Phase Transition and Second Harmonic Generation in

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Phase Transition and Second Harmonic Generation in Thiophosphates Ag2Cd(P2S6) and AgCd3(PS4)S2 Containing Two Second-Order Jahn−Teller Distorted Cations Yu-Hang Fan, Xiao-Ming Jiang,* Bin-Wen Liu, Shu-Fang Li, Wei-Huan Guo, Hui-Yi Zeng, Guo-Cong Guo,* and Jin-Shun Huang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’ s Republic of China S Supporting Information *

ABSTRACT: Two new phases in the Ag−Cd−P−S system containing two second-order Jahn−Teller (SOJT) distorted d10 cations (Cd2+ and Ag+), namely, Ag2Cd(P2S6) (1) and AgCd3(PS4)S2 (2), are obtained via mediumtemperature solid-state synthesis. Compound 1 exhibits a two-dimensional layered structure and undergoes a first-order structural phase transition at approximately 280 °C. This outcome can be ascribed to the significant mismatch in the expansion coefficients between Cd−S (Ag−S) and P−P (P− S) bonds evaluated through bond valence theory. The three-dimensional noncentrosymmetric (NCS) framework of 2 is constructed by two types of tetrahedral layers consisting of corner-shared CdS4, AgS4, and PS4 tetrahedra. Compound 2 exhibits second harmonic generation (SHG) intensity of 0.45 times that of commercial AgGaS2 (AGS) at a laser irradiation of 1.85 μm and an optical band gap of 2.56 eV, and no intrinsic vibrational absorption of chemical bonds is observed in the range of 2.5−18.2 μm. Both phase transition in 1 and SHG properties in 2 are closely related to the SOJT distorted d10 cations and diverse phosphorus−sulfur polyanions (PaSb)n−, which together can easily result in NCS distorted structures and interesting properties.

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which possess two types of metals bonding to (PaSb)n− anions, have richer structural and property features. Quaternary metal thiophosphates can be mainly categorized into two subgroups. The first one is the A-M−P−S system (A = alkaline or alkaline earth metal cations; M = Ag+, Zn2+, Cd2+, In3+, Bi3+, Ti4+, Zr4+, Nb5+, and Ta5+), in which the polyanions (PaSb)n− are combined with the M−S polyhedra to form the (MxPaSb)m− units,17 such as (BiP2S6)−, [In(PS4) (PS5)2]6−,18 (Nb2PS10)−,19 and (MPS6)m− (M = Zr4+, Nb5+, and Ta5+).20 The second one is the B−M−P−S system (B = neither alkaline nor alkaline earth metal cations; M = Ag+, Zn2+, Cd2+, Ti4+, and Nb5+), in which the (MxPaSb)m− groups can be formed only when the atomic ratio of S: P (the valence of P is +5) is greater than four. The M atoms are bonded to the S atoms of PS4 tetrahedra and the S atoms, which do not bond to the P atom, to construct (MxPaSb)m− groups, such as (Nb2PS10)− in Ag(Nb2PS10) and (Ti2P2S11)2− in Ag2(Ti2P2S11).21 The d10 closed-shell cations, such as Ag+, Cu+, and Cd2+, usually exhibit particular behaviors because of the second-order Jahn−Teller (SOJT) effect.22 The SOJT effect can couple their filled d orbitals and empty s orbitals of similar energy to lower the energy barrier to form different coordination geometries,

INTRODUCTION Metal thiophosphates,1 which have technologically promising properties such as semiconductor light emitting,2 ion migration,3 nonlinear optical (NLO) behaviors,4 phase transitions,3b,4a,5,6 and ferroelectricity,5b have received much attention recently. Diverse physical properties of thiophosphates are partially derived from the structural flexibility of various phosphorus−sulfur polyanions (PaSb)n−, such as (PS4)3−, (P2S6)4−, (P2S7)4−, (P3S10)5−, and (P4S13)6−. These phosphorus−sulfur polyanions appear alone or are integrated with each other or with (S2)2− and metal cations in compounds, such as AgTi2(PS4)3,7a SnP2S6,5b KAuP2S7,7c Ag7(PS4)(P2S7),3b Cs8U5(P3S10)2(PS4)6,8 Rb3Ti3(P4S13) (PS4)3,9 Ag2Nb(P2S6)(S2),10 and Ag(Nb2PS10).11 Ternary metal thiophosphates have interesting physical properties. For instance, two-dimensional (2D) MPS3 (M = first-row transition metal) systems 12 have a potential application for high-energy density lithium batteries. Li3PS4 and Li7P3S11 can be used as ionic conductors.13 InPS414 has large NLO susceptibility and piezoelectric coefficient, GaPS415 has a considerable birefringence, and Sn2P2S6 is a promising ferroelectric material for memory devices.16 Even so, the structural diversities of ternary metal thiophosphates are limited because of their relatively simpler components than those of quaternary ones. By contrast, quaternary metal thiophosphates, © 2016 American Chemical Society

