New Polar Oxysulfide for Nonlinear Optical Applications

Abstract

Single crystals of a new noncentrosymmetric polar oxysulfide SrZn.sub.2S.sub.2O (s.g. Pmn2.sub.1) grown in a eutectic KF-KCl flux with unusual wurtzite-like slabs consisting of close-packed corrugated double layers of ZnS.sub.3O tetrahedra vertically separated from each other by Sr atoms and methods of making same.

Claims

1. A method for synthesizing the compound SrZn.sub.2S.sub.2O comprising: loading a silver tube with 1 mmol of SrO, 1 mmol of Zn, 1 mmol of S, 3.4 mmol of KF, and 4.1 mmol of KCl; placing the silver tube in a silica tube that was evacuated to 10.sup.−4 Pa and sealed; heating the silver tube in a tube furnace to 900° C. at 5° C./min, held for 24 h, and cooled to 600° C. at 0.17° C./min; and making a molten salt from a eutectic KF-KCl mixture, wherein SrO, Zn and S are dissolved in the eutectic KF-KCL mixture to form a flux.

2. The method of claim 1, further comprising, extracting the compound from the flux by dissolving the flux in water followed by sonication.

3. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound contains corrugated double layers of ZnS.sub.3O tetrahedra vertically separated by Sr.sup.2+ ions wherein an O/S anion ordered arrangement provides two distinct orientations of the ZnS.sub.3O tetrahedra.

4. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound crystalizes in noncentrosymmetric polar space group Pmn2.sub.1.

5. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound forms colorless, transparent crystals.

6. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound has a band gap of 3.86 eV.

7. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound is stable up to 650° C. in O.sub.2 gas atmosphere.

8. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound is phase matchable with twice a SHG intensity of potassium dihydrogen phosphate (KDP).

9. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound has a Sr:Zn:S molar ratio of approximately 1.0:2.0:2.3.

10. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein the compound has a band gap energy of 3.86 eV.

11. The method for synthesizing the compound SrZn.sub.2S.sub.2O of claim 1, wherein temperatures greater than 1000° C. decompose the SrZn.sub.2S.sub.2O compound.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The construction designed to carry out the invention will hereinafter be described, together with other features thereof. The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein:

[0013] FIG. 1 shows Table 1—Results of Structural Refinement of SrZn.sub.2S.sub.2O Using Single-Crystal XRD Data.

[0014] FIG. 2 shows Table 2—Selected Interatomic Distances (A) and Bond Angles (deg) of SrZn.sub.2S.sub.2O at Room Temperature.

[0015] FIG. 3 shows at (a, b) a perspective view of the SrZn.sub.2S.sub.2O crystal structure along the a and c axes and at (c) O/S anion-ordered arrangement in the close-packed layers of ZnS.sub.3O tetrahedra.

[0016] FIG. 4 shows the local coordination environment around Sr and Zn atoms.

[0017] FIG. 5 shows a thermogravimetric curve of SrZn.sub.2S.sub.2O in O.sub.2 gas atmosphere.

[0018] FIG. 6 shows the UV-vis-NIR optical absorption spectrum of SrZn.sub.2S.sub.2O.

[0019] FIG. 7 shows at (a) total and partial density of states and at (b) band dispersions of SrZn.sub.2S.sub.2O.

[0020] FIG. 8 shows SHG intensities of SrZn.sub.2S.sub.2O and KDP are plotted against particle size.

[0021] FIG. 9 shows Table 3—Direction and Magnitude of the Dipole Moments (in Debye) of the Polyhedral Building Units in SrZn.sub.2S.sub.2O, CaZnSO, and Wurtzite ZnS.

[0022] FIG. 10 shows a photograph of a colorless transparent single crystal of SrZn.sub.2S.sub.2O.

[0023] FIG. 11 shows a comparison of the powder XRD pattern of purified SrZn.sub.2S.sub.2O with a theoretical pattern based on the single-crystal XRD analysis.

[0024] FIG. 12 shows SXRD patterns for purified SrZn.sub.2S.sub.2O measured at room temperature.

[0025] FIG. 13 shows a room-temperature XRD pattern of the products after TG measurement.

[0026] FIG. 14 shows a comparison of the crystal structure of SrZn.sub.2S.sub.2O (Pmn2.sub.1) with that of CaFeSeO (C.sub.mc2.sub.1).

