Structural dimension modulation in a new oxysulfide system of Ae₂Sb₂O₂S₃ (Ae = Ca and Ba)

Abstract

Inspired by the abundant structural diversity and potential applications of Sb-based oxysulfides, two new compounds with the same stoichiometry, Ae₂Sb₂O₂S₃ (Ae = Ca, Ba), were successfully synthesized via solid state reactions. Both crystallize in the C2/c space group (no. 15) of a monoclinic system, however, possess different structural dimensions induced by different sizes of template alkaline-earth ions. Ca₂Sb₂O₂S₃ is composed of one-dimensional anionic [Sb₂O₂S₃]⁴⁻ chains isolated by Ca²⁺ ions, while Ba₂Sb₂O₂S₃ consists of zero-dimensional anionic [Sb₂O₂S₃]⁴⁻ clusters separated by Ba²⁺ ions. Higher structural dimension endows the 1D Ca₂Sb₂O₂S₃ with a higher melting point (m.p. = 720 °C) and a narrower band gap (E_g = 2.36 eV), compared with the m.p. = 692 °C and E_g = 2.78 eV of Ba₂Sb₂O₂S₃. Both exhibit interesting photoluminescence properties with multiemission characteristics. DFT calculations reveal that the band edges of Ae₂Sb₂O₂S₃ are mainly composed of Sb−S bond orbitals, while O 2p orbitals also contribute to the valence band maximum of Ba₂Sb₂O₂S₃.

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Introduction

Owing to their manifold structural chemistry and novel physical properties, oxychalcogenides containing the 15-group metal Sb or Bi have shown great potential in aspects of superconductivity,^1–3 optoelectronics,^4–8 thermoelectricity^9–12 and nonlinear optics (NLO).¹³ Because of the existence of stereochemically active lone-pair (SALP) hybridized s/p states, Sb³⁺ ions possess various coordination environments with low symmetry, like a 3-fold triangle pyramid, a 5-fold square pyramid and a 6-fold distorted octahedron. Besides, the moderate bonding affinity of antimony towards S and O makes it possible to form mixed-anion coordination polyhedra, such as [SbOS₂]³⁻,^14–16 [SbO₃S]⁵⁻,¹⁷ and [SbOQ₄]⁷⁻ (Q = S, Se) units,^4,9,13,18 further increasing the structural diversity of Sb-based oxychalcogenides. Heteroanionic coordination geometries with low symmetry can induce high local polarization, which can not only facilitate carrier separation, benefiting optoelectronic and photocatalysis applications,^5,19 but also achieve large second harmonic generation (SHG) responses for nonlinear optics.^13,20–23 However, the number of Sb-based oxysulfides containing Sb/O/S mixed-anion units is still limited, as is the investigation towards their potential applications.

Highly ionic cations like alkali metal (A) and alkaline-earth metal (Ae) ions are widely used as structural templates for the construction of new chalcogenides. The spatial arrangement of building units and the structural dimensions of crystals are directly determined by the radii and coordination environments of template ions. Oxychalcogenides containing alkali ions as templates have rarely been reported because the hard-Lewis-acid alkali ions tend to bond with the highly electronegative O²⁻ instead of the soft S²⁻, inducing phase separation. In contrast, relatively softer alkaline-earth metal ions are more suitable as templates for the construction of oxychalcogenides, and have been used in many material systems like AeZnOS (Ae = Ca, Ba),^24,25 Sr₂Cu₂MO₂S₂ (M = Mn, Co, Zn)^26,27 and AeGeOQ₂ (Ae = Sr, Ba; Q = S, Se).^20,22 For Sb-based oxychalcogenides, early exploration of the Ae–O–Sb-Q system started from the structural determination of CaSb₁₀O₁₀S₆, a mineral named sarabauite in 1978.^14–16 Their subsequent synthesis attempts also implied the existence of other new phases in this system. However, over the past four decades, only one oxyselenide, Sr₂Sb₂O₂Se₃, in this system was reported in 2017.¹⁸ By adding another metal element into the quaternary Ae–O–Sb–Q system, some oxychalcogenides with novel crystal structures have been discovered such as SrOCuSbQ₂ (Q = S, Se),^4,9 Sr₆Cd₂Sb₆O₇S₁₀¹³ and Sr_3.5Pb_2.5Sb₆O₅S₁₀.⁷ These materials are promising candidates for photoelectric, thermoelectric or NLO frequency conversion, which urged us to explore new members in the Ae–O–Sb–Q system.

