.68 eV)8c and α-HgS (1.86 and 2.08 eV).44 Intensity of the strongest peak of spectrum B is about 7% to that of spectrum A.

performed photoconductivity measurements on cleaved Cs2Hg3S4 crystals using laser light of 405 nm (Figure 14).

Figure 12. Electronic band structure (a) and projected density of states (PDOS) of Cs2Hg3S4. In both plots, Fermi level is set to 0 eV. In panel b, red, blue, pink, cyan, and green lines correspond to s-orbitals of Hg(1), p-orbitals of Hg(1), s-orbitals of Hg(2), p-orbitals of Hg(2), and p-orbitals of S, respectively.

Figure 14. Photoconductivity response to laser irradiation from a crystal of Cs2Hg3S4 measured at room temperature with the field applied along c-axis. Mobility-lifetime products (μτ) for (left) electrons and (right) holes extracted by fitting the data.

Different voltage polarities were applied to the illuminated electrode. For strongly absorbed light, the photocurrent can be modeled by41

shown in PDOS at around −3.5 and +4.5 eV, as bonding and antibonding states, respectively. Photoluminescence and Photoconductivity. Analysis of the photoluminescent (PL) behavior of Cs2Hg3S4 is useful in providing information about electronically active impurity and defect centers.39 Figure 13 shows the PL spectrum measured at 10 K on a polished sample. The positions of the two peaks at 2.99 and 2.98 eV are consistent with the bandgap and its phonon replica, respectively.40 A broad peak around 1.86 eV is observed in the spectra (intensity ∼7% of the strongest band at 2.99 eV), along with even smaller peaks at 1.68 and 2.08 eV. Detailed analyses, peak assignments, correlations with donor and acceptor levels, and temperature dependence of the PL peaks is ongoing and will be reported elsewhere. The grown crystals of Cs2Hg3S4 exhibited a room temperature resistivity of ∼108 ohm·cm. To determine the mobilitylifetime products (μτ) for both holes and electrons, we

2

I μτV (1 − e−L / μτV ) I (V ) = 0 2 L s L 1+ Vμ

(2)

where I0 is saturation current, L is the sample thickness, s is the surface recombination velocity, and V is the applied voltage. The electron and hole photoconductivity curves for Cs2Hg3S4 are shown in Figure 14. Using fits of the data of Cs2Hg3S4 to eq 2, the μτ values are attained as ∼4.2 × 10−4 cm2 V−1 (electrons) and ∼5.8 × 10−5 cm2 V−1 (holes). The saturation current I0 assigned as 1.53 and 5.48 nA for electrons and holes, respectively. The μτ products for the Cs2Hg3S4 samples are promising and comparable to other detector-grade crystals such as HgI2 [(μτ)e = 3 × 10−4 cm2 V−1, (μτ)h = 4 × 10−5 cm2 V−1),8e PbI2 [(μτ)e = 8 × 10−6 G

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Chemistry of Materials cm2 V−1; (μτ)h = 9 × 10−7 cm2 V−1],8e CsCdInTe3 [(μτ)e = 1.07 × 10−4 cm2 V−1; (μτ)h = 1.32 × 10−5 cm2 V−1],21c Tl6SeI4 [(μτ)e = 7 × 10−3 cm2 V−1; (μτ)h = 6 × 10−4 cm2V−1],8b CsPbBr3 [(μτ)e = 1.69 × 10−3 cm2 V−1; (μτ)h = 1.33 × 10−3 cm2 V−1];13 Cs2Hg3Se4 [(μτ)e ∼ >8 × 10−4 cm2 V−1],8a and Cs2Hg6S7 [(μτ)e = 1.200 × 10−3 cm2 V−1; (μτ)h = 1.01 × 10−4 cm2 V−1].8c The μτ products for Cs2Hg3S4, however, are lower than those of CZT (μτ)e = 4.5 × 10−2 cm2 V−1; (μτ)h = 1 × 10−4cm2 V−1),42 the leading material for room temperature Xray and γ-ray detection. For Cs2Hg3S4, higher values of μτ of the electrons and holes should be obtainable with further efforts for purification and optimization of crystal growth.

