Thiophosphates Containing Ag+ and Lone-Pair Cations with

Article pubs.acs.org/IC

Thiophosphates Containing Ag+ and Lone-Pair Cations with Interchiral Double Helix Show Both Ionic Conductivity and Phase Transition Yu-Hang Fan, Hui-Yi Zeng, Xiao-Ming Jiang,* Ming-Jian Zhang, Bin-Wen Liu, 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: Quaternary metal thiophosphates containing second-order Jahn− Teller distorted d10 Ag+ and lone-pair cations, Ag3Bi(PS4)2 (1), Ag7Sn(PS4)3 (2), and Ag7Pb(PS4)3 (3), were obtained by solid-state synthesis. The structural frameworks of 2 and 3 feature an infinite 1-D interchiral double helix 1∞(Ag3P2S11), which is rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C. This can be ascribed to the significant mismatch in the expansion coefficients between Pb−S (Ag−S) and P− S bonds evaluated by bond valence theory. The three compounds are Ag+ ionic conductors, and Ag+ ion migration pathways are proposed by calculating maps of low bond valence mismatch. Moreover, the optical properties of the three compounds were studied, and electronic structure calculations were performed. The combination of second-order Jahn−Teller distorted d10 cation and lone-pair cation provides a new strategy to explore new metal thiophosphates with interesting structures and promising properties.

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INTRODUCTION

One main subgroup of quaternary metal thiophosphates is the A−M−P−S system (A = alkali metal cations; M = Zn2+, Cd2+, Sn2+, Pb2+, Al3+, In3+, Bi3+, Ti4+, etc.), a majority of which can be prepared by the alkali metal−thiophosphate-flux method.1b,c,11 Moreover, the very electropositive alkali metals often lead to their low-dimensional (0-D, 1-D, and 2-D) structures.12 In another main subgroup, the B−M−P−S system (B = non-alkali-metal cations; M = Zn2+, Cd2+, In3+, Bi3+, Ti4+, or Nb5+ cations, etc.),13 many compounds were prepared by directly combining the elements, and their structures are more covalent and tend to be high-dimensional (3-D or 2-D) compared with those of the A−M−P−S system.14 Cations with d10 closed-shell configuration, like Ag+ and Cu+, generally exhibit interesting specific behaviors as a result of the second-order Jahn−Teller (SOJT) effect.15 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, generally resulting in large thermal parameters and positional (static and dynamic) disorders. The dynamic disorder of d10 closed-shell cations in some compounds, such as Ag5Te2Cl, Ag4Zr3S8, and CuHgSX (X = Cl, Br),16 plays a key role in their fast ion conductivity. Ionic conductors are of special interest due to their potential technological applications in solid-state batteries and electrochemical cells.17 Normally, different structures with low-energy barriers between them can be easily overwhelmed by thermal

1

Metal thiophosphates have attracted increasing attention because of their wide compositional and structural diversities and remarkable properties, such as ionic conductivity,2 thermoelectricity,3 nonlinear optical behavior,4 piezoelectricity, and ferroelectric phase transition.5 Multitudinous physical properties found in such systems partially result from the combination of metal−S and P−S bonds with distinctly different bonding characters. These properties also result from the structural flexibility of P−S polyanions (PaSb)n−, such as (PS4)3− in CePS4,1a (P2S6)4− in SnP2S64c and Ag2 Nb(P2 S6 )(S 2),6 (P2 S7 )4− in KBiP 2S 71b and K 10Th 3(P2S7)4(PS4)2,1c (P3S10)5− in Cs8U5(P3S10)2(PS4)6,1c (P2S10)4− in Cs4P2S10,1d and (P4S13)6− in Rb3Ti3(P4S13)(PS4)3.1e Ternary metal thiophosphates containing only one kind of metal show a variety of physical properties. For instance, InPS4 exhibits a large nonlinear optical susceptibility and piezoelectric coefficient,7 and Li3PS4 and Ag7P3S11 can be used as ionic conductors.8 Sn2P2S6 is a promising ferroelectric material for application in memory devices,9 and the two-dimensional (2D) MPS3 (M = first-row transition metal) systems are potential electrode materials for high-energy density batteries.10 Even so, the structure and property of ternary metal thiophosphates can be further enriched by the incorporation of a different kind of metal. Quaternary metal thiophosphates containing two kinds of metals especially with significantly different chemical characters will certainly show richer structural and property features. © 2017 American Chemical Society

Received: November 3, 2016 Published: January 5, 2017 962

DOI: 10.1021/acs.inorgchem.6b02667 Inorg. Chem. 2017, 56, 962−973

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Parameters for 1−3 chemical formula temp fw cryst color cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (Mg/m3) μ (mm−1) θ range (deg) index range

indep reflns/Rint GOF on F2 R1a (I > 2σ(I)) wR2b (all data) Δρmax/Δρmin (e/Å3) a

Ag3BiP2S8 (1)

Ag7SnP3S12 (2)

Ag7PbP3S12 (3)

Ag7PbP3S12 (3)

25 °C 851.01 dark red 0.10 × 0.09 × 0.07 monoclinic P21/c 6.715(4) 20.426(11) 10.767(5) 120.08(3) 1277.9(12) 4 4.424 19.768 2.96−25.50 −6 ≤ h ≤ 8 −24 ≤ k ≤ 24 −13 ≤ l ≤ 13 2368/0.0734 0.953 0.0516 0.1280 3.316/−2.214

