Phase-Change Behavior and Nonlinear Optical Second and Third

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Phase-change behavior and nonlinear optical second and third harmonic generation of the one-dimensional K CsPSe and metastable #-CsPSe (1-x)

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6

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Alyssa S Haynes, Felix O Saouma, Calford O Otieno, Daniel J Clark, Daniel P. Shoemaker, Joon I. Jang, and Mercouri G. Kanatzidis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00065 • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Chemistry of Materials

Phase-change behavior and nonlinear optical second and third harmonic generation of the one-dimensional K(1-x)CsxPSe6 and metastable β-CsPSe6 Alyssa S. Haynes1, Felix O. Saouma2, Calford O. Otieno2, Daniel J. Clark2, Daniel P. Shoemaker3,4, Joon I. Jang2 and Mercouri G. Kanatzidis1,3

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Department of Chemistry, Northwestern University, Evanston, Illinois, 60208

Department of Physics, Applied Physics, and Astronomy, Binghamton University, Binghamton,

New York, 13902 3

Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439

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Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801

Abstract The APSe6 (A = K, K(1-x)Csx, Cs) family of one-dimensional (1D) materials was studied to examine the effects of the cation size on the nonlinear optical (NLO) response. The family has high-performing NLO properties with the noncentrosymmetric parent material, KPSe6, having infinite 1D chains of 1/∞[PSe6–]. This structure has been successfully substituted with cesium up to K0.6Cs0.4PSe6 while retaining the polar character. All compounds crystallize in the space group Pca21 and have band gaps of 2.1 eV. In situ powder X-ray diffraction experiments using synchrotron radiation were used to determine the specifics of the amorphous to crystalline behavior and the crystallization and melting kinetics of the APSe6 system. These measurements

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revealed a new phase, β-CsPSe6, which is metastable and crystallizes in the noncentrosymmetric tetragonal space group P421c with a = 12.526(2) Å, c = 12.781(3) Å, V = 2005.3(6) Å3, and Z = 8. The structure is composed of 2 sets of mutually perpendicular 1/∞[PSe6–] chains chargebalanced by Cs cations, and the band gap of β-CsPSe6 is 1.9 eV. Second harmonic generation (SHG) measurements demonstrate that substitution of Cs into KPSe6 maintains the strong NLO signal with a very high SHG coefficient (χ(2)) of ~150 pm/V for K(1-x)CsxPSe6 and 30 pm/V for βCsPSe6. Laser-induced damage threshold analysis reveals APSe6 exhibits two-photon absorption (2PA) with input laser intensity greater than 1 GW/cm2 and optical damage from 2PA at ~2 GW/cm2. The materials also exhibit strong third harmonic generation (THG) with THG coefficients (χ(3) × 105) for KPSe6, K0.6Cs0.4PSe6, α-CsPSe6, and β-CsPSe6 to be 2.6, 3.1, 1.8, and 1.1 pm2/V2, respectively.


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Introduction Nonlinear optical (NLO) crystals are important because they can produce coherent light sources in difficult-to-reach frequency regions of the electromagnetic spectrum. For example, when an intense laser beam of frequency ω is directed onto a second-order NLO medium, some of the light will be converted into a frequency of 2ω via second harmonic generation (SHG).1 Second-order NLO finds application for generating infrared (IR) frequencies, which is the fingerprint region for both organic and inorganic molecules such as biohazards,2 chemical warfare agents,3 pollutants and trace gases.4 Also, generating a coherent beam in the IR is essential for minimally invasive medical surgery5 and telecommunications.6,7 Wide band gap chalcogenides are promising candidates for application in the IR because they are transparent in this region.8 Additionally in the mid-IR, amorphous chalcogenides can have high third-order nonlinearity, which can be characterized by third harmonic generation (THG), frequency tripling of ω. These materials can be utilized for numerous optoelectronic applications such as signal amplification and optical switching, and it is used in many modern NLO devices.9,10 For practicality, NLO materials should also possess phase-matchability for wave mixing applications, high thermal stability, and a large second- or third-order NLO coefficient (χ(2) or χ(3), respectively).11 A noncentrosymmetric structure is a prerequisite for a material to possess a nonzero χ(2) value, but this is not a requirement for third-order NLO processes.11 For both SHG and THG, materials containing highly polarizable atoms are favorable for high-intensity NLO response.12-15 Low-dimensional structures with strong covalent character have been shown to be beneficial for high-intensity SHG.16,17 Third-order NLO effects have been less investigated in chalcogenide systems, but because SHG and THG have similar origin it is likely that low dimensionality increases third-order nonlinearity as well. Substituting in larger counter-cations to


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separate the anionic chains in the structure can be a fine-tuned way to lower dimensionality and/or alter the Se…Se interchain interaction. This phenomena is known as the counter-cation effect and occurs due to steric consequences of the cation on the spacing of anionic frameworks.18 19 Recently discovered NLO chalcogenides include AAsQ2 (A = Li, Na; Q = S, Se),13,17 A3Ta2AsS11,20 Ln4GaSbS9 (Ln = Pr, Nd, Sm, Gd-Ho),21 Ba2BiInS5,22 Ba23Ga8Sb2S38,23 βK2Hg3Ge2S8,24 BaGa4Se7,25 LiGaGe2Se6,26 Li2In2GeSe6,27 BaGa2GeQ6 (Q = S, Se),28 Li2CdGeS4,29 Li2CdSnS4,30 Cs5BiP4Se12,31 CsZrPSe6,14 K2P2Se6,32 and APSe6 (A = K, Rb).33 KPSe6 and RbPSe6 feature 1/∞[PSe6–] chains along the a-axis with significant Se…Se interchain interactions, and they crystallize in the polar space group Pca21. α-CsPSe6 on the other hand is composed of the same polymeric chains as KPSe6, but the chains crystallize in a centrosymmetric space group, P2/n.34 All APSe6 (A = K, Rb, Cs) form stable amorphous structures which interconvert reversibly with their crystalline counterparts, and the K and Rb salts demonstrate strong SHG responses in both the crystalline and glassy forms. Theoretical calculations show crystalline KPSe6 and RbPSe6 possess χ(2) values of 151.3 and 149.4 pm V-1, respectively.16 In the near IR, glassy KPSe6 and RbPSe6 without poling exhibit SHG response comparable to AgGaSe2, a benchmark mid-IR NLO material.33,35 Herein we investigate how the Cs cation influences the structure and SHG performance as compared to KPSe6 by examining the K(1-x)CsxPSe6 series. We are motivated from previous combined theoretical and experimental research that has shown increasing the chain separation in 1D materials (i.e. weakening the interaction strength between the chains), which also modifies the associated electronic structures, and may enhance the overall SHG response.16 This idea can


