Phase Transition, Conformational Exchange, and Nonlinear Optical

Mar 17, 2016 - The soluble molecular selenophosphate salts ACsP2Se8 (A = K, Rb, Cs) crystallize in the orthorhombic space group Ccce with a = 14.982(3...
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Phase Transition, Conformational Exchange, and Nonlinear Optical Third Harmonic Generation of ACsP2Se8 (A = K, Rb, Cs) Alyssa S. Haynes,† Abhishek Banerjee,† Felix O. Saouma,‡ Calford O. Otieno,‡ Joon I. Jang,‡ and Mercouri G. Kanatzidis*,†,§ †

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Physics, Applied Physics, and Astronomy, Binghamton University, Binghamton, New York 13902, United States § Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: The soluble molecular selenophosphate salts ACsP2Se8 (A = K, Rb, Cs) crystallize in the orthorhombic space group Ccce with a = 14.982(3) Å, b = 24.579(5) Å, and c = 13.065(3) Å for the Cs salt and a = 14.782(3) Å, b = 23.954(5) Å, and c = 13.044(3) Å for the K analogue. ACsP2Se8 is composed of the molecular 6-membered ring, [P2Se8]2−, in the twist conformation charge balanced by alkali metals. The band gaps of these compounds are 2.44 ± 0.2 eV for Cs2P2Se8, 2.41 ± 0.2 eV for RbCsP2Se8, and 2.36 ± 0.2 eV for KCsP2Se8. The amorphous versions of these materials can be made by water quenching the melt and have band gaps for all ACsP2Se8 of 2.12 ± 0.2 eV. Raman spectroscopic studies exhibit active modes of PSe4 and Se−Se in the compound. Solution 31P NMR studies shed light into the interesting conformational fluxionality of the [P2Se8]2− anion, including a conformation that has not been previously observed. Thermal analysis reveals ACsP2Se8 exhibits a phase transition, which we investigate by in situ synchrotron powder X-ray diffraction. Third harmonic generation (THG) nonlinear optical measurements determined the THG coefficient, χ(3), for amorphous and crystalline Cs2P2Se8 of 1.8 ± 0.2 × 105 pm2/V2 and 2.4 ± 0.1 × 105 pm2/V2, respectively.

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INTRODUCTION Chalcophosphates are compounds with oxidized phosphorus and at least one P-Q bond, where Q is sulfur, selenium, or tellurium. They are generally ternary (A/P/Q and M/P/Q) and quaternary (A/M/P/Q) where A is alkali metal and M is a metal. These species have been primarily isolated using the polychalcogenide molten flux method, which can stabilize metastable phases at low temperatures,1 but congruently melting phases can also be made via direct combination reactions. This class of materials has great structural diversity because of the vast number of stable [PxQy]z− building blocks and various binding modes in which they can engage. Selenophosphate anions specifically range from one-dimensional, polymeric chains such as 1/∞[P3Se4−],2 1/∞[PSe6−],3 and 1/∞[P2Se62−],4 to discrete molecular species including [PSe4]3−,5−7 [P8Se18]6−,8 [P2Se10]4−,9 [P2Se9]4−,7 [P2Se6]4−,7,10 [P5Se12]5−,11 [P6Se12]4−,11,12 and [P3Se7]3−.13 Chalcophosphates can be used as starting materials in solidstate reactions or coordination chemistry in solution. Additionally, these materials can possess technologically significant properties such as reversible phase transitions,3,11,14,15 ferroelectricity,16−18 photoluminescence,19,20 and second harmonic generation (SHG).4,11,15,21−25 As compared to SHG, © XXXX American Chemical Society

third-order nonlinear optical (NLO) effects have been much less explored in the literature of crystalline chalcogenide chemistry. Third-order NLO mechanisms including twophoton absorption and self-focusing are utilized in emerging photonic and optoelectronic applications including optical switching,26 ultrafast all-optical signal processing for telecommunications,27 frequency-resolved optical-gating to fully characterize laser pulses,28 and laser scanning microscopy.29 Third harmonic generation (THG) is a useful technique to characterize the third-order nonlinearity of pristine powdered samples,30,31 although it is not used directly for practical frequency-tripling applications. Herein, we report the synthesis, structure, optical properties, thermal behavior, 31P NMR, and THG of ACsP2Se8 (A = K, Rb, Cs). ACsP2Se8 is composed of the molecular [P2Se8]2− anion charge balanced by alkali metal cations. The [P2Se8]2− anion has been previously reported in the mixed organicinorganic salts [PPh 4 ] 2 [P 2 Se 8 ], [Li(py) 4 ] 2 [P 2 Se 8 ], [(nBu)4N]2[P2Se8]·2MeCN, [C8H15N2]2[P2Se8], and [LiReceived: February 8, 2016 Revised: March 16, 2016

A

DOI: 10.1021/acs.chemmater.6b00551 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials (MeCN)4]2[P2Se8].32,33 Fully inorganic Cs2P2Se8 has been reported as unpublished results,13 and here we present the first characterization of Cs2P2Se8. The crystalline and amorphous band gaps of ACsP2Se8 are ∼2.4 eV and ∼2.1 eV, respectively. These materials possess a high-temperature phase transition, which we examined by conducting in situ powder X-ray diffraction (PXRD) experiments using synchrotron radiation. Variable temperature solution 31P NMR exhibits a dynamic equilibrium between the parent twist conformation of the [P2Se8]2− anion to the chair and boat conformers, the presence of the boat conformer being observed for the first time. Interesting structural transformation of the erstwhile reported 1 /∞[PSe6−] polymeric chain and the [P2Se8]2− anion is also observed and elucidated using temperature-dependent NMR and liquid chromatography−mass spectrometry (LC-MS) studies. THG measurements demonstrate a higher THG coefficient, χ(3), for both glassy and crystalline Cs2P2Se8, compared to the reference material, AgGaSe2, even though their band gaps are higher. NLO coefficients typically are inversely scalable to a power law in the band gap, which makes glassy and crystalline Cs2P2Se8 interesting candidates for further high-order nonlinear optical investigation.