25 °C 1351.49 yellow 0.28 × 0.17 × 0.13 monoclinic P21/c 10.590(6) 10.206(5) 21.165(3) 119.873(15) 1983.6(15) 4 3.879 9.487 2.22−25.50 −12 ≤ h ≤ 12 −12 ≤ k ≤ 12 −25 ≤ l ≤ 25 3673/0.0315 1.031 0.0456 0.1134 2.717/−2.761

25 °C 1439.98 orange 0.10 × 0.09 × 0.07 monoclinic P21/c 10.740(3) 9.899(3) 21.078(5) 118.678(11) 1966.0(10) 4 4.865 16.853 2.20−27.48 −12 ≤ h ≤ 13 −12 ≤ k ≤ 12 −27 ≤ l ≤ 25 4451/0.0470 1.046 0.0445 0.1454 3.646/−3.110

212 °C 1439.98 red 0.10 × 0.09 × 0.07 hexagonal P63/m 10.721(3) 10.721(3) 10.083(5) 1003.7(8) 2 4.766 2.20−26.15 −7 ≤ h ≤ 10 −9 ≤ k ≤ 12 −12 ≤ l ≤ 11 700/0.0293 1.013 0.0596 0.1823 5.259/−3.322

R1 = ||Fo| − |Fc||/|Fo|. bwR2 = [w(Fo2 − Fc2)2]/[w(Fo2)2]1/2.

An inorganic double helix is very rare, and only ANb2P2S12 (A = K, Rb, Cs), Cs2PdSe8, Ag4SiO4, and Na3Ir3O8,22 which contain a double helix made of two same-handed helixes, have been reported. The interchiral double helix, built by intercrossing left-handed and right-handed helixes, is very rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C. All three compounds are Ag+ ionic conductors. In compounds 1 and 2, partial Ag+ ions are situated in statistically split sites, implying the dynamic disorder of Ag+ and ionic conductivity as confirmed by the measurements of ac impedance spectroscopy at low frequencies. In this Article, we report their syntheses, crystal structures, ionic conductivity, phase transition, and optical properties, as well as the calculation of bond valence and first-principles electronic structure.

perturbation, leading to phase transition. Phase-change materials have important applications in the fields of nonvolatile optical and electronic data storage and memories, e.g., DVDs and Blu-ray disks.18 Metal thiophosphates containing the so-called lone-pair cations with stereochemically active electron pairs, such as Sb3+, Bi3+, Sn2+, and Pb2+, have been proven to form a group of compounds presenting second-order nonlinear optical (NLO) properties with high probability. For instance, α-Na6Pb3P4S16, β-NaSbP2S6, KSbP2S6, and KBiP2S6 exhibit relatively strong SHG responses.4b,19 The active electron pairs of lone-pair cations in thiophosphates always push the sulfide ligands toward one side of the cations, leading to a highly asymmetric coordination geometry. Some interesting structures and properties are predicted to be created by the combination of the second-order Jahn−Teller distorted d10 cation and the lonepair cation in metal thiophosphates. Many quaternary mixed metal thiophosphates such as KBiP2S6, Tl3Bi(PS4)2, Na6Pb3(PS4)4, TlPbPS4, TlSnPS4, and ASnPS4 (A = K, Rb, Cs) have been synthesized and investigated.20 However, until now, only AgBiP2S6,14a,21 the first quaternary thiophosphate containing a lone-pair cation, has been reported in the Ag−VA−P−S (VA = As, Sb, Bi) system, whereas no compound in the Ag−IVA−P−S (IVA = Ge, Sn, Pb) system has been reported. On the basis of these ideas, during the exploration of quaternary metal thiophosphates containing both d10 and lonepair cations, three new metal thiophosphates Ag3Bi(PS4)2 (1), Ag7Sn(PS4)3 (2), and Ag7Pb(PS4)3 (3) were obtained by medium-temperature solid-state synthesis. Compound 1 crystallizes in a 3-D centrosymmetric structure, which is built by infinite one-dimensional 1∞(Ag2BiP2S9) chains connected by Ag−S bonds and AgS4 units. Compounds 2 and 3 are isostructural and feature infinite 1-D interchiral double helixes 1 ∞(Ag3P2S11) assembled in a nearly hexagonal honeycomb style.

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EXPERIMENTAL SECTION

Reagents and Synthesis. All starting materials were used as received without further purification. Single crystals of the three compounds were obtained by solid-state reactions. Dark red crystals of compound 1 were synthesized from the reactions containing 0.2 mmol of Bi (Aladdin Chemistry Co. Ltd., 99.999%), 0.6 mmol of Ag (Macklin Biochemical Co. Ltd., 99.95%), 1.6 mmol of S (Sinopharm Chemical Regent Co. Ltd., 99.999%), and 0.4 mmol of red phosphorus (Aladdin Chemistry Co. Ltd., 99.99%). Yellow crystals of compound 2 were synthesized from the reactions containing 0.2 mmol of Sn (Aladdin Chemistry Co. Ltd., 99.99%), 1.4 mmol of Ag (99.95%), 2.4 mmol of S (99.999%), and 0.6 mmol (99.99%) of red phosphorus. Orange crystals of compound 3 were synthesized from the reactions containing 0.2 mmol of Pb (Aladdin Chemistry Co. Ltd., 99.0%), 1.4 mmol of Ag (99.95%), 2.4 mmol of S (99.999%), and 0.6 mmol of red phosphorus (99.99%). Compounds 1−3 were prepared by the following procedure: the starting materials were ground into a fine powder in an agate mortar, pressed into a pellet, loaded into Pyrex tubes, evacuated to 1 × 10−4 Torr, and then flame-sealed. The tubes were then placed into a computer-controlled furnace. The tubes were 963