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be thought of as lowering the dimensionality of the structure, however, the structures remain 1D and the most important factor is how the strength of the short interchain interactions is changed. In the 1D system, AAsS2 (A = Li, Li0.6Na0.4, Na), the sodium analogue is centrosymmetric whereas the lithium and Li0.6Na0.4 analogues are noncentrosymmetric. It was found that LiAsS2 and Li0.6Na0.4AsS2 had SHG responses 10× and 30× of AgGaSe2, respectively.13 The additional 300% increase in the SHG response of Li0.6Na0.4AsS2 is attributed to its weaker interchain interaction as compared to LiAsS2 arising from the increased chain separation due to the countercation effect.16,17 The goal of this work was to determine if the same SHG altering effect can occur with a different noncentrosymmetric 1D chalcogenide system. In our investigation of K(1-x)CsxPSe6 we were interested in studying how the partial substitution of Cs for K affects the structure and NLO response of the material. Interestingly, we find that the interchain interactions are sufficiently strong, so that the incorporation of Cs+ ions does not have a significant effect in weakening them by further separating the 1/∞[PSe6–] chains. Therefore, we found that the impact of Cs substitution is rather insignificant across the compounds although all of them exhibit strong NLO responses. Thermal behavior, characterized through in situ synchrotron powder X-ray diffraction experiments of APSe6 (A = K, K0.6Cs0.4, Cs), uncovered a new phase of the Cs analogue (βCsPSe6), which is metastable and crystallizes as a new structure type. This phase is elusive and difficult to detect by conventional synthetic studies and could clearly be detected only through the in situ synchrotron experiments. The SHG and THG performance and laser-induced damage threshold (LIDT) of the APSe6 materials are described in detail. THG characterization of inorganic materials is scarce in the literature, and we present here one of the few THG reports of crystalline chalcogenide compounds.


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Experimental and Physical Measurements The experimental section detailing the synthesis of all materials described in this work and the physical measurements section are included in the supporting information.

Results and Discussion Synthesis of KPSe6, K(1-x)CsxPSe6, and α-CsPSe6. The synthesis of crystalline KPSe6, K(1-x)CsxPSe6, and α-CsPSe6 was achieved via direct combination by mixing stoichiometric amounts of K2Se, Cs2Se2, P, and Se and slow heating to 250 ºC or 280 ºC to give an orange, crystalline product. Slow heating was utilized because all compounds uniquely prefer to form on heating. The amorphous forms of the compounds were created by water quenching the same mixtures from 800 ºC, unless otherwise noted. Difficulties in synthesizing highly crystalline product were encountered because this system of materials favors the glassy state. For this reason, we crystallized these compounds on heating the amorphous phase to provide enough energy for crystallization. Using longer annealing times than those described in the synthesis section did not increase crystallization. On the other hand, if the experimental samples were not heated long enough, they did not have time to fully crystallize and had a significantly large glassy fraction. Therefore, much effort was put into controlling the crystallization process and determining an intermediate heating time for optimal crystallinity. Motivation and Structure of K(1-x)CsxPSe6. The motivation for this work is to analyze how small changes in the cation influence the structure and therefore properties of APSe6 (A = K, Rb, Cs). Cesium is larger and more polarizable than potassium and rubidium, and α-CsPSe6 has the greatest interchain separation of the three APSe6 compounds (i.e. weakest Se…Se interchain


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strength), which may yield the highest second-order NLO activity if it were noncentrosymmetric. Here, we investigate the substitution limit of cesium into KPSe6 while retaining the noncentrosymmetric unit cell. The Se…Se interchain distance in the 1/∞[PSe6–] chains will potentially increase with increasing size of the alkali metal cation. The larger the Se…Se interchain separation, the lower the interchain interaction, which may act to modulate the NLO efficiency based on previous theoretical calculations on 1D materials.16 K(1-x)CsxPSe6 samples with x = 0.1 – 0.4 increasing by 0.1 increments reveal through powder X-ray diffraction (PXRD) analysis that the noncentrosymmetric structure of KPSe634 is retained (Figure 1). The PXRD peaks are shifted to lower 2θ angles with increasing x value, demonstrating a larger unit cell consistent with the incorporation of Cs. Energy-dispersive X-ray spectroscopy (EDS) reveals that both K and Cs are present in the approximate desired stoichiometric ratios for K(1-x)CsxPSe6 (x = 0.1 – 0.4). PXRD measurements of K(1-x)CsxPSe6 glasses show no crystalline peaks within the detection limit. The single crystal refinement of K0.6Cs0.4PSe6 supports the PXRD and shows the 1/∞[PSe6–] chains are charge-balanced by a solid solution of K+ and Cs+ cations. For the synthesis of K(1-x)CsxPSe6 with x > 0.4, the trend in increasing unit cell size does not continue. For x = 0.5 – 0.6, K0.6Cs0.4PSe6 and the α-CsPSe6 phase most likely consisting of both K+ and Cs+ ions co-crystallized. For x = 0.7 – 0.9 only α-CsPSe6 with mixed K and Cs cations formed. These results show that the stability limit of the noncentrosymmetric form of K(1x)CsxPSe6

is with x ~ 0.4. After this point, the average cation size is too large to stabilize the

noncentrosymmetric arrangement of the 1/∞[PSe6–] chains, and the centrosymmetric chain packing conformation becomes more stable.