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EXPERIMENTAL SECTION

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PHYSICAL MEASUREMENTS

Scanning Electron Microscopy. A Hitachi S-3400 scanning electron microscope equipped with a PGT energy-dispersive X-ray analyzer was used to conduct semiquantitative microprobe analyses and energy dispersive spectroscopy (EDS). The parameters used for EDS spectra were 25 kV accelerating voltage, 60 mA probe current, and 60 s acquisition time. Single Crystal X-ray Diffraction. Single crystals of Cs2P2Se8 and KCsP2Se8 were adhered to the tip of a glass fiber with glue. A STOE IPDS II single crystal diffractometer operating at 50 kV and 40 mA was used to conduct X-ray diffraction measurements with Mo Kα radiation (λ = 0.71073 Å). Data collection was performed using X-Area software, integration was carried out in X-RED, and a numerical absorption correction was performed within X-SHAPE, all of which are programs provided by STOE. The crystal structures were solved via direct methods and refined in the SHELXTL program package.36 Solid-State UV/vis/Near-IR Spectroscopy. BaSO4 was used as the reference and taken to have 100% reflectance. Samples were ground into a fine powder and placed on top of a bed of compressed BaSO4. Diffuse-reflectance measurements were taken on a Shimadzu UV-3600 PC double-beam, double-monochromator spectrophotometer in the range of 200−2000 nm. The collected reflectance data were converted to absorbance via the Kubelka−Munk equation: α/S = (1− R)2/2R, where R is reflectance, α is the absorption coefficient, and S is the scattering coefficient.34 The fundamental absorption edge was estimated by linearly fitting the absorbance of the converted data. Raman Spectroscopy. Samples were crushed into a fine powder and loaded into borosilicate glass capillaries for measurement. Raman experiments were conducted using a DeltaNU Advantage NIR spectrometer using an excitation wavelength of 785 nm from a diode laser equipped with a charge-coupled device (CCD) camera detector. The laser was estimated to have a maximum power of 60 mW and a beam diameter of 35 μm. Spectra were collected by averaging 16 5-s frames taken from 100−1000 cm−1 at room temperature. NMR Spectroscopy. In Solid-State. 31P magic angle spinning (MAS) NMR spectra were collected on a Varian VNMRS 400 MHz NMR. Chemical shifts are referenced with respect to NH4H2PO4 (δ = 0.8 ppm). Pure samples of the crystalline and glassy forms of Cs2P2Se8 were loaded into a 5 mm zirconia rotor, and experiments were performed using a pulse width (pwx90) of 40 ms and relaxation delay of 5 min. A total of 64 scans were collected for the crystalline and glassy samples, respectively, with a spin rate of 10000 rpm for both samples. In Solution. Variable temperature 31P, 77Se, and 31P−31P 2D Exchange Spectroscopy (EXSY) solution NMR spectra were recorded on an Agilent DD MR-400 system, containing wide variable temperature capability. Spectra were recorded on a solution of 100 mg of crystalline or glassy phase samples of Cs2P2Se8 in 1 mL of 10% DMF-d7/DMF. Each spectrum was collected for a period of approximately 2 h consisting of 2048 scans. The spectra were referenced with respect to an 85% solution of H3PO4 in D2O for 31P (δ = 0 ppm) and Me2Se in D2O for 77Se (δ = 0 ppm). Mass Spectroscopy. Solution phase mass spectroscopic studies were performed on a Bruker autofelx III smartbeam MALDI-TOF instrument. Measurements were performed on a 1 mM concentrated sample in 5% DMF/CHCl3. For MALDI measurements, a solution of approximately 10% sample to the matrix of 2,5-dihydroxybenzoic acid (DHB) by mass was prepared. One μL of this solution was placed on the sample plate, and the solvent was evaporated using mild heating. Measurements were done in negative ion mode. Differential Thermal Analysis (DTA). Coarsely ground samples of ∼90 mg were placed in a silica ampule, evacuated to ∼10−4 mbar, and flame sealed. A similar mass of α-Al2O3 used as a reference was placed in a separate ampule, evacuated to ∼10−4 mbar, and flame sealed. DTA measurements were conducted on a Shimadzu DTA-50 thermal analyzer using a temperature rate of ±5 °C/min and a maximum temperature of 400 °C. Melting and crystallization temperatures were recorded at the maximum of exothermic peaks and minimum of endothermic valleys.

Reagents. All chemicals were used as obtained: potassium metal (98%, Sigma Aldrich, St. Louis, MO); rubidium metal (99.9+%, Strem Chemicals, Inc., Newburyport, MA); cesium metal (99.9+%, Strem Chemicals, Inc., Newburyport, MA); red phosphorus powder (99%, Sigma Aldrich, St. Louis, MO); selenium pellets (99.99%, Sigma Aldrich, St. Louis, MO). K2Se, Rb2Se2, and Cs2Se2 were synthesized by reacting stoichiometric amounts of the elements in liquid ammonia as described elsewhere.34,35 Synthesis. Stoichiometric amounts of K2Se, and/or Rb2Se2, and/or Cs2Se2, P, and Se were combined together (∼0.5 g total mass) in a 9 mm fused silica tube in a dry, nitrogen-filled glovebox. The tubes were evacuated to ∼10−4 mbar and flame sealed. The tubes were then placed in a programmable furnace and followed the specified heating profiles below. Crystallinity or lack thereof and phase purity were determined by PXRD. All samples have 100% purity unless otherwise noted. Both crystalline and amorphous materials are air and moisture sensitive and hence stored in an evacuated desiccator. Both crystalline and glassy ACsP2Se8 materials are soluble in degassed, anhydrous DMF and NMF but insoluble in acetonitrile and methanol. Crystalline ACsP2Se8 (A = K, Rb, Cs). Direct combination ratios of the needed starting materials were used for each compound. The mixtures were heated to 500 °C and soaked there for 10 h. The system was then cooled to 250 °C over 10 h at which point the furnace was turned off. The yellow product was isolated by opening the tubes in air then washing for ∼2 h in a solution of 20 mL of ether with 0.1 mL of triethylphosphine to remove a slight amount of selenium as a second phase. Amorphous ACsP2Se8 (A = K, Rb, Cs). Direct combination ratios of the needed starting materials were used for each compound. The mixtures were heated to 550 °C, soaked there for 12 h, and then water quenched. The red-orange product was isolated by opening the tubes in air.

Powder X-ray Diffraction (PXRD) Analysis. PXRD measurements were performed on a computer-controlled INEL CPS 120 powder diffractometer equipped with a graphite monochromator and Cu Kα radiation, which operated at 40 kV and 20 mA. The system used flat sample geometry and collected data in reflection mode with a positive-sensitive detector. Calculated PXRD spectra were created using the CIFs of refined structures in the Visualizer software package of the program FINDIT. B

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Chemistry of Materials Table 1. Crystal Data and Structure Refinement for Cs2P2Se8 and KCsP2Se8 at 293(2) Ka empirical formula formula weight temperature wavelength crystal system space group unit cell dimensions

volume Z density (calculated) absorption coefficient F(000) θ range for data collection index ranges reflections collected independent reflections completeness to θ = 25.00° refinement method data/restraints/parameters goodness-of-fit final R indices [>2σ(I)] R indices [all data] largest diff peak and hole a

Cs2P2Se8

KCsP2Se8

959.44 293(2) K 0.71073 Å orthorhombic Ccce a = 14.982(3) Å, α = 90.00° b = 24.579(5) Å, β = 90.00° c = 13.065(3) Å, γ = 90.00° 4811.0(17) Å3 12 3.974 g/cm3 22.862 mm−1 4944 2.23 to 25.00° −17 ≤ h ≤ 17, −29 ≤ k ≤ 29, −15 ≤ l ≤ 15 14837 2125 [Rint = 0.0543] 99.90% full-matrix least-squares on F2 2125/0/83 1.094 Robs = 0.0344 wRobs = 0.0573 Rall = 0.0490, wRall = 0.0601 1.558 and −0.596 e·Å−3

865.63 293(2) K 0.71073 Å orthorhombic Ccce a = 14.782(3) Å, α = 90.00° b = 23.954(5) Å, β = 90.00° c = 13.044(3) Å, γ = 90.00° 4618.7(16) Å3 12 3.735 g/cm3 21.754 mm−1 4512 2.25 to 25.00° −17 ≤ h ≤ 17, −28 ≤ k ≤ 28, −15 ≤ l ≤ 15 14141 2038 [Rint = 0.0404] 99.70% full-matrix least-squares on F2 2038/0/85 1.13 Robs = 0.0282 wRobs = 0.0524 Rall = 0.0375, wRall = 0.0544 0.647 and −0.364 e·Å−3

R = ∥Fo|−|Fc∥/Σ|Fo|, wR = {Σ[w(|Fo|2−|Fc|2)2]/Σ[w(|Fo|4)]}1/2 and calc w = 1/[σ2(Fo2)+(0.0261P)2+10.8883P] where P = (Fo2+2Fc2)/3.