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Figure 1. (a) Coordination polyhedra of Ag, Bi, and P atoms in 1. (b) The crystal structure of 1 viewed along the a-direction. Blue distorted tetrahedra represent the Ag(3)S4 unit. (c) Infinite one-dimensional 1∞(Ag2BiP2S9) chain viewed along the b-direction, also identified by the reddotted rectangle in part b. The AgS3, AgS4, BiS5, and PS4 units are, respectively, represented by blue triangle pyramids, blue tetrahedra, red tetragonal pyramids, and pink tetrahedra. heated from room temperature to 250 °C at a rate of 50 °C/h, and maintained at that temperature for 1 day, and then heated to 600 °C at a rate of 50 °C/h, and maintained that temperature for 4 days, and then slowly cooled to 250 °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 product yields of 90% (based on Bi) for 1, 95% (based on Sn) for 2, and 95% (based on Pb) for 3. Pure crystals of the compounds were hand-picked under a microscope for physical property measurements. Their purities were confirmed by powder X-ray diffraction (XRD) studies (Figure S2 in the Supporting Information (SI)). Crystal Structure Elucidation. Single crystals were mounted on glass fibers for single-crystal XRD analysis. The measurements were performed on a mercury/Pilatus CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 25 °C. Variable-temperature single-crystal XRD experiments were carried out on an Oxford Diffraction/Agilent Supernova diffractometer (graphitemonochromated Mo Kα radiation, λ = 0.71073 Å) fitted with an Oxford Cryosystems 700 open flow cooling device.23 The intensity data sets were collected with an ω-scan technique and reduced using CrystalClear software.24 The structures of 1−3 were solved by direct methods and refined by full-matrix least-squares techniques on F2 with anisotropic thermal parameters for all atoms. All calculations were performed with the Siemens SHELXL version 5 package of crystallographic software.25 Relevant crystallographic data and details of the experimental condition for 1−3 are summarized in Table 1. Atomic coordinates and selected interatomic distances are reported in Tables S1−S5 in the Supporting Information. Powder XRD, Energy-Dispersive X-ray (EDS), IR, and UV−Vis Diffuse-Reflectance Spectroscopic Analyses. The powder XRD patterns were collected with a Rigaku Miniflex II diffractometer

powered 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 variable-temperature powder XRD measurements were performed using Rigaku Ultima-IV with Cu Kα radiation and variable-temperature apparatus. Semiquantitative microscope EDS analysis using a JSM6700F scanning electron microscope was performed on the single crystals of 1−3. This confirmed the presence of Ag, Bi, P, and S in the approximate molar ratio of 2.6:1.0:2.3:6.0, Ag, Sn, P, and S in the approximate molar ratio of 7.2:0.7:3.0:10.2, and Ag, Pb, P, and S in the approximate molar ratio of 7.6:1.0:3.6:10.8, for 1, 2, and 3, respectively. These results are in agreement with their formulas as determined by XRD. Furthermore, no other elements were detected. The IR spectra were recorded using a Vertex 70 FT-IR spectrophotometer in the range 4000−400 cm−1. Powdered samples were pressed into pellets with KBr. The diffuse reflectance spectra were recorded at room temperature on a computercontrolled Lambda 950 UV−vis−NIR spectrometer equipped with an integrating sphere in the wavelength range 200−800 nm. A BaSO4 plate was used as reference, on which the finely ground powdery samples were coated. The absorption spectra were calculated from reflection spectra using the Kubelka−Munk function: α/S = (1 − R)2/ 2R, where α is the absorption coefficient, S is the scattering coefficient, and R is the reflectance. Thermal Analysis. Thermogravimetric analyses of 1−3 were carried out with a Mettler−Toledo TGA/DSC 1 apparatus under a nitrogen atmosphere. The samples and reference were held in Al2O3 crucibles and heated from room temperature to 800 °C at a rate of 10 °C/min. Low-temperature differential scanning calorimetry (DSC) was performed using a Netzsch DSC 204F1 thermal analyzer, with powder samples of 1−3 heated from −100 to 220 °C at 10 °C/min. Alternating Current Electrical Conductivity Measurements. The thin disks (∼0.25 mm) were prepared by pressing powder 964


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Figure 2. Coordination polyhedra of Ag, Pb, and P atoms in 3.

Figure 3. (a) The 1∞(Ag3P2S11) interchiral double helix viewed along the b-direction, with its central tunnel filled by Pb2+ cations. (b) The 1 ∞(Ag3P2S11) interchiral double helix viewed along the a-direction. The right- and left-handed helixes are, respectively, guided by the green and yellow helical lines. (c) The overview structure of 3 viewed along the b-direction. The Pb2+ cations, P(1)S4 tetrahedra, and Ag(5−7)S3 trigons are omitted for clarity. (d) The 1∞(Ag3P2S11) interchiral double helix and the interlinkage between the right- and left-handed helixes are consolidated by sharing corners with P(1)S4 tetrahedra, and Ag(5)S3, Ag(6)S3, and Ag(7)S3 trigons. The blue tetrahedra, pink tetrahedra, turquoise tetrahedra, and turquoise trigons represent Ag(1−3)S4, PS4, Ag(4)S4, and Ag(5−7)S3 units, respectively. samples of 1−3 under a pressure of ∼20 MPa. The top and bottom surfaces were poled and sandwich-coated with Au layers as two electrodes using an ion sputtering coater. Impedance spectra of 1−3 were recorded with a Solartron SI 1260 impedance gain-phase analyzer. After an equilibration time of 30 min, impedance spectra (5 mHz to 10 MHz) were recorded at a temperature range 26−50 °C. The samples were kept under an oxygen- and moisture-free argon atmosphere during the measurements. Computational Procedures. Electronic band structures, densities of states (DOS), and optical properties of 1−3 were studied by