[0027] FIG. 15 shows Crystal Orbital Hamiltonian Population (COHP) for (Zn1/Zn2)-O and (Zn1/Zn2)-S interactions in SrZn.sub.2S.sub.2O.

[0028] FIG. 16 shows Table 4—Atomic coordinates and equivalent isotropic displacement parameters Ueq for SrZn.sub.2S.sub.2O at 300 K.

[0029] FIG. 17 shows Table 5—Anisotropic displacement parameters U.sub.ij (10.sup.2×Å.sup.2) for SrZn.sub.2S.sub.2O at 300 K.

[0030] FIG. 18 shows Table 6—Lattice constants and atomic coordinates obtained by the first principle calculations for SrZn.sub.2S.sub.2O

[0031] FIG. 19 shows Table 7—Atomic coordinates and isotropic displacement parameters Biso refined from SXRD data collected from SrZn.sub.2S.sub.2O at room temperature.

[0032] It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all its respects, to every aspect of this invention. As such, the preceding objects can be viewed in the alternative with respect to any one aspect of this invention. These and other objects and features of the invention will become more fully apparent when the following detailed description is read in conjunction with the accompanying figures and examples. However, it is to be understood that both the foregoing summary of the invention and the following detailed description are of a preferred embodiment and not restrictive of the invention or other alternate embodiments of the invention. In particular, while the invention is described herein with reference to a number of specific embodiments, it will be appreciated that the description is illustrative of the invention and is not constructed as limiting of the invention. Various modifications and applications may occur to those who are skilled in the art, without departing from the spirit and the scope of the invention, as described by the appended claims. Likewise, other objects, features, benefits and advantages of the present invention will be apparent from this summary and certain embodiments described below, and will be readily apparent to those skilled in the art. Such objects, features, benefits and advantages will be apparent from the above in conjunction with the accompanying examples, data, figures and all reasonable inferences to be drawn therefrom, alone or with consideration of the references incorporated herein.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0033] With reference to the drawings, the invention will now be described in more detail. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the presently disclosed subject matter belongs. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the presently disclosed subject matter, representative methods, devices, and materials are herein described.

[0034] Unless specifically stated, terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise.

[0035] Furthermore, although items, elements or components of the disclosure may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent.

[0036] The current disclosure focused on the Sr—Zn—S—O system and synthesized the new composition, SrZn.sub.2S.sub.2O, crystallizing in the polar space group Pmn2.sub.1. SrZn.sub.2S.sub.2O is compositionally, but not structurally, related to SrFe.sub.2S.sub.2O and SrFe.sub.2Se.sub.2O, which crystallize in the centrosymmetric space group Pmmn. The current disclosure provides the synthesis, crystal structure, and SHG behavior of this new polar oxysulfide material.

[0037] Single crystals of a new zinc oxysulfide SrZn.sub.2S.sub.2O were grown in a eutectic KF-KCl flux, and the structure was determined by single-crystal X-ray diffraction. SrZn.sub.2S.sub.2O crystallizes in the noncentrosymmetric polar space group Pmn2.sub.1 with lattice parameters of a=3.87440(10) Å, b=9.9847(3) Å, and c=6.0916(2) Å. In the crystal structure, close-packed corrugated double layers of ZnS.sub.3O tetrahedra, which are derived from the wurtzite structure, are vertically separated by Sr.sup.2+ ions. In addition, the O/S anion ordered arrangement in each close-packed layer yields two distinct orientations of the Zn-centered tetrahedra. The crystals of SrZn.sub.2S.sub.2O are colorless and transparent, and the oxysulfide has a band gap of 3.86 eV, based on UV-vis-NIR diffuse reflectance measurements. Thermogravimetric measurements showed that SrZn.sub.2S.sub.2O is stable up to 650° C. in O.sub.2 gas atmosphere. First-principle calculations indicate that the valence band maximum is mainly composed of O-2p and S-3p states, whereas the conduction band minimum is derived from Zn-4s, Zn-4p, and Sr-4d states. The calculated band dispersion reveals a direct band gap corresponding to a transition between S-3p and Zn-4s energy levels. Second harmonic generation (SHG) measurements determined that SrZn.sub.2S.sub.2O is phase matchable with twice the SHG intensity of potassium dihydrogen phosphate (KDP) in contrast to CaZnSO with similar ZnS.sub.3O building units, which exhibits non-phase matching behavior.