Received: April 23, 2016 Published: December 16, 2016 114

DOI: 10.1021/acs.inorgchem.6b01016 Inorg. Chem. 2017, 56, 114−124

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Inorganic Chemistry

powdery impurities was found in the products of both reactions and could be easily cleaned up mechanically. Pure crystals of the compounds were hand picked under a microscope to measure their physical properties, and their purities were confirmed through powder X-ray diffraction (XRD) studies (Figure S2 in the Supporting Information). Crystal Structure Determinations. Single crystals of 1 and 2 were mounted on glass fibers for single-crystal XRD analysis. The measurements were performed on a Pilatus CCD diffractometer equipped with graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å) at 293 K. The intensity data sets were collected with a ωscan technique and reduced using CrystalClear software.28 The structures of 1 and 2 were computed through direct methods and refined using full-matrix least-squares techniques on F2, with anisotropic thermal parameters for all atoms. All of the calculations were performed with the Siemens SHELXL version 5 of crystallographic software.29 The formulas collectively consider the crystallographically refined compositions and the requirements of charge neutrality. Relevant crystallographic data and details of the experimental condition for 1 and 2 are summarized in Table 1.

which generally result in large thermal parameters and positional (static and dynamic) disorder. The dynamic disorder of d10 closed-shell cations in some compounds, such as Ag5Te2Cl, Ag10Te4Br3, and CuHgSX (X = Cl, Br),23 has a key role in their fast ion conductivity. The different lattice structures with low-energy barrier can be easily overwhelmed by thermal perturbation, which leads to phase transition. Phasechange materials have been widely used for nonvolatile optical and electronic data storage and memory applications, e.g., DVDs and Blu-ray disks.24 The high-symmetry coordination of d10 cations is usually unstable and distorts to low-symmetry geometries, particularly to the NCS symmetry, which is indispensable for NLO properties. NLO materials are of current interest because of their uses in optical signal processing and as new laser sources.25 Much progress has been made particularly in the ultraviolet (UV)-visible region, and many oxide materials have been discovered and even commercialized, including KH2PO4 (KDP), KTiOPO4 (KTP), β-BaB2O4 (BBO), and LiB3O5 (LBO), over the past few decades. The commercially available NLO crystals, such as ZnGeP2 and AgGaS2, which can be used in the mid- and far-infrared (IR) regions, are not adequate for high-power applications mainly because of their low laser damage thresholds.26 Therefore, finding new efficient IR NLO materials has become one of the research focuses in NLO material science and laser technology.27 One approach to design IR NLO materials is the simultaneous incorporation of SOJT distorted d10 cations and NCS PS4 tetrahedra in a single nonoxygen compound. Using these ideas, two new silver cadmium thiophosphates, namely, Ag2Cd(P2S6) (1) and AgCd3(PS4)S2 (2), were obtained through medium-temperature solid-state synthesis. These two compounds are the first ones in the Ag−Cd−P−S system to simultaneously contain two SOJT distorted d10 cations (Ag+ and Cd2+) in a single compound. Interestingly, compound 1 exhibits a significant phase transition at approximately 280 °C, and compound 2 shows an NLO response in the IR region. In this paper, we report the syntheses and crystal structures of Ag2Cd(P2S6) (1) and AgCd3(PS4)S2 (2), their phase transition and NLO properties, and the calculation of bond-valence and first-principles electronic structure.

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Table 1. Crystal Data and Structure Refinement Parameters for 1 and 2 empirical formula formula weight crystal color and shape crystal size (mm) crystal system/space group a (Å) b (Å) c (Å) β (deg) V (Å3), Z Dc (Mg/m3) μ (MoKα) (mm−1) θ range (deg) index range

reflections collected/unique/ Rint data/parameters GOF on F2 Flack parameter final R indicesa (all data)