theoretical calculations. Before the calculations, the structures of 1 and 2 were modified by fixing the split Ag atoms with partial occupancy at reasonable positions with full occupancy, and were then used after further geometric optimization, i.e., for 1, the split Ag(3A, SOF = 0.17) and Ag(3B, SOF = 0.83) atoms were fixed at their major split position Ag(3B) with an occupancy of 1, and for 2, the split Ag(4A, SOF = 0.5) and Ag(4B, SOF = 0.5) atoms were fixed at their midpoint with an occupancy of 1. The structure of 3, determined from single-crystal XRD analysis, was directly adopted for the calculation. The electronic structure calculations based on DFT were performed using the 965


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Figure 4. (a) TG curve showing that compound 3 can be stable up to ∼392 °C. Inset: DSC curve of 3 between −100 and 220 °C. (b) DSC curve of 3 obtained upon heating, and subsequent cooling.

Figure 5. (a) Crystal structure of 3 at 25 °C. (b) Crystal structure of 3 at 212 °C; the Pb and Ag positions are mixed with partial P and S atoms, whose occupancies are not exactly determined because of the high disorder at high temperature. CASTEP package.26 The generalized gradient approximation27 was chosen as the exchange-correlation functional, and a norm-conserving pseudopotential was used. The plane-wave cutoff energy was 800 eV for 1−3, and a threshold of 10−5 eV was set for the self-consistent field convergence of the total electronic energy. The valence electronic configurations for Ag, Bi, Sn, Pb, P, and S were 4s24p64d105s1, 5d106s26p3, 5s25p2, 5s25p65d106s26p2, 3s23p3, and 3s23p4, respectively, while the numerical integration of the Brillouin zone was performed utilizing 3 × 1 × 2, 2 × 2 × 1, and 2 × 2 × 1 Monkhorst−Pack κ-point meshes for 1−3, respectively. The Fermi level (Ef = 0 eV) was selected as the reference.

tetrahedron; the Ag(2) atom is surrounded by three S atoms, forming a triangle pyramid with the Ag atom in a vertex; and Ag(3) atom is tetrahedrally coordinated by four S atoms, with the Ag(3) atom split into two adjacent positions, Ag(3A) and Ag(3B). The Bi atom is coordinated by five S atoms to form a distorted BiS5 tetragonal pyramid, while each P atom adopts a tetrahedral coordination with four S atoms. An overview of the 3-D structure of 1 is presented in Figure 1b. The structure is built by infinite one-dimensional 1 ∞(Ag2BiP2S9) chains connected by Ag(1)−S(3) bonds and Ag(3)S4 units. Each BiS5 tetragonal pyramid shares edges with one P(1)S4 and one P(2)S4 tetrahedra forming a BiP2S9 unit. Subsequently, all BiP2S9 units share corners with Ag(1)S4 tetrahedra and Ag(2)S 3 triangle pyramids forming 1 ∞(Ag2BiP2S9) chains (Figure 1c) along the a-direction. The 1 ∞(Ag2BiP2S9) chains are assembled via Ag(1)−S(3) bonds and corner- and edge-sharing with Ag(3)S4 units along the b- and cdirections to construct the 3-D structure of 1. Crystal Structure of 2 and 3. Compounds 2 and 3 are isostructural, with the structure of 3 discussed as representative. The Ag atoms in 3 adopt two different types of distorted coordination geometry with S atoms, i.e., the Ag(1−4)S4 tetrahedra type and the Ag(5−7)S3 trigon type (Figure 2).

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RESULTS AND DISCUSSION Crystal Structure of 1. There are, respectively, three, one, two, and eight crystallographically independent positions for Ag, Bi, P, and S atoms in 1. As presented in Figure 1a, the Ag(1) atom is surrounded by four S atoms, forming a distorted Table 2. Used Bond Valence Parameters B and R0 for the Bond Valence Calculation of 335 bond valence param

