Wide band gap design of new chalcogenide compounds: KSrPS₄ and CsBaAsS₄

Abstract

Recently, the exploration of infrared nonlinear optical (IR NLO) materials has mainly focused on chalcogenide compounds. However, their practical applications are often hampered by the low laser damage thresholds (LDTs). It is known that wide band gaps can significantly enhance the LDTs of materials, and the introduction of alkali and alkaline earth cations would broaden the band gap. Accordingly, in this work two new compounds KSrPS₄ and CsBaAsS₄ with both alkali and alkaline earth cations were synthesized successfully. Both compounds crystallize in the space group Pnma (62) of the orthorhombic system, and the structures consist of isolated PnS₄ (Pn = P, As) tetrahedra with the interstices occupied by K (or Cs) and Sr (or Ba) atoms, respectively. The band gaps of compounds were determined by different methods. The UV-visible diffuse reflectance spectra revealed that the band gaps of KSrPS₄ and CsBaAsS₄ are larger than 3.62 eV and 2.86 eV, respectively. The band gaps are primarily determined by the PnS₄ tetrahedra.

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Introduction

It is well known that coherent IR lasers have been widely applied in civil and military fields such as laser guidance, signal communication and industrial processing. A number of NLO materials have been applied to generate IR lasers. The commercial IR NLO materials are mainly focused on chalcogenide compounds, such as AgGaS₂ and AgGaSe₂,^1,2 which have been applied in optoelectronic and thermoelectric areas because of their diverse structures and physical properties.^3–8 However, these materials suffer from the low LDTs, which limits their further application in various areas. Here, the low LDT is principally caused by the relatively small band gap of the material. Materials with higher LDTs can be obtained based on broader band gaps. Numerous research has been conducted to search for novel, promising mid-IR NLO materials that exhibit broad band gaps and strong NLO responses.^9–29 In our previous research for Ag₃Ga₃SiSe₈ and AgGaSiSe₄ compounds, it mentioned that the band gap of material can be enlarged through the introduction of the Ge or Si element.^24,25 And compared to the band gap of AgGaSe₂ (ref. 2) with 1.8 eV, the band gaps of AgGaSiSe₄ (ref. 24) and Ag₃Ga₃SiSe₈ (ref. 25) are broadened to 2.63 eV and 2.30 eV, respectively. The introduction of alkali or alkaline earth cations which usually are electropositive elements also contributes to broaden band gap of materia.^26–28 For example, the band gaps of BaGa₄S₇ (ref. 26) and LiGaS₂ (ref. 27) increased to 3.54 eV and 4.15 eV, respectively, compared to that of AgGaS₂ with 2.75 eV. Those of BaGa₄Se₇ (ref. 28) and LiGaSe₂ (ref. 27) increased to 2.64 eV and 3.34 eV, respectively, compared to that of AgGaSe₂ with 1.8 eV. And with the introduction of both alkali and alkaline earth cations, the band gap of Na₂BaGeS₄ (ref. 29) reached 3.7 eV. Wherefore in this work, we attempted to simultaneously introduce the alkali and alkaline earth cations to broaden the band gap of material. Meanwhile the introduction of PnS₄ (Pn = P, As) groups are conducive to improve the NLO properties.³⁰ Through the combination of alkali and alkaline earth cations with the PnS₄ groups, we conducted the research to the A/AL/Pn/S (A = alkali cations, AL = alkaline earth cations) system, and fortunately, the two new compounds KSrPS₄ and CsBaAsS₄ were successfully synthesized. The syntheses, crystal structures, electronic structures and optical properties of the materials were also investigated.

Experimental section

Synthesis

The compound KSrPS₄ was prepared by combining K₂S₃ (prepared by stoichiometric reaction of the K (Sinopharm chemical Reagent Co., Ltd., 98%) and S (Sinopharm chemical Reagent Co., Ltd., 99.5%) elements in liquid NH₃), SrS (prepared by stoichiometric reaction of Sr (Sinopharm chemical Reagent Co., Ltd., 99%) and S elements), Nd (Sinopharm chemical Reagent Co., Ltd., 99.9%), P₂S₅ (Sinopharm chemical Reagent Co., Ltd., 99%) and additional S (Sinopharm chemical Reagent Co., Ltd., 99.5%) in the molar ratio 2 : 1 : 1 : 1 : 6. The sample was heated in a computer-controlled furnace to 973 K, then kept there for 5 d, then cooled to 373 K at the rate of 3 K h⁻¹. The resulting melts were washed with dimethylformamide (DMF) and acetone in turn. The product consisted of colorless transparent platelet KSrPS₄. This compound was stable in dry air for several days.