[0038] Experimental Section

[0039] Reagents. Zn (Alpha Aesar, 99.9%) and S (Fisher, 99.99%) in powder form were used as received. SrO was prepared by heating SrCO.sub.3 in air at 1050° C. overnight. KF (Alpha Aesar, 99.9%) and KCl (Alpha Aesar, 99.9%) were dried at 260° C. prior to use.

[0040] Crystal Growth. Single crystals of SrZn.sub.2S.sub.2O were obtained by a molten salt method using a eutectic KF-KCl mixture. A silver tube welded closed on one end was loaded with 1 mmol of SrO, 1 mmol of Zn, 1 mmol of S, 3.4 mmol of KF, and 4.1 mmol of KCl. The top of the tube was crimped, and the tube was placed inside a silica tube that was evacuated to 10.sup.−4 Pa and sealed. The starting materials were heated in a tube furnace to 900° C. at 5° C./min, held for 24 h, and cooled to 600° C. at 0.17° C./min at which point the furnace was turned off and allowed to cool to room temperature. The product was extracted from the flux by dissolving the flux in water, aided by sonication. Colorless transparent plate-like crystals of SrZn.sub.2S.sub.2O (typical dimension 0.1×0.2×0.05 mm.sup.3, see FIG. 10) were isolated via vacuum filtration. Semiquantitative analyses by energy-dispersive X-ray analysis (EDS) indicated a Sr:Zn:S molar ratio of approximately 1.0:2.0:2.3. The structure of SrZn.sub.2S.sub.2O was solved by single crystal X-ray diffraction.

[0041] Solid-State Synthesis. Polycrystalline powder samples of SrZn.sub.2S.sub.2O were synthesized using SrO, Zn, and S in a stoichiometric ratio. The starting materials were thoroughly mixed in an agate mortar, pressed into a pellet, and heated in an evacuated silica tube at 1000° C. for 24 h. Powder XRD measurement using a Bruker D2 Phaser equipped with an LYNXEYE silicon strip detector and a Cu Kα source identified SrZn.sub.2S.sub.2O as the main phase and SrS as a minor phase. Additional heat treatment under the same condition did not improve the relative amount of SrZn.sub.2S.sub.2O to SrS. Heating at higher temperatures (>1000° C.) decomposed SrZn.sub.2S.sub.2O. A pure sample was obtained by sonicating the product prepared at 1000° C. with water in which SrS readily decomposes and the remaining SrZn.sub.2S.sub.2O was isolated by vacuum filtration. The powder XRD data of SrZn.sub.2S.sub.2O after removal of SrS is shown in FIG. 11. A Rietveld refinement of the synchrotron powder XRD data of SrZn.sub.2S.sub.2O after removal of SrS is shown in FIG. 12, and the atomic coordinates and atomic displacement parameters are listed in FIG. 19, which shows Table 7—Atomic coordinates and isotropic displacement parameters Biso refined from SXRD data collected from SrZn.sub.2S.sub.2O at room temperature. The space group is Pmn2.sub.1 (No. 31) with a=3.87466(1) Å, b=9.98582 Å, and c=6.09207 Å. R indices are Rwp=2.44%, Rp=1.65%, RB=7.129%, and RF=5.47%. Site occupancy for each atom was fixed to unity. The Biso parameters for S1 and S2 were constrained to the same value during refinements. The c coordinate of Sr atom (the heaviest element) along the polar axis was fixed to those obtained by the single-crystal structure analysis.

[0042] Single Crystal Structure Determination. X-ray intensity data were collected from a clear colorless crystal using a Bruker D8 QUEST diffractometer. The D8 utilizes an Incoatec microfocus source (Mo Kα radiation, λ=0.71073 Å) and a Photon II CMOS area detector. The detector was operated in the shutterless mode, and fast scans with 1 s exposure times were used to quantify peaks that were too intense for the detector to measure during the full data collection. Data collection covered 98.8% of reciprocal space up to θ.sub.max=36.308°, with an average redundancy of 15. The raw area detector frames were reduced and corrected for Lorentz, polarization, and absorption effects using the SAINT+ and SADABS programs. Initial structural models and subsequent least-squares refinements were performed with the SHELX package, through the OLEX.sup.2 GUI.