Herein, we used the alkaline-earth metal ions Ca²⁺ and Ba²⁺ as templates and successfully synthesized two novel oxysulfides Ae₂Sb₂O₂S₃ (A = Ca, Ba). Both compounds crystallize in the C2/c space group, while they possess different coordination chemistries and structural dimensions. Ca₂Sb₂O₂S₃ is composed of one-dimensional (1-D) anionic [Sb₂O₂S₃]⁴⁻ chains isolated by Ca²⁺ ions, while Ba₂Sb₂O₂S₃ consists of zero-dimensional (0-D) anionic [Sb₂O₂S₃]⁴⁻ clusters separated by Ba²⁺ ions. Due to their distinct structural dimensions, the two compounds showed different optical absorption properties and thermal stabilities. First-principles calculations were also performed to gain insight into the electronic structures of the two compounds.

Experimental section

Synthesis

Ca₂Sb₂O₂S₃. All operations are performed in an Ar-protected glove box. 0.0721 g of CaS (1 mmol), 0.0972 g of Sb₂O₃ (1/3 mmol) and 0.0566 g of Sb₂S₃ (1/6 mmol) were weighted and ground. The mixture was loaded in a carbon-coated silica tube, and it was subsequently flame-sealed under vacuum below 10^–3 mbar. The tube was heated to 650 °C in 10 h, held for 48 h, and then cooled to 350 °C in 72 h. Yellow rod-like single crystals were obtained after the above process.

Ba₂Sb₂O₂S₃. 0.1694 g of BaS (1 mmol), 0.0972 g of Sb₂O₃ (1/3 mmol) and 0.0566 g of Sb₂S₃ (1/6 mmol) were weighted and loaded in a carbon-coated silica tube. The tube was flame-sealed under vacuum, then heated to 600 °C in 10 h, held for 48 h, and then cooled to 300 °C in 72 h. Yellow bulk single crystals were obtained after the above process.

Single crystal X-ray crystallography

Single crystals of Ca₂Sb₂O₂S₃ and Ba₂Sb₂O₂S₃ were picked out, and data collection was performed under 180 K on a Rigaku XtaALB PRO 007HF single crystal X-ray diffractometer equipped with mirror-monochromated Mo-Kα radiation and an Oxford Cryo stream (80–500 K). The crystal structures were solved by direct methods and refined by full-matrix least-square on F² using the SHELXTL program package.²⁸ The crystallographic data and structure refinement details of A₂Sb₂O₂S₃ (A = Ca and Ba) are summarized in Table S1.† Selected bond lengths and angles are shown in Table S2.† Atomic coordinates, equivalent isotropic displacement parameters and anisotropic displacement parameters are shown in Tables S3–S6.†

Powder X-ray diffraction and scanning electron microscopy

The polycrystalline samples were fully ground for the collection of powder X-ray diffraction (PXRD) data. PXRD analysis was performed at 298 K in the range of 2θ from 5° to 80° at a scan rate of 1.2° min⁻¹ on a Bruker D2 phaser diffractometer equipped with a monochromatized source of Cu Kα radiation (λ = 0.15406 nm) at 4 kW (40 kV, 100 mA). Images of the single crystals and semi-quantitative energy dispersive X-ray (EDX) spectra were obtained using a Phenom Pro scanning electron microscope (SEM) equipped with a Princeton Gamma Tech (PGT) energy-dispersive X-ray analyzer. Single crystals were placed on the surface of a double-sided carbon/aluminium tape attached on an aluminium SEM substrate. EDX data were collected at an accelerating voltage of 15 keV with a 60 s accumulation time.