High-Resolution Synchrotron PXRD. Samples were ground in air, sieved to 63−90 μm, placed in 0.3 mm diameter borosilicate capillaries, evacuated to ∼10−4 mbar, and flame sealed. Variable temperature PXRD patterns were collected using beamline 17-BM-B at the Advanced Photon Source (APS) in Argonne National Laboratory, running at 18 keV (λ = 0.727 Å) with a Perkin-Elmer aSi C-window. The capillaries were heated using an electrical resistance furnace described by Chupas et al.37 The samples were heated from 50 to 150 °C at a rate of 10 °C/min, further heated to 400 °C at 1 °C/ min, cooled to 200 °C at 1 °C/min, and cooled to the final temperature of 50 °C at 10 °C/min. The detector distance was 600 mm, and six 5-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.38 Rietveld refinement of the data was conducted using the EXPGUI graphical interface for GSAS.39,40 Sample Preparation for Third Harmonic Generation (THG) Measurements. Samples were crushed into a powder and then manually sieved into seven different particle size ranges: 150−125 μm, 125−106 μm, 106−90 μm, 90−63 μm, 63−45 μm, 45−20 μm, and < 20 μm. The various size ranges were placed into different 1.6−1.7 × 90 mm borosilicate capillary tubes in air, capped with clay under dry nitrogen environment, then flame sealed to prevent the samples from being exposed to moisture and oxygen during measurement. THG Measurements. The description of our tunable picosecond laser system is described elsewhere.41 The incident pulse energy was tuned to 20 μJ before being mildly focused onto samples with a spot size of ∼0.5 mm in diameter by a CaF2 lens ( f = 75 mm) far away from the Z-scan focus. Here we determined the beam spot size in order to i) properly average the THG signals from powders of random orientations and ii) to minimize the change in the spot size when we vary the fundamental wavelength λ over a broad range; the beam waist w0 at the Z-scan focus undergoes a significant λ-dependent variation via w0 = (λ/π)( f/σ), where f and σ are the focal length and the Gaussian width of the incident beam, respectively.42 The THG signal from the sample was collected using reflection geometry by a fiber-

optic bundle, which was coupled to a selective-grating spectrometer (set to 600 grooves/mm) equipped with a charge-couple device (CCD) camera. THG signals from other optical components were confirmed to be negligible. The relative THG signals spectrally resolved in a broad wavelength range were precisely calibrated with the known and measured efficiencies of all optical components. The data presented below were all normalized to the same exposure time, 1 s.

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RESULTS AND DISCUSSION Synthesis of ACsP2Se8 (A = K, Rb, Cs). Crystalline and glassy ACsP2Se8 (A = K, Rb, Cs) can be made pure by slowly cooling or water quenching the melt, respectively, of stoichiometric starting materials from 500 °C. ACsP2Se8 is a reversible phase-change material and can be switched between the crystalline and glassy forms by heating the material to 500 °C and then controlling the speed of cooling from the melt. Efforts to make the pure K analogue resulted in a mixture of KPSe6 and K2P2Se6. Attempts to synthesize the pure Rb analogue resulted in mixture of RbPSe 6 and Rb 2 P 2 Se 8 microcrystals. The Rb 2 P 2 Se 8 phase was identified by qualitatively matching the experimental PXRD to the calculated KCsP2Se8 PXRD pattern. From previous reports of the [P2Se8]2− anion32 and current experimental observations, we can infer that the [P2Se8]2− anion prefers to crystallize with larger cations. Structure of ACsP2Se8 (A = K, Rb, Cs). Cs2P2 Se8 crystallizes in the centrosymmetric, orthorhombic space group Ccce (Table 1). The unit cell is made up of 12 [P2Se8]2− anions charge balanced by Cs cations (Figure 1). The [P2Se8]2− anions are two distorted PSe4 tetrahedra fused via bridging Se atoms in the twist conformation with bond angles ranging from 96.816(30)−125.149(30)°. The P−Se terminal bond lengths (2.130(1)−2.136(2) Å) are shorter than the P−Se bridging bond lengths (2.271(2)−2.288(2) Å), as expected. There are C

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Figure 3. Coordination environment of cesium in Cs2P2Se8 with thermal ellipsoids set at 90%. Color scheme: cesium (green), selenium (red), phosphorus (black).

Figure 1. Unit cell of Cs2P2Se8 along the c-axis with thermal ellipsoids set at 90%. The six layers of [P2Se8]2− molecular ions are labeled on the right. Color scheme: cesium (green), selenium (red), phosphorus (black).

2-fold rotation axes. The two Cs2P2Se8 units within each layer are made up by c-glides perpendicular to the a-axis. Layer 4 is made up by e-glides (e means can glide along either the a- or baxis) perpendicular to the c-axis from Layer 1. The remainder of the unit cell (Layers 3, 5, and 6) is formed by c-glides perpendicular to the b-axis. Since the pure K and Rb analogues were inaccessible and there are two crystallographically distinct Cs sites in Cs2P2Se8, we aimed to make RbCsP2Se8 and KCsP2Se8 to see if there was ordering of the alkali metal site. Both syntheses were successful; however, the single crystals of RbCsP2Se8 were small and too weakly diffracting to obtain data of high enough quality to solve and refine the structure. We fully refined the structure of KCsP2Se8, which crystallizes in the same phase as Cs2P2Se8 (Table 1). Therefore, the Rb structure can be assumed to also crystallize in the same phase with parameters in between the Cs and K analogues. PXRD spectra of all three compounds also imply the same phase is present with peak shifting as expected based on the cation size (Figure S1). In KCsP2Se8, both alkali metal sites are partially occupied as a solid solution, but the site with closer Se−A contacts has 34% higher occupancy of K (Table 2). The unit cell contracts most along the b-axis with the substitution of 50% K for Cs (Table S3). This contraction allows for three additional intermolecular Se···Se van der Waals contacts (≤3.80 Å) between [P2Se8]2− molecular ions from different layers (along b-axis) in KCsP2Se8 as compared to Cs2P2Se8 (Table S4). There are no Se···Se interactions between [P2Se8]2− molecular ions in the same layer in the ac plane within the van der Waals radii sum for either KCsP2Se8 or Cs2P2Se8. This demonstrates that the intermolecular Se···Se contacts get stronger as the “chemical pressure” exerted in the unit cell increases from Cs2P2Se8 to KCsP2Se8 Optical Absorption. The crystalline band gaps of Cs2P2Se8, RbCsP2Se8, and KCsP2Se8 are 2.44 ± 0.2 eV, 2.41 ± 0.2 eV, and 2.36 ± 0.2 eV, respectively, which agrees well with their yellow color (Figure 4a). This shows that the size of the alkali metal slightly influences the band structure near the band gap. In contrast, the band gap for all glassy ACsP2Se8 is 2.12 ± 0.2 eV (Figure 4b). The red-shift observed in the amorphous phase is expected because structural defects present in glassy materials create midgap states leading to so-called Urbach band tailing and therefore a lower band gap.33,43,44

two crystallographically distinct [P2Se8]2− molecular ions in the asymmetric unit (Figure 2). The first [P2Se8]2− contains two