[0043] The title compound crystallizes in the NCS orthorhombic space group Pmn2.sub.1 with lattice parameters a=3.87440(10) Å, b=9.9847(3) Å, and c=6.0916(2) Å. The asymmetric unit contains one Sr site, two Zn sites, two S sites, and one 0 site, all of which lie on Wyckoff site 2a with mirror symmetry. Each site was individually allowed to freely refine, and there were no significant deviations from unity. PLATON modules TwinRotMat and Addsym were used to check for minor twin components and missed symmetry elements. The twin law (−1 0 0 0 −1 0 0 0 −1) with volume fraction 0.032(6) was implemented, and the model was refined as a two-component inversion twin. No missed symmetry elements were found, confirming the NCS space group. The final refinement based on the model resulted in an excellent R.sub.1 value of 0.0133 and the small maximum, and minimum residual electron densities of 1.109 and −0.758 indicate an excellent fit of the model to the data.

[0044] Powder XRD, TGA, and UV-Vis. Powder X-ray diffraction (PXRD) patterns were collected at room temperature using a Bruker D2 Phaser instrument. Patterns were collected in the 5-65° 2θ angular range with a step size of 0.02°. Thermogravimetric analysis (TGA) was performed using a PerkinElmer Pyris 1 TGA system under O.sub.2 gas flow (60 mL/min). The sample was loaded in an alumina crucible and heated to 1000° C. at 10° C./min. The thermal products were analyzed by PXRD. UV-vis-NIR absorption measurements on SrZn.sub.2S.sub.2O were carried out using a Shimazu UV2600 UV-vis-NIR spectrometer (used in the diffuse reflectance mode) equipped with an integrating sphere. Deuterium and halogen lamps were used as sources of UV and visible-NIR light, respectively. Spectra were recorded over the range of 220-1200 nm. The diffuse reflectance data were converted to absorbance internally by the instrument by use of the Kubelka-Munk function.

[0045] SHG Measurements. The powder SHG measurements were carried out with the Kurtz-Perry method using a pulsed Nd:YAG Quantel Ultra laser (Model: Ultra 50) with a wavelength of 1064 nm. KH.sub.2PO.sub.4 (KDP) was used as a benchmark material. SrZn.sub.2S.sub.2O and KDP were ground and sieved into distinct particle size ranges (<20, 20-45, 45-63, 63-75, 75-90, 90-125 μm). The intensities of the frequency-doubled output emitted from the sieved samples were detected using a photomultiplier tube and recorded on the Tektronix oscilloscope (Model: TDS3032).

[0046] Density Functional Theory (DFT) Calculations. First-principles DFT calculations were performed for SrZn.sub.2S.sub.2O using the Vienna Ab Initio Simulation Package. The Perdew-Burke-Ernzerchof generalized gradient approximation (GGA) was employed for the exchange and correlation function. Projector augmented-wave potentials were used for Sr, Zn, S, and O atoms. The cell parameters and atomic positions were optimized until the maximum force on each atom was less than 0.02 eV/A, based on the experimentally determined crystal structure. Then, a single point calculation was carried out to calculate a band structure and density of states. Plane wave basis sets with a cutoff of 500 and 520 eV were used for the optimization and single point calculations, respectively. In both calculations, the self-consistent field tolerance was 1.0×10.sup.−7 eV/atom and the k-point mesh was 9×5×7. The crystal orbital Hamilton population (COHP) analysis was performed using the LOBSTER code.

[0047] Results and Discussion

[0048] The single-crystal structure solution established that SrZn.sub.2S.sub.2O crystallizes in the NCS polar space group of Pmn2.sub.1 (No. 31). The details of the structural refinement are listed in Table 1, selected interatomic distances and angles are compiled in Table 2, and atomic coordinates and atomic displacement parameters are listed in Tables 4 and 5.

[0049] SrZn.sub.2S.sub.2O crystallizes in a unique two-dimensional structure, shown in FIG. 3, in which wurtzite-like slabs, consisting of close-packed corrugated double layers of ZnS.sub.3O tetrahedra, are located in the ac plane. The slabs are separated from each other vertically by Sr.sup.2+ ions positioned parallel to the ac plane. The O.sup.2− and S.sup.2− ions, both of which occupy two inequivalent crystallographic positions, are crystallographically ordered and located in each layer such that the oxide ions are connected to both electropositive Sr.sup.2+ and electropositive Zn.sup.2+ ions. Furthermore, the ZnS.sup.3O tetrahedra have two distinct orientations in the ab plane and are connected by a common shared sulfur apex that creates the corrugation. This type of layered structure is substantially different from other wurtziterelated intergrowth structures, where the corner-shared tetrahedra form single layers of MS.sub.3O (M=Fe, Co, Zn) tetrahedra in which all tetrahedra have the same orientation.