Pb−S

Ag−S

P−S

B R0

0.37 2.49

0.37 2.36

0.37 2.08 966


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double helixes (Figure 3a). Furthermore, the interlinkages between the right- and left-handed helixes are also consolidated by corner-sharing P(1)S4 tetrahedra, and Ag(5)S3, Ag(6)S3, and Ag(7)S3 trigons (Figure 3d) between their neighboring screw 1 (Ag3P2S11) are threads. The interchiral double helixes ∞ assembled in a nearly hexagonal honeycomb style, forming the 1-D tunnels along the b-direction between the different double helixes. These tunnels are then embedded with tetrahedrally coordinated Ag(4) atoms by four S atoms from the double helixes (Figure 3c). In 1−3, the bond distances of Ag−S (2.442−2.888 Å) and P−S (2.018−2.083 Å) are comparable to Ag−S and P−S bond lengths in Ag7P3S1128 and Ag3PS4,29 respectively. The Bi−S bond distances in 1, varying from 2.665 to 2.970 Å, are in agreement with the corresponding values in BiPS430 and AgBiP2S6.14a In addition, the Sn−S bond distances in 2 (ranging from 2.612 to 2.658 Å) and Pb−S bond distances in 3 (ranging from 2.809 to 2.918 Å) are also close to the Sn−S bond lengths in KSnPS420d and Pb−S bond lengths in Pb3(PS4)2,31 respectively. Due to the lone electron pairs surrounding the central atoms Bi (6s2), Pb (6s2), and Sn (5s2), Bi3+ in 1 adopts a distorted tetragonal-pyramidal geometry (Figure 1a), which can be viewed as a sulfur-deficient BiS6 octahedron. Similarly, the tricoordinated, not tetrahedrally coordinated, Sn2+ in 2 and Pb2+ in 3 (Figure 2h) can also be attributed to the loneelectron-pairs’ role in pushing the sulfide ligands toward one side of the cations, resulting in a highly asymmetric coordination geometry. Moreover, the interchiral double helix constructed by AgS4 and PS4 tetrahedra is very rare in inorganic compounds, indicating that some novel structures can be created by combining the second-order Jahn−Teller distorted d10 cation and the lone-pair cation in metal thiophosphates. Phase Transition of 3. The thermogravimetric (TG) curves (Figures S3a and S4a and Figure 4a) show that compounds 1, 2, and 3 can be stable up to 520, 465, and 392 °C, respectively. The DSC curve of 3 measuring from −100 to 220 °C (inset in Figure 4a) shows an endothermic peak at ∼204 °C, which is far below the decomposition temperature (392 °C), indicating a phase transition. As can be seen from the DSC thermogram of 3 upon heating from room temperature to 300 °C and then cooling (Figure 4b), the onset temperature of the phase transition upon heating is ∼204 °C, whereas the cooling cycle yields an onset temperature of ∼196 °C. The hysteresis effect is a common feature typical for first-order structural phase transition, and the phase transition enthalpy is estimated to be 995.4 J mol−1 by integrating the DSC peak area. The temperature dependence of powder XRD patterns of 3 from 170 to 275 °C were measured. As shown in Figure S5, the XRD patterns below 205 °C are almost unchanged. With a further increase in temperature, the XRD patterns at 205 °C show significant abrupt changes compared with those below 205 °C. Several main Bragg peaks like (121), (211), and (300) disappear, and several new ones such as peaks at 34.5°, 37.7°, and 50.8° are created. This further indicates a lattice phase transition of 3 at ∼204 °C, which is consistent with the critical temperature determined from DSC measurement. No significant difference between the powder XRD patterns (Figure S5) of the same sample before heating and after undergoing the phase transition and cooling down to room temperature can be found. Similarly, EDS results of single crystals of 3 before and after undergoing phase transition are almost the same, indicating that the phase transition of 3 is reversible.

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

atom 2

Pb(1)

S(8) S(10) S(2) S(5) S(3) S(9) S(12) S(1) S(3) S(8) S(4) S(2) S(4) S(6) S(11) S(4) S(6) S(9) S(3) S(1) S(7) S(10) S(6) S(10) S(12) S(7) S(11) S(9) S(4) S(5) S(7) S(1) S(3) S(10) S(2) S(12) S(6) S(8) S(9) S(11)

Ag(1)

Ag(2)

Ag(3)

Ag(4)

Ag(5)

Ag(6)

Ag(7)

P(1)

P(2)

P(3)

distance (Å) 2.809 2.918 2.876 2.664 2.524 2.567 2.608 2.507 2.551 2.608 2.682 2.658 2.534 2.563 2.685 2.546 2.805 2.705 2.548 2.613 2.462 2.471 2.461 2.527 2.538 2.522 2.540 2.538 2.063 2.033 2.082 2.032 2.054 2.071 2.049 2.023 2.074 2.050 2.063 2.024

(3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (4) (4) (3) (4) (4) (4) (4) (4) (4) (4) (4) (4)

dR/dT (10−6 Å K−1) 21.84 36.18 29.96 61.79 31.97 39.31 47.58 29.63 36.46 47.46 67.00 59.92 33.60 38.40 67.87 35.58 118.98 74.89 35.91 48.63 23.99 25.01 23.97 32.58 34.35 31.87 34.53 34.20 3.27 2.85 3.58 2.83 3.14 3.41 3.07 2.71 3.47 3.09 3.28 2.73

The Pb atom is tricoordinated by three S atoms forming a PbS3 triangle pyramid with the Pb atom in a vertex. All P atoms are coordinated by four S atoms forming weakly distorted PS4 tetrahedra. The crystal structure of 3 features infinite one-dimensional interchiral double helixes 1∞(Ag3P2S11) assembled in a nearly hexagonal honeycomb style (Figure 3c). The tetrahedral units Ag(2)S4, P(2)S4, and Ag(3)S4 share corners with each other in a central atom sequence of Ag(2)P(2)Ag(3)Ag(2)P(2)Ag(3) forming a right-handed helix 1∞(Ag2PS9) (Figure 3b). Moreover, the tetrahedral units P(3)S4, Ag(1)S4, and Ag(2)S4 share corners with each other in a central atom sequence of P(3)Ag(1)Ag(2)P(3)Ag(1)Ag(2) forming a left-handed helix 1 1 ∞(Ag2PS9). A right-handed helix ∞(Ag2PS9) and a left-handed helix 1∞(Ag2PS9) are then intercrossed by sharing the Ag(2)S4 tetrahedra forming an interchiral double helix 1∞(Ag3P2S11) along the b-direction with the central tunnels filled by Pb2+ cations, each of which is coordinated by three S atoms from the 967


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Figure 6. Representative impedance spectra of crystalline samples of 1 (a−c), 2 (d−f), and 3 (g−i) at three temperatures: 26, 40, and 50 °C.