UV-Vis diffuse reflectance spectroscopy

Optical diffuse-reflectance measurements were performed at room temperature using a UV-4100 spectrophotometer operating from 1000 nm to 250 nm. BaSO₄ was used as a 100% reflectance standard. The powder samples were spread on a compacted base of BaSO₄ powder. The generated reflectance versus wavelength data were used to calculate the band gaps of the compounds using the Kubelka–Munk equation.²⁹

Photoluminescence (PL) spectroscopy

The PL emission properties of the two compounds were investigated using a steady-state/transient fluorescence spectrometer (Edinburgh FLS980). Emission spectra were collected at room temperature under excitation wavelengths of 270, 280, 300, 320 and 340 nm, respectively.

Thermal analysis

The thermostability of Ae₂Sb₂O₂S₃ (Ae = Ca and Ba) was studied by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) using a Thermal Analysis SDT2960 thermal analyzer under N₂ flow. A silica crucible containing 5.0 mg of the powder sample was placed on the sample side of the detector, with another empty crucible on the reference side. The sample was heated to 850 °C (Ca₂Sb₂O₂S₃) or 900 °C (Ba₂Sb₂O₂S₃) and cooled down at a rate of ±15 °C min⁻¹ for two cycles.

Electronic structure calculations

First-principles calculations were performed using the projected augmented wave method (PAW)³⁰ within density functional theory (DFT) as implemented in the Vienna Ab Initio Simulation Package (VASP).^31–33 The exchange correlation functional was treated within the spin-polarized generalized gradient approximation (GGA) and parameterized by the Perdew–Burke–Ernzerhof (PBE) version.³⁴ The cutoff energy of the plane wave basis was set to 560 eV. The Monkhorst–Pack k-point grids of 4 × 12 × 4 for Ca₂Sb₂O₂S₃ and 5 × 10 × 10 for Ba₂Sb₂O₂S₃ were used for Brillouin zone (BZ) sampling. The crystal structure and lattice parameters were fixed as the values observed in experiments during structural optimization, while the positions of atoms were relaxed until the atomic forces on each atom were less than 0.01 eV Å⁻¹. The crystal orbital Hamiltonian population (COHP) curves were extracted using the Lobster program.³⁵

Results and discussion

Synthesis

The two compounds were obtained by the solid-state reactions of CaS or BaS, Sb₂O₃ and Sb₂S₃. The phase purities of the as-synthesized products were validated by comparing the experimental PXRD patterns with the simulated ones, as shown in Fig. 1a and c. The yellow Ca₂Sb₂O₂S₃ crystals show preferred orientation growth with a rod-like form, while the pale yellow Ba₂Sb₂O₂S₃ crystals possess bulk shapes, as shown in the SEM images in Fig. 1b and d. All the four elements distributed uniformly in the crystals revealed by EDX mapping analysis. The semiquantitative EDX spectra give molar ratios of Ca : Sb : S = 1 : 1.4 : 1.6 and Ba : Sb : S = 1 : 1.2 : 1.3 in Ca₂Sb₂O₂S₃ and Ba₂Sb₂O₂S₃ crystals, respectively (Fig. S1†).

Fig. 1 Simulated (red) and experimental (black) PXRD patterns of (a) Ca₂Sb₂O₂S₃ and (c) Ba₂Sb₂O₂S₃. The SEM images and EDX mapping analysis of (b) Ca₂Sb₂O₂S₃ and (d) Ba₂Sb₂O₂S₃.

Crystal structures

The crystal structures of Ae₂Sb₂O₂S₃ (Ae = Ca, Ba) were determined by single crystal X-ray diffraction and structural refinements, and the crystallographic data and refinement details are shown in Table S1.† Both compounds crystallize in the C2/c space group (no. 15) of the monoclinic system, with one independent alkaline-earth Ca or Ba site (8f Wyckoff position), one independent Sb site (8f), one independent O site (8f) and two independent S sites (4e, 8f). They share the same stoichiometry, nevertheless, possess distinct crystal structures with different structural dimensions, as shown in Fig. 2.

