English

• 1. INTRODUCTION

  In the past decades, many achievements have been made in exploring functional materials in chalcogenides, which can be used as nonlinear optics, electro optics, superionic conductors, and pyroelectrics^[1-10]. As an important subgroup of chalcogenide, thiophosphates exhibit rich structural diversity as well as unique physical properties, and have received broad attention^[11-15]. Thiophosphates are typically composed of tetrahedral [PQ₄]^3– (Q = S, Se, Te) and ethane-like [P₂Q₆]^4– units, the combination of which could further generate more complex building blocks such as [P₂Se₆]^4–[16], [P₂Se₉]^4–[17], and infinite chains like [P₂Se₆]^2–[18], [PSe₆]^–[19], [P₅Se₁₀]^5–[20]. Moreover, discrete [P_xQ_y]^n– fragments can be assembled with other metals to form a variety of extended frameworks with fascinating properties. For example, A₄GeP₄Se₁₂ (A = K, Rb, Cs) are excellent IR NLO materials exhibiting large second-harmonic-generation effect which is ∼30 times that of bench AgGaSe₂ at 730 nm^[21]. AZrPS₆ (A = K, Rb, Cs) are unique examples of stable inorganic semiconductors with band gap emission very attractive for technological applications^[22]. Rb₄Sn₅P₄Se₁₀ is a semimetallic selenophosphate and displays high conductivity of 51 S/cm at 300 K^[23]. Li₁₀SnP₂S₁₂ is an affordable lithium superionic conductor with very high values of 7 mS/cm for the grain conductivity^[24]. Although many thiophosphates have been found, investigations on thiophosphates containing Ga are rare. During our attempts to explore A–Ga–P–Q system, a new phase, Cs₂Ga₃PS₈ (1), has been synthesized. Herein, the syntheses, structures, and thermal and optical properties of 1 are presented. Interestingly, the compound exhibits a broad photoluminescent emission band at 420 nm. To gain further insights on its luminescent properties, the calculations of electronic band structure and density of states were performed.

  2. EXPERIMENTAL

  2.1 Syntheses

  The following reagents were used as obtained: Ba metal (99.9%), Ga metal (99.99%), P powder (99.99%), S powder (99.99%), and CsCl powder (99.99%). All operations were handled under an Ar atmosphere in a glove box. The title compound was synthesized by the stoichiometric mixture of Ba, Ga, P, S, and CsCl with total mass of 500 mg in a molar ratio of 1:3:1:8:2. The mixture was loaded into quartz tubes and then flame-sealed. The tubes were placed into a computer-controlled furnace, heated to 750 ℃ over 24 hours, subsequently dwelled for 4 days, and finally cooled down to room temperature at 3 ℃/hour. After the products were washed with deionized water and dried with methanol, lamellar colorless single crystals of 1 were observed, and the samples for further property measurements were obtained by hand picking under a microscope.

  2.2 Single-crystal X-ray diffraction

  Single-crystal X-ray diffraction measurement was performed on a Rigaku Pilatus CCD diffractometer using a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The intensity dataset of the title compound was collected using an ω-scan technique and reduced using the CrysAlisPro^[25]. The structure was solved by direct methods and refined with full-matrix least-squares methods on F² with anisotropic thermal parameters for all atoms^[26].

  2.3 X-ray powder diffraction

  Powder X-ray diffraction (XRD) data were recorded on an automated Rigaku MiniFlex II X-ray diffractometer equipped with a diffracted monochromator set for Cu-Kα radiation (λ = 1.54057 Å), operating at 30 kV and 40 mA. The observed powder pattern of the title compound was well-suited to the simulated one (Fig. S1b).

  2.4 Elemental analysis

  Selected crystals were fixed on the sample platform and analyzed by energy dispersive analyses X-ray spectroscopy (EDX) by using an EDX-equipped Hitachi S-3500 SEM spectrometer. Energy dispersive spectroscopy (EDS) analysis of the crystals of the title compound confirmed the presence of Cs/Ga/P/S with a molar ratio of 2.0/2.9/1.1/7.8, which is close to that determined from the single-crystal X-ray diffraction analysis (Fig. S1a).