the hexagonal system at high temperature. From the structure comparison (Figure 5) and orientation matrix analysis of diffraction geometry, upon phase transition, the neighboring interchiral double helix layers in the ab-plane shift a little relative to each other along the c-direction so that the unit-cell β angle of 61.3° at low temperature decreases to exactly 60° at high temperature. The c-axis length is also halved, leading to a change in the lattice symmetry. In addition, the structure of 3 at high temperature is highly disordered; only heavy atoms like Pb and Ag are assigned to the largest electron density peaks during structure determination and refinement. These positions should then be mixed with partial P and S atoms to meet charge neutrality of the whole structure. To further study the structural origin of the phase transition of 3, thermal expansion coefficients of chemical bonds were calculated on the basis of bond valence theory.32 Generally, the bond lengths (R) in a crystal structure depend on temperature (T). Therefore, the lattice phase transition of a compound can be mainly attributed to serious thermal expansion coefficient mismatches in chemical bonds. The thermal expansion coefficients (dR/dT) of all bonds in 3 can be evaluated by dR/dT = 1.35k/G,33 where k is the Boltzmann constant and G is the force constant of the chemical bonds. The force constant G can be calculated by G = (k0q2/Re2)(1/B − 2/Re),34 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

Figure 7. Conductivity data (T = 40−100 °C) of crystalline samples of 1, 2, and 3.

The structure of 3 at 212 °C above the critical temperature, defined as the high-temperature phase, was also determined by single-crystal XRD using the same single crystal used for structure determination at room temperature, defined as the low-temperature phase. As shown in Table 1, the structure of 3 transforms from the space group of P21/c (No. 14) in the monoclinic system at room temperature to P63/m (No. 176) in 968


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Figure 8. Representative impedance spectra of crystalline samples of 1 (a), 2 (b), and 3 (c) at three temperatures, 26, 40, and 50 °C.

Figure 9. Absorption spectra of 1 (a), 2 (b), and 3 (c), respectively, converted from their diffuse reflectance spectra.

electrode−sample interfacial impedances which are associated with ionic polarization and diffusion-limited phenomena at the electrode. The presence of this low-frequency spike supports the idea that conduction occurs mainly through ions.36 The real and imaginary parts of the impedance of the three compounds decrease with the increase in temperature from 26 to 50 °C. This is normal for the semiconductor (insulator) and the ionic conductor, because both the number of available charge carriers (electron and ion) and the drift velocity of carriers increase as the temperature increases. Temperature dependence of the conductivities of 1−3 are shown in Figure 7. Activation energies of 0.038 eV for 1, 0.095 eV for 2, and about 0.11 eV for 3 were extracted from the extrapolated slopes in the Arrhenius type log σ versus 1/T plots. The activation energies of 1−3 are well below 0.3 eV, which may make them interesting for application in electrochemical devices.16d There is a good correlation between the topology of ionconduction pathways and maps of low bond valence mismatch, which can be used for exploring ion diffusion in a crystal structure.37 Pathways of the lowest activation energy for Ag+ migration in compounds 1−3 are assumed to be those

supposed to be the experimental bond lengths determined from XRD, and q is the point charges estimated by (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 Pb−S, Ag−S, and P−S bonds in 3 are listed in Table 2. The calculated expansion coefficients (dR/dT) of all bonds in 3 are shown in Table 3. The dR/dT of the P−S bond is 1 order smaller than those of Pb−S and Ag−S bonds, while the dR/dT values of Pb−S and Ag−S bonds are close to each other. Thus, the phase transition of 3 at high temperature can be ascribed to the significant mismatch in the expansion coefficients between Pb−S (Ag−S) and P−S bonds. Ionic Conductivity Measurements. The disordered Ag sites in the structures of 1−3 prompt us to measure their ionic conductive properties. Typical impedance spectra of polycrystalline samples of 1−3 at three temperatures (26, 40, and 50 °C) plotted in the complex impedance plane are shown in Figure 6. The high-frequency arc that dominates the impedance spectra can be attributed to the intra- and intergranular responses of the pelleted samples. All impedance spectra for 1− 3 clearly show the presence of low-frequency spikes at three different temperatures. The low-frequency spike is attributed to 969


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Figure 10. Band structures of 1 (a), 2 (b), and 3 (c) (bands are shown only between −2.0 and 3.2 eV for clarity). Total and partial densities of state for 1 (d), 2 (e), and 3 (f). The Fermi level is set at 0 eV for all the band structures and DOS.

−7.5 eV to the Fermi level are composed of Ag-4d, P-3p, and S3p states mixed with small amounts of Bi-6p and S-3s states. The states from −17.5 to −7.5 eV originate predominately from Bi-6s, P-3s, P-3p, and S-3s states. Therefore, the optical absorptions of 1 can be mainly ascribed to the charge transitions from Ag-4d, P-3p, and S-3p states to Bi-6p and S3p states. For 2, the Sn-5p, P-3s, P-3p, and S-3p states, hybridized with a small amount of Ag-5s and Ag-4p states, create the CBs between the Fermi level (0.0 eV) and 5.0 eV. Moreover, the VBs between −7.5 eV and the Fermi level are mostly formed by Ag-4d, P-3p, and S-3p states mixed with a small amount of P-3s, Sn-5s, Sn-5p, and S-3s states, while the VBs between −17.5 and −7.5 eV are mostly formed by S-3s, P3s, and P-3p states mixed with a small amount of S-3p states. Therefore, the optical absorptions of 2 are mainly ascribed to the charge transitions from Ag-4d, P-3p, and S-3p states to Sn5p, P-3s, P-3p, and S-3p states. For 3, the Pb-6p, P-3s, P-3p, and S-3p states, mixed with a small amount of Ag-5s and Ag-4p states, form the CBs between the Fermi level (0.0 eV) and 5.0 eV. Furthermore, the VBs between −7.5 eV and the Fermi level are mostly created by Ag-4d, P-3p, and S-3p states, while the VBs between −17.5 and −7.5 eV are predominantly formed by Pb-5d, S-3s, P-3s, and P-3p states. Therefore, the optical absorptions of 3 can be mainly attributed to charge transitions from Ag-4d, P-3p, and S-3p states to Pb-6p, P-3s, P-3p, and S3p states.