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The structural parameters of K(1-x)CsxPSe6 (x = 0.0 – 0.4) were examined further by obtaining high-resolution synchrotron PXRD from 11-BM at the APS at Argonne National Laboratory and fitting the data through Rietveld refinement in GSAS (Supporting Information Figure S1).36,37 Following Végard’s law, the lattice parameters of the unit cells increase linearly with increasing x (Figure 2a).38 These results show that the unit cell is enlarging. To examine the interchain interaction strength, however, we must analyze the shortest interchain contacts in the K(1-x)CsxPSe6 (x = 0.0 – 0.4) series. KPSe6 can be thought of as the starting point with its pseudo-1D character and interchain interactions between Se3 of one chain with Se2 and Se4 of the next chain, which are both less than the van der Waals radii sum of 3.80 Å (Figure 1b). Although these are van der Waals contacts, in selenide systems they tend to be strong and can give rise to band broadening. If the chains are pried apart by the larger Cs+ ions, this is expected to show in the Se…Se distances and the optical band gaps.17,32,34,39,40 Interestingly, with increasing Cs fraction, the Se3…Se4 distance, which is the shortest interchain separation, does not show a clear trend with increasing x (Figure 2b and Table 1). The same is true of the other short van der Waals interchain distance, Se2…Se3. However, by averaging these two interchain contacts there is almost no difference in overall chain separation. These observations suggest that the interchain Se…Se contacts are strong enough that the Cs substitution is not sufficient to weaken them; instead of decreasing the Se…Se contacts with increasing Cs substitution, the chains torque in the presence of the larger cation to keep the same interchain separation. When the fraction of Cs entering the structure is large enough to destabilize the Se…Se contacts, the entire 1/∞[PSe6–] chains change their packing to a centrosymmetric conformation, as is the case of α-CsPSe6.


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Thermal Properties and Crystallization Progression of APSe6 (A = K, K0.6Cs0.4, Cs). Differential thermal analysis (DTA) was utilized to examine the thermal properties of K0.6Cs0.4PSe6 and compare them to KPSe6 (Figure 3a). Both materials crystallize and melt at very similar temperatures. Amorphous K0.6Cs0.4PSe6 and KPSe6 exhibit a broad exothermic peak on heating corresponding to crystallization at 250 ºC and 261 ºC, respectively, and a subsequent endothermic peak corresponding to melting at 286 ºC and 300 ºC, respectively. There are no thermal events on cooling the melt, and PXRD analysis of the DTA product reveals a glass. DTA of amorphous CsPSe6 was also conducted to examine any thermal differences with the noncentrosymmetric and centrosymmetric structures and revealed that it too crystallizes only on heating at 197 ºC and melts at 317 ºC (Figure 3b). Therefore, a slow heating technique is utilized to form crystalline K0.6Cs0.4PSe6, KPSe6, and CsPSe6 since these materials prefer to crystallize on heating. To investigate the crystallization, melting, and amorphous nature of this unique system in greater detail, in situ synchrotron X-ray diffraction experiments were conducted at the Advanced Photon Source at Argonne National Laboratory. These experiments clearly show that on heating both amorphous KPSe6 and K0.6Cs0.4PSe6, first elemental selenium and then the desired compound crystallize (Figure 4a and b). By carefully analyzing the intensity variation of the KPSe6 or K0.6Cs0.4PSe6 Bragg peaks as the temperature increases, the exact temperature range of crystallization (161 – 199 ºC and 169 – 197 ºC, respectively) and melting (293 – 307 ºC and 279 – 309 ºC, respectively) were determined. The formation of the selenium phase is surprising. The selenium disappears at 221 ºC, and this does not affect the intensity of the KPSe6 or K0.6Cs0.4PSe6 peaks. All of the crystalline KPSe6 or K0.6Cs0.4PSe6 congruently melts by 307 ºC or 309 ºC, respectively, on heating and does not recrystallize on cooling. The temperatures for


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crystallization and melting of KPSe6 and K0.6Cs0.4PSe6 are slightly shifted as compared to the DTA results due to the temperature-rate difference of the two experiments. The in situ PXRD experiment for CsPSe6, however, tells a different story. On heating, αCsPSe6 initially does not crystallize, but rather a previously unknown phase of CsPSe6 (we call it β-phase) crystallizes beginning at 167 ºC followed by a phase transition from this compound into α-CsPSe6 from 241 – 265 ºC (Figure 4c). This phase transition is unexpected because this thermal event was not observed in the DTA experiments. α-CsPSe6 melts congruently by 331 ºC. This synchrotron experiment revealed the existence of a new phase, which prompted the exploration of isolating β-CsPSe6 for structural analysis. This also demonstrates the importance of in situ crystallization experiments to describe the full story of the crystallization kinetics of this material.41 Additionally, differential scanning calorimetry (DSC) was employed to determine if the CsPSe6 phase transition was exothermic or endothermic because DTA did not provide helpful results (Figure 4d). The DSC curve shows that β-CsPSe6 exothermically crystallizes peaking at 171 ºC. At 221 ºC, there is a minor endothermic event corresponding to the melting of a slight second phase of selenium below the detection limit of the PXRD. At 273 ºC, there is another minor endothermic event, which can be attributed to the phase transition from β-CsPSe6 into αCsPSe6. At 316 ºC, there is a major endothermic event, which corresponds to melting of αCsPSe6. The DSC results match well to the in situ PXRD data of CsPSe6. Synthesis of Metastable β-CsPSe6. Using the in situ synchrotron PXRD results as a guide, a new heating profile was established to isolate β-CsPSe6 in bulk at room temperature in order to determine its crystal structure and other physical properties. By slow heating to 220 ºC rather than 250 ºC and using shorter heating and annealing times, β-CsPSe6 was successfully