Fig. 2 The basic structural units and crystal structures of (a) Ca₂Sb₂O₂S₃ and (b) Ba₂Sb₂O₂S₃ viewed down the b axis.

Ca₂Sb₂O₂S₃ is composed of 1-D anionic [Sb₂O₂S₃]⁴⁻ chains isolated with each other by Ca²⁺ ions, which shares a similar structure with recently reported oxychalcogenides A₂Bi₂O₂Se₃ (A = Sr and Ba) and Sr₂Sb₂O₂Se₃.¹⁸ The edge-sharing [SbOS₄]⁷⁻ square pyramids extend in the b direction to form the [Sb₂O₂S₃]⁴⁻ chains that are truncated to two pyramidal units in the transverse direction. Within a single chain, two neighboring rows of [SbOS₄]⁷⁻ point to the opposite orientations. The O atoms are surrounded by three Ca atoms and one Sb atom to form edge-sharing tetrahedral chains (Fig. S2a†), which is similar to the PbO-type coordination environments of O atoms in oxychalcogenides such as CeOSbS₂, Sr₆Cd₂Sb₆O₇S₁₀ and SrOCuSbQ₂ (Q = S, Se).^4,9,13,36 In comparison, Ba₂Sb₂O₂S₃ possesses a novel structure type which consists of 0-D anionic [Sb₂O₂S₃]⁴⁻ clusters separated by Ba²⁺ ions. The neighboring [SbOS₂]³⁻ trigonal pyramids connect each other by sharing the S atom to form the bi-trigonal pyramid [Sb₂O₂S₃]⁴⁻. This kind of oxysulfide building unit in Ba₂Sb₂O₂S₃ is reported for the first time as far as we know. The tetrahedra of the centered O atom and the surrounding three Ba atoms and one Sb atom are corner-sharing or edge-sharing with each other to form an ionic layer in the bc plane (Fig. S2b†). According to our computational analysis discussed below, Ba²⁺ ions interact with the oxygen atoms through ionic bonding, and in doing so interrupt the connection of the [Sb₂O₂S₃]³⁻ units via the bridging of the oxygen atoms to form 1D chains.

Alkaline-earth metal ions are structure-directing agents to modulate the connection modes and spatial arrangements of covalent building units, as well as the dimension of the crystal structures. In the oxychalcogenide Ae₂Sb₂O₂Q₃ (Ae = Ca, Sr, Ba; Q = S, Se) system, the coordination environments of Sb atoms and the connected modes of Sb/O/Q groups are determined by Ae ions. Furthermore, the structural dimensions of compounds in this system can be adjusted by changing the Ae ions (Fig. 3). In the three compounds Ca₂Sb₂O₂S₃, Sr₂Sb₂O₂Se₃ and Ba₂Sb₂O₂S₃, the coordination numbers of Ae ions become higher along with the increasing ionic radii, from 6-fold of Ca²⁺ (99 pm), to 8-fold of Sr²⁺(113 pm) and 9-fold of Ba²⁺ (135 pm). Larger ionic cations have stronger capability to ‘cut’ the covalent framework into smaller pieces, resulting in lower structural dimension.

Fig. 3 Structure comparison, and coordination environments of alkaline-earth ions and Sb atoms of (a) Ca₂Sb₂O₂S₃, (b) Sr₂Sb₂O₂Se₃ and (c) Ba₂Sb₂O₂S₃.