connections between equilibrium positions, for which the maximum valence mismatch is defined as ΔV = |V − Vequilibrium|. Here, the simple valence sum V = ∑SAg−S and SAg−s = exp[(2.36 − RAg−S)/0.37], and RAg−S is the distance between the trial Ag+ positions and well-defined S2− positions. Pathways of Ag+ migration in the three compounds were estimated by setting the maximum valence mismatch to 0.3, while the valence sum for Ag+ is calculated for a grid of points throughout the asymmetric units. As can be seen from the calculated models of Ag+ ion migration pathways shown in Figure 8, a 1-D continuous region of low bond valence mismatch connects Ag(3)S4 units along the a-direction in 1. Moreover, a 2-D continuous region connects Ag(1, 2, 4)S4 and Ag(5, 6)S3 units in the ab-plane in 2 and 3, which are proposed to be Ag+ ion migration pathways. Optical Spectrum Studies. The IR spectra of 1−3 did not show any intrinsic vibrational absorption of chemical bonds in the range 4000−600 cm−1 (Figure S6). However, minor peaks at ∼1600 and 3400 cm−1 originate from the water, while the absorptions at around 550 cm−1 can be attibuted to the P−S asymmetric stretching vibrations. 38 The optical diffuse reflectance spectra of 1, 2, and 3 reveal optical band gaps of 1.74, 2.25, and 2.09 eV, respectively (Figure 9), which agree well with their dark red (1), yellow (2), and orange (3) colors. Theoretical Studies. To further study the optical properties of 1−3, calculations of the band structures and DOS were performed using the CASTEP program.26 From the band structure plots (Figure 10), compounds 1−3 are all indirect band gap materials. The calculated band gaps of 1, 2, and 3 are 1.73, 1.73, and 1.77 eV, respectively, which are close to the corresponding experimental values of 1.74, 2.25, and 2.09 eV for 1−3, respectively. As shown in Figure 10, for 1, the conductive bands (CBs) above the Fermi level (0−5.0 eV) are mainly derived from Bi6p and S-3p states mixed with small amounts of Ag-5s, Ag-4p, P-3s, and P-3p states. Moreover, the valence bands (VBs) from

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CONCLUSIONS

Three new quaternary metal thiophosphates containing secondorder Jahn−Teller distorted d10 Ag+ and lone-pair cations, Ag3Bi(PS4)2 (1), Ag7Sn(PS4)3 (2), and Ag7Pb(PS4)3 (3), were prepared by solid-state reactions, with compounds 2 and 3 being the first two cases found in the Ag−IVA−P−S family. The 3-D structure of 1 is built by infinite one-dimensional 1 ∞(Ag2BiP2S9) chains connected by Ag−S bonds and AgS4 units. Compounds 2 and 3 are isostructural, and their structures 970


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Inorganic Chemistry feature infinite 1-D interchiral double helixes 1∞(Ag3P2S11) assembled in a nearly hexagonal honeycomb style. The interchiral double helix, composed of intercrossing left- and right-handed helixes, is very rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C as determined by DSC and variable-temperature powder XRD measurements. High-temperature single-crystal XRD analysis showed that the structural origin can be explained by a little shift in the ab-plane of the neighboring interchiral double helix layers relative to each other along the c-direction so that the unit-cell β angle of 61.3° at low temperature decreases to exactly 60° at high temperature. The structural phase transition can be ascribed to the significant mismatch in the expansion coefficients between Pb−S (Ag−S) and P−S bonds evaluated by bond valence theory. The low-frequency spikes on the ac impedance spectra indicate that compounds 1−3 are Ag+ ionic conductors. Ag+ ion migration pathways are proposed to be 1D connected regions along the a-direction in 1 and 2-D connected regions in the ab-plane in 2 and 3, on the basis of the calculation of maps of low bond valence mismatch. The optical diffuse reflectance spectra reveal band gaps of 1.74, 2.25, and 2.09 eV for 1, 2, and 3, respectively. Electronic structure calculations show that the optical absorptions for 1 are mainly derived from charge transitions from Ag-4d, P-3p, and S-3p states to Bi-6p and S-3p states. Moreover, optical absorptions for 2 and 3 are mainly derived from charge transitions from Ag4d, P-3p, and S-3p states to Sn-5p (or Pb-6p), P-3s, P-3p, and S-3p states. Several new compounds with novel structures and promising properties may be created by combining the secondorder Jahn−Teller distorted d10 cation and the lone-pair cation in metal thiophosphates.