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isolated at room temperature. The synthesis of β-CsPSe6 proved very challenging because if heated above 220 ºC or annealed at this temperature for longer than two hours, it would transform into the more stable α-phase. Though ultimately successful, this caused significant difficulty in synthesizing high-quality, strongly diffracting single crystals for structure refinement. Structure of β-CsPSe6. β-CsPSe6 adopts the noncentrosymmetric tetragonal space group P421c and a new structural arrangement. Similar to all other known compounds in the APSe6 family, β-CsPSe6 is composed of linked PSe4 tetrahedra with diselenide bridges, creating the same infinite 1/∞[PSe6–] chains (Figure 5a). The structure has the same chain conformation as in centrosymmetric α-CsPSe6, however, the 4 rotoinversion symmetry axis of the space group makes up 2 sets of mutually perpendicular 1/∞[PSe6–] chains yielding noncentrosymmetric chain packing and twice the size of the asymmetric unit per unit cell as compared to α-CsPSe6. Half of the 1/∞[PSe6–] chains are parallel to the b-axis (Figure 5b) and the other half are along the symmetry equivalent a-axis. The overall structure is analogous to a waffle fry (Figure 6). The PSe4 tetrahedra are distorted with Se-P-Se angles ranging from 101.489(346) – 126.947(431) º. The P-Se and Se-Se distances are normal and range from 2.131(9) – 2.294(9) Å and 2.349(4) – 2.386(5) Å, respectively. The volume of β-CsPSe6 is expected to be twice that of α-CsPSe6 due to the fact that there are twice as many asymmetric units per unit cell resulting from the perpendicular chain packing in the β-phase. The volumes of α-CsPSe6 and β-CsPSe6 are 981.8(6) Å3 and 2005.3(6) Å3, respectively. The volume of β-CsPSe6 is slightly greater than twice the volume of α-CsPSe6, attributing to its lower density of 4.224 g/cm3 as compared to 4.236 g/cm3 for α-CsPSe6. Generally, the most stable form of a compound has the highest density, which points to α-CsPSe6


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being more stable than β-CsPSe6. The metastability of β-CsPSe6 can be realized by examining the Se…Se interchain distances within the sum of the van der Waals radii between the α- and βphases. Between parallel chains in β-CsPSe6, there is one short contact of 3.221(5) Å, and between perpendicular chains there are two longer separations of 3.570(5) Å and 3.695(5) Å. The weaker interchain interactions between the mutually perpendicular chains are easiest to disrupt to transform into the α-phase where all van der Waals interchain Se…Se distances range from 3.160(2) – 3.280(7) Å, which stabilizes the structure. It is interesting to contemplate how the perpendicular chains of the β-form change to become parallel in the α-form. Given the facile transformation and relative low temperature we suggest that the most plausible mechanism is via temporal Se-Se bond breaking, cleaving the old chain and reforming them in parallel fashion rather than intact infinite chains actually rotating by 90 º in the solid state. Optical Absorption. The experimental band gaps of K(1-x)CsxPSe6 with x = 0.1-0.4 and KPSe6 are all 2.1 eV, which agrees well with their orange color (Figure 7). The invariability in the band gap as a function of x additionally supports that the interchain interaction strength of these materials does not change with increased cation size. If the interchain interaction strength was lowered, the band gap should blue-shift due to the increased Se…Se interchain separation.13,17 The band gaps of α- and β-CsPSe6 are 2.1 and 1.9 eV, respectively (Figure 7). Glassy KPSe6, K(1-x)CsxPSe6 (x = 0.1-0.4), and CsPSe6 all have band gaps of 2.0 eV, which is consistent with their deep red color. Red-shifted amorphous band gaps are typical due to the fact that irregularities in the overall solid-state structure of the glass as compared to the crystalline phase causes defects and midgap states below and above the conduction and valence band, respectively, (Urbach tailing) and therefore a lower band gap.42,43


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NLO Measurements SHG Phase-Matchability of Crystalline KPSe6 and K0.6Cs0.4PSe6. Prior to observing broadband NLO properties of the samples, the phase-matchability of KPSe6 and K0.6Cs0.4PSe6 was checked at both λ = 1800 nm and 3300 nm as fundamental beam. We assume that if KPSe6 and K0.6Cs0.4PSe6 demonstrate the same phase-matching behavior, then this is also the case for K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3). We found that all samples are non-phase-matchable within our experimental range. Previous reports of NLO measurements on crystalline KPSe6 showed phasematchable behavior, which disagrees with these results.33 Because phase-matchability can be affected by defects and is therefore sensitive to sample quality, new syntheses were conducted with additional annealing steps of 16 and 40 hours for both KPSe6 (KPSe6 A and KPSe6 B, respectively) and K0.6Cs0.4PSe6 (K0.6Cs0.4PSe6 A and K0.6Cs0.4PSe6 B, respectively) to see if this would remove defects and give the expected phase-matchable behavior (Figure 8a). The extra-annealed samples showed slightly higher amounts of crystallinity through PXRD, however, we found them to be non-phase-matchable. Therefore, for all further characterizations, the samples are compared to AgGaSe2, a benchmark IR chalcogenide that is non-phase-matchable up to λ = 3.0 µm.44 In future studies, thin films of KPSe6 will be examined for advanced NLO characterization based on the Maker fringe method.11,45,46 The phase-matching behavior then should be more clearly observed in the film form. If the compounds are not truly phasematchable, quasi-phase-matching via poling can be utilized for more efficient NLO response.47-49 Broadband SHG of Crystalline KPSe6, K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4), α- and βCsPSe6. Broadband SHG measurements of the samples were taken from λSHG = λ/2 = 550–1550 nm at particle size (d) = 32 – 45 µm for direct comparison. Figure 8b displays the semi-


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logarithmic (semi-log) plots of wavelength-dependent SHG counts from crystalline KPSe6, K(1x)CsxPSe6

(x = 0.1, 0.2, 0.3, 0.4), and AgGaSe2. The KPSe6 and K(1-x)CsxPSe6 materials did not

undergo additional annealing. It is clear from Figure 8b that KPSe6 and K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4) outperform AgGaSe2 for the entire examined wavelength range, and K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4) all have similar performance to KPSe6. In order to estimate the SHG coefficients and determine if there is any SHG enhancement with increasing Cs substitution, the relative SHG photon counts were compared at 1800 nm rather than the static limit (plateau region). This is because the samples are non-phase-matchable in the static limit but AgGaSe2 is. The χ(2) values of the samples were calculated for the nonphase-matching case:

