LI Meng-Yue XIE Xiu-Yuan WU Xin-Tao LI Xiao-Fang② LIN Hua②

a (Department of Chemistry, Fuzhou University, Fuzhou 350116, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT New functional materials of metal chalcophosphates have been receiving increasing attention due to their wide structural diversity and technologically promising properties. In this work, a quaternary selenophosphate, Cs2ZnP2Se6, has been successfully prepared by the high-temperature solid state reactions with a modified reactive CsCl flux. Single-crystal X-ray diffraction analyses show that Cs2ZnP2Se6 crystallizes in triclinic space group with a = 7.66000(10), b = 7.712(7), c = 12.7599(3) Å, α = 96.911(18)°, β = 104.367(14)°,γ = 109.276(13)°, V = 672.16 Å3 and Z = 2. The major structure feature is the one-dimensional (1D) chain comprised of alternating units of tetrahedrally coordinated Zn2+ ions to the ethane-like [P2Se6]4- units, in which counterbalanced Cs cations are accommodated. Significantly, photo-electrochemical measurement indicated that the title compound was photo-responsive under visible-light illumination. Moreover, the optical gap of 2.67 eV for Cs2ZnP2Se6 was deduced from the UV/Vis reflectance spectroscopy and theoretical calculation shows an indirect band gap with an electronic transfer excitation of Se-4p to Zn-3d/4p and P-3p orbitals. This work presents not only a novel potential application of metal chalcophosphates, but also a facile approach to prepare alkali metal-containing chalcogenides.

Keywords: selenophosphate, zinc, one-dimensional chain, photo-electrochemistry, semiconductor;

1 INTRODUCTION

As an important component of multifunctional materials,metal chalcophosphates have been receiving increasing attention for their potential applications in reversible phase-change transitions[1,2], ferroelectricity[3,4], nonlinear optics[5-8], photoluminescence[9], γ-ray detection[10], thermoelectrics[11], and magnetism[12]. In terms of structure, [PxQy]z-anion groups are formed by tetrahedral or pyramidal phosphorus and some have complex structures such as (1D)[P2Q6]2-[7,9,12,15], [PQ6]-[8,11,14], [P3Q4]-[13], [P5Q10]5-[16-18],and zero-dimensional (0D) [PQ4]3-[10,19,20], [P2Q6]4-[21-23],[P2Q9]4-[24], [P2Q10]4-[25], [P3Q7]3-[26], [P5Q12]5-[5], [P6Q12]4-[5,27]and [P8Q18]6-[28]. In the case of selenium, the ethane-like[P2Se6]4-units are the most dominant building block[15].Furthermore, these [P2Se6]4-units coordinate with metal atoms to form structures of different dimensions. Typical examples include 0D MII2P2Se6(MII= Ba, Ca, Eu, Pb, Sn,Sr)[29-31]and MI4P2Se6(MI= Ag, K, Na, Tl)[19,32-34]; 1D A2P2Se6(A = K, Rb)[7], A2MI2P2Se6(A = K, Cs; MI= Cu, Ag,Au)[20,22], KInP2Se6[35], K4Sc2(PSe4)2(P2Se6)[36],K4In2(PSe5)2(P2Se6)[37], Rb3Sn(PSe5)(P2Se6)[37],K5In3P6Se19[38], K4In4P6Se20[38]and Rb2MIIIP2Se7(MIII= Ce,Gd)[39]; two-dimensional (2D) NaCeP2Se6[40], K2LaP2Se7[41],Rb4Sn5P4Se20[42], Cs4ThP5Se17[43], MII2P2Se6(MII= Fe, Hg,Mg, Zn)[23,44,45], KMP2Se6(M = Sb, Bi)[46], and A2ThP3Se9(A = K, Rb)[43]; and three-dimensional (3D) K10Sn3(P2Se6)4[47]and KREP2Se6(RE = Y, La, Ce, Pr, Gd)[12].

To date, most metal chalcophosphates containing alkali metals have been prepared by the traditional solid-state reactions at high temperature using the molten alkali metal polychalcogenide flux techniques[48]. Recently, we have used the alkali metal halide mixtures as reactive fluxes and this synthetic approach appears to be of general utility in preparing new multinary chalcogenides with various alkali metals[49-69].