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T. J.; Kanatzidis, M. G. Alkali-metal polythiophosphate fluxes for synthesis at intermediate temperatures. Isolation and structural characterization of ABiP2S7 (A = K, Rb). Chem. Mater. 1993, 5, 1061−1063. (c) Hess, R. F.; Abney, K. D.; Burris, J. L.; Hochheimer, H. D.; Dorhout, P. K. Synthesis and characterization of six new quaternary actinide thiophosphate compounds: Cs8U5(P3S10)2(PS4)6, K10Th3(P2S7)4(PS4)2, and A5An(PS4)3, (A = K, Rb, Cs; An = U, Th). Inorg. Chem. 2001, 40, 2851−2859. (d) Aitken, J. A.; Canlas, C.; Weliky, D. P.; Kanatzidis, M. G. [P2S10]4‑: A novel polythiophosphate anion containing a tetrasulfide fragment. Inorg. Chem. 2001, 40, 6496− 6498. (e) Wu, Y. D.; Bensch, W. Rb3Ti3(P4S13) (PS4)3 and Cs2Ti2(P2S8) (PS4)2: Two polar titanium thiophosphates with complex one-dimensional tunnels. Inorg. Chem. 2007, 46, 6170− 6177. (f) Mercier, R.; Malugani, J. P.; Fahys, B.; Robert, G.; Douglade, J. Structure of lithium tetrathiophosphate. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1982, B38, 1887−1890. (g) Mercier, R.; Malugani, J. P.; Fahys, B.; Douglande, J.; Robert, G. Synthesis, crystalline-structure and vibrational analysis of lithium hexathiohypodiphosphate-Li4P2S6. J. Solid State Chem. 1982, 43, 151−162. (h) Gutzmann, A.; Naether, C.; Bensch, W. Synthesis, crystal structure and vibrational properties of K0.38TaPS6 and Rb0.46TaPS6: Chemical intercalation and deintercalation of the ternary TaPS6 phase. Solid State Sci. 2004, 6, 1155−1162. (i) Komm, T.; Schleid, T. The first rubidium rare-earth (III) thiophosphates: Rb3M3[PS4]4 (M = Pr, Er). J. Solid State Chem. 2005, 178, 454−463. (j) Banerjee, S.; Szarko, J. M.; Yuhas, B. D.; Malliakas, C. D.; Chen, L. X.; Kanatzidis, M. G. Room temperature light emission from the low-dimensional semiconductors AZrPS6 (A = K, Rb, Cs). J. Am. Chem. Soc. 2010, 132, 5348−5350. (2) (a) Adams, S.; Rao, R. P. Structural requirements for fast lithium ion migration in Li10GeP2S12. J. Mater. Chem. 2012, 22, 7687−7691. (b) Kuhn, A.; Koehler, J.; Lotsch, B. V. Single-crystal X-ray structure analysis of the superionic conductor Li10GeP2S12. Phys. Chem. Chem. Phys. 2013, 15, 11620−11622. (c) Jansen, M.; Henseler, U. Synthesis, structure determination, and ionic conductivity of sodium tetrathiophosphate. J. Solid State Chem. 1992, 99, 110−119. (3) McGuire, M. A.; Reynolds, T. K.; DiSalvo, F. J. Exploring thallium compounds as thermoelectric materials: Seventeen new thallium chalcogenides. Chem. Mater. 2005, 17, 2875−2884. (4) (a) Cleement, R.; Lacroix, P. G.; O'Hare, D.; Evans, J. Multiproperty materials: Magnetism and NLO properties. Adv. Mater. 1994, 6, 794−797. (b) Zhou, M. L.; Kang, L.; Yao, J. Y.; Lin, Z. S.; Wu, Y. C.; Chen, C. T. Midinfrared nonlinear optical thiophosphates from LiZnPS4 to AgZnPS4: A combined experimental and theoretical study. Inorg. Chem. 2016, 55, 3724−3726. (c) Cleary, D. A.; Willett, R. D.; Ghebremichael, F.; Kuzyk, M. G. Temperature dependent second harmonic generation in tin phosphorus sulfides. Solid State Commun. 1993, 88, 39−41. (5) (a) Carpentier, C. D.; Nitsche, R. Ferroelectricity in Sn2P2S6. Mater. Res. Bull. 1974, 9, 1097−1100. (b) Rogach, E. D.; Sviridov, E. V.; Arnautova, E. A.; Savchenko, E. A.; Protsenko, N. P. Reverse switching effect in ferroelectric-films of Sn2P2S6. Zh. Tekh. Fiz. 1991, 61, 201−204. (c) Bourdon, X.; Grimmer, A. R.; Cajipe, V. B. 31P MAS NMR study of the ferrielectric-paraelectric transition in layered CuInP2S6. Chem. Mater. 1999, 11, 2680−2686. (d) Kroupa, J.; Tyagur, Y. I.; Grabar, A. A.; Vysochanskii, Y. M. Electro-optic properties of Sn2P2S6. Ferroelectrics 1999, 223, 421−428. (6) Wu, Y. D.; Bensch, W. Syntheses, Crystal structures and spectroscopic properties of Ag2Nb[P2S6][S2] and KAg2[PS4]. J. Solid State Chem. 2009, 182, 471−478. (7) (a) Diehl, R.; Carpentier, C. D. The structural chemistry of indium phosphorus chalcogenides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1978, 34, 1097−1105. (8) (a) Homma, K.; Yonemura, M.; Kobayashi, T.; Nagao, M.; Hirayama, M.; Kanno, R. Crystal structure and phase transitions of the lithium ionic conductor Li3PS4. Solid State Ionics 2011, 182, 53−58. (b) Brinkmann, C.; Eckert, H.; Wilmer, D.; Vogel, M.; Schmedt auf der Guenne, J.; Hoffbauer, W.; Rau, F.; Pfitzner, A. Re-entrant phase transition of the crystalline ion conductor Ag7P3S11. Solid State Sci. 2004, 6, 1077−1088.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02667. Atomic coordinates, select bond distances, powder XRD patterns, DSC curves, IR spectra, and additional information (PDF) Details of crystallographic studies (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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REFERENCES

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