EXPERIMENTAL SECTION

Reagents and Synthesis. All the starting materials were used as received without further purification. Single crystals of the two compounds were obtained through solid-state reactions. Light-yellow crystals of compound 1 were crystallized from a reaction mixture containing 1.0 mmol of Cd (Aladdin Chemistry Co. Ltd., 99.999%), 2.0 mmol of Ag (Macklin Biochemical Co. Ltd., 99.95%), 6.0 mmol of S (Sinopharm Chemical Regent Co. Ltd., 99.999%), and 2.0 mmol of red phosphorus (Aladdin Chemistry Co. Ltd., 99.99%). Olive crystals of compound 2 were crystallized from the reaction containing 3.0 mmol of Cd (99.99%), 1.0 mmol of Ag (99.95%), 6.0 mmol of S (99.999%), and 1.0 mmol of red phosphorus (99.99%). The starting materials were ground into fine powder in an agate mortar, pressed into pellets, and then loaded into Pyrex tubes. The pellets were then evacuated to 1 × 10−4 Torr and flame-sealed. Then the tubes were placed into a computer-controlled furnace and heated from room temperature to 250 °C at a rate of 30 °C/h and kept at 250 °C for 1 day. The tubes were then heated to 650 °C at 30 °C/h, maintained at that temperature for 4 days, and then slowly cooled to 300 °C at a rate of 2.5 °C/h. They were finally cooled to room temperature in 12 h. Crystals of the title compounds were obtained with the product yields of approximately 90% for 1 and 85% for 2. A small amount of white

R indices (I > 2σ(I)) largest diff peak/hole (e/Å3) a

Ag2Cd(P2S6) (1)

AgCd3(PS4)S2 (2)

582.51 light yellow, lamellar 0.14 × 0.10 × 0.02 monoclinic, C2/c 6.435(4) 11.079(4) 13.433(5) 97.926(9) 948.5(7), 4 4.079 7.891 3.1−27.5 −7 ≤ h ≤ 7 −10 ≤ k ≤ 13 −16 ≤ l ≤ 16 3801/881/0.0513

668.47 olive, block 0.14 × 0.11 × 0.09 monoclinic, Cc 12.288(4) 7.1164(17) 12.280(4) 110.669(7) 1004.7(5), 4 4.419 8.665 3.4−27.5 −15 ≤ h ≤ 15 −9 ≤ k ≤ 8 −13≤ l ≤ 15 4457/2047/0.0169

881/52 0.950

2047/101 0.996 0.00(3) R1 = 0.0220 wR2 = 0.0487 R1 = 0.0217 wR2 = 0.0486 1.76/−1.53

R1 = 0.0629 wR2 = 0.1645 R1 = 0.0495 wR2 = 0.1378 1.56/−1.54

R = Σ∥Fo| − |Fc∥/ Σ|Fo|, wR = (Σ(w(Fo2 − Fc2)2)/ Σ(w(Fo2)2))1/2.

Atomic coordinates and selected interatomic distances are reported in Tables S1−S4. Bond valence theory30 is reliable and useful for the structural analyses of inorganic compounds. The bond valence sum calculations were performed for all the crystallographically independent positions in 1 and 2; the results are shown in Tables S5 and S6. The calculated bond valence sum for all the ions is close to their normal valence, and the agreement factors of the sum defined as bondvalence-sum/Normal-valence are close to 1. Powder XRD. The powder XRD patterns (Figure S2) were collected with a Rigaku Miniflex II diffractometer at 30 kV and 15 mA for Cu-Kα radiation (λ = 1.5418 Å) with a scan speed of 2°/min at room temperature. The measurements of the variable-temperature powder XRD were performed using Rigaku Ultima-IV with Cu-Kα radiation (λ = 1.5418 Å) and a variable-temperature apparatus. The 115


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Figure 1. (a) Coordination polyhedra of Ag, Cd, and P atoms in 1. (b) 2∞(Ag2CdP2S6) layer in the ab plane. The AgS3, CdS6, and P2S6 units are represented by the blue trigon, distorted red and pink octahedron, respectively. (c) Crystal structure of 1 viewed along the a direction. Computational Procedures. The calculation models for 1 and 2 were built directly from their crystallographic data determined by a single-crystal XRD analysis. The electronic structure calculations based on density functional theory (DFT) were performed using the CASTEP package.32 The generalized gradient approximation33 was chosen as the exchange−correlation functional, and the normconserving pseudo potential34 was used. The plane-wave cutoff energy was 765 eV for both 1 and 2, and the threshold of 10−5 eV was set for the self-consistent field convergence of the total electronic energy. The electronic configurations for Ag, Cd, P, and S were 4s24p64d105s1, 4d105s2, 3s23p3, and 3s23p4, respectively. The numerical integration of the Brillouin zone was performed using 2 × 4 × 2 and 4 × 2 × 2 Monkhorst−Pack κ-point meshes for 1 and 2. The Fermi level (Ef = 0 eV) was selected as the reference. Optical properties were calculated and described in terms of the complex dielectric function ε(ω) = εRe (ω) + iεIm (ω). The imaginary part of the dielectric function εRe (ω) is given in the following equation 1:35