/

= /

,

≅ /

(1)

where and are the SHG counts from the sample and reference at λ = 1800 nm,

respectively, and

are the SHG coherence lengths of the sample and reference, respectively,

and = 66 pm/V.50 In Eq. (1), we simply assumed that ~ , since the SHG coherence lengths were smaller than the minimum particle size achievable (~ 20 µm) by sieving. Experimental χ(2) values are listed in Table 2 below. The determined χ(2) values of all materials demonstrate that the high SHG efficiency is stable with Cs-substitution of KPSe6. This is consistent with the unchanged interchain interaction strength of the material upon Cs incorporation into the structure. We also wanted to compare the SHG response of KPSe6 and K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4) with the pure cesium counterparts. Due to the fact that there is no significant difference between the SHG efficiency of K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4), SHG measurements of KPSe6 A, KPSe6 B, K0.6Cs0.4PSe6 A, K0.6Cs0.4PSe6 B, α- and β-CsPSe6 and AgGaSe2 were


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evaluated (Figure 8c). For this comparison, all four KPSe6 and K0.6Cs0.4PSe6 samples exhibit the highest signal and outperform all other materials throughout the entire broadband range tested. The SHG counts of α- and β-CsPSe6 are lower than those from AgGaSe2. SHG from all examined materials except for α-CsPSe6 is expected because KPSe6, K0.6Cs0.4PSe6, and β-CsPSe6 are noncentrosymmetric. The observation of a weak but finite SHG response from samples of αCsPSe6 is attributed to the partial presence of the β-CsPSe6 phase. β-CsPSe6 itself possesses a significantly stronger SHG response, consistent with its noncentrosymmetric structure, but it is relatively weaker than those from KPSe6 and K0.6Cs0.4PSe6. The χ(2) values of these materials were calculated with the same method as above using Eq. (1) and are listed in Table 3. It is interesting to note that the 16 and 40 hours of additional annealing did not affect the χ(2) value of KPSe6 or K0.6Cs0.4PSe6. Laser-Induced Damage Threshold of Crystalline KPSe6, K0.6Cs0.4PSe6, α- and β-CsPSe6. The laser-induced damage thresholds (LIDTs) of KPSe6 A, KPSe6 B, K0.6Cs0.4PSe6 A, K0.6Cs0.4PSe6 B, α- and β-CsPSe6 were determined to further characterize the NLO properties of these materials. To determine the LIDT, the SHG response at the fundamental Nd:YAG line (1064 nm), which is the radiation source for typical picosecond difference frequency generation, was measured as a function of input intensity. The input intensity, I, was varied in the range of 0.2 – 5 GW/cm2, and the spectrally integrated SHG counts from the samples and AgGaSe2 were recorded to check the I-dependence. For example, Figure 9 shows the plots for K0.6Cs0.4PSe6 A and AgGaSe2. The black lines represent the maximum SHG case in which fundamental depletion is absent, i.e., = , where and I are the SHG and fundamental intensities with being a proportionality constant that incorporates 2. The values were determined by fitting the low-intensity regime where two-photon absorption (2PA) is minimal.29


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The measured SHG counts observed from the K0.6Cs0.4PSe6 A deviate from the solid line at I > 1 GW/cm2, indicating that the fundamental beam is significantly depleted by 2PA. We estimated the corresponding 2PA coefficient, β, by fitting the observed SHG power dependence using a modified fundamental intensity by 2PA, , = ⁄1 + , =

(2)

where d = 32 – 45 µm is the particle size for our reflection geometry. The red trace is the fit to the experimental measurements using Eq. (2) where the β value is shown in Table 3 along with the 2PA coefficients of the other samples. Similarly, the value of β = 43.5 cm/GW at 1064 nm for AgGaSe2 was determined, which is consistent with previous reports.29 This value is also consistent with a two-band model within a factor of 2 but likely overestimated due to optical damage induced by significant 2PA as the data points indeed deviate from the red traces for I > ~2 GW/cm2 for all APSe6 (A = K, K0.6Cs0.4, α-Cs, β-Cs), and this can be seen for K0.6Cs0.4PSe6 A in Figure 9. The higher the β value, the more susceptible a material is to optical damage via 2PA. Even with lower β values and higher band gaps, Figure 9 shows that the LIDTs of APSe6 materials are rather similar to that of AgGaSe2. The laser damage observed likely contains an extrinsic contribution because slight impurities and defects can reduce the LIDT of a material. More purified samples of APSe6 should exhibit better resistance to laser damage. Broadband THG of Crystalline KPSe6, K0.6Cs0.4PSe6, α- and β-CsPSe6. Determining the third-order nonlinearity is important for a comprehensive NLO characterization of a material. Additionally, third-order NLO processes such as self-focusing and multiphoton absorption are important for up-and-coming optoelectronic and photonic applications including optical switching,9 laser scanning microscopy,51 frequency-resolved optical-gating to fully characterize laser pulses,52 and ultrafast all-optical signal processing for future high-speed optical


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communications.53 However, as compared to SHG, THG is much less studied in the literature of chalcogenide chemistry. Although THG is not used directly for practical applications, it is a useful method for characterizing the third-order nonlinearity of pristine powdered samples. All materials possess a nonzero χ(3) value, and it is the lowest-order optical nonlinearity for centrosymmetric compounds. The THG properties of crystalline KPSe6 A, KPSe6 B, K0.6Cs0.4PSe6 A, K0.6Cs0.4PSe6 B, α- and β-CsPSe6 were examined from λTHG = λ/3 = 367–1033 nm. All THG of these materials are non-phase-matchable because the THG phase-mismatch is larger than the SHG phasemismatch. THG signal is expected from all samples because there are no rules related to crystalline symmetry forbidding THG. The broadband THG responses from the samples d = 32 – 45 µm are plotted in Figure 10 together with AgGaSe2. By simply assuming similar THG coherence lengths for all samples and

reference and comparing with = 1.6 × 10$ pm /V of AgGaSe2,54 we estimated values of APSe6 (A = K, K0.6Cs0.4, α-Cs, β-Cs) using

,

≅ ( / (

(3)

where ( and ( are the averaged THG counts from the samples and reference at λ = 2700−3100 nm, respectively. Experimental χ(3) values are also listed in Table 3. Interestingly, these results demonstrate that the polar conformation of the 1/∞[PSe6–] chains and parallel chain packing (Pca21) yield higher THG response than the centrosymmetric (P2/n) and perpendicularly packed chains (P421c). Additionally, the size of the counter-cation does not significantly influence the THG response within the same Pca21 structure, which is also consistent with the case for SHG.


