KNa_0.78Eu_0.27In_3.80B₁₂S₁₂: a novel hexanary thioborate featuring a B₁₂S₁₂ cluster and diverse InS_x (x = 4, 5, 6) units

Abstract

Thioborates combine the advantages of sulfides and borates, and are a new type of multifunctional inorganic material with versatile structural features. Here, a thioborate KNa_0.78Eu_0.27In_3.80B₁₂S₁₂ crystallizing with a new type of compound was synthesized by a high-temperature flux method. Its structure features a rarely discovered B₁₂S₁₂ cluster and diverse InS_x (x = 4, 5, 6) units built in a three-dimensional {[In_3.80B₁₂S₁₂]^2.6−}_∞ polyanionic framework. The structural complexity and novelty are well elaborated. Its optical band gap was determined to be 2.28 eV, and a theoretical calculation of its electronic structure was also performed. This work not only enriches the structural chemistry of chalcogenoborates but also provides a new potential RE–B–Q family (RE = rare-earth metal; Q = S, Se). The latter may produce some interesting functionals.

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1. Introduction

The boron element features structural complexity, electron deficiency, unusual bonding situations, and builds a rich boron chemistry.¹ Boron and boron-based materials have important applications in modern industry, agriculture, biology, medicine and national defense.^2–8 There are many variants of crystalline elemental boron, and all of them are built by the basic building unit B₁₂ icosahedron. Each B₁₂ icosahedron has three B–B σ single bonds and ten three-center two-electron (3c–2e) bonds.⁹ This specific bonding style yields a rich boron chemistry, in which the closo-polyhedral borane anions have drawn attention to these species as precursors of a new and rapidly expanding field of chemistry enriched with novel applications in contemporary materials research.^10–12closo-borane [B₁₂H₁₂]²⁻ is a cluster exhibiting a three-dimensional (3D) aromatic electron delocalization and a remarkable chemical stability.^13–15 In contrast to the highly reactive borane with a small molecular weight, [B₁₂H₁₂]²⁻-based salts are the most stable molecules; this anion survives extended heating in strongly acidic or alkaline solutions.^16,17

Attracted by the specialty of B₁₂, many efforts have been made to enrich this special entity.¹⁸ Completely saturated with hydrogen, halogens, or even hydroxyl groups, the boron icosahedra can be obtained in solution.^10,19–24 All of these bond with B₁₂ to form an anion cluster. Different from this style, B₁₂ icosahedron can also connect with twelve Q (Q = S, Se) atoms to form a unique [B₁₂Q₁₂]¹⁴⁻ cluster.^25–32 To date, around thirty chalcogenoborates containing B₁₂Q₁₂ cluster have been discovered. In their structures, there are two types of coordination environments around each B₁₂ icosahedron. One is B₁₂ saturated with six BQ₃ units to form the [B₁₂(BQ₃)₆]⁸⁻ anion,²⁵ and the other is B₁₂ connected with six MQ₄ and two M′Q₆ units to build a [MB₁₂(M′Q₄)₃]^x− (x = 0, 2; M = Sm, Gd, Ga, In, Sn, Cr; M′ = Ga, In, Sn, Si) open framework.^31,33–37 Specifically, the noncentrosymmetric (NCS) ones of the latter show Kleinman-forbidden second-harmonic generation activity.^33–36

Chalcogenoborates containing B₁₂Q₁₂ clusters are very rare when compared with those containing B₁₂X₁₂, B₁₂(OH)₁₂ or B₁₂H₁₂ clusters.^38–42 Encouraged by the structural diversity for these, it is necessary to explore more B₁₂Q₁₂-based ones to enrich their structural chemistry and explore their application potentials. Based on this consideration, our continuous investigation of the [B₁₂Q₁₂]¹⁴⁻-based system has led to the discovery of a novel hexanary thioborate, KNa_0.78Eu_0.27In_3.80B₁₂S₁₂ (1). Its 3D structure features two types of B₁₂ icosahedron and diverse InS_x (x = 4, 5, 6) units, and B₁₂ icosahedra are consolidated by eight or four InS_x units, representing a new type of chalcogenoborate. Here, the synthesis, crystal structure, and diverse characterizations are reported.