simulated patterns were produced using the Mercury program and structures from single-crystal diffraction. Energy-Dispersive X-ray Spectroscopy (EDS). Semiquantitative microscope analyses of single crystals using EDS were performed on a JSM6700F scanning electron microscope and confirmed the presence of Ag, Cd, P, and S in the approximate molar ratio of 2.1:1.0:2.1:4.9 for (1) and 1.0:3.1:1.3:5.4 for (2). The molar ratios were consistent with the stoichiometric ratios, and no other elements were detected. Thermal Analysis. Thermogravimetric (TG) analyses of 1 and 2 were conducted with a Mettler−Toledo TGA/DSC 1 apparatus under a nitrogen atmosphere. The samples and reference were held in Al2O3 crucibles and heated at a rate of 10 °C/min from room temperature to 800 °C. Differential scanning calorimetry (DSC) of 1 was performed with the same apparatus. The sample was heated to 300 °C at 10 °C/ min and then cooled to 25 °C at a rate of 10 °C/min. Reproducibility of the results was confirmed by running multiple heating/cooling cycles. Low-temperature DSC of 1 and 2 was performed using the Netzsch DSC 204F1 thermal analyzer. The samples of ground crystalline material were heated from −100 to 300 °C at 10 °C/min. IR and UV−Vis−Near−IR (NIR) Diffuse-Reflectance Spectroscopy. The IR spectra were recorded using a Vertex 70 FT−IR spectrophotometer in the range of 4000−400 cm−1. Powdered samples were pressed into pellets with KBr. The diffuse reflectance spectra were recorded at room temperature on a computer-controlled Lambda 950 UV−Vis−NIR spectrometer equipped with an integrating sphere in the wavelength range of 200−800 nm. A BaSO4 plate was used as a reference on which the finely ground powders of the samples were coated. The absorption spectra were calculated from reflection spectra using the Kubelka−Munk function.31 Second-Harmonic Generation (SHG) Measurements. The measurement of powder SHG of 2 was investigated using a modified Kurtz−Perry powder technique under laser irradiation at 1.85 μm. The crystalline sample of AGS with a similar particle size (approximately 100 μm) served as the standard. The frequency-doubling signals (925 nm) were detected through an Andor DU420A-BR-DD CCD.

ε Im(ω) =

2e 2π Ωε0

∑

| < ΨCK |u·̂ r|ΨVK > |2 δ(EKC − EKV − E) (1)

K ,V ,C

where δ(ECK − EVK − E) defines the energy difference between the conduction and valence bands (VBs) at the k point with absorption of energy E, û is the polarization of the incident electric field, Ω is the volume of the primitive cell, e is the electric charge, and ΨCK and ΨVK are the vectors defining the conduction and VB wave functions at k. εRe(ω) can be obtained by using the dispersion relationship of Kramers−Kronig:

ε Re(ω) = 1 +

2 P π

∫0

∞

ω′ε2(ω′) d ω′ ω′2 − ω2

(2)

The P in front of the integral means the principal value. The firstorder nonresonant susceptibility at the low-frequency region is given by χ(1) ii (ω) = εii(ω) − 1, and the second-order susceptibilities are 116


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Figure 2. (a) Coordination polyhedra of Cd, Ag, and P atoms in 2. (b) Crystal structure of 2 viewed along the a direction. The CdS4, AgS4, and PS4 units are represented by distorted red, blue, and pink tetrahedron, respectively. Type-I and type-II tetrahedral layers are denoted by dotted boxes. (c) Type-I tetrahedral layer. (d) Type-II tetrahedral layer. calculated by using the “velocity-gauge” formula derived by Sipe et al.:36