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In general, NLO coefficients (i.e. χ(2) and χ(3)) scale inversely with a power law in the band gap.16 In this study, it is interesting that the larger band gap but low-dimensional materials, APSe6 (A = K, K0.6Cs0.4, α-Cs, β-Cs), possess larger NLO coefficients compared with AgGaSe2, which has a smaller bandgap but a three-dimensional structure. This indicates that the SHG coefficient of a material is not solely determined by its band gap, but other parameters as well such as structure and dimensionality where the latter can especially affect the joint density of states for transitions.16 Future theoretical calculations examining the electronic structure of these materials would bring more insight into their NLO properties. Additionally, it would be beneficial to have future studies of various NLO materials focused on determining these other parameters that affect the magnitude of SHG and THG.

Conclusion The substitution limit of K(1-x)CsxPSe6 that still retains the noncentrosymmetric structure is x ~ 0.4. Although the unit cell of K(1-x)CsxPSe6 enlarged following Végard’s law with increasing Cs substitution, the interaction strength between the chains did not decrease, as supported by the similar band gaps of 2.1 eV and the unchanging SHG and THG responses between KPSe6 and K(1-x)CsxPSe6.

These

results

demonstrate

that

any

counter-cation

effect

in

the

noncentrosymmetric KPSe6 phase is too small to significantly influence the NLO efficiencies. Most importantly, the SHG efficiency of K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4) is very strong, with χ(2) ~150 pm/V. In situ synchrotron PXRD measurements were key in providing invaluable data on the kinetics of the crystallization and amorphization of these materials. In addition these


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measurements were responsible for the surprising discovery of a new metastable form of CsPSe6, the β-phase, which upon further heating transforms into the stable α-phase. All crystalline samples of APSe6 (A = K, K0.6Cs0.4, α-Cs, and β-Cs) produced strong NLO responses. Based on these measurements, χ (2), χ (3), and β were determined for all materials. When prepared into practical film forms with heat and/or poling treatment, our results indicate that the compounds have potential for various NLO applications in the mid-IR.

Supporting Information Available: X-ray crystallographic data of K0.6Cs0.4PSe6 and β-CsPSe6 in CIF format. Experimental methods and physical measurements sections; tables of single crystal refinement details, atomic coordinates and displacement parameters of K0.6Cs0.4PSe6 and β-CsPSe6; figures of the Rietveld refinement of K(1-x)CsxPSe6 (x = 0, 0.1, 0.2, 0.3, 0.4); figures of α-CsPSe6 and β-CsPSe6 PXRD; pair distribution function analysis and Raman spectroscopy discussions; figure and discussion of SHG of amorphous KPSe6 prepared with different synthetic methods; broadband SHG and THG comparisons of KPSe6 A, KPSe6 B, K0.6Cs0.4PSe6 A, and K0.6Cs0.4PSe6 B, and LDT of KPSe6 A, KPSe6 B, K K0.6Cs0.4PSe6 B, α-CsPSe6 and β-CsPSe6. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work is supported by the National Science Foundation Grant DMR-1410169. Use of 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, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. This work also made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support under the


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State of Illinois, Northwestern University, and the National Science Foundation with grants DMR-1121262 through the MRSEC program at the Materials Research Center, and EEC0118025/003 through The Nanoscale Science and Engineering Center. The Raman experiments were performed in the IMSERC facility at Northwestern University and supported through the university. A.S.H. gratefully acknowledges support by the National Science Foundation through a Graduate Research Fellowship under Grant No. DGE-1324585. A.S.H. additionally appreciates the mentorship from Dr. Greg Halder, Dr. Christos D. Malliakas, Dr. Collin D. Morris, Dr. Amy Sarjeant, and Ms. Charlotte Stern that helped make this work possible. References (1) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Chem. Soc. Rev. 2006, 35, 710-717. (2) Pestov, D.; Xi, W.; Ariunbold, G. O.; Murawski, R. K.; Sautenkov, V. A.; Dogariu, A.; Sokolov, A. V.; Scully, M. O. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 422-427. (3) Pushkarsky, M. B.; Webber, M. E.; Macdonald, T.; Patel, C. K. N. Appl. Phys. Lett. 2006, 88, 044103. (4) Pushkarsky, M.; Tsekoun, A.; Dunayevskiy, I. G.; Go, R.; Patel, C. K. N. Proc. Natl. Acad. Sci. U.S.A 2006, 103, 10846-10849. (5) Serebryakov, V. A.; Boiko, E. V.; Petrishchev, N. N.; Yan, A. V. J. Opt. Tech. 2010, 77, 6-17. (6) Eggleton, B. J.; Luther-Davies, B.; Richardson, K. Nat. Photonics 2011, 5, 141148. (7) Dorn, R.; Baums, D.; Kersten, P.; Regener, R. Adv. Mater. 1992, 4, 464-473. (8) Chung, I.; Kanatzidis, M. G. Chem. Mater. 2014, 26, 849-869. (9) Zakery, A.; Elliott, S. R. In Springer Series Opti.; Rhodes, Ed.; Springer: 2007; Vol. 135, p 129-150. (10) Asobe, M. Opt. Fiber Technol. 1997, 3, 142-148. (11) Jang, J. I.; Chung, I.; Kanatzidis, M. G.; Ketterson, J. B. New Developements in Photon and Materials Research; NOVA Scientific Publishers, 2013. (12) Stucky, G. D.; Phillips, M. L. F.; Gier, T. E. Chem. Mater. 1989, 1, 492-509. (13) Bera, T. K.; Song, J. H.; Freeman, A. J.; Jang, J. I.; Ketterson, J. B.; Kanatzidis, M. G. Angew. Chem. Int. Ed. Engl. 2008, 47, 7828-7832. (14) Banerjee, S.; Malliakas, C. D.; Jang, J. I.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2008, 130, 12270-12272. (15) Wang, T.; Gai, X.; Wei, W.; Wang, R.; Yang, Z.; Shen, X.; Madden, S.; LutherDavies, B. Opt. Mater. Express 2014, 4, 1011-1022. (16) Song, J.-H.; Freeman, A. J.; Bera, T. K.; Chung, I.; Kanatzidis, M. G. Phys. Rev. B 2009, 79, 245203.