The Sb atoms in Ca₂Sb₂O₂S₃ and Sr₂Sb₂O₂Se₃ possess similar coordination modes. However, the [SbOS₄]⁷⁻ square pyramids in Ca₂Sb₂O₂S₃ are highly distorted with two short Sb–S bonds (2.479 Å and 2.608 Å) and two longer ones (3.007 Å and 3.057 Å), compared with the more uniform Sb–Se bonds (2.788–3.041 Å) in Sr₂Sb₂O₂Se₃. This difference indicates that the 1-D structure containing Ca²⁺ template is loosely packed. The Sb–S bond lengths in Ca₂Sb₂O₂S₃ correspond to those in other oxysulfides containing [SbOS₄]⁷⁻ units like SrOCuSbS₂ (2.67–2.95 Å) and Sr₆Cd₂Sb₆O₇S₁₀ (2.435–2.951 Å). With the increase of template size, the Sb/O/S covalent framework is divided into isolated [Sb₂O₂S₃]⁴⁻ clusters in the 0-D Ba₂Sb₂O₂S₃. The Sb–S bond lengths of the [SbOS₂]³⁻ trigonal pyramids are 2.415 Å and 2.608 Å, which are comparable to those in compounds such as CaSb₁₀O₁₀S₆ (2.468–2.485 Å) containing [SbOS₂]³⁻,¹⁵ KCu₂SbS₃ (2.448 Å)³⁷ and Ba₂SbS₃I³⁸ (2.389–2.408 Å) containing [SbS₃]³⁻ units. Therefore, this work provides a simple strategy to adjust the spatial arrangement of building units, structural dimension, and further the dimension-related transport properties and electronic structures in the Sb-based oxychalcogenide system.

Thermal behavior

The thermal behavior of the two compounds was investigated using DSC-TGA analysis under N₂ flow, and the results are shown in Fig. 4. For Ca₂Sb₂O₂S₃, a slight weight loss of 5% occurred in the range of 580–720 °C in the TGA curve. Subsequently, it showed a significant weight loss on heating above 720 °C, accompanied by two endothermic peaks at 721 °C and 777 °C in the first cycle of the DSC curve. In the second cycle, both endothermic peaks disappeared. No exothermic peak was observed in the cooling process. The results indicate that Ca₂Sb₂O₂S₃ melts incongruently around 720 °C. The TGA curve of Ba₂Sb₂O₂S₃ showed a slight weight loss of 2% around 550 °C. A single endothermic peak at 692 °C appeared during the heating process of the first cycle of the DSC curve with an obvious weight loss. No other peak was observed in the DSC curve, indicating an incongruent-melting behavior of Ba₂Sb₂O₂S₃. The melting point of Ba₂Sb₂O₂S₃ (692 °C) is lower than that of Ca₂Sb₂O₂S₃ (720 °C), which reflects weaker interactions between the Sb/O/S units in the 0-D structure than those in the 1-D structure.

Fig. 4 The TGA curves (blue) and DSC curves (first cycle: black and second cycle: red) of (a) Ca₂Sb₂O₂S₃ and (b) Ba₂Sb₂O₂S₃.

Optical properties

The UV-Vis optical absorption properties of Ca₂Sb₂O₂S₃ and Ba₂Sb₂O₂S₃ were investigated by UV-Vis diffuse reflectance spectroscopy, and the results are shown in Fig. 5a. Both spectra exhibit obvious adsorption edges, indicating the semiconducting nature of the two compounds. The band gap values were determined by the extrapolation method using the Kubelak–Munk equation (Fig. 5b).

Fig. 5 (a) UV-Vis absorption spectra of Ae₂Sb₂O₂S₃, and (b) a plot of (αhν)²vs. energy obtained using the Kubelka–Munk equation. PL emission spectra of (c) Ba₂Sb₂O₂S₃ and (d) Ca₂Sb₂O₂S₃ excited at 270, 280, 300, 320, 340 nm.

The (αhν)²versus hν curves (where α is the absorption coefficient, and h is Planck's constant, ν is the frequency) reveal the band gaps of 2.36 eV and 2.78 eV for Ca₂Sb₂O₂S₃ and Ba₂Sb₂O₂S₃, respectively. The 0.42 eV band gap difference reflects the different electronic structures of the two compositionally close compounds. The narrower band gap of Ca₂Sb₂O₂S₃ mainly origins from the extending bandwidth brought by the stronger orbital interactions in the 1-D structure. The different coordination environments around the Sb atoms in the two compounds also lead to different contributions of Sb orbitals near the band edges, which would also affect the band gap values. The band gap of Ba₂Sb₂O₂S₃ is slightly larger than that of Ba₂SbS₃I (2.64 eV) containing isolated [SbS₃]⁶⁻ units,³⁸ which could be induced by the incorporation of highly electronegative O²⁻.