Figure 2. Two crystallographically distinct [P2Se8]2− molecular ions in Cs2P2Se8 with thermal ellipsoids set to 90% and atom labels. Color scheme: selenium (red), phosphorus (black).

perpendicular 2-fold rotation axes, and the second contains one 2-fold rotation axis to make up the molecular ion. Each Cs+ cation coordinates to 10 selenium atoms from four different [P2Se8]2− molecular ions (Figure 3). The Cs−Se bonds range from 3.648(1)−4.018(1) Å with Se−Cs−Se bond angles ranging from 49.832(15)−168.537(22)°. The entire unit cell can be visualized as six layers of Cs2P2Se8 in the ac-plane containing two Cs2P2Se8 units per layer, and these layers are labeled in Figure 1. Layers 1, 3, 5, and 6 contain the [P2Se8]2− molecular ions with one 2-fold rotation axis, and Layers 2 and 5 consist of the [P2Se8]2− molecular ions with two D

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Table 2. Comparison of K and Cs Percent Composition in Alkali Metal Sites and A−Se Bond Ranges in KCsP2Se8 and Cs2P2Se8 compound

site

Cs composition (%)

K composition (%)

A−Se bond range (Å)

KCsP2Se8

K1/Cs1 K1/Cs2 Cs1 Cs2

74.4 40.4 100 100

25.6 59.6 0 0

3.606(1)−3.927(1) 3.533(1)−4.034(1) 3.684(1)−3.971(1) 3.648(1)−4.018(1)

Cs2P2Se8

Figure 5. Raman spectra of crystalline (black) and glassy (red) Cs2P2Se8.

The main distinction between the phases is the difference in energy for the second major Raman shift (273 cm−1 for crystalline and 259 cm−1 for glassy). Since this peak is shifted, we were curious to see if the glassy phase is polymeric rather than molecular. The density of crystalline Cs2P2Se8 was calculated to be 3.974 g/cm3 from the single crystal structure refinement, and the density of glassy Cs2P2Se8 was measured to be 3.9561 g/cm3 using an Accupyc 1340 Gas Pycnometer for the volume and a 4-digit balance for the mass. The lower density of the glassy phase shows the most likely scenario is that it is also molecular in nature; if the glass polymerized the molecular [P2Se8]2− anion we would expect a higher density for the glassy phase compared to its crystalline, molecular counterpart. NMR Spectroscopy. We decided to perform multinuclear NMR studies, in the solid-state as well as in solution, in order to better understand the correlation between the solid-state structure with respect to its solution phase behavior. In the paragraphs below, we discuss our results from such measurements. Solid-State Measurements. Solid-state NMR has been used as an important tool to characterize inorganic chalcophosphates.3,4,15,41,48 As discussed earlier, the asymmetric unit of Cs2P2Se8 consists of two crystallographically independent [P2Se8]2− units. Comparing the solid-state 31P NMR spectra of the crystalline versus amorphous phase, as expected, the crystalline spectrum is better−resolved under identical measurement parameters (Figure S5). In the crystalline phase, the presence of these two independent [P2Se8]2− units is clearly evidenced, with the observation of two distinct peaks having 31P chemical shifts at −10.2 and −14.9 ppm, respectively (Figure S5a). To the best of our knowledge this is the first report of solid-state NMR for {P2Se8} units, and the chemical shifts are observed to lie in the same range as observed for the twist conformation in solution (vide inf ra). Measurements in Solution. Cs2P2Se8 was observed to be readily soluble in DMF, with the rate of solubility dependent on

Figure 4. Band gaps of ACsP2Se8. (a) Crystalline KCsP2Se8 (2.36 ± 0.2 eV), RbCsP2Se8 (2.41 ± 0.2 eV), and Cs2P2Se8 (2.44 ± 0.2 eV); (b) Amorphous KCsP2Se8, RbCsP2Se8, and Cs2P2Se8 (all 2.12 ± 0.2 eV).

Raman Spectroscopy. All ACsP2Se8 Raman spectra looked identical between crystalline or glassy spectra, so glassy and crystalline Cs2P2Se8 Raman spectra are discussed and compared as representative materials. Several shifts are observed between 100−550 cm−1, and the results are summarized in Table 3.45−47 The glassy and crystalline Raman spectra are similar, but the glassy phase overall has broader peaks (Figure 5). This implies that although the longrange crystalline order is lost the local structural building blocks are conserved in the glass. Table 3. Raman Stretches of Crystalline and Glassy Cs2P2Se8 crystalline Cs2P2Se8 PSe4 bending PSe4 symmetric stretch Se−Se stretch PSe4 asymmetric stretches

120 165 220 273 396 420 515

−1

cm cm−1 cm−1 cm−1 cm−1 cm−1 cm−1

(w) (w) (s) (s) (w) (w) (w)

glassy Cs2P2Se8 157 cm−1 (w) 220 cm−1 (s) 259 cm−1 (s) 420 cm−1 (w) 513 cm−1 (w) E