As shown in Figure 1b, all CdS6 and P2S6 units edge share with each other to build a nearly hexagonally distorted octahedron ring, forming honeycomb-type voids in the center. The voids are filled with AgS3 trigons, which all share sulfur corners with CdS6 and P2S6 units to form a neutral 2 ∞(Ag2CdP2S6) layer. This layer is constructed by AgS3, CdS6, and P2S6 units with a ratio of 2:1:1. The 2∞(Ag2CdP2S6) layers are further assembled along the c direction with an interlayer distance of 3.18 Å by electrostatic forces (Figure 1c). Compound 1 is isostructural with Ag2MgP2S67b and Ag2MnP2S6.22a The distortion of CdS6 octahedron with the Cd−S bond lengths ranging from 2.651(2) Å to 2.754(2) Å in 1 can be ascribed to the SOJT effect of d10 closed-shell cations Cd2+.22c,d The Cd2+ cation is coordinated by six S atoms with two long Cd−S bond lengths at 2.75 Å and four short ones at 2.65 Å (× 2) and 2.66 Å (× 2). Therefore, the Cd2+ cation tends to move away from the octahedron center to achieve an asymmetrical environment. The ZnS6 octahedron in the Ag2ZnP2S6 analogue38 also expresses apparent distortion attributed to the SOJT effect, and positional splitting, which does not occur in 1, can be found for two Ag atoms in Ag2ZnP2S6. The main bond distances and angles in 1 are summarized in Tables S1 and S2. The P−S bond distances vary from 2.035(3) Å to 2.040(3) Å, and the P−P bond distance is 2.252(5) Å, which is in the normal range for P−S and P−P bond lengths in the known Ag2MgP2S6 and Ag2MnP2S6. The Cd−S bond lengths range from 2.651(2) Å to 2.753(2) Å, which are comparable with the values found in Cd2P2S6.22c The Ag−S

3

i e 2 mω f jl ⎤ dk pij pjl plj ⎡⎢ fil ⎥ + 3 E − Ejl ⎥⎦ 4π 2E − Eji ⎢⎣ E − Eli

χ (2) (− 2ω ; ω , ω) =

∑∫ i ,j,l

BZ

(3)

where the optical transition dipole moment p is taken from dielectric function of CASTEP optical properties calculation. The refractive index n and birefringence Δn were obtained according to the following formula:37a

n(ω) =

2 2 εRe (ω) + εIm (ω) + εRe

2

(4)

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RESULTS AND DISCUSSION Crystal Structure of 1. Only one crystallographically independent Ag, Cd, or P atom and three crystallographically independent S atoms exist in 1. Figure 1a presents the coordination environments of these atoms. Each Ag+ cation is surrounded by three S atoms, forming a nearly regular triangle. Each Cd2+ cation is coordinated by six S atoms to form distorted CdS6 octahedron. Each of the two P atoms possesses a tetrahedral coordination of one P atom and three S atoms and combines into pairs via a P−P bond. Each pair is surrounded by six S atoms, forming a distorted P2S6 octahedron. 117


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Figure 3. (a) TG curve shows that compound 1 can be stable up to approximately 450 °C. Inset: DSC curve of 1 between −100 and 300 °C. (b) The DSC curve of 1 was obtained upon heating and then cooling.

Figure 4. Simulated and variable-temperature XRD powder patterns of 1 from 240 to 320 °C. The black and red asterisks represent the disappeared and new Bragg peaks above 280 °C with increasing temperature.

ring to form the 2D type-II tetrahedral layer. All the CdS4, AgS4, and PS4 tetrahedra in type-I and type-II layers use three of the four tetrahedral vertices to form extended 2D layers, leaving one vertex for each tetrahedron. Type-I and type-II tetrahedral layers are stacked alternately along the c direction by sharing the S atoms to form the 3D framework of 2 (Figure 2b). The bond lengths observed in 2 are summarized in Table S4. The Cd−S bond lengths ranging from 2.459(2) Å to 2.802(2) Å are comparable with those in Cd3.5PS6,22d Cd4GeS6,39 and Ag4CdGe2S7.40 The Ag−S bond distances range from 2.502(3) Å to 2.803(3) Å, consistent with the corresponding values in Ag3PS441 and Ag3Y(PS4)2.42 The P−S distances varying from 2.038(3) Å to 2.064(3) Å are also relatively similar to the P−S bond lengths found in Cd3.5PS6, AgZnPS4,43 and AgTi2(PS4)3.7a As shown from the wide bond-length ranges of Cd−S and Ag− S, the CdS4 and AgS4 tetrahedra are significantly distorted, whereas PS4 tetrahedra are almost regular, which can be attributed to the SOJT effect of Cd2+ and Ag+ with d10 closed shells.44 Although 1 and 2 contain the same component

bond distances in 1 range from 2.470(3) Å to 2.480(3) Å, which are also in agreement with the corresponding values in Ag2ZnP2S6. Crystal Structure of 2. In compound 2, Cd atoms occupy three crystallographically independent positions, and each one is surrounded by four S atoms to form distorted CdS4 tetrahedra (Figure 2a). Only one crystallographically independent position exists for Ag and P atoms, and each position is coordinated by four S atoms to form distorted AgS4 and PS4 tetrahedra. The three-dimensional (3D) framework of 2 (Figure 2b) is constructed by two types of tetrahedral layers. One Cd(3)S4, one AgS4, and one PS4 tetrahedron share two corners of each tetrahedron with each other to form a trigonallike (CdS4) (AgS4) (PS4) unit, which further shares three vertices of the trigonal-like units to form a two-dimensional (2D) type-I tetrahedral layer (Figure 2c). The type-II tetrahedral layer (Figure 2d) consists of only CdS4 tetrahedra. Three Cd(1)S4 and three Cd(2)S4 alternately share two corners with two neighbors to form a hexagonal-like (CdS 4 ) 6 tetrahedral ring, which further shares six vertices of such a 118