With the above considerations in mind, we focused our investigations on the quaternary A/M/P/Q system and successfully obtained one Zn-containing chalcophosphate,Cs2ZnP2Se6. Crystal structure of this compound was first characterized by Kanatzidis et al. in 2016, but there were not any physicochemical properties reported[70]. In this paper,the synthesis, optical gap and theoretical calculation are systemically presented. Moreover, photo-responsive under visible-light illumination are discovered in the metal chalcophosphate for the first time.

2 EXPERIMENTAL

2. 1 Synthesis of Cs2ZnP2Se6

All manipulations were performed in a dry Ar-filled glovebox (H2O content ＜ 0.1 ppm, O2content ＜ 0.1 ppm). Se(99.999%, Aladdin), P (99%, ABCR), Zn (99.95%, Alfa-Aesar)and CsCl (99.99%, Aladdin) were used as received.Cs2ZnP2Se6was prepared from a mixture of CsCl, Zn, P, and Se in the molar ratio of 1.75:2:4:9. The reactants were loaded into a silica crucible and then transferred into a silica jacket.This jacket was flame sealed under a vacuum of 10−3Pa, and then heated in a tube furnace from room temperature to 723 K in 20 h and annealed at this temperature for 20 h, and then heated to 1173 K at a rate of 20 K/h, following a hold time of 50 h, finally subsequently cooled to 573 K at 5 K/h before the furnace was turned off. After washing with distilled water and absolute ethanol, the products consisting of yellow rods of the title compound were obtained. Based on the single-crystal X-ray structural analysis and energy dispersive X-ray spectroscope (EDX) elemental results (Fig. 1), the chemical formula “Cs2ZnP2Se6” was given. The homogeneity of samples was obtained as indicated by the powder X-ray diffraction data (PXRD) shown in Fig. 2. The title compound was insoluble in water and stable in air for more than two months.

Fig. 1. EDX results of Cs2ZnP2Se6

Fig. 2. Experimental (red) and simulated (black) PXRD patterns of Cs2ZnP2Se6

2. 2 Single-crystal X-ray crystallography determination

A yellow rod-shaped crystal was mounted on a glass fiber.The single-crystal X-ray diffraction data were collected on a Rigaku Mercury CCD diffractometer equipped with a graphite-monochromated Mo-Ka radiation source (λ =0.71073 Å) at 293 K. The data were corrected for Lorentz and polarization factors. Absorption correction was performed by the multi-scan method[71]. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2by SHELXL-2014[72]. All atoms were refined with anisotropic thermal parameters. The coordinates were standardized using STRUCTURE TIDY[73]. The structure was solved and refined successfully in the triclinicspace group with a =7.66000(10), b = 7.712(2), c = 12.7599(3) Å, α = 96.911(18)°,β = 104.367(14)°, γ = 109.276(13)°, V = 672.16(3) Å3and Z =2. The final R = 0.0416 and wR = 0.0998 (w = 1/[σ2(Fo2) +(0.0443P)2+ 8.7453P], where P = (Fo2+ 2Fc2)/3), (Δρ)max=1.598, (Δρ)min= -1.529 and S = 1.029 for 3045 observed reflections (I ＞ 2σ(I)) with 151 parameters and generated a formula of Cs2ZnP2Se6, which agreed well with the EDX results. The parameters of atomic positions and anisotropic displacement are shown in Table 1. The selected key lengths are listed in Table 2.

Table 1. Atomic Coordinates and Equivalent Isotropic Displacement Parameters of Cs2ZnP2Se6

Table 2. Selected Bond Lengths (Å) of Cs2ZnP2Se6

2. 3 Powder X-ray diffraction (PXRD)

Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku MiniFlex II powder diffractometer by using Cu-Kα radiation at room temperature. The measurement range of 2θ is 10～70° and the scan step width was 0.02°.

2. 4 Elemental analysis

The elemental analysis data were collected on a field emission scanning electron microscope (FESEM, JSM6700F)equipped with an energy dispersive X-ray spectroscope (EDX,Oxford INCA) on clean single crystal surfaces.

2. 5 UV-Vis-near IR spectroscopy

The optical diffuse reflectance spectra of Cs2ZnP2Se6powdery samples were measured by a Perkin-Elmer Lambda 950 UV-vis spectrometer equipped with an integrating sphere over a 200～2000 nm wavelength range at room temperature and a BaSO4plate as a reference, on which the finely ground sample powders were coated. The absorption spectrum was calculated using the Kubelka-Munk function:α/S = (1 - R)2/2R (α: absorption coefficient, S: scattering coefficient, R: reflectance)[74].