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further supported by the observations from 2D 31P EXSY measurements performed at −60 °C, where cross peaks arise between peaks at −5 and −90 ppm, 9.5 and −90 ppm, but no such cross peaks from peaks at 9.5 and −5 ppm (Figure S6). Interestingly cross peaks are also observed in the 31P EXSY spectrum between the broad features occurring at 3.5 and −41.0 ppm, strongly indicating the presence of possible intermediates between the interconversion of the different conformers of the [P2Se8]2− anion. Hence from such observations we can infer the chair conformation to be the intermediate species, converting dynamically to the twist and boat conformations with no interconversion occurring between the twist and boat conformers (Figure 6b). We believe these interconversions occur via thermally activated rotations of the Se−Se and P−Se bonds within the ring similar to cyclohexane.49 In order to understand the interconversion of the different conformers of the [P2Se8]2− anion via the intermediates observed at varied temperatures in the NMR spectrum, we also performed 31P solution NMR studies on the previously reported polymeric 1/∞[PSe6−] anionic chain, which is also soluble in DMF.3 Interestingly, the 31P solution NMR of the anionic one-dimensional chain of 1/∞[PSe6−] showed an identical spectrum (Figure S7) as observed with the [P2Se8]2− anion, indicating a dynamic interconversion of the 1 /∞[PSe6−] chain to the more robust [P2Se8]2− discrete units in solution. This observation is further elucidated with mass spectroscopic studies (vide inf ra). The solution 77Se NMR spectrum of the [P2Se8]2− anion, recorded at −50 °C, also revealed the presence of multiple conformers of the anion (Figure S8). The spectrum is observed to match previous literature reports.32 As expected peaks from the parent twist conformation were observed to have greater intensity, occurring at chemical shifts of 743.6 and 448.2 ppm. The chair conformation shows peaks at 899.5, 383.2, and 220.7 ppm, respectively. The peaks from the newly observed boat conformation have the lowest-relative intensity and are observed to have chemical shifts at 383.2, 355.8, and 307.7 ppm, respectively. Mass Spectroscopy. Mass spectroscopic measurements of the [P2Se8]2− anion and 1/∞[PSe6−] anionic chain were performed using MALDI-TOF, in order to prevent the further decomposition of the anion possible for ESI measurements. The MS data of the [P2Se8]2− anion (Figure S9a) shows the presence of the parent anion with m/z centered at 690 Da, confirming the integrity of the parent anion in solution. The corresponding MS data from the solution of the 1/∞[PSe6−] anionic chain showed the presence of features similar to those observed for the [P2Se8]2− anion (Figure S9b). Such studies give further evidence of the interconversion of the 1/∞[PSe6−] anionic chain into the discrete units of the [P2Se8]2− anion in solution. Thermal Properties and Crystallization Progression of Cs2P2Se8. The thermal behavior of Cs2P2Se8 was first investigated using differential thermal analysis (DTA) (Figure 7). This experiment revealed that there is one endotherm at 374 °C on heating, which corresponds to congruent melting of the title compound. Then on cooling, there were two exotherms, a major peak at 317 °C and a minor one at 282 °C. These results stay consistent over multiple DTA cycles. Discovering two exotherms on cooling was a surprise, and to determine if the sample contained additional phases or a phase transition, we conducted in situ high-resolution synchrotron

the morphology. While for crystalline samples, NMR solutions required overnight preparation; glassy phase samples were observed to be readily soluble. The solution NMR spectra of the crystalline and glassy samples were observed to be similar and will be discussed as one. Out of the three possible conformations of the [P2Se8]2− anion expected to exist in solution, viz. the parent twist conformation as well as the rearranged chair and boat (Figure 6b), literature reports so far have shown the observation of only

Figure 6. (a) Temperature-dependent 31P NMR of [P2Se8]2− anion in solution showing the evolution of the different conformers at different temperatures. (b) Scheme showing the conversion of the different conformers of the [P2Se8]2− anion in solution.

two of such conformations, respectively the twist and the chair.32 From our measurements performed with Cs2P2Se8 in DMF, however, we can confirm the observation of all possible conformers at sufficiently low temperatures. Temperaturedependent 31P NMR measurements show the slow evolution of one conformation from another (Figure 6a). As expected from the solid-state structure, the twist conformation was observed to exist abundantly at room temperature, having a chemical shift at −5.4 ppm, apart from small amounts of the chair conformation observed at −89.6 ppm,32 with a twist:chair relative abundance ratio of 4.5:1. Upon a gradual decrease in the temperature from room temperature, a subsequent increase in the relative abundance ratio of the twist vs chair conformation was observed, such that at −50 °C the value of the relative abundance was observed to increase to 7:1 with respect to twist. Appearance of the boat conformation, not observed previously,32 having a chemical shift of 9.5 ppm, was detected to start from a temperature of −30 °C, with abundance increasing with lowering of the temperature (Figure 6a). This growth in the amount of boat conformation with decreasing temperature was observed to be unabated compared to increasing relative amounts of the twist conformation with respect to the chair conformation. Since the relative abundance of the twist and chair conformers continued to increase with decreasing temperature, even after the appearance of the boat conformer, we can infer that the boat conformer forms exclusively from the chair conformer and not from the twist (Figure 6a). This inference is F

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expect that they have similar third-order NLO properties. As a representative, therefore, the THG properties of amorphous and crystalline Cs2P2Se8 were investigated in order to better understand their potential for NLO applications. The fundamental wavelength ranged from 2300−3100 nm at increments of 200 nm such that the corresponding THG photon energy was well below the band gaps of the compounds. The absolute NLO efficiencies of the samples were directly compared with the commercially available mid-IR NLO reference material, AgGaSe2, obtained by grinding an optical-quality single crystal provided by Gooch and Housego. First, the type-I phase-matchability (PM) of the materials was examined by comparing the THG counts to particle size.25,30,50,51 Figure 10a shows THG particle size dependence for crystalline Cs2P2Se8, amorphous Cs2P2Se8, and AgGaSe2 at λ = 2900 nm, all of which indicate non-PM THG with the corresponding coherence length, = lc ∼ 20−45 μm. This result was expected due to a large refractive index mismatch between the fundamental and THG beams. We found that the THG coherence lengths for the Cs2P2Se8 samples slightly increase at 3100 nm (Figure S10), while that for AgGaSe2 remains the same as shown in Figure 10a. The wavelength-dependent THG response from crystalline and amorphous Cs2P2Se8, respectively, with d = 20−45 μm plotted on a semilog scale is shown in Figure 10b. Wavelengthdependent THG counts from the reference of the same particle size are also plotted for direct comparison. The THG responses from crystalline and amorphous Cs2P2Se8 are significantly larger as compared to that from AgGaSe2. We estimated the THG coefficients of amorphous and crystalline Cs2P2Se8 at λ = 2900 nm, where both the sample and the reference are non-PM with minimal multiphoton absorption effects and have the same coherence lengths. With 52 5 2 2 (3) χ(3) values ref = 1.6 × 10 pm /V of AgGaSe2, we estimated χ of amorphous and crystalline Cs2P2Se8 using

Figure 7. DTA of crystalline Cs2P2Se8; first cycle in black and second cycle in red.

PXRD experiments at the Advanced Photon Source at Argonne National Laboratory. On heating crystalline Cs2P2Se8, in situ synchrotron PXRD agrees with the DTA and shows no activity until the material is completely melted at 395 °C. On cooling, unexpectedly, four short-lived unknown phases crystallize starting at 345 °C and then melt by 301 °C (Table 4 and Table 4. Different Phases Seen in the in Situ Synchrotron PXRD of Cs2P2Se8 with Their Temperature Range of Crystallization on Cooling from the Melt

a

phase

initial crystallization (°C)

final melting (°C)

red stara green stara blue stara brown stara Cs2P2Se8

345 333 345 333 331

313 301 331 327 N/A

Unknown phases.