  2.5 UV-Vis diffuse reflectance spectroscopy

  Optical diffuse reflectance measurement was made to measure the band gap of the title compound by Perkin-Elmer Lambda 900 UV-Vis spectrophotometer accompanied with an integrating sphere attachment, with BaSO₄ used as a reference. Absorption spectrum was calculated from the reflection spectrum using the Kubelka-Munk formula: α/S = (1 – R)²/2R^[27], in which α is the absorption coefficient, S the scattering coefficient, and R the reflectance.

  2.6 Photoluminescence

  The photoluminescence (PL) measurement of 1 was conducted on a single-grating Edinburgh EI920 fluorescence spectrometer equipped with a 450 W Xe lamp and a PMT detector.

  2.7 Thermal analysis

  Thermal properties of the title compound were measured by differential scanning calorimetry (DSC) with a TGA/DSC Mettler Toledo thermal analyzer. Polycrystalline sample (approximately 10 mg) was put into a quartz tube, then evacuated to ~10^–4 Torr and sealed. Finally, the tube experienced a heating/cooling cycle at a rate of 10 ℃/min.

  2.8 Electronic structure calculation

  The electronic band structure and density of state (DOS) of 1 were calculated by the CASTEP code^[28] on the basis of density functional theory (DFT)^[29], using a plane-wave expansion of the wave functions and an ultra-soft pseudo potential. The orbital electrons of Cs 5s²5p⁶6s², Ga 3d¹⁰4s²4p¹ and S 3s²3p⁴ were treated as valence electrons. A plane-wave cutoff energy was set to be 295 eV with a grid of Monkhorst-Pack k-points of 4×4×2.

  3. DISCUSSION

  3.1 Structure description

  Compound 1 crystallizes in monoclinic space group of P$ \overline 1 $ (No. 2) with a = 7.22730(10), b = 7.64670(10), c = 14.2671(3) Å, α = 91.005(2), β = 91.146(2), γ = 106.016(2)º, V = 757.50(2) ų and Z = 2. The asymmetric unit is depicted in Fig. 1a. There are two crystallographically independent Cs atoms, two Ga atoms, eight S atoms, and two mixed positions with equal occupancy of Ga and P. The title compound exhibits a two-dimensional layer structure (Fig. 2a). All Ga and P atoms are tetrahedrally coordinated by S atoms to form GaS₄ and (Ga/P)S₄ tetrahedra. GaS₄ tetrahedra share two corners with each other to form 13 tetrahedra chains extending along the a direction, which are further bridged by (Ga/P)S₄ tetrahedra dimers alternately, forming a [Ga₃PS₈]^2– layer in the ac plane (Fig. 2b). The (Ga/P)S₄ tetrahedra dimers are constructed by two edge-shared (Ga/P)S₄ tetrahedra. The counter Cs⁺ are embedded between [Ga₃PS₈]^2– layers.

  Figure 1

  

  Figure 1.  Coordination environments of Ga and P atoms (a), and ionic interactions around Cs atoms (b) in the asymmetric unit of 1

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  Figure 2

  

  Figure 2.  (a) Crystal structure of 1 viewed along the a axis. (b) A [Ga₃PS₈]^2– layer. Green and purple tetrahedra represent (Ga/P)S₄ and GaS₄ units, respectively. Black balls in (a) are Cs atoms

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  Compound 1 belongs to Cs₂M₃^IIIM^VQ₈ (Q = S, Se, Te) family (type-I)^[30], which can be derived from AM^IIIM^IVQ₄ family (type-II)^{[31, 32]} by replacing all M^IV atoms with equal amounts of M^III and M^V atoms. The modification of AM^IIIM^IVQ₄ family can also lead to A₂M^IIM₃^IVQ₈ (Q = S, Se, Te) family (type-III) via the substitution of two M^III atoms by one M^II and one M^IV atoms^[33-35]. The structures of type-I, II and III compounds are similar, but they exhibit different structure disorders of tetrahedrally coordinated centers. In type-II compounds, the trivalent and tetravalent metal ions are disordered over all tetrahedral sites. Type-III family of compounds is completely ordered, whereas in the type-I compounds, all M^V and partial M^III positions are disordered. The flexible substitution behavior of AM^IIIM^IVQ₄ family makes it a good platform for exploring new materials with rich structure features and physical properties^{[31, 32]}.