The compound CsBaAsS₄ was prepared by combining Cs₂S₃ (prepared by stoichiometric reaction of the Cs (Alfa Aesar, 99.8%) and S (Sinopharm chemical Reagent Co., Ltd., 99.5%) elements in liquid NH₃), BaS (Sinopharm chemical Reagent Co., Ltd., 99%), Nd (Sinopharm chemical Reagent Co., Ltd., 99.9%), As₂S₃ (Sinopharm chemical Reagent Co., Ltd., 99%) and additional S (Sinopharm chemical Reagent Co., Ltd., 99.5%) in the molar ratio 2 : 0.5 : 1 : 1 : 6. The sample was heated in a computer-controlled furnace to 973 K, then kept there for 5 d, then cooled to 373 K at the rate of 3 K h⁻¹. The resulting melts were washed with DMF and acetone in turn. The product consisted of yellow transparent platelet crystal of CsBaAsS₄. This compound was stable in dry air for several days.

Energy dispersive X-ray fluorescence test

The analysis of the compounds was carried out with Shimadzu EDX-720 Energy Dispersive X-ray Fluorescence spectrometer. The tests for each compound were performed with 5 times, and the spectra of number 1 and 6, as shown in Fig. 1 (the rest of the number 2–5 and 7–10 spectra are shown in the ESI†), manifested the presence of K, Sr, P, S and Ca, Ba, As, S in the approximate molar ratio of 1 : 1 : 1 : 4 and 1 : 1 : 1 : 4, respectively.

Fig. 1 (a) Energy dispersive X-ray fluorescence spectra of KSrPS₄, (b) energy dispersive X-ray fluorescence spectra of CsBaAsS₄.

Single crystal data collection

Single crystal X-ray diffraction data were collected with graphite-monochromatized Mo K radiation (λ = 0.71073 Å) at 220 K on a STOE Imaging Plate Diffraction System (IPDS-1).

UV-vis diffuse reflectance test

UV-vis diffuse reflectance spectroscopy test was performed on a Shimadzu UV-3600 spectrophotometer. The samples and BaSO₄ (totally reflected) were ground together at room temperature. Then the mixture was prepared as a flat specimen, and in UV/vis range, the resolution was 0.1 nm. The data were collected at 200–800 nm.

Second harmonic generation test

The optical second harmonic generation test was performed on the powder sample of KSrPS₄ and CsBaAsS₄ by means of the Kurtz–Perry method,³¹ with a 1.06 μm Q-switch laser. The samples were ground and sieved by using a series of mesh sizes in the range of 80–100 μm. Similar size of AgGaS₂ was chosen as the reference.

Laser damage threshold test

The laser damage thresholds of title compounds were evaluated on powder sample with a pulsed YAG laser (1.06 μm, 10 ns, 10 Hz).³² Similar size of AgGaS₂ was chosen as the reference.

Structure determination

The structure model was obtained by direct methods and was refined by full-matrix least-squares refinement based on F2 using the Shelxtl package.³³ The positions were standardized with the Structure Tidy program within the Platon package.³⁴ The final refinement converged with a residual factor of wR₂ = 0.095 (all data). Technical details of the data acquisition as well as some refinement results are summarized in Table 1.

Table 1 Crystal data and structure refinement for KSrPS₄ and CsBaAsS₄

	KSrPS4	CsBaAsS4
a R(F) = ∑∣∣Fo∣ − ∣Fc∣∣/∑∣Fo∣ for Fo^2 > 2σ(Fo^2).b Rw(Fo^2) = {∑[w(Fo^2 − Fc^2)^2]/∑wFo^4}^1/2 for all data.
Fw (g mol^−1)	285.93	473.41
a (Å)	16.8214(9)	11.9066(6)
b (Å)	6.6274(5)	6.9184(5)
c (Å)	6.5585(4)	10.0338(5)
V (Å^3)	731.16(8)	826.53(8)
Space group	Pnma (62)	Pnma (62)
Z	4	4
Index ranges	−21 ≤ h ≤ 22	−15 ≤ h ≤ 15
	−8 ≤ k ≤ 8	−9 ≤ k ≤ 9
	−8 ≤ l ≤ 8	−13 ≤ l ≤ 12
Theta range	2.42–28.08	2.65–28.08
Number of reflection collected	873	1029
ρc (g cm^−3)	2.598	3.804
μ (cm^−1)	91.84	140.24
R(F)^a	0.0292	0.0309
Rw(Fo^2)^b	0.0770	0.0775

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The first-principles calculations

The first-principles calculations for the KSrPS₄ and CsBaAsS₄ crystals are performed by the plane-wave pseudo potential method implemented in the CASTEP package based on the density functional theory (DFT).³⁵ The ion–electron interactions are modeled by the optimized normal-conserving pseudo potentials for all elements. The adopted density functional method is exchange-correlation (XC) functional of local density approximation (LDA).³⁶ The kinetic energy cutoffs of 800 eV and Monkhorst–Pack k-point meshes³⁷ with density of (2 × 3 × 2) and (1 × 3 × 3) points in the Brillouin zone are chosen for KSrPS₄ and CsBaAsS₄ crystals, respectively. Our tests reveal that the above computational set ups are sufficiently accurate for the present purposes.