[0050] The heteroleptic zinc coordination environment creates the asymmetric tetrahedral coordination, as illustrated in FIG. 4. The Zn(1)-O and Zn(2)-O bond distances of 1.9196(17) Å and 1.9468(19) Å, respectively, are somewhat longer than the Zn—O bond distances in CaZnSO (1.8997(5) Å) and SrZnSO (1.9000(7) Å) having similar Zn-centered coordination, but are shorter by 2-3% than the average Zn—O bond distance of 1.979 Å in wurtzite ZnO. The Zn(1)-S(2) and Zn(2)-S(1) bond distances are similar to the Zn—S bond distances found in CaZnSO (2.3718(5) Å) and SrZnSO (2.4173(9) Å) and are longer by 1.4 and 2.5%, respectively, than the average Zn—S bond distance of 2.343 Å in wurtzite ZnS. The Zn(1)-S(1) and Zn(2)-S(2) bond distances are within 1% of the distances observed in wurtzite. The bond angles of O—Zn—S and S—Zn—S are within ±4% of the ideal value(109.5°) for a regular tetrahedron. The Sr.sup.2+ ions are found in a distorted octahedral coordination environment consisting of two oxide ions at 2.3985(10) Å and four sulfide ions at 3.0087(8)-3.1102(8) Å, yielding a second asymmetric unit in the structure. The bond-valence-sum (BVS) calculations resulted in 2.02 for Sr, 1.99 for Zn(1), and 1.90 for Zn(2), which are in good agreement with the expected oxidation states.

[0051] The long-range order and coexistence of mixed anions in the same layer in SrZn.sub.2S.sub.2O are quite unique because, typically, oxygen and the other chalcogens tend to be separated in different layers. Recently, similar anion order in a different framework was reported for the iron-based oxychalcogenides, AFe.sub.2Q.sub.2O (A=Sr, Ba; Q=S, Se) and CaFeSeO. In the structure of AFe.sub.2Q.sub.2O two infinite chains of FeQ.sub.3O tetrahedra share oxygen apexes to create double chains, which in turn are connected by sharing three sulfur atoms with another double chain, resulting in layers. These close packed layers are stacked parallel with A.sup.2+ layers perpendicular to the hexagonal c-axis, unlike SrZn.sub.2S.sub.2O, where the layers are parallel to the hexagonal c-axis. Another example is CaFeSeO, crystallizing in the space group Cmc2.sub.1 which can be considered to be a member of a homologous series (AO)(MQ)n (M=Fe, Zn; n=1, 2) to which CaFeSeO and SrZn.sub.2S.sub.2O belong. CaFeSeO contains puckered sheets consisting of FeSe.sub.2O.sub.2 tetrahedra located in the ac plane that are also vertically separated by Ca.sup.2+ layers oriented parallel to the hexagonal c axis, see FIG. 14. A major reason for the structural difference between SrZn.sub.2S.sub.2O and SrFe.sub.2S.sub.2O may be the electronic configuration of these d transition metal cations. While the Zn.sup.2+ ion has the closed-shell (spherical) d.sup.10 electronic configuration, tetrahedrally coordinated Fe.sup.2+ ion possesses a doubly occupied e.sub.g orbital, which favors a more distorted coordination geometry. A distortion index (D) defined by Baur is useful for evaluating the magnitude of distortion in polyhedra. D is calculated using bond lengths in a polyhedron, using equation 1, below:

[00001]$\begin{array}{cc}D=\frac{1}{4}\underset{1}{\overset{4}{.Math.}}\left(\frac{.Math.l_i-l_{\mathrm{av}}.Math.}{l_{\mathrm{av}}}\right)& \left(1\right)\end{array}$

[0052] where l.sub.i is the length between the metal center and the ith surrounding atom, and l.sub.av is the average bond length. A larger D value indicates a higher magnitude of polyhedral distortion. The D value of the FeS.sub.3O tetrahedra in SrFe.sub.2S.sub.2O is 0.08474, much larger than the 0.07400 and 0.07115 for the Zn(1)S.sub.3O and Zn(2)S.sub.3O tetrahedra, respectively, in SrZn.sub.2S.sub.2O. This is consistent with the difference in electronic configuration between Zn.sup.2+ and Fe.sup.2+. Note that CaFeSO and CaZnSO adopt similar crystal structures despite the large difference between their D values (0.08609 for FeS.sub.3O and 0.07845 for ZnS.sub.3O). This suggests that the difference in the anion ordered arrangement should be another important factor for maintaining the same crystal symmetry.