χS(3) = χR(3)

Figure 8). No unknown phase is present for more than 32 °C of temperature change. Cs2P2Se8 starts to crystallize at 331 °C and remains present through cooling to room temperature. There were not enough peaks to index the unit cells of any of the unknowns. We attempted to synthesize them ex situ in the laboratory by heating to 450 °C to form a melt; then on cooling, we water quenched immediately or annealed the reactions up to 1 week at temperatures ranging from 275 to 350 °C. All experiments resulted in crystalline Cs2P2Se8 when quenching at ≤300 °C and amorphous Cs2P2Se8 for >300 °C. This leads us to believe that these unknown phases are not stable at room temperature. To further investigate the influence of the unknown phases on the crystallization of Cs2P2Se8 we determined its in situ synchrotron PXRD peak intensity versus temperature for hkl = 191 and 260 on cooling, which is depicted in Figure 9. The diffraction data demonstrates that the Cs2P2Se8 phase crystallizes from 331−295 °C. There is a large jump in Bragg peak intensity (i.e., amount of crystallization) from 315−313 °C. This temperature corresponds to the final melting of the unknown red star phase (Table 4). It is likely that the Cs2P2Se8 and red star phases have similar composition and potentially structural units, and once the red star phase melts there is a larger amount of melt that can crystallize into Cs2P2Se8. NLO THG Measurements. Since the title compounds have almost identical band gaps within the same crystal structure, we

IS(3ω) IR(3ω)

1/2 ref lc lcs

(1)

where IS(3ω) and IR(3ω) are the THG counts from the samples and the reference, respectively. Based on eq 1, our calculation yields χ(3) for amorphous and crystalline Cs2P2Se8 of ∼1.8 ± 0.2 × 105 pm2/V2 and 2.4 ± 0.1 × 105 pm2/V2, respectively. In general, NLO coefficients (i.e., χ(2) and χ(3)) scale inversely with a power law in the band gap.53,54 Therefore, it is rather interesting that the larger band gap Cs2P2Se8 materials possess larger THG coefficients compared with that of AgGaSe2.52 This may indicate that the new materials have a larger dipole matrix element as well as an enhanced joint density of states for the optical transition.54,55 A slightly higher THG coefficient of crystalline Cs2P2Se8 as compared to glassy is likely due to a coherent NLO effect originating from crystallinity, which is consistent with our previous studies.25,30 As evidenced from the obtained results, both crystalline and glassy Cs2P2Se8 have stronger third-order NLO coefficients than the benchmark NLO material of AgGaSe2. Glassy As2S3, due to its high refractive index and nonlinearity (χ(3) of 5.6 × 105 pm2/V2),56 is the most studied chalcogenide for third-order optical nonlinearity, which is of the same order of magnitude as crystalline and glassy Cs2P2Se8. Third-order nonlinear studies of chalcogenide glasses are widely known in the literature, and reports of third-order nonlinear susceptibilities in chalcogenide G

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Figure 8. In situ synchrotron PXRD pattern of Cs2P2Se8 on cooling emphasizing the four unknown short-lived phases in Table 4: (a) green star phase peaks marked; (b) red star phase peaks marked; (c) blue star phase peaks marked; (d) brown star phase peaks marked.

Figure 9. Intensity vs temperature from in situ synchrotron PXRD of Cs2P2Se8 on cooling for hkl peaks 191 (black) and 260 (red).

glasses, such as Ge-Ga-Sb-S, 57 GeS 2 -Ga 2 S 3-CdS, 58 and GeS2In2S3,59 have yielded χ(3) values of the same or lesser magnitude than Cs2P2Se8.

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CONCLUSION The reversible phase-change materials ACsP2Se8 have been synthesized by direct combination reactions. This family of materials contains the cyclic molecular anion [P2Se8]2−, which prefers to crystallize with large cations. The sizable difference in the crystalline and amorphous band gaps of these compounds makes them attractive candidates for investigations of their switching properties and optical storage applications. The unique thermal behavior of Cs2P2Se8 was analyzed by in situ high-resolution synchrotron PXRD, where four high-temperature short-lived species were discovered. NMR studies in solid-

Figure 10. THG measurements of crystalline Cs2P2Se8 (blue), glassy Cs2P2Se8 (green), and reference AgGaSe2 (red): (a) THG particle size dependence showing non-type-I phase-matching with a fundamental wavelength of 2900 nm; (b) broadband THG showing relative efficiencies.

state confirm the presence of two independent [P2Se8]2− anions in the asymmetric unit. Temperature-dependent solution 31 P NMR shows the progression of several H

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Chemistry of Materials conformations of the [P2Se8]2− anion, including the interesting observation of the boat conformation for the first time. Further NMR and MS studies of the 1/∞[PSe6−] anionic chain shows its conversion to the discrete units of the [P2Se8]2− anions in solution. Even with their higher band gaps, both glassy and crystalline Cs2P2Se8 have stronger THG response than AgGaSe2, which was used as the reference material. Therefore, these materials could be potentially important for future NLO applications.