Results and discussion

The two compounds KSrPS₄ and CsBaAsS₄ crystallize in space group Pnma (62) of the orthorhombic system and adopt the TlEuPS₄ (ref. 38) structure type, As shown in Fig. 2. The structures consist of isolated PnS₄ (Pn = P, As) tetrahedra separated by the K (or Cs) and Sr (or Ba) atoms, and the atoms locate in the tunnel formed by the tetrahedra, respectively. The asymmetric units for both compounds contain one crystallographically independent K (or Cs) atom, one independent Sr (or Ba) atom, one independent P (or As) atom and three independent S atoms.

Fig. 2 (a) Unit cell structure of KSrPS₄, (b) PS₄ tetrahedron in KSrPS₄ structure, and (c) AsS₄ tetrahedron in CsBaAsS₄ structure.

For KSrPS₄, K atom is coordinated to a bicapped trigonal prism of eight S atoms with K–S distances ranging from 3.336(1) to 3.603(2) Å, which is close to those of 3.2058(14)–3.5239(14) Å for K–S distances in KBiSiS₄ (ref. 39) and 3.194(3)–3.601(3) Å for K–S distances in KBiGeS₄.³⁹ The Sr atom is coordinated to 9 S atoms with Sr–S distances ranging from 2.992(1) to 3.685(1) Å, which is close to those of 2.928–3.242 Å for Sr–S distances in Sr₂ZnS₃.⁴⁰ Each P atom is bonded to four S atoms to form a slightly distorted tetrahedron, with the bond lengths ranging from 2.023(2) to 2.049(8) Å, similar to those of 2.010(1)–2.090(1) Å for P–S distances in LiZnPS₄.⁴¹ Since S–S bonds are not observed in the structure, the oxidation states of 1+, 2+, 5+, 2− can be distributed to K, Sr, P and S, respectively.

For CsBaAsS₄, Cs atom is bonded to 10 S atoms with Cs–S distances ranging from 3.468(2) to 3.895(1) Å, which is close to those of 3.4022(10)–4.0982(13) Å for Cs–S distances in Cs₃Bi(AsS₄)₂.⁴² While Ba is coordinated to 9 S atoms with Ba–S distances ranging from 3.182(2) to 3.588(0) Å, similar to those of 2.896(2)–3.331(2) Å for Ba–S distances in Ba₂₃Ga₈Sb₂S₃₈.⁴³ Each As atom is bonded to four S atoms to form a slightly distorted tetrahedron, with the As–S bond lengths ranging from 2.153(4) to 2.161(1) Å, which is similar to those of 1.918(3)–2.390(3) Å for As–S distances in Cs₃Bi(AsS₄)₂ and 2.229(2)–2.266(2) Å for As–S distances in Cs₃Ta₂AsS₁₁.⁴⁴ Since S–S bonds are not observed in the structure, the oxidation states of 1+, 2+, 5+, 2− can be distributed to Cs, Ba, As and S, respectively.

For these compounds, the larger content of alkali cations (K, Cs) and alkaline cations (Ba, Sr) is, the sparser the PS₄ and AsS₄ tetrahedra connectivity will be, which can be referred to our previous work on Ba₅Al₂Se₈ and Ba₅Ga₂Se₈.⁴⁵ And the tetrahedra would be completely separated from each other by the K (or Cs) and Sr (or Ba) atoms.

From the UV-visible diffuse reflectance spectra of the compounds in Fig. 3(a), the absorption edges are about 343 nm and 434 nm for KSrPS₄ and CsBaAsS₄, respectively. The band gaps of KSrPS₄ and CsBaAsS₄ obtained by direct extrapolation method⁴⁶ with baseline tangents were 3.62 and 2.86 eV, respectively. To correctly compared the band gaps of different compounds, the measurement methods need to be consistent. And in the Fig. 3(b), the direct extrapolation method with upper tangents were carried out on the spectra, and the results showed that the band gaps of compounds are 4.23 eV and 3.25 eV, respectively. These two compounds band gaps are significantly larger than that of the commercial materials AgGaS₂ (ref. 1) with 2.75 eV and AgGaSe₂ (ref. 2) with 1.8 eV. The band gaps of KSrPS₄ and CsBaAsS₄ are also larger than that of BaGa₄S₇ (ref. 26) with 3.54 eV and BaGa₄Se₇ (ref. 28) with 2.64 eV, respectively, indicated that compounds with large band gaps were obtained. According to the single crystal growth experiment results, the colors for KSrPS₄ and CsBaAsS₄ crystals are colorless and yellow (see in Fig. 4), respectively, which was in good accordance with the UV-visible reflectance spectra. In the Fig. 3(c) and (d), the direct and indirect band gaps are measured by Tauc–Davis Mott expressions.⁴⁷ The measurement of direct band gap indicated that KSrPS₄ and CsBaAsS₄ is 3.92 eV and 2.97 eV, respectively. And the measurement of indirect band gap indicated that KSrPS₄ and CsBaAsS₄ is 3.84 eV and 2.91 eV, respectively.