[0053] The thermogravimetric (TG) analysis of SrZn.sub.2S.sub.2O, shown in FIG. 5, demonstrates a high thermal stability in air, up to 650° C. An irreversible weight increase is observed once the temperature exceeds 650° C. and continues to 1000° C. The TG curve shows a weight gain starting at 650° C. that reaches a maximum value at 970° C. and then decreases on further heating up to 1000° C. The net weight gain is 14.2%. Based on a powder the X-ray diffraction analysis of the products, FIG. 13, the net gain results from the decomposition of SrZn.sub.2S.sub.2O into ZnO and SrSO.sub.4. The calculated weight gain is 16.1%, which is consistent with the experimental value.

[0054] The UV-vis-NIR absorption spectrum of SrZn.sub.2S.sub.2O powder is shown in FIG. 6. The absorption curve exhibits a steep increase close to 4 eV, and an extrapolation of the linear portion of the absorption plot to the x-axis indicates a band gap of E.sub.g=3.86 eV. The large band gap is consistent with the clear, colorless crystals grown out of the flux. The weak absorption noticeable between 3 and 3.8 eV is likely due to impurity states. The 3.86 eV band gap of SrZn.sub.2S.sub.2O is significantly larger than the 3.3 eV band gap of ZnO.sub.4 as well as the 3.1 eV band gap of SrZnSO and is comparable with those found in zinc-based sulfides and oxysulfides, such as ZnS (3.8 eV), CaZnSO (3.7 eV), and BaZnSO (3.9 eV).

[0055] FIG. 7 shows the total and partial DOS and the band dispersion of SrZn.sub.2S.sub.2O, which are qualitatively similar to those of (Ca/Sr)ZnSO with similar local coordination environments around the metal cations. The computationally optimized lattice constants and atomic coordinates are consistent with those determined by the single-crystal XRD analysis, see Table 6. The band dispersion reveals that SrZn.sub.2S.sub.2O is a direct band gap semiconductor with the band gap energy of 2.26 eV at the Γ point. Underestimation of the band gap energy is typical of DFT calculations. The valence band maximum is mainly composed of the O-2p and S-3p states. The S-3p band is located at a higher energy level than the O-2p band, as observed in the other oxysulfide compounds. The partial DOS plots indicate the strong hybridization between Zn-3d/Zn-4s/Zn-4p or Sr-4d/Sr-5s/Sr-5p and O-2p/S-3p states in the valence band ranging from −6 eV to the Fermi energy level, resulting in the (Zn-3d/Zn-4s/Zn-4p)-(O-2p/S-3p) bonding states and the (Zn-3d)-(O-2p/S-3p) antibonding state, see FIG. 15. The conduction band maximum is composed of Zn-4s and the Zn-4p and Sr-4d states that reside at higher energy regions. These states form antibonding states with O-2p/S-3p. Based on these results, the direct band gap corresponds to a transition between S-3p and Zn-4s energy levels.

[0056] Materials that crystallize in polar NCS space groups may exhibit SHG behavior, and since SrZn.sub.2S.sub.2O crystallizes in the NCS polar space group Pmn2.sub.1, powder SHG measurements with a 1064 nm laser were performed. FIG. 8 shows the SHG intensities as a function of particle size, which are compared with the known SHG intensity of KDP. The results are consistent with type-I phase-matching, as proposed by Kurtz and Perry. The SHG intensities increase with increasing particle size and reach a plateau at an SHG efficiency of approximately 2 times that of KDP. It should be noted that SrZn.sub.2S.sub.2O is the first example of an oxysulfide material to demonstrate a phase-matching behavior, in contrast to the nonphase matching behaviors in CaZnSO and α-Na.sub.3PO.sub.4S.