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(3) Chung, I.; Do, J.; Canlas, C. G.; Weliky, D. P.; Kanatzidis, M. G. APSe6 (A = K, Rb, and Cs): polymeric selenophosphates with reversible phase-change properties. Inorg. Chem. 2004, 43, 2762−2764. (4) Chung, I.; Malliakas, C. D.; Jang, J. I.; Canlas, C. G.; Weliky, D. P.; Kanatzidis, M. G. Helical polymer [P2Se62−]: strong second harmonic generation response and phase-change properties of its K and Rb salts. J. Am. Chem. Soc. 2007, 129, 14996−15006. (5) Garin, J.; Parthe, E. The crystal structure of Cu3PSe4 and other ternary normal tetrahedral structure compounds with composition 13564. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28, 3672−3674. (6) Dickerson, C. A.; Fisher, M. J.; Sykora, R. E.; Albrecht-Schmitt, T. E.; Cody, J. A. Solvothermal Synthesis and Structure of a New Selenium-Rich Selenophosphate K3PSe4·2Se6. Inorg. Chem. 2002, 41, 640−642. (7) Knaust, J.; Dorhout, P. Synthesis and structures of Na4P2Se6, Cs3PSe4, and Rb4P2Se9. J. Chem. Crystallogr. 2006, 36, 217−223. (8) Chondroudis, K.; Kanatzidis, M. G. [P8Se18]6‑: A New Oligomeric Selenophosphate Anion with P4+ and P3+ Centers and Pyramidal [PSe3] Fragments. Inorg. Chem. 1998, 37, 2582−2584. (9) Gave, M. A.; Canlas, C. G.; Chung, I.; Iyer, R. G.; Kanatzidis, M. G.; Weliky, D. P. Cs4P2Se10: A new compound discovered with the application of solid-state and high temperature NMR. J. Solid State Chem. 2007, 180, 2877−2884. (10) Francisco, R. H. P.; Tepe, T.; Eckert, H. A Study of the System Li-P-Se. J. Solid State Chem. 1993, 107, 452−459. (11) Chung, I.; Jang, J. I.; Gave, M. A.; Weliky, D. P.; Kanatzidis, M. G. Low valent phosphorous in the molecular anions [P5Se12]5− and β[P6Se12]4−: phase change behavior and near infrared second harmonic generation. Chem. Commun. 2007, 4998−5000. (12) Chung, I.; Karst, A. L.; Weliky, D. P.; Kanatzidis, M. G. [P6Se12]4‑: A Phosphorus-Rich Selenophosphate with Low-Valent P Centers. Inorg. Chem. 2006, 45, 2785−2787. (13) Chung, I.; Holmes, D.; Weliky, D. P.; Kanatzidis, M. G. [P3Se7](3‑): a phosphorus-rich square-ring selenophosphate. Inorg. Chem. 2010, 49, 3092−3094. (14) Breshears, J. D.; Kanatzidis, M. G. β-KMP2Se6 (M = Sb, Bi): Kinetically Accessible Phases Obtained from Rapid Crystallization of Amorphous Precursors. J. Am. Chem. Soc. 2000, 122, 7839−7840. (15) Morris, C. D.; Chung, I.; Park, S.; Harrison, C. M.; Clark, D. J.; Jang, J. I.; Kanatzidis, M. G. Molecular germanium selenophosphate salts: phase-change properties and strong second harmonic generation. J. Am. Chem. Soc. 2012, 134, 20733−20744. (16) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Bulk Characterization methods for non-centrosymmetric materials: second-harmonic generation, piezoelectricity, pyroelectricity, and ferroelectricity. Chem. Soc. Rev. 2006, 35, 710−717. (17) Carpentier, C. D.; Nitsche, R. Ferroelectricity in Sn2P2S6. Mater. Res. Bull. 1974, 9, 1097−1100. (18) Vysochanskii, Y. Ferroelectricity in complex chalcogenides M′M″P2X6 (M′, M″ - Sn, Pb, Cu, In, Cr; X - S, Se). Ferroelectrics 1998, 218, 275−282. (19) Chung, I.; Song, J.-H.; Kim, M. G.; Malliakas, C. D.; Karst, A. L.; Freeman, A. J.; Weliky, D. P.; Kanatzidis, M. G. The Tellurophosphate K4P8Te4: Phase-Change Properties, Exfoliation, Photoluminescence in Solution and Nanospheres. J. Am. Chem. Soc. 2009, 131, 16303− 16312. (20) Banerjee, S.; Szarko, J. M.; Yuhas, B. D.; Malliakas, C. D.; Chen, L. X.; Kanatzidis, M. G. Room temperature light emission from the low-dimensional semiconductors AZrPSe6 (A = K, Rb, Cs). J. Am. Chem. Soc. 2010, 132, 5348−5350. (21) Banerjee, S.; Malliakas, C. D.; Jang, J. I.; Ketterson, J. B.; Kanatzidis, M. G. [ZrPSe6−]: a soluble photoluminescent inorganic polymer and strong second harmonic generation. J. Am. Chem. Soc. 2008, 130, 12270−12272. (22) Chung, I.; Kanatzidis, M. G. Metal Chalcogenides: A Rich Source of Nonlinear Optical Materials. Chem. Mater. 2014, 26, 849− 869.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00551. Atomic coordinates and displacement parameters of Cs2P2Se8 and KCsP2Se8; PXRD spectra, SEM images, and EDS spectra of KCsP2Se8, RbCsP2Se8, and Cs2P2Se8; further characterization of NMR, mass spectroscopy, in situ PXRD, and THG (PDF) X-ray crystallographic data of Cs2P2Se8 and KCsP2Se8 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Grant DMR-1410169. Part of this work utilized the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. SEM experiments were conducted in the EPIC facility (NUANCE CenterNorthwestern University), supported by the MRSEC program at the Materials Research Center with the National Science Foundation Grant DMR-1121262, the International Institute for Nanotechnology (IIN), and the State of Illinois through the IIN. Raman, NMR, and MS measurements were performed in the IMSERC facility at Northwestern University and supported through the university. We thank Gooch and Housego for supplying the AgGaSe2 crystal. A.S.H. gratefully acknowledges support through a Graduate Research Fellowship by the National Science Foundation under Grant No. DGE-1324585. A.S.H. also appreciates the mentorship from Drs. Greg Halder and Daniel Shoemaker related to the synchrotron in situ PXRD measurements, Drs. Yongbo Zhang and Yuyang Wu for solution and solid-state NMR experiments, and Saman Shafaie for mass spectroscopy studies.

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REFERENCES

(1) Kanatzidis, M. G. New directions in synthetic solid state chemistry: chalcophosphate salt fluxes for discovery of new multinary solids. Curr. Opin. Solid State Mater. Sci. 1997, 2, 139−149. (2) Chondroudis, K.; Kanatzidis, M. G. 1/∞[P3Se4−]: A Novel Polyanion in K3RuP5Se10; Formation of Ru-P Bonds in a Molten Polyselenophosphate Flux. Angew. Chem., Int. Ed. Engl. 1997, 36, 1324−1326. I

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Article

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(42) Jang, J. I.; Park, S.; Clark, D. J.; Saouma, F. O.; Lombardo, D.; Harrison, C. M.; Shim, B. Impact of two-photon absorption on second-harmonic generation in CdTe as probed by wavelengthdependent Z-scan nonlinear spectroscopy. J. Opt. Soc. Am. B 2013, 30, 2292−2295. (43) Kolobov, A. V.; Tominaga, J. Properties of Amorphous Chalcogenides. In Chalcogenides; Springer Series in Materials Science: 2012; Vol. 164, pp 35−47. (44) Tanaka, K. The Charged Defect Exists? J. Optoelectron. Adv. Mater. 2001, 3, 189−198. (45) Chung, I.; Jang, J. I.; Malliakas, C. D.; Ketterson, J. B.; Kanatzidis, M. G. Strongly nonlinear optical glass fibers from noncentrosymmetric phase-change chalcogenide materials. J. Am. Chem. Soc. 2010, 132, 384−389. (46) Choi, K.-Y.; Kanatzidis, M. G. New Uranium Chalcoanimonates, RbU2SbS8 and KU2SbSe8, with a Polar Noncentrosymmetric Structure. Chem. Mater. 1999, 11, 2613−2618. (47) Evenson, C. R., IV; Dorhout, P. K. Selenophosphate Phase Diagrams Developed in Conjunction with the Synthesis of the New Compounds K2La(P2Se6)1/2(PSe4), K3La(PSe4)2, K4La0.67(PSe4)2, K9‑xLa1+x/3(PSe4)4 (x = 0.5), and KEuPSe4. Inorg. Chem. 2001, 40, 2875−2883. (48) Canlas, C. G.; Kanatzidis, M. G.; Weliky, D. P. 31P Solid State NMR Studies of Metal Selenophosphates Containing [P2Se6]4‑, [P4Se10]4‑, [PSe4]3‑, [P2Se7]4‑, and [P2Se9]4‑ Ligands. Inorg. Chem. 2003, 42, 3399−3405. (49) Wade, L. G., Jr. Cyclohexane Conformations. In Organic Chemistry; Pearson: Upper Saddle River, NJ, 2012. (50) Jang, J. I.; Chung, I.; Kanatzidis, M. G.; Ketterson, J. B. New Developements in Photon and Materials Research; NOVA Scientific Publishers: 2013. (51) Kurtz, S. K.; Pery, T. T. A powder technique for the evaluation of nonlinear optical materials. J. Appl. Phys. 1968, 39, 3798−3813. (52) http://marzenell.de/Research/Z-Scan/z-scan.html (accessed Mar 1, 2016). (53) Jackson, A. G.; Ohmer, M. C.; LeClair, S. R. Relationship of the second order nonlinear optical coefficient to energy gap in inorganic non-centrosymmetric crystals. Infrared Phys. Technol. 1997, 38, 233− 244. (54) Song, J.-H.; Freeman, A. J.; Bera, T. K.; Chung, I.; Kanatzidis, M. G. First-principles prediction of an enhanced optical secondharmonic susceptibility of low-dimensional alkali-metal chalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 245203. (55) Boyd, R. W. Nonlinear Opt., 3rd ed.; Academic Press: San Diego, CA, 2008. (56) Kobayashi, H.; Kanbara, H.; Koga, M.; Kubodera, K. Thirdorder nonlinear optical properties of As2S3 chalcogenide glass. J. Appl. Phys. 1993, 74, 3683−3687. (57) Kohoutek, T.; Mizuno, S.; Suzuki, T.; Ohishi, Y.; Matsumoto, M.; Misumi, T. Third-harmonic generation measurement of nonlinear optical susceptibility χ(3) of Ge-Ga-Sb-S chalcogenide glasses proposed for highly nonlinear photonic fibers. J. Opt. Soc. Am. B 2011, 28, 298− 305. (58) Wang, X. F.; Wang, Z. W.; Yu, J. G.; Liu, C. L.; Zhao, X. J.; Gong, Q. H. Large and ultrafast third-order optical nonlinearity of GeS2−Ga2S3−CdS chalcogenide glass. Chem. Phys. Lett. 2004, 399, 230−233. (59) Dong, G.; Tao, H.; Chu, S.; Wang, S.; Zhao, X.; Gong, Q.; Xiao, X.; Lin, C. Study on the structure dependent ultrafast third-order optical nonlinearity of GeS2−In2S3 chalcogenide glasses. Opt. Commun. 2007, 270, 373−378.