2. 6 Photocurrent measurement

To investigate the optical behavior of Cs2ZnP2Se6, a photo-electrochemical cell consisting of three electrodes (A glassy carbon electrode (GCE, 3.00 mm in diameter),saturated Ag/AgCl electrode and Pt wires were served as the working electrode, reference electrode and counter electrode,respectively) was constructed. Then, the reaction was carried out by irradiating the solution using a 300 W Xe lamp(PLS-SXE300CUV, λ ＞ 420 nm). And the result of the transient photocurrent was recorded by a CHI660E electrochemical workstation (Shanghai Chen-Hua Instrument Corporation, China) at 298 K in 1.0 M KOH (aq) electrolyte.

2. 7 Computational methods

Utilizing density functional theory (DFT) as implemented in the Vienna ab-initio simulation package (VASP) code[75],we investigate the electronic structures of the title compound.We used projector augmented wave (PAW) method[76]for the ionic cores and the generalized gradient approximation(GGA)[77]for the exchange-correlation potential, in which the Perdew-Burke-Ernzerhof (PBE) type[78]exchangecorrelation was adopted. The reciprocal space was sampled with 0.03 Å−1spacing in the Monkhorst-Pack scheme for structure optimization, while denser k-point grids with 0.01 Å−1spacing were adopted for property calculation. We used a mesh cutoff energy of 600 eV to determine the selfconsistent charge density. All geometries are fully relaxed until the Hellmann-Feynman force on atoms is less than 0.01 eV/Å and the total energy change is lower than 1.0 × 10−5eV.

3 RESULTS AND DISCUSSION

3. 1 Structure description

Single-crystal XRD data reveal that compound Cs2ZnP2Se6crystallizes in the triclinic space group(No.2) with a = 7.66000(10), b = 7.712(7), c = 12.7599(3) Å, α =96.911(18)°, β = 104.367(14)°, γ = 109.276(13)°, V = 672.16 Å3and Z = 2. In a symmetric unit, there are 11 crystallographically unique atoms, including two Cs (Cs(1),Cs(2)) sites, one Zn site, two P (P(1), P(2)) sites and six Se(Se(1), Se(2), Se(3), Se(4), Se(5), Se(6)) sites, respectively,and all of them are at the Wyckoff sites of 2i. The detailed refinement data are listed in Table 1, and the selected bond distances are shown in Table 2.

The crystal structure is shown in Fig. 3, in which the 1D chain structure is composed of [ZnP2Se2]2-chains extending along the c axis and scattering with Cs cations located in the chains. Two types of ethane-like structures where the[P2Se6]4−group ([(P1)2Se6]4−and [(P2)2Se6]4−) and the[ZnSe4]6−group with four different bond lengths intersect and share Se atoms make this 1D [ZnP2Se2]2-anion chain.The Zn2+ions form 4-fold tetrahedral [ZnSe4] containing one Se(1), one Se(2), one Se(4) and one Se(6) atoms. As depicted in Fig. 4, the Zn atom shows usual Zn-S interatomic lengths, ranging from 2.4708(13) to 2.4989(14)Å. P(1) atom bridges with P(1) atom to form a P−P bond in 2.252 Å, and each P(1) atom forms a bond with three Se atoms. Similarly, two P(2) atoms form a P-P bond with a bond length of 2.257, and each P(2) atom and the surrounding Se atoms form a tri-coordination environment.The P atom shows the customary P-Se interatomic lengths,ranging from 2.135(2) to 2.210(2) Å. The Cs(1) cation has a coordination environment with ten Se atoms, while Cs(2)cation coordinates with eight Se atoms. For all this, the Cs-Se distances vary from 3.6096(11) to 4.0356(13) Å,which is common in CsSbSe2(3.570 ～4.222 Å)[79],Cs8Ga4Se10(3.379～4.268 Å)[80]and Cs10Ga6Se14(3.444～4.329 Å)[80].