[0057] Based on the anion group theory described by Chen, dipole moments of anionic polyhedra play a crucial role in the SHG response. Therefore, to evaluate the contribution to the SHG from the asymmetric anionic groups in SrZn.sub.2S.sub.2O, the dipole moments of [SrS.sub.4O.sub.2] and [ZnS.sub.3O] building units in the unit cell were calculated using a simple bond-valence approach proposed by Poeppelmeier. For comparison, the dipole moments in wurtzite ZnS and CaZnSO were also calculated. As detailed in Table 3, see FIG. 9, the magnitude of the dipole moments of SrS.sub.4O.sub.2, Zn(1)S.sub.3O, and Zn(2)S.sub.3O building units are estimated to be 12.342, 4.375, and 3.032 D (Debye: 10.sup.−18 esu cm), respectively. Both Zn-centered tetrahedra have a dipole moment much larger than the one observed for ZnS.sub.4 in wurtzite, ZnS, but somewhat smaller than the dipole moment of ZnS.sub.3O in CaZnSO. The dipole moments of Zn(1)S.sub.3O and Zn(2)S.sub.3O tetrahedra point to the S3 triangular planes, similar to what is observed for CaZnSO. Given the mm.sup.2 point group for SrZn.sub.2S.sub.2O, only the z components of the dipole moments of Sr- and Zn-centered polyhedra contribute to the polarization in the unit cell. The net dipole moments along the polar c axis in the unit cell are 4.695, 2.344, and 2.609 D for SrS.sub.4O.sub.2, Zn(1)S.sub.3O, and Zn(2)S.sub.3O building units, respectively. Because the net dipole moments of Zn(1)S.sub.3O and Zn(2)S.sub.3O tetrahedra are directed antiparallel to each other, the dipole moment of the SrS.sub.4O.sub.2 octahedra appears to be primarily responsible to the SHG response of SrZn.sub.2S.sub.2O.

[0058] Understanding the relationship between phase matchability and NCS structures is a challenging subject, especially for nonmolecular solids. Although SrZn.sub.2S.sub.2O and CaZnSO possess similar chemical and electronic structures, the former is phase matching and the latter is non-phase matching. The underlying mechanism of the phase matchability in SrZn.sub.2S.sub.2O is not understood at this point; however, the different anion order and arrangements of the polar anionic units should yield birefringence (Δn) satisfying the phase matching conditions. Recently, Lin et al. discussed the SHG properties of phasematchable AM.sub.3Se.sub.6 (A=Cs, Ba, M=Ga, In, Si, Ge, Sn) and nonphase-matchable CsM.sub.9Se.sub.12 (M=0.56Ga/0.44Cd) crystallizing in the polar NCS R3 space group. They suggested that Δn is correlated with the magnitude of the dipole moments in AM.sub.3Se.sub.6, with one type of dipole moment derived from MSe.sub.4. By contrast, in CsM.sub.9Se.sub.12, three types of dipole moments of the MSe.sub.4 units are aligned along different directions resulting in small Δn values, which are responsible for the nonphase matching behaviors. The relationship between the orientation and the magnitude of acentric building units and the phase matchability in the selenide phases does not seem to be consistent with that in the zinc oxysulfide phases. In phase matchable SrZn.sub.2S.sub.2O, the dipole moments of SrS.sub.4O.sub.2, Zn(1)-S.sub.3O, and Zn(2)S.sub.3O, which lie in the be plane, are inclined relative to the c axis by −22°, 32°, and −59°, respectively, see FIG. 9 Table 3. The orientation preference of the building units is weaker than for those in the nonphase matchable CaZnSO, where the dipole moments of the CaS.sub.3O.sub.3 and ZnS.sub.3O units are aligned along the c direction. The difference in phase matchability between the selenide and oxysulfide phases may be related to the biaxial structure of SrZn.sub.2S.sub.2O, which cannot simply be compared with the uniaxial CaZnSO and selenide phases. Detailed future examinations of the optical properties of larger sized single crystals combined with theoretical calculations should lead to an improved understanding of the phase matchability in these oxysulfide materials.

[0059] Accordingly, single crystals of a new noncentrosymmetric polar oxysulfide SrZn.sub.2S.sub.2O (s.g. P.sub.mn2.sup.1) have been grown in a eutectic KF-KCl flux. The oxysulfide has unusual wurtzite-like slabs consisting of close-packed corrugated double layers of ZnS.sub.3O tetrahedra; the slabs are vertically separated from each other by Sr atoms. These structural features are significantly different from the hexagonal AZnSO (A=Ca, Sr, Ba) which contains similar ZnS.sub.3O tetrahedra. SrZn.sub.2S.sub.2O exhibits phase-matching behavior with SHG efficiencies twice that of KDP, which likely results from the presence of the acentric ZnS.sub.3O and SrS.sub.4O.sub.2 anionic units. This is the first realization of a phase matchable oxysulfide material. The large band gap energy of 3.86 eV and the high thermal stability up to 650° C. in an O.sub.2 gas atmosphere is beneficial for practical applications. Understanding of the difference in phase matchability between SrZn.sub.2S.sub.2O and CaZnSO will serve as a guide for the chemical design of additional functional mixed anion materials.