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Table 2. Used Bond Valence Parameters B and R0 for the Bond Valence Calculation of 147

elements, they have different structural and property features. The 2D centrosymmetric structure of 1 is composed of AgS3, CdS6, and P2S6 fundamental units, and the 3D NCS structure of 2 is composed of AgS4, CdS4, and PS4 units. The phenomenon may be partially attributed to the flexible coordination characteristics of Ag, Cd, and P by S atoms. Phase Transition of 1. The TG curves (Figures 3a and S3b) show that compounds 1 and 2 can be stable up to 450 and 530 °C, respectively. The DSC curve of 1 between −100 and 300 °C (inset in Figure 3a) shows an endothermic peak at approximately 280 °C, which indicates phase transition. As observed in the DSC thermogram of 1 upon heating from room temperature to 300 °C and then cooling (Figure 3b), the onset temperature of the phase transition upon heating is at 280 °C, whereas the cooling cycle yields an onset temperature of 267 °C. The hysteresis effect is a common feature of a first-order structural phase transition, and the phase transition enthalpy is estimated to be 1859.6 J mol−1 by integrating the DSC peak area. The temperature dependence of powder XRD of 1 from 240 to 320 °C was measured at an interval of 10 °C. As shown in Figure 4, the XRD patterns below 270 °C are nearly unchanged. The main Bragg peaks, such as (002), (112), (1̅13), (004), (223), and (008), from 240 to 270 °C shift to low Bragg angles with increasing temperature because of the thermal expansion of the crystal lattice. With the temperature increasing further, the XRD pattern at 280 °C shows significantly abrupt changes compared with those below 270 °C. Several main Bragg peaks, such as (004), (223), and (008), disappear, and several new ones at 33.9°, 38.2°, 45.1°, and 52.5° are generated. The (002) peak does not shift too much. These results further indicate the structural phase transition of 1 at approximately 280 °C, which is consistent with the critical temperature determined from the DSC measurement. After undergoing phase transition, no significant difference in the powder XRD patterns (Figure S2a) for the same sample before heating and after cooling down to room temperature can be found, thus indicating that the phase transition of 1 is reversible. The high-temperature structure of 1 above 280 °C cannot be obtained because of the temperature limit of our single-crystal XRD. Nevertheless, the structural origin of phase transition of 1 can be proposed based on bond valence theory.30 Generally, the bond length (R) in the crystal structure of a compound is temperature (T) dependent, and the structural phase transition can be mainly attributed to the significant mismatch of the thermal expansion coefficients of chemical bonds. The thermal expansion coefficients (dR/dT) of all bonds in 1 can be evaluated through dR/dT = 1.35k/G,45 where k is the Boltzmann constant and G is the force constant of the chemical bonds. The force constant G can be calculated through G = (k0q2/Re2)(1/B − 2/Re),46 where k0 = 1/4πε0 = 23 nN Å2 electrons−2, B is the bond valence parameter, Re is the equilibrium length of the bond and is supposed to be the experimental bond lengths determined from XRD, and q is the point charges that can be estimated through (8S/3)3/4, where the bond valence S is defined as exp [(R0 − Re)/B]. The used bond valence parameters B and R0 for Cd−S, Ag−S, P−S, and P−P bonds in 1 are listed in Table 2. The calculated expansion coefficients (dR/dT) of all bonds in 1 are shown in Table 3. The dR/dT of P−S and P−P bonds is one order smaller than those of Cd−S and Ag−S bonds, and the dR/dT of Cd−S and Ag−S bonds is close to each other.

bond valence parameters

Cd−S

Ag−S

P−S

P−P

B R0

0.37 2.304

0.37 2.119

0.37 2.145

0.35 2.22

Table 3. Calculated Expansion Coefficients (dR/dT) of Cd− S, Ag−S, P−S, and P−P Bonds in 1 atom 1

atom 2

mult

distance (Å)

dR/dT (10−6 Å K−1)

Cd(1)

S(1) S(2) S(3) S(1) S(2) S(3) S(1) S(2) S(3) P(1)