The PL emission properties of the two compounds were examined at various excitation wavelengths. As shown Fig. 5c and d, both compounds exhibit multi-emission characters. Two emission bands centered at 406 and 550 nm appeared in the spectrum of Ba₂Sb₂O₂S₃ excited at 270 nm, while the former showed higher intensity. Once the excitation wavelength increased to 280 nm, the emission intensity at 406 nm sharply decreased while the band at 550 nm showed relatively slight weakness. Both emissions became weaker along with the increasing excitation wavelengths, and a new emission band centered around 395 nm appeared under 320 nm and 340 nm excitation. The PL emission properties of Ca₂Sb₂O₂S₃ are similar to those of Ba₂Sb₂O₂S₃. It also revealed two emission bands centered at 406 nm and 550 nm once excited at 270 nm, while the latter showed higher intensity than that of Ba₂Sb₂O₂S₃. The above-band-gap PL emissions of the two compounds should originate from the processes involving defect levels below the valence band maximum or above the conduction band minimum. Such above-band-gap PL emissions were also observed in other compounds, such as CH₃NH₃PbI₃,³⁹ CsPbCl₃, CsPbBr₃,⁴⁰ TlPbI₃,⁴¹ GaAs_1−xN_x⁴² and GaAs_1−xP_x.⁴³ The similar PL emission behaviors indicate similar types of defects in the two closely-related compounds.

Electronic structures

In order to reveal the origin of different optical absorption properties of the two compounds, DFT calculations were performed. The band structures shown in Fig. 6a and b reveal that Ca₂Sb₂O₂S₃ possesses a direct band gap (Γ to Γ) of 1.42 eV, while the minimum band gap of Ba₂Sb₂O₂S₃ is indirect (C to Γ) with an energy of 2.05 eV. As the energy at Γ near the valence band edge is just 0.05 eV lower than that at C, Ba₂Sb₂O₂S₃ can be considered as a quasi-direct-band gap semiconductor. The underestimation of calculated band gaps compared with the experimental values originates from the limitations of DFT calculations we performed. It is worth noting that the valence band dispersion of Ca₂Sb₂O₂S₃ near the Fermi level is more obvious than that of Ba₂Sb₂O₂S₃, reflecting the stronger bond interactions in the 1-D [Sb₂O₂S₃]⁴⁻ chains of the former. It also indicates a better hole-transport capability in Ca₂Sb₂O₂S₃.

Fig. 6 Band structures of (a) Ca₂Sb₂O₂S₃ and (b) Ba₂Sb₂O₂S₃, and total DOS and partial DOS of Sb, O and S in (c) Ca₂Sb₂O₂S₃ and (d) Ba₂Sb₂O₂S₃.

Total and partial density of states (DOS) reveals that the valence band maximum (VBM) and the conduction band minimum (CBM) of Ca₂Sb₂O₂S₃ are mainly contributed by the S and Sb orbitals (Fig. 6c). The VBM is composed of S 3p states, Sb 5s and 5p states, while the CBM is composed of Sb 5p and S 3p states. The localized Sb 5s states near the VBM indicate the lone-pair character of Sb 5s² electrons. The main part of O 2p states is located at −1 eV below the Fermi level, making little contribution to the band edge. For Ba₂Sb₂O₂S₃, the VBM is mainly composed of S 3p, Sb 5s and 5p states, and some contribution of O 2p states (Fig. 6d). The CBM consists of Sb 5p and S 3p states, which is similar to that of Ca₂Sb₂O₂S₃. The lone-pair character of Sb 5s² in Ba₂Sb₂O₂S₃ is not that obvious, which is reflected by the almost equal contribution of Sb 5s and 5p states near the VBM. In addition, the main distribution of Sb 5p states appears above 2.5 eV in the CB of Ba₂Sb₂O₂S₃ along with the increase O 2p density in this region, while the density of Sb 5p states in Ca₂Sb₂O₂S₃ appears near the CBM. These differences could be induced by O orbital participation near the band edges in Ba₂Sb₂O₂S₃. Therefore, the wider band gap of Ba₂Sb₂O₂S₃ origins not only from the narrow bandwidth caused by suppressed band dispersion in the 0-D structure, but also from the participation of O states in the band edges.