[0060] In a further embodiment, crystal growth was achieved via single crystals of SrZn.sub.2S.sub.2O grown from KF/KCl eutectic molten salt. 1 mmol of SrO, 1 mmol of Zn, 1 mmol of S, 3.4 mmol of KF, and 4.1 mmol of KCl were loaded in a silver tube that had been welded on one end. The top of the tube was crimped and sealed in an evacuated silica tube under 10-4 Pa. The starting materials were heated in a tubular furnace to 900° C. at 5° C./min, held for 24 h, and cooled to 600° C. at 0.17° C./min. Then the furnace was turned off and cooled down naturally to room temperature. The product was sonicated in water and isolated via vacuum filtration. Colorless transparent plate-like crystals (typical dimension 0.1′0.2′0.05 mm3. See FIG. 10, which shows a photograph of a colorless transparent single crystal of SrZn.sub.2S.sub.2O, which were characterized as SrZn.sub.2S.sub.2O by single-crystal XRD, were isolated from the filtered product containing ZnS and ZnO crystals.

[0061] Polycrystalline powder samples of SrZn.sub.2S.sub.2O were synthesized using SrO, Zn, and S in a stoichiometric ratio. The starting materials were thoroughly mixed in an agate mortar, pressed into a pellet, and heated in an evacuated silica tube 1000° C. for 24 h. Powder XRD measurement using a Bruker D2 Phaser equipped with an LYNXEYE silicon strip detector and a Cu-Ka source identified SrZn.sub.2S.sub.2O as the main phase and SrS as a minor phase. Additional heat treatment on the same condition did not improve the relative amount of SrZn.sub.2S.sub.2O to SrS. Heating at higher temperatures (>1000° C.) decomposed SrZn.sub.2S.sub.2O. The pure sample was obtained by sonicating the product prepared at 1000° C. with water. SrS was easily decomposed by water and removed via vacuum filtration. The powder XRD data of SrZn.sub.2S.sub.2O after washing is presented in FIG. 11, which shows Comparison of the powder XRD pattern of purified SrZn.sub.2S.sub.2O with a theoretical pattern based on the single-crystal XRD analysis. A small amount of SrS was detected.

[0062] Rietveld refinement. Synchrotron X-ray powder diffraction (SXRD) data on the purified SrZn.sub.2S.sub.2O powder sample was collected at room temperature using a one-dimensional X-ray detector installed on BL15XU, NIMS beamline at SPring-8 in Japan. The synchrotron radiation X-rays were mono-chromatized to the wavelength of 0.65298 Å. The sample was loaded in 0.2 mm diameter glass capillary, and the diffraction data was recorded in 0.003° increments in the range of 6<20<50°. As presented in FIG. 12, which shows SXRD patterns for purified SrZn.sub.2S.sub.2O measured at room temperature, the SXRD data was analyzed by Rietveld refinement using the program RIETAN-FP [1]. The refined crystallographic data are given in FIG. 19, see Table 7. With respect to the SXRD patterns for purified SrZn.sub.2S.sub.2O measured at room temperature, obtained, calculated, and difference values are presented by cross marks (red), upper solid lines (black), and bottom solid lines (blue), respectively. The upper and lower vertical lines (green) represent the Bragg peak positions of SrZn.sub.2S.sub.2O and SrS, respectively. SrS is present at 0.3 weight %. FIG. 13 shows room-temperature XRD pattern of the products after TG measurement, which could be assigned to ZnO and SrSO.sub.4. SrZn.sub.2S.sub.2O powder was heated in an aluminum pan up to 1000° C. in O.sub.2 gas atmosphere. FIG. 14 shows a comparison of the crystal structure of SrZn.sub.2S.sub.2O (Pmn2.sub.1) with that of CaFeSeO (Cmc2.sub.1). FIG. 15 shows Crystal Orbital Hamiltonian Population (COHP) for (Zn1/Zn2)-O and (Zn1/Zn2)-S interactions in SrZn.sub.2S.sub.2O. FIG. 16 shows Table 4—Atomic coordinates and equivalent isotropic displacement parameters Ueq for SrZn.sub.2S.sub.2O at 300 K. FIG. 17 shows Table 5—Anisotropic displacement parameters Uij (102′Å2) for SrZn.sub.2S.sub.2O at 300 K. FIG. 18 shows Table 6—Lattice constants and atomic coordinates obtained by the first principle calculations for SrZn.sub.2S.sub.2O.

[0063] While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art using the teachings disclosed herein.