Shim, Y.; Bag, S.; Douvalis, A. P.; Wasielewski, M. R.; Kanatzidis, M. G. J. Am. Chem. Soc. 2011, 133, 7252−7255. (2) (a) Beal, A. R.; Hughes, H. P.; Liang, W. Y. J. Phys. C Solid State Phys. 1975, 8, 4236−4248. (b) Kam, K. K.; Parkinson, B. A. J. Phys. Chem. 1982, 86, 463−467. (c) Terrones, H.; Lopez-Urias, F.; Terrones, M. Sci. Rep. (U. K.) 2013, 3, 1549−1555. (d) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Nat. Nanotechnol. 2012, 7, 699−712. (3) (a) Fontana, M.; Deppe, T.; Boyd, A. K.; Rinzan, M.; Liu, A. Y.; Paranjape, M.; Barbara, P. Sci. Rep. 2013, 3, 1634−1638. (b) Fortin, E.; Sears, W. M. J. Phys. Chem. Solids 1982, 43, 881−884. (c) JaegerWaldau, A.; Lux-Steiner, M. C.; Bucher, E. Solid State Phenom. 1994, 37−38, 479−484. (4) (a) Fisher., Ø.; Maple, M. B. Topics in Current Physics: Superconductivity in Ternary Compounds I; Springer-Verlag: Berlin, 1982. (b) Hampshire, D. P. Handbook of Superconducting Materials; Taylor & Francis: New York, 2002; Vol. 2. (c) Maaren, M. H. v.; Schaeffer, G. M. Phys. Lett. 1966, 20, 131−131. (d) Yvon, K. I. Current Topics in Material Science; Elsevier: Amsterdam, 1979; Vol. 3. (e) Zhang, Y.; Yang, L. X.; Xu, M.; Ye, Z. R.; Chen, F.; He, C.; Xu, H. C.; Jiang, J.; Xie, B. P.; Ying, J. J.; Wang, X. F.; Chen, X. H.; Hu, J. P.; Matsunami, M.; Kimura, S.; Feng, D. L. Nat. Mater. 2011, 10, 273− 277. (5) (a) Chung, I.; Jang, J. I.; Malliakas, C. D.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2010, 132, 384−389. (b) Chung, I.; Kanatzidis, M. G. Chem. Mater. 2014, 26, 849−869. (c) Isaenko, L.; Yelisseyev, A.; Lobanov, S.; Petrov, V.; Rotermund, F.; Slekys, G.; Zondy, J. J. J. Appl. Phys. 2002, 91, 9475−9480. (d) Liao, J. H.; Marking, G. M.; Hsu, K. F.; Matsushita, Y.; Ewbank, M. D.; Borwick, R.; Cunningham, P.; Rosker, M. J.; Kanatzidis, M. G. J. Am. Chem. Soc. 2003, 125, 9484−9493. (e) Vogel, E. M.; Weber, M. J.; Krol, D. M. Phys. Chem. Glasses 1991, 32, 231−254. (f) Chen, M. C.; Wu, L. M.; Lin, H.; Zhou, L. J.; Chen, L. J. Am. Chem. Soc. 2012, 134, 6058−60. (6) (a) Biswas, K.; He, J. Q.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. Nature 2012, 489, 414−418. (b) Biswas, K.; He, J. Q.; Zhang, Q. C.; Wang, G. Y.; Uher, C.; Dravid, V. P.; Kanatzidis, M. G. Nat. Chem. 2011, 3, 160−166. (c) Chung, D. Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C.; Bastea, M.; Uher, C.; Kanatzidis, M. G. Science 2000, 287, 1024− 1027. (d) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004, 303, 818−821. (e) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005, 127, 9177−9190. (f) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem. Int. Ed. 2009, 48, 8616−8639. (g) Kanatzidis, M. G.; McCarthy, T. J.; Tanzer, T. A.; Chen, L.-H.; Iordanidis, L.; Hogan, T.; Kannewurf, C. R.; Uher, C.; Chen, B. Chem. Mater. 1996, 8, 1465− 1474. (7) (a) Chrissafis, K.; Kyratsi, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G. Chem. Mater. 2004, 16, 1932−1937. (b) Pandey, A.; Brovelli, S.; Viswanatha, R.; Li, L.; Pietryga, J. M.; Klimov, V. I.; Crooker, S. A. Nat. Nanotechnol. 2012, 7, 792−7. (8) (a) Androulakis, J.; Peter, S. C.; Li, H.; Malliakas, C. D.; Peters, J. A.; Liu, Z. F.; Wessels, B. W.; Song, J. H.; Jin, H.; Freeman, A. J.; Kanatzidis, M. G. Adv. Mater. 2011, 23, 4163−4167. (b) Johnsen, S.; Liu, Z. F.; Peters, J. A.; Song, J. H.; Nguyen, S.; Malliakas, C. D.; Jin, H.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. J. Am. Chem. Soc. 2011, 133, 10030−10033. (c) Li, H.; Peters, J. A.; Liu, Z. F.; Sebastian, M.; Malliakas, C. D.; Androulakis, J.; Zhao, L. D.; Chung, I.; Nguyen, S. L.; Johnsen, S.; Wessels, B. W.; Kanatzidis, M. G. Cryst. Growth Des. 2012, 12, 3250−3256. (d) Nguyen, S. L.; Malliakas, C. D.; Peters, J. A.; Liu, Z. F.; Im, J.; Zhao, L. D.; Sebastian, M.; Jin, H.; Li, H.; Johnsen, S.; Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Chem. Mater. 2013, 25, 2868−2877. (e) Owens, A. J. Synchrotron Radiat. 2006, 13, 143−150. (9) (a) Del Sordo, S.; Abbene, L.; Caroli, E.; Mancini, A. M.; Zappettini, A.; Ubertini, P. Sensors (Basel) 2009, 9, 3491−3526. (b) Rahman, R.; Plater, A. J.; Nolan, P. J.; Appleby, P. G. Radiat. Prot. Dosim. 2013, 154, 477−482.