(23) Chung, I.; Kim, M. G.; Jang, J. I.; He, J.; Ketterson, J. B.; Kanatzidis, M. G. Strongly nonlinear optical chalcogenide thin films of APSe6 (A = K, Rb) from spin-coating. Angew. Chem., Int. Ed. 2011, 50, 10867−10870. (24) Chung, I.; Song, J.-H.; Jang, J. I.; Freeman, A. J.; Ketterson, J. B.; Kanatzidis, M. G. Flexible Polar Nanowires of Cs5BiP4Se12 from Weak Interactions between Coordination Complexes: Strong Nonlinear Optical Second Harmonic Generation. J. Am. Chem. Soc. 2009, 131, 2647−2656. (25) Jang, J. I.; Haynes, A. S.; Saouma, F. O.; Otieno, C. O.; Kanatzidis, M. G. Broadband studies of the strong mid-infrared nonlinear optical responses of KPSe6. Opt. Mater. Express 2013, 3, 1302−1312. (26) Zakery, A.; Elliott, S. R. Optical Switching in Chalcogenide Glasses. In Optical Nonlinearities in Chalcogenide Glasses and Their Applications; Rhodes, W. T., Ed.; Springer: 2007; Vol. 135, pp 129− 150. (27) Fuentes-Hernandez, C.; Ramos-Ortiz, G.; Tseng, S.-Y.; Gaj, M. P.; Kippelen, B. Third-harmonic generation and its applications in optical image processing. J. Mater. Chem. 2009, 19, 7394−7401. (28) Tsang, T.; Krumbügel, M. A.; DeLong, K. W.; Fittinghoff, D. N.; Trebino, R. Frequency-resolved optical-gating measurements of ultrashort pulses using surface third-harmonic generation. Opt. Lett. 1996, 21, 1381−1383. (29) Barad, Y.; Eisenberg, H.; Horowitz, M.; Silberberg, Y. Nonlinear scanning laser microscopy by third harmonic generation. Appl. Phys. Lett. 1997, 70, 922−924. (30) Haynes, A. S.; Saouma, F. O.; Otieno, C. O.; Clark, D. J.; Shoemaker, D. P.; Jang, J. I.; Kanatzidis, M. G. Phase-Change Behavior and Nonlinear Optical Second and Third Harmonic Generation of the One-Dimensional K(1−x)CsxPSe6 and Metastable β-CsPSe6. Chem. Mater. 2015, 27, 1837−1846. (31) Jang, J. I.; Park, S.; Harrison, C. M.; Clark, D. J.; Morris, C. D.; Chung, I.; Kanatzidis, M. G. K4GeP4Se12: a case for phase-change nonlinear optical chalcogenide. Opt. Lett. 2013, 38, 1316−1318. (32) Rotter, C.; Schuster, M.; Kidik, M.; Schoen, O.; Klapoetke, T. M.; Karaghiosoff, K. Structural and NMR spectroscopic investigaions of chair and twist conformers of the P2Se8(2‑) anion. Inorg. Chem. 2008, 47, 1663−1673. (33) Zhao, J.; Pennington, W. T.; Kolis, J. W. Chemistry of phosphorus selenide anions: Synthesis and structure of P2Se82‑ and [Fe2(CO)4(PSe5)2]2‑. J. Chem. Soc., Chem. Commun. 1992, 265−266. (34) McCarthy, T. J.; Ngeyi, S. P.; Liao, J. H.; Degroot, D. C.; Hogan, T.; Kannewurf, C. R.; Kanatzidis, M. G. Molten salt synthesis and properties of three new solid-state ternary bismuth chalcogenides, βCsBiS2, γ-CsBiS2, and K2Bi8Sel3. Chem. Mater. 1993, 5, 331−340. (35) McCarthy, T. J.; Kanatzidis, M. G. Synthesis in molten alkali metal polyselenophosphate fluxes: A new family of transition metal selenophosphate compounds, A2MP2Se6 (A = K, Rb, Cs; M = Mn, Fe) and A2M′2P2Se6 (A = K, Cs; M′ = Cu, Ag). Inorg. Chem. 1995, 34, 1257−1267. (36) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (37) Chupas, P. J.; Chapman, K. W.; Kurtz, C.; Hanson, P. L.; Grey, C. P. A versatile sample-environment cell for non-ambient X-ray scatering experiments. J. Appl. Crystallogr. 2008, 41, 822−824. (38) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Häusermann, D. Two-dimensional detector software: From real detector to idealized image or two-theta scan. High Pressure Res. 1996, 14, 235−248. (39) Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (40) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); 2000. (41) Syrigos, J. C.; Clark, D. J.; Saouma, F. O.; Clarke, S. M.; Fang, L.; Jang, J. I.; Kanatzidis, M. G. Semiconducting Properties and PhaseMatching Nonlinear Optical Response of the One-Dimensional Selenophosphates ANb2PSe10 (A = K, Rb, and Cs). Chem. Mater. 2015, 27, 255−265. J

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