Fig. 3 Diffuse spectra of KSrPS₄ and CsBaAsS₄: (a) the spectra with upper tangents; (b) the spectra with baseline tangents; (c) the spectra of (F(R) hυ)² versus hυ; (d) the spectra of (F(R) hυ)^0.5 versus hυ.

Fig. 4 The single crystals of KSrPS₄ and CsBaAsS₄ compounds.

The laser damage test was performed on the powder sample with a pulsed YAG laser (1.06 μm, 10 ns, 10 Hz). AgGaS₂ was served as the reference. The results showed that both the laser damage thresholds of KPSrS₄ and CsBaAsS₄ are above 5 times of that of AgGaS₂. The optical second harmonic generation test was performed on KSrPS₄ and CsBaAsS₄ with a 1.06 μm Q-switch laser. The compounds KSrPS₄ and CsBaAsS₄ did not give any second harmonic generation signal because of their centrosymmetric structure.

The electronic band structures of the KSrPS₄ and CsBaAsS₄ crystals are shown in Fig. 5. The calculations with sX-LDA functional were carried out to investigate the experimental values of band gaps and the results showed the values of 3.58 eV and 2.73 eV for KSrPS₄ and CsBaAsS₄, respectively.

Fig. 5 (a) The band structure of KSrPS₄, and (b) band structure of CsBaAsS₄.

The partial density of state (PDOS) projected on the constitutional atoms of the KSrPS₄ and CsBaAsS₄ are shown in Fig. 6, from which several electronic characteristics are shown: (i) the region lowered than −5 eV consist of the isolated inner-shell states with K 2s2p (or Cs 5s5p), Sr 4s4p (or Ba 5s5p), P 3s2p (or As 4s3p) and S 3s2p orbitals, which have little interaction with neighbor atoms. (ii) The upper part of the valence band (about −4 eV) is mainly composed of the p orbitals of P 3p (As 4p) and S 3p orbitals, but the extra top of the VB is occupied by the S 3p orbitals. (iii) Although all the elements contribute to the states on the bottom of conduct band, states on the bottom of conduct band mostly come from the S and P (As) atoms.

Fig. 6 (a) The PDOS of KSrPS₄, and (b) PDOS of CsBaAsS₄ crystals.

In summary, the upper part of valence band and the bottom of conduct band are principally determined by the P (As) and S elements, which indicated that the optical absorption is mostly determined by (PnS₄)³⁻ (Pn = P, As) groups.

To further investigate the relationship between the structures and properties of the compounds, the contours of electronic density difference on the PS₄ and AsS₄ groups were drawn for the title compounds in Fig. 7, which illustrate charge redistribution due to the formation of chemical bonds. It is obvious that more charges are located on the P–S bonds than on the As–S bonds, indicating the much stronger covalent characteristic of the former chemical bonds.

Fig. 7 (a) The electronic charge density difference plotting on the PS₄ groups, and (b) AsS₄ groups.

Conclusions

To obtain compounds with wide band gaps, in this work, introduction of both the alkali and alkaline earth cations was carried out. Two new compounds KSrPS₄ and CsBaAsS₄ were successfully synthesized. The compounds crystallize in space group Pnma (62) of the orthorhombic system and adopt the TlEuPS₄ structure type. The band gaps of compounds were determined by four methods. The results revealed that the band gaps of KSrPS₄ and CsBaAsS₄ are larger than 3.62 eV and 2.86 eV, respectively, which imply that the band gaps of compounds were successfully broadened compared with the commercial materials AgGaS₂ and AgGaSe₂, respectively. The band gaps are primarily determined by the PnS₄ (Pn = P, As) tetrahedra. Such compounds may arouse further interest in exploring new IR NLO materials with wide band gaps, through introducing the alkali or alkaline earth cations.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 51502169), China “863” project (No. 2015AA034203), Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1503), Shanghai University of Engineering Science “Innovation Training Project of Undergraduate” (No. cs1504002) and Shanghai University of Engineering Science “Scientific Research Training Project of Undergraduate” (No. HZJY20160029).

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data in CIF format for KSrPS₄ and CsBaAsS₄. CCDC 1502143 and 1503947. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ra01142c

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