Fig. 3. Structure view of Cs2ZnP2Se6 viewed down the c-direction with the unit cell marked (left).A single 1D chain built of corner sharing [P2Se6] and [ZnSe4] units (right)

Fig. 4. Basic building units of [(P(1))2Se6], [ZnSe4], [(P(2))2Se6], [Cs(1)Se10] and [Cs(2)Se10]in Cs2ZnP2Se6 with the atom numbers and bond lengths marked

3. 2 Optical properties

The optical absorption spectrum (Fig. 5) shows the band gap (Eg) is 2.67 eV for Cs2ZnP2Se6, which is consistent with its yellow color. Such value is well-consistent with the previously reported result in Cs2ZnP2Se6(Eg= 2.63 eV) by Kanatzidis et al. in 2016[70]. Moreover, these data are comparable to those of selenophosphates, such as Cs5P5Se12(Eg= 2.17 eV)[5], K2P2S6(Eg= 2.08 eV)[7], CsZrPSe6(Eg=2.0 eV)[14], and RbPSe6(Eg= 2.18 eV)][8]. The result of the transient photocurrent in Fig. 6 shows an obvious photocurrent generating when the conductive glass coated with Cs2ZnP2Se6is subjected to excitation with regular visible light. Moreover, the reproducibility of the transient photocurrent response was confirmed by the on-off cycles of illumination. The photocurrent data are around 100 nA/cm2that is weaker than those of [(Ba19Cl4)(Ga6Si12O42S8)] (150 nA/cm2)[81]and Lu5GaS9(150 nA/cm2)[82], but stronger than those of Yb6Ga4S15(50 nA/cm2)[82], BaCuSbS3(55 nA/cm2)[83]and BaCuSbSe3(30 nA/cm2)[83]. As far as we know, this is the first metal chalcophosphate with remarkable photo-electrochemical response.

Fig. 5. UV-vis diffuse reflectance of Cs2ZnP2Se6

Fig. 6. Transient photocurrent response of Cs2ZnP2Se6 under simulated solar light irradiation

3. 3 Electronic structure calculation

The electronic band structure of Cs2ZnP2Se6was calculated and shown in Fig. 7 with the highest occupied state set as EF= 0 eV. The valence band maximum (VBM) resides at the Y point whereas the conduction band minimum (CBM) was found along the Γ point. The electronic band structures (Fig. 7)reveals that Cs2ZnP2Se6is an indirect band-gap semiconductor with VBM and CBM at different k points.The fundamentally calculated Eg(1.87 eV) is less than the experimental Eg(2.67 eV), which is expected because of the well-known fact that the density functional theory (DFT)underestimates the Egof bulk solids[84−86].

Fig. 7. Calculated electronic band structure of Cs2ZnP2Se6.Inset: the first Brillouin zone with symmetry points (red)

Fig. 8 depicts the calculation of partial density of states(PDOS) by the Perdew-Burke-Ernzerhof (PBE). PDOS of Cs,Zn, P, and Se in the energy range of −6～6 eV were divided into four sub-sections VB-I (−6 to −2.5 eV), VB-II (−2.5 to 0 eV), CB-I (1.5 to 4.5 eV), and CB-II (4.5 to 6 eV)according to different orbital features. VB-I has a large contribution from the valence electrons of the P-3p and Zn element (4s and 3d states) that mix with the Se-4p states.VB-II is dominated by the p orbitals of Zn (4p), P (3p), and Se (4p), indicating that VB-II absorption is not only determined by the charge transitions in [ZnSe4]6−tetrahedral units but also [P2Se6]4−units. The bottom of CB, which contains CB-I and CB-II sections, is primarily derived from Zn element (4s and 4p states) and P-3p and partial P-3s especially in the 1.5～4.5 eV energy part with mixing from the Se-4p states. As the filling atom, the electron states of Cs contribute only to the higher energy region (4 to 6 eV).Because the optical absorption of a material can be mostly ascribed to the change in transitions between the states of VB and CB near Eg, that is VB-I and CB-I, the [ZnSe4] and[P2Se6] groups should make absolute contributions to the optical properties.

4 CONCLUSION

In summary, by using a facile high-temperature fluxing method, we successfully obtained a quaternary centrosymmetric compound Cs2ZnP2Se6, which possesses a onedimensional [ZnP2Se6]2-chain running down the [001]direction separated by isolated Cs+cations. UV/Vis/NIR diffuse reflectance spectroscopy study shows its semiconducting behavior with an indirect optical gap of around 2.67 eV conformed by the theoretical study. Significantly,Cs2ZnP2Se6is the first reported metal chalcophosphate with remarkable photo-electrochemical response. These results of this work not only provide a facile approach to prepare alkali metal-containing chalcogenides, but also expand a novel potential application of metal chalcophosphates.

Fig. 8. Total and partial density of states (DOSs) of Cs2ZnP2Se6