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(17) Bera, T. K.; Jang, J. I.; Song, J. H.; Malliakas, C. D.; Freeman, A. J.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2010, 132, 3484-3495. (18) Kim, K.-W.; Kanatzidis, M. G. J. Am. Chem. Soc. 1998, 120, 8124-8135. (19) Kanatzidis, M. G. Phosphorus, Sulfur Silicon Relat. Elem. 1994, 93, 159-172. (20) Bera, T. K.; Jang, J. I.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2009, 131, 75-77. (21) Chen, M.-C.; Li, L.-H.; Chen, Y.-B.; Chen, L. J. Am. Chem. Soc. 2011, 133, 4617-4624. (22) Geng, L.; Cheng, W.-D.; Lin, C.-S.; Zhang, W.-L.; Zhang, H.; He, Z.-Z. Inorg. Chem. 2011, 50, 5679-5686. (23) Chen, M.-C.; Wu, L.-M.; Lin, H.; Zhou, L.-J.; Chen, L. J. Am. Chem. Soc. 2012, 134, 6058-6060. (24) 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. (25) Yao, J.; Mei, D.; Bai, L.; Lin, Z.; Yin, W.; Fu, P.; Wu, Y. Inorg. Chem. 2010, 49, 9212-9216. (26) Mei, D.; Yin, W.; Feng, K.; Lin, Z.; Bai, L.; Yao, J.; Wu, Y. Inorg. Chem. 2011, 51, 1035-1040. (27) Yin, W.; Feng, K.; Hao, W.; Yao, J.; Wu, Y. Inorg. Chem. 2012, 51, 5839-5843. (28) Lin, X.; Guo, Y.; Ye, N. J. Solid State Chem. 2012, 195, 172-177. (29) Brant, J. A.; Clark, D. J.; Kim, Y. S.; Jang, J. I.; Zhang, J. H.; Aitken, J. A. Chem. Mater. 2014, 26, 3045-3048. (30) Lekse, J. W.; Moreau, M. A.; L., M. K.; Yeon, J.; Halasyamani, P. S.; Aitken, J. A. Inorg. Chem. 2009, 48, 7516-7518. (31) Chung, I.; Song, J.-H.; Jang, J. I.; Freeman, A. J.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2009, 131, 2647-2656. (32) Chung, I.; Malliakas, C. D.; Jang, J. I.; Canlas, C. G.; Weliky, D. P.; Kanatzidis, M. G. J. Am. Chem. Soc. 2007, 129, 14996-15006. (33) Chung, I.; Jang, J. I.; Malliakas, C. D.; Ketterson, J. B.; Kanatzidis, M. G. J. Am. Chem. Soc. 2010, 132, 384-389. (34) Chung, I.; Do, J.; Canlas, C. G.; Weliky, D. P.; Kanatzidis, M. G. Inorg. Chem. 2004, 43, 2762-2764. (35) Jang, J. I.; Haynes, A. S.; Saouma, F. O.; Otieno, C. O.; Kanatzidis, M. G. Opt. Mater. Express 2013, 3, 1302-1312. (36) Toby, B. H. J. Appl. Cryst. 2001, 34, 210-213. (37) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), 2000. (38) Denton, A. R.; Ashcroft, N. W. Phys. Rev. A 1991, 43, 3161-3164. (39) Chung, I.; Jang, J. I.; Gave, M. A.; Weliky, D. P.; Kanatzidis, M. G. Chem. Commun. 2007, 4998-5000. (40) Chung, I.; Karst, A. L.; Weliky, D. P.; Kanatzidis, M. G. Inorg. Chem. 2006, 45, 2785-2787. (41) 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, 1092210927.


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(42) Kolobov, A. V.; Tominaga, J. In Chalcogenides; Springer Series in Materials Science: 2012; Vol. 164, p 35-47. (43) Tanaka, K. J. Optoelectron. Adv. M. 2001, 3, 189-198. (44) Byer, R. L.; Choy, M. M.; Herbst, R. L.; Chemla, D. S.; Feigelson, R. S. Appl. Phys. Lett. 1974, 24, 65-68. (45) Chung, I.; Kim, M. G.; Jang, J. I.; He, J.; Ketterson, J. B.; Kanatzidis, M. G. Angew. Chem. Int. Ed. Engl. 2011, 50, 10867-10870. (46) Maker, P. D.; Terhune, R. W.; Nisenoff, M.; Savage, C. M. Phys. Rev. Lett. 1962, 8, 21-23. (47) Meyn, J.-P.; Fejer, M. M. Opt. Lett. 1997, 22, 1214-1216. (48) Hoyt, C. W.; Sheik-Bahae, M. Opt. Lett. 2002, 27, 1543-1545. (49) Canagasabey, A.; Corbari, C.; Gladyshev, A. V.; Liegeois, F.; Guillemet, S.; Hernandez, Y.; Yashkov, M. V.; Kosolapov, A.; Dianov, E. M.; Ibsen, M.; Kazansky, P. G. Opt. Lett. 2009, 34, 2483-2485. (50) Bhar, G. C. Jpn. J. Appl. Phys. Part I, Supplement 32-3 1993, 32, 653-659. (51) Barad, Y.; Eisenberg, H.; Horowitz, M.; Silberberg, Y. Appl. Phys. Lett. 1997, 70, 922-924. (52) Tsang, T.; Krumbügel, M. A.; DeLong, K. W.; Fittinghoff, D. N.; Trebino, R. Opt. Lett. 1996, 21, 1381-1383. (53) Fuentes-Hernandez, C.; Ramos-Ortiz, G.; Tseng, S.-Y.; Gaj, M. P.; Kippelen, B. J. Mater. Chem. 2009, 19, 7394-7401. (54) http://marzenell.de/Research/Z-Scan/z-scan.html.