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CONCLUDING REMARKS The new compound Cs2Hg3S4 has a layered structure and a direct bandgap of 2.8 eV. Although it exhibits a mildly incongruent melting behavior, nevertheless large single-crystals could still be grown using the Bridgman crystal growth method described above. Cs2Hg3S4 is a direct bandgap semiconductor with high mass density, suitable optical bandgaps and high resistivity. The compound is photoconductive and the carrier mobility-lifetime products (μτ) of its single-crystals are comparable to other hard radiation detector semiconductors such as α-HgI2 or unoptimized CZT. These findings suggest that Cs2Hg3S4 is a promising candidate for further investigations related to X-ray and γ-ray detection.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic file (CIF), crystallographic refinement details, atomic coordinates with equivalent isotropic displacement parameters, anisotropic displacement parameters, and selected bond distances for Cs2Hg3S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. G. Kanatzidis. E-mail: [email protected]. Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Homeland Security ARI program with grant 2014-DN-077-ARI086-01. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. A.H. acknowledges financial support from the National Science Foundation (Fellowship DGE-1324585 and grant DMR1410169).

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REFERENCES

(1) (a) Bag, S.; Gaudette, A. F.; Bussell, M. E.; Kanatzidis, M. G. Nat. Chem. 2009, 1, 217−224. (b) Bag, S.; Kanatzidis, M. G. J. Am. Chem. Soc. 2008, 130, 8366−8376. (c) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.; Kanatzidis, M. G. Science 2007, 317, 490−493. (d) Chianelli, R. R.; Daage, M.; Ledoux, M. J. Adv. Catal. 1994, 40, 177−232. (e) Prins, R.; Debeer, V. H. J.; Somorjai, G. A. Catal. Rev. 1989, 31, 1−41. (f) Yuhas, B. D.; Smeigh, A. L.; Samuel, A. P. S.; H