2× 2× 2×

2.753(2) 2.651(2) 2.661(2) 2.480(3) 2.470(3) 2.474(3) 2.040(3) 2.035(3) 2.037(3) 2.252(5)

44.24 27.45 28.75 26.04 24.84 25.41 2.94 2.88 2.90 5.82

Ag(1)

P(1)

The phase transition of 1 at high temperature can be ascribed to the significant mismatch in the expansion coefficients between Cd−S (Ag−S) and P−P (P−S) bonds. SHG Property of 2. The NCS structure of 2 prompts us to measure its SHG properties. Powder SHG on the hand-selected crystalline sample was measured by using the Kurtz and Perry method48 under laser irradiation at 1.85 μm. A sample of AGS (approximately 100 μm) was prepared as a reference material. The SHG efficiency of 2 is approximately 0.45 times that of AGS (deff = 12.5 pm V−1),49 as shown in Figure 5a. The SHG signal intensity measured through the Kurtz and Perry powder method is proportional to the square of the second-order nonlinear deff coefficient, and the second-order susceptibility χ(2) eff is twice the SHG coefficient deff. Therefore, the derived second-order susceptibilities χ(2) eff for 2 is 16.77 pm/V. The IR spectrum of 2 did not show any intrinsic vibrational absorption of chemical bonds in the wavelength of 2.5−18.2 μm (Figure S5), which covers the important band ranges 3−5 and 8−14 μm of the atmospheric transparent window. The absorptions at 550 cm−1 can be assigned to the ν (P−S) vibrations.50 The optical diffuse reflectance spectra reveal optical band gaps of 2.49 eV for 1 and 2.56 eV for 2 (Figure 5b), which agree well with their light yellow and olive crystals, respectively. Generally, laser damage thresholds of NLO materials, which increase with an increase in band gap, are relative to their band gaps.37 The band gap of 2 (2.56 eV) is comparable with those of commercial AGS (2.73 eV) and ZnGeP2 (2.0 eV),51 implying that 2 has comparable laser damage thresholds with them. Theoretical Studies. To further study the optical properties of 1 and 2, the band structures and densities of states (DOS) were calculated using the CASTEP program.32 As shown in the band structure plots (Figure 6a,c), both compounds exhibit semiconductor characters with band gaps of 2.052 eV for 1 and 1.701 eV for 2. The calculated band gaps of both compounds are significantly smaller than the corresponding experimental values of 2.49 eV (1) and 2.56 eV (2) because of the well-known limitation of the DFT methods. The lowest energy of the conductive bands (CBs) and the highest energy of the VBs are both localized at the G 119


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Figure 5. (a) SHG signals of 2 and reference AGS. (b) Diffuse reflection spectra of 1 and 2.

Figure 6. Band structures of 1 (a) and 2 (c) (shown bands are only between −2 and 4 eV for clarity). Total and partial density of state of 1 (b) and 2 (d). The Fermi level is set to 0 eV for the band structures and DOS.

point, thus indicating that both compounds 1 and 2 are direct band gap materials. As shown in the DOS and partial density of states (PDOS) diagrams (Figure 6b,d), for 1, the CB above the Fermi level (0−5.0 eV) is mainly derived from the P-3p and S-3p states mixed with small amounts of Ag-4s, Ag-4p, Cd-5s, and P-3s states. The VB from −5.0 eV to the Fermi level (0.0 eV) is composed of Ag-4d and S-3p states mixed with small amounts of Cd-5s and Cd-4d states. The state from −15.0 eV to −5.0 eV originates predominantly from S-3s and Cd-4d states. Therefore, the optical absorption for 1 can mainly be ascribed to the

charge transitions from Ag-4d and S-3p states to P-3p and S-3p states. For 2, similar to 1, the P-3p and S-3p states hybridized with a small amount of Ag-4s, Ag-4p, Cd-5s, and P-3s states create CBs above and close to the Fermi level, whereas the VBs just below the Fermi level are mostly formed by Ag-4d and S3p states mixed with a small amount of Cd-5s and Cd-4d states. The VBs farther below the Fermi level are mostly derived from the S-3s and Cd-4d states. Therefore, the optical absorptions of 2 are also mainly ascribed to the charge transitions from the Ag4d and S-3p states to the P-3p and S-3p states. 120


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Figure 7. Calculated real and imaginary parts of the optical dielectric constants of 1 (a: real part, b: imaginary part) and 2 (c: real part, d: imaginary part) along the a, b, and c axes.

Figure 8. Calculated second-order susceptibilities (a) and calculated birefringence Δn (b) of 2.