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Table 1. Lattice parameters, volume, and shortest interchain Se…Se distances in K(1-x)CsxPSe6. x values were determined from EDS in SEM, and all other values were determined using Rietveld analysis in GSAS. x (mol) 0 (0) 0.102(3) 0.204(9) 0.312(15) 0.391(7)

a (Å) 11.5635(1) 11.6148(1) 11.6758(1) 11.7392(1) 11.7804(1)

b (Å) 6.8309(1) 6.8400(1) 6.8502(1) 6.8605(1) 6.8677(1)

c (Å) 11.3529(1) 11.3893(1) 11.4278(1) 11.4643(1) 11.4864(1)

V (Å3) 896.75(1) 904.83(1) 914.02(1) 923.30(1) 929.31(1)

Se3…Se4 (Å) 3.1514(52) 3.1262(51) 3.1646(33) 3.1511(31) 3.1599(70)

Se2…Se3 (Å) 3.2923(46) 3.3633(59) 3.3117(28) 3.2934(28) 3.2917(70)

Average Se…Se (Å) 3.2219(49) 3.2448(55) 3.2382(31) 3.2223(30) 3.2258(70)

Table 2. Experimental second-order nonlinear susceptibilities of KPSe6 and K(1-x)CsxPSe6 (x = 0.1, 0.2, 0.3, 0.4). These samples did not have additional annealing. Material KPSe6 K0.9Cs0.1PSe6 K0.8Cs0.2PSe6 K0.7Cs0.3PSe6 K0.6Cs0.4PSe6

χ(2) pm/V 157.5 ± 7.0 166.2 ± 10.2 139.0 ± 8.5 159.1 ± 9.7 163.4 ± 10.0

Table 3. Experimental band gap energies, second-order nonlinear susceptibilities, third-order nonlinear susceptibilities, and 2PA coefficients of KPSe6 and K0.6Cs0.4PSe6 with extra-annealing (samples labeled A or B had 16 or 40 hours of additional annealing, respectively), α-CsPSe6, βCsPSe6, and AgGaSe2. Material KPSe6 A KPSe6 B K0.6Cs0.4PSe6 A K0.6Cs0.4PSe6 B α-CsPSe6 β-CsPSe6 AgGaSe2

Band Gap (eV) 2.1 2.1 2.1 2.1 2.1 1.9 1.7

χ(2) pm/V 151.7 ± 6.7 143.4 ± 7.2 172.8 ± 10.3 167.9 ± 10.5 5.0 ± 1.3 29.6 ± 5.6 66

χ(3) × 105 pm2/V2 2.6 ± 0.1 2.3 ± 0.1 3.1 ± 0.2 2.8 ± 0.2 1.8 ± 0.4 1.1 ± 0.2 1.6


β cm/GW 15.5 ± 5 16.9 ± 5 24.8 ± 5 20.8 ± 5 43.9 ± 5 59.8 ± 5 43.5

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Figure 1. (a) Ball and stick representation of the unit cell of K(1-x)CsxPSe6 viewed along the baxis. (b) Zoomed-in view of unit cell in green box from (a) labeling all atoms with van der Waals interacting interchain Se…Se labeled in bold. Color scheme: K/Cs (blue), P (black), and Se (red).


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Figure 2. (a) Lattice parameter (c-axis) vs. x value of K(1-x)CsxPSe6 displaying a linear trend from Végard’s law. (b) Interchain distance of Se3…Se4 (black), Se2…Se3 (red), and their average (blue) vs. x value of K(1-x)CsxPSe6. Error bars are on all points. If no error bar can be seen, the error is within the size of the point.


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Figure 3. (a) DTA traces of amorphous K0.6Cs0.4PSe6 (black) and KPSe6 (red). (b) DTA trace of amorphous CsPSe6.


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Figure 4. (a) In situ PXRD of amorphous KPSe6 on heating from 150-307 ºC. Peaks from selenium as a second phase marked with (*). (b) In situ PXRD of amorphous K0.6Cs0.4PSe6 on heating from 150-309 ºC. Peaks from selenium as a second phase marked with (*). (c) In situ PXRD of amorphous CsPSe6 on heating from 163-331 ºC with β-CsPSe6 from 167-239 ºC (red), phase transition including both α- and β-CsPSe6 from 241-265 ºC (green), and α-CsPSe6 from 267-329 ºC (blue). (d) DSC curve of amorphous CsPSe6 on heating from 75-400 ºC.


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Figure 5. Crystal structure of β-CsPSe6 with thermal ellipsoids set at 90%. Color scheme: Cs (green), P (black), and Se (red). (a) Propagating chain of 1/∞[PSe6–] down the b-axis. (b) Unit cell viewed down the a-axis showing noncentrosymmetric chain packing.


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Figure 6. View of β-CsPSe6 down the c-axis with thermal ellipsoids set to 90% next to analogous waffle fry depiction. Color scheme: Cs (green), Se (red), and P (black).

Figure 7. Diffuse reflectance UV-vis spectra of crystalline K0.6Cs0.4PSe6 (black), KPSe6 (red), αCsPSe6 (blue), and β-CsPSe6 (green), with band gaps of 2.1, 2.1, 2.1, and 1.9 eV, respectively.


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Figure 8. Samples labeled A or B had 16 or 40 hours of additional annealing, respectively. Samples without an A or B label did not have additional annealing. (a) Semi-log plot of SHG intensity vs. particle size showing non-phase-matching behavior of crystalline KPSe6 and K0.6Cs0.4PSe6 at λ = 1800 nm. (b) Broadband SHG comparison of crystalline KPSe6, K0.9Cs0.1PSe6, K0.8Cs0.2PSe6, K0.7Cs0.3PSe6, K0.6Cs0.4PSe6, and reference AgGaSe2. (c) Broadband SHG comparison of crystalline KPSe6 A, K0.6Cs0.4PSe6 A, α-CsPSe6, β-CsPSe6, and reference AgGaSe2.


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Figure 9. SHG power dependence of K0.6Cs0.4PSe6 A (blue) and AgGaSe2 (green) plotted on a log-log scale, superimposed by square-law fits (black lines). Both materials undergo 2PA as indicated by 2PA fits (red traces). Optical decomposition begins to occur when materials deviate from the red 2PA traces.

Figure 10. Semi-log broadband THG comparison of crystalline KPSe6, K0.6Cs0.4PSe6, α-CsPSe6, β-CsPSe6, and AgGaSe2 reference.


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Figure TOC


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Phase-Change Behavior and Nonlinear Optical Second and Third

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