DOI: 10.1021/cm504089r Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials (10) Eberth, J.; Simpson, J. Prog. Part. Nucl. Phys. 2008, 60, 283−337. (11) (a) Burger, A.; Groza, M.; Cui, Y.; Hillman, D.; Brewer, E.; Bilikiss, A.; Wright, G. W.; Li, L.; Lu, F.; James, R. B. J. Electron. Mater. 2003, 32, 756−760. (b) Owens, A.; Peacock, A. Nucl. Instrum. Methods Phys. Res., Sect. A 2004, 531, 18−37. (c) Schlesinger, T. E.; Toney, J. E.; Yoon, H.; Lee, E. Y.; Brunett, B. A.; Franks, L.; James, R. B. Mater. Sci. Eng., R 2001, 32, 103−189. (12) (a) McGregor, D. S.; Hermon, H. Nucl. Instrum. Methods Phys. Res., Sect. A 1997, 395, 101−124. (b) Lachish, U. J. Cryst. Growth 2001, 225, 114−117. (13) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z. F.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Cryst. Growth Des. 2013, 13, 2722−2727. (14) Hammersley, A. P.; Sevensson, S. O.; Hanfland, M.; Fitch, A. N.; Häusermann, D. High Pressure Res. 1996, 14, 235−248. (15) Wang, J.; Toby, B. H.; Lee, P. L.; Ribaud, L.; Antao, S. M.; Kurtz, C.; Ramanathan, M.; Von Dreele, R. B.; Beno, M. A. Rev. Sci. Instrum. 2008, 79, 085105-1−085105-7. (16) Lee, P. L.; Shu, D.; Ramanathan, M.; Preissner, C.; Wang, J.; Beno, M. A.; Von Dreele, R. B.; Ribaud, L.; Kurtz, C.; Antao, S. M.; Jiao, X.; Toby, B. H. J. Synchrotron Radiat. 2008, 15, 427−432. (17) Chupas, P. J.; Chapman, K. W.; Kurtz, C.; Hanson, J. C.; Lee, P. L.; Grey, C. P. J. Appl. Crystallogr. 2008, 41, 822−824. (18) Dalesio, L. R.; Hill, J. O.; Kraimer, M.; Lewis, S.; Murray, D.; Hunt, S.; Watson, W.; Clausen, M.; Dalesio, J. Nucl. Instrum. Methods Phys. Res., Sect. A 1994, 352, 179−184. (19) Sheldrick, G. M. SHELXS97: Program for Crystal Structure Solution; University of Gottingen: Gottingen, Germany, 1997. (20) (a) Liao, J. H.; Varotsis, C.; Kanatzidis, M. G. Inorg. Chem. 1993, 32, 2453−2462. (b) Mccarthy, T. J.; Ngeyi, S. P.; Liao, J. H.; Degroot, D. C.; Hogan, T.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1993, 5, 331−340. (21) (a) Cho, N. K.; Peters, J. A.; Liu, Z.; Wessels, B. W.; Johnsen, S.; Kanatzidis, M. G.; Song, J. H.; Jin, H.; Freeman, A. Semicond. Sci. Technol. 2012, 27, 015016-1−015016-4. (b) Li, H.; Malliakas, C. D.; Liu, Z. F.; Peters, J. A.; Jin, H.; Morris, C. D.; Zhao, L. D.; Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Chem. Mater. 2012, 24, 4434− 4441. (c) Li, H.; Malliakas, C. D.; Peters, J. A.; Liu, Z. F.; Im, J.; Jin, H.; Morris, C. D.; Zhao, L. D.; Wessels, B. W.; Freeman, A. J.; Kanatzidis, M. G. Chem. Mater. 2013, 25, 2089−2099. (d) Liu, Z. F.; Peters, J. A.; Wessels, B. W.; Johnsen, S.; Kanatzidis, M. G. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 659, 333−335. (22) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (23) (a) Kresse, G.; Furthmuller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (b) Kresse, G.; Hafner, J. Phys. Rev. B Condens. Matter Mater. Phys. 1994, 49, 14251−14269. (24) (a) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Perderson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 42, 6671−6687. (b) Perdew, J. P.; Levy, M. Phys. Rev. Lett. 1983, 51, 1884−1887. (25) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. J. Chem. Phys. 2003, 118, 8207−8215. (26) Heyd, J.; Scuseria, G. E. J. Chem. Phys. 2004, 121, 1187−1192. (27) Shoemaker, D. P.; Hu, Y. J.; Chung, D. Y.; Halder, G. J.; Chupas, P. J.; Soderholm, L.; Mitchell, J. F.; Kanatzidis, M. G. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 10922−7. (28) Ohmiya, T. J. Appl. Crystallogr. 1974, 7, 396−397. (29) Axtell, E. A., III; Liao, J.-H.; Pikramenou, Z.; Kanatzidis, M. G. Chem.Eur. J. 1996, 2, 656−666. (30) (a) Bronger, W.; Hardtdegen, H.; Kanert, M.; Mueller, P.; Schmitz, D. Z. Anorg. Allg. Chem. 1996, 622, 313−318. (b) Bronger, W.; Hendriks, U.; Mueller, P. Z. Anorg. Allg. Chem. 1988, 559, 95−105. (31) Narducci, A. A.; Ibers, J. A. J. Alloys Comp. 2000, 306, 170−174. (32) (a) Axtell, E. A.; Park, Y.; Chondroudis, K.; Kanatzidis, M. G. J. Am. Chem. Soc. 1998, 120, 124−136. (b) Kanatzidis, M. G.; Park, Y. Chem. Mater. 1990, 2, 99−101. (33) Islam, S. M.; Im, J.; Freeman, A. J.; Kanatzidis, M. G. Inorg. Chem. 2014, 53, 4698−704.