The calculated real εRe(ω) and imaginary εIm(ω) parts of the optical dielectric constants of 1 and 2 along three different crystallographic axes are illustrated in Figure 7. As shown in the dispersion of the calculated εIm(ω) spectra, the onset energy of absorption is located at approximately 2.50 eV for both 1 and 2 corresponding to their experimental band gaps. Some major and minor absorption peaks can be found on the calculated εIm(ω) spectra of 1 and 2 along the a, b, and c polarization directions. The peaks are located at approximately 4.74, 6.56, and 8.80 eV for 1 and 5.02 and 7.30 eV for 2, and they are derived from the charge transfers from the Ag-4d and S-3p

states to the P-3p and S-3p states according to the DOS analysis. To gain further insights into the NLO properties of 2, the second-order NLO susceptibility was also calculated to explain the SHG efficiencies of 2. Under the restriction of Kleinman’s symmetry, compound 2 has six nonvanishing independent (2) (2) (2) (2) second-order susceptibility tensors (χ(2) 111, χ122, χ133, χ113, χ223, (2) χ333) because of its m point group. As shown in Figure 8a, among all the second-order susceptibility tensors, χ(2) 113 is the largest one at the energy below 1.30 eV. The calculated χ(2) 113 is 11.6 pm/V at a wavelength of 1.85 μm (0.67 eV), which is close to our experimentally derived χ(2) eff coefficient for 2 (16.77 pm/ 121


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V). In addition, the birefringence value of 2 (Figure 8b) ranges from 0.065 to 0.12 with energy from 0 to 3 eV. This result indicates that compound 2 may favorably achieve the phasematching condition in the SHG process. The SHG effect of 2 can be mainly attributed to the AgS4 and PS4 tetrahedral units based on the DOS analysis.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.-C.G.). *E-mail: [email protected] (X.-M.J.). ORCID

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Guo-Cong Guo: 0000-0002-7450-9702

CONCLUSIONS In summary, two new silver cadmium thiophosphates containing two second-order SOJT-distorted d10 cations (Cd2+ and Ag+), namely, Ag2CdP2S6 (1) and AgCd3(PS4)S2 (2), have been prepared through solid-state reactions. Compound 1 features a centrosymmetrically layered structure, in which all edge-sharing CdS6 and P2S6 units form honeycomb-type voids. These voids are filled with AgS3 trigons, which all share corners with CdS6 and P2S6 units to form a neutral 2 ∞(Ag2CdP2S6) layer. The 3D NCS framework of 2 is constructed by two types of tetrahedral layers, which consist of corner-sharing CdS4, AgS4, and PS4, or only CdS4 tetrahedra stacked alternately along the c direction by sharing the S atoms. Compound 1 undergoes a significant first-order structural phase transition at approximately 280 °C, as determined through the DSC and variable-temperature powder XRD measurements. The transition can be ascribed to the significant mismatch in the expansion coefficients between Cd−S (Ag−S) and P−P (P−S) bonds evaluated through bond valence theory. Compound 2 exhibits an SHG intensity of 0.45 times that of commercial AGS at laser irradiation of 1.85 μm, which is determined from the powder SHG measurement. No intrinsic vibrational absorption of chemical bonds is observed in the range of 2.5−18.2 μm. The optical diffuse reflectance spectra of 1 and 2 reveal band gaps of 2.49 and 2.56 eV. The band gap of 2 is close to that of commercial AGS (2.73 eV) and ZnGeP2 (2.0 eV), thus implying that 2 has comparable laser damage thresholds with them. Electronic structure calculations indicate that the optical absorptions for both 1 and 2 are mainly derived from charge transitions from Ag-4d and S-3p states to P-3p and S-3p states. The calculated largest coefficient of 2 (11.6 pm/V) at a wavelength of 1.85 μm, based on DFT, is close to the experimentally derived χ(2) eff coefficient (16.77 pm/V). Both phase transition of 1 and NLO properties of 2 are closely related to the SOJT distorted d10 cations and diverse (PaSb)n− units, of which the simultaneous incorporation into a single compound can easily result in the distorted NCS structure. Studies on the title compounds show that new quaternary thiophosphates with interesting physical and chemical properties could be designed from the effective incorporation of two SOJT distorted cations in thiophosphates.

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Article

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the financial support by the NSF of China (91222204, 21403231, 21303203, 21403237, and 21401052) and the NSF of Fujian Province (2014J05025 and 2014J05034).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01016. CIF data (CIF) Microscopic elemental analyses, experimental and simulated powder X-ray diffraction patterns, TG, DSC, and IR curves and selected bond lengths and angles of 1−2 (PDF) 122


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