As the soft Lewis acid Ba²⁺ in Ba₂Sb₂O₂S₃ can presumably facilitate the formation of isolated [SbOS₂]³⁻ trigonal pyramids by interacting with the soft base lone pair on Sb³⁺, we first look at the local environment of Sb. As shown in Fig. S3,† Sb has four closest Ba neighbors at distances less than 4 Å (3.777–3.924 Å), while none of them is in the direction of the Sb lone pair. The reason is that three Ba atoms are close to the O atom at distances around 2.7 Å. The remaining one is close to the S atom of another [Sb₂O₂S₃]⁴⁻ cluster. In this fashion, the ionic attraction is maximized at the expense of the Lewis acid–base interaction. The COHP curves for the Sb–O, Sb–S (Fig. S4a†), Ba–Sb, Ba–O and Ba–S bonds (Fig. S4b†) were also calculated to analyze the strength and nature of the bonds. The COHP bonding values for the Sb–O and Sb–S bonds are about 10 times larger than those of the Ba–Sb, Ba–O and Ba–S contacts, indicating the dominant ionic interaction among the latter contacts. Furthermore, the Ba–O interaction is much stronger than those of the Ba–S and Ba–Sb contacts. All these confirm that the interaction of Ba²⁺ with the host lattice is ionic with negligible contribution from the Ba–Sb interaction.

Conclusions

In summary, we have successfully synthesized two novel Sb-based oxysulfides Ae₂Sb₂O₂S₃ (Ae = Ca, Ba) via solid state reactions. Both crystallize in the C2/c space group (no. 15) with different structure types. Ca₂Sb₂O₂S₃ is composed of 1-D anionic [Sb₂O₂S₃]⁴⁻ chains isolated by Ca²⁺ ions. The [Sb₂O₂S₃]⁴⁻ chain is constructed by edge-sharing [SbOS₄]⁷⁻ square pyramids. Ba₂Sb₂O₂S₃ consists of 0-D anionic [Sb₂O₂S₃]⁴⁻ clusters separated by Ba²⁺ ions. The higher structural dimensions of Ca₂Sb₂O₂S₃ induce a higher m.p. (720 °C) and a narrower band gap (E_g = 2.36 eV), compared with those of Ba₂Sb₂O₂S₃ (m.p. = 692 °C and E_g = 2.78 eV). Both compounds show interesting multi-emission characters. The calculated electronic structures reveal that the band edges of both compounds mainly consist of Sb–S bond orbitals, while O 2p orbitals also make some contribution to the VBM of Ba₂Sb₂O₂S₃. We suppose that this work provides an example of the template effect on group arrangement and structural dimension, and can serve as a guide for the structural design of new functional Sb-based oxychalcogenides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant 21871008, 22005006, and 22001263) and the China Postdoctoral Science Foundation (Grant 2019M660298 and 2020T130009). We would thank Prof. Chong Zheng for his support in the theoretical calculations.

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Footnote

† Electronic supplementary information (ESI) available: EDX spectra, oxygen coordination environments, crystallographic data and structure refinement details, selected bond lengths and angles, atomic coordinates, equivalent isotropic displacement parameters and anisotropic displacement parameters of Ca₂Sb₂O₂S₃ and Ba₂Sb₂O₂S₃. CCDC 1985461 and 1985463. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi00698g

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