(34) Marking, G. A.; Hanko, J. A.; Kanatzidis, M. G. Chem. Mater. 1998, 10, 1191−1199. (35) Krishnan, N. G.; Delgass, W. N.; Robertson, W. D. J. Phys. F Met. Phys. 1977, 7, 2623−2635. (36) Ma, S.; Shim, Y.; Islam, S. M.; Subrahmanyam, K. S.; Wang, P.; Li, H.; Wang, S.; Yang, X.; Kanatzidis, M. G. Chem. Mater. 2014, 26, 5004−5011. (37) (a) Pratt, A. R.; Muir, I. J.; Nesbitt, H. W. Geochim. Cosmochim. Acta 1994, 58, 827−841. (b) Smart, R. S. C.; Skinner, W. M.; Gerson, A. R. Surf. Interface Anal. 1999, 28, 101−105. (c) Islam, S. M.; Subrahmanyam, K. S.; Malliakas, C. D.; Kanatzidis, M. G. Chem. Mater. 2014, 26, 5151−5160. (38) Orgel, L. E. An Introduction to Transition-Metal Chemistry: Ligand-Field Theory; Methuen: London, 1966; Vol. 2. (39) (a) Belas, E.; Grill, R.; Franc, J.; Moravec, P.; Bok, J.; Horodysky, P.; Hoschi, P. I. T. N. S. IEEE Trans. Nucl. Sci. 2007, 54, 786−791. (b) Yang, J.; Yu, Z. Z.; Liu, J. M.; Tang, D. Y. J. Crytal Growth. 1990, 101, 281−284. (40) Grundmann, M. The Physics of Semiconductors; Springer: New York, 2006. (41) Many, A. J. Phys. Chem. Solids. 1965, 26, 575−578. (42) Kargar, A.; Ariesanti, E.; McGregor, D. S. Nucl. Technol. 2011, 175, 131−137. (43) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51 (Pt 1), 33−38. (44) Simpson, C. T.; Imaino, W. I.; Becker, W. M.; Faile, S. P. Solid State Commun. 1978, 28, 39−41.

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