  As listed in Table S2, Ga–S distances of fully occupied GaS₄ tetrahedra in 1 are in the range of 2.2484~2.3361 Å, which are close to those in β-LaGaS₃ (2.194~2.325 Å)^[36] and SnGa₄S₇ (2.214~2.337 Å)^[37]. In (Ga/P)S₄ tetrahedra, the Ga/P–S distances range from 2.1087 to 2.2309 Å, which are between the typical P–S and Ga–S bond lengths. Two crystallographically independent Cs atoms are surrounded by nine and eleven S atoms, respectively, with ionic interactions. The Cs–S distances in the range of 3.469~4.118 Å (Fig. 1b) are consistent with those in Cs[Lu₇S₁₁]^[38].

  3.2 Experimental band gap and photoluminescent spectra

  The UV-Visible-NIR diffuse reflectance spectrum of 1 exhibits obvious absorption edge and the band gap is estimated to 3.05 eV (Fig. 3a), which is consistent with its colorless feature. The band gap of 1 is comparable to those of some other thiophosphates, such as KAg₂[PS₄] (3.02 eV)^[39] and K₄GeP₄S₁₂ (3.0 eV)^[21]. The photoluminescent spectra of 1 were studied in the solid state at room temperature, and its excitation and emission spectra are plotted in Fig. 3b. Compound 1 exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm.

  Figure 3

  

  Figure 3.  (a) UV-Vis diffuse reflectance spectrum and (b) excitation and emission spectra of 1

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  3.3 Differential thermal analysis

  The differential scanning calorimetry (DSC) was used to examine the thermal properties of 1 (Fig. 4), which showed that the compound exhibits a broad endothermic peak on the heating curve, that is, crystals of 1 melt at 645 ℃. Correspondingly, there is an exothermic peak at 626 ℃ for crystallization during the cooling process.

  Figure 4

  

  Figure 4.  DSC curves of 1

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  3.4 Electronic structure calculation

  To better understand the optical properties, theoretical calculations including electronic band structures and partial density of states (PDOS) of 1 are calculated by DFT. The calculated electronic band structure is plotted in Fig. 5a, indicating a direct band gap of 1.839 eV. The PDOS (Fig. 5b) shows that the conductive band (CB) close to the Fermi level is mostly composed of S-3p and P-3p states, as well as a small portion of P-3s state. While the valence band (VB) from –4.0 eV to the Fermi level originates predominately from S-3p and Ga-4p states. The contributions of Cs atom states to bands from –6 to 9 eV are negligible, so luminescent properties of 1 can be mainly ascribed to electron transfer from S-3p and Ga-4p states to the S-3p and P-3p ones.

  Figure 5

  

  Figure 5.  Electronic band structure (a) and the partial density of states (b) of 1

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  4. CONCLUSION

  In summary, a new phase, Cs₂Ga₃PS₈, in triclinic space group of P$ \overline 1 $ has been successfully synthesized by high-temperature reactant flux method. Its structure is built from 2D infinite 2 ∞[Ga₃PS₈]^2– layers, separated by Cs⁺. UV-vis-NIR spectroscopy measurement indicated that Cs₂Ga₃PS₈ shows a wide band gap of 3.08 eV. The melting point of this compound is 645 ℃. Cs₂Ga₃PS₈ exhibits a broad photoluminescent emission band at 420 nm upon excitation at 295 nm. Theoretical calculation of electronic band structure indicated that fluorescent properties of Cs₂Ga₃PS₈ origin charge transfer from S-3p and Ga-4p states to S-3p and P-3p states.

  
  
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• 

  Figure 1  Coordination environments of Ga and P atoms (a), and ionic interactions around Cs atoms (b) in the asymmetric unit of 1

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  Figure 2  (a) Crystal structure of 1 viewed along the a axis. (b) A [Ga₃PS₈]^2– layer. Green and purple tetrahedra represent (Ga/P)S₄ and GaS₄ units, respectively. Black balls in (a) are Cs atoms

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  Figure 3  (a) UV-Vis diffuse reflectance spectrum and (b) excitation and emission spectra of 1

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  Figure 4  DSC curves of 1

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  Figure 5  Electronic band structure (a) and the partial density of states (b) of 1

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