2. Experimental section

2.1. Synthesis and analyses

A high-temperature solid-state method was used to synthesize the crystals of KNa_0.78Eu_0.27In_3.80B₁₂S₁₂ (1). All the starting materials were used as received without further purification. Considerable efforts were devoted to synthesize 1, and eventually an optimized routine with a high yield was found. For the optimized synthetic route, a 500 mg mixture of Eu₂O₃ (99.9%), In₂O₃ (99.9%), S (99.95%) and B (99%) with 1 : 3 : 16 : 12 of Eu : In : B : S, and an additional 400 mg of NaI and 200 mg of KI (99%) as flux, were used as the starting materials, which were hand-ground in an agate mortar for 30 minutes. The obtained mixture was pressed into a pellet and then loaded into an evacuated quartz ampoule with a vacuum degree of 1 × 10⁻⁴ Torr. The quartz tube was put in a muffle furnace and heated to 950 °C in 25 h and kept for 7 days, and then cooled down to 300 °C in 5 days. The dark-red crystals of 1 are stable without any change after exposure to air for a few months at room temperature.

A semiquantitative microscopic elemental analysis on the as-prepared single-crystals of 1 was performed on a field-emission scanning electron microscope (FESEM, Zeiss-Supra55) equipped with an energy dispersive X-ray spectroscope (EDS, Bruker, Quantax), which confirmed the presence of K, Na, Eu, In and S. Specifically, the average ratio of In and Eu is 13, close to that of the single-crystal structure analysis, determined as 14.07. The B element could not be detected in view of its light mass (Table S1 and Fig. S1†).

The inductively coupled plasma (ICP) spectroscopy of 1 was measured on an Optima 7300 DV spectrometer. Polytetrafluoroethylene(PTFE) beakers were used to dissolve the powder of 1 and blank samples were also measured to eliminate background effects. The results show that the mass ratios of Eu/In (14.67) and K/Na (1.26) are close to the theoretical values (Eu/In = 14.07 and K/Na = 1.33).

The powder X-ray diffraction measurement of 1 was carried out with a Bruker D8 Advance diffractometer at 40 kV and 100 mA for Cu-Kα radiation (λ = 1.5406 Å) at room temperature. The 2θ range was 10–60° with a scan step width of 2°. The calculated pattern was generated using the Mercury program with its single-crystal structure data. JANA2006 software was employed to perform the Rietveld refinement (Table S2† and Fig. 1).

Fig. 1 Rietveld refinement for the powder X-ray diffraction pattern of 1.

2.2. Crystal data

Several suitable single crystals of 1 were selected using an optical microscope for the single-crystal X-ray diffraction analysis. The diffraction data were collected by using a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature on a Bruker D8 QUEST X-ray diffractometer. The structure was solved by direct methods and refined on F² by full-matrix least-squares methods using the Shelxtl2014 crystallographic software package.⁴³ The crystallographic data are listed in Table 1, and the atom positions and anisotropic displacement parameters (ADP), and bond distances are listed in Tables S3 and S4,† respectively. The CIF document has also been deposited with the CCDC 2086848.†

Table 1 Crystal data and structure refinement parameters for 1

Chemical formula	KNa0.78Eu0.27In3.80B12S12
a R1 = ||Fo| − |Fc||/|Fo|. b wR2 = [w(Fo^2 − Fc^2)^2]/[w(Fo^2)^2]^1/2.
FW	1048.21
T (K)	296
Crystal system	Monoclinic
Space group	C2/m
Z	4
a (Å)	15.2538(12)
b (Å)	10.4468(7)
c (Å)	14.6131(10)
β (°)	105.036(3)
V (Å^3)	2248.9(3)
D calcd (g cm^−3)	3.096
μ (mm^−1)	5.888
F(000)	1930.0
Crystal size/mm^3	0.136 × 0.12 × 0.05
2θ range (°)	4.78 to 50.942
Indep. reflns./Rint	2108/0.0245
Data/restraints/parameters	2108/0/164
GOF on F^2	1.097
R1^a (I > 2σ(I))	0.0306
wR2^b (all data)	0.0701

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Considering that the structure of 1 is heavily disordered, involving partial occupation and spilt sites, it is necessary to make a detailed description about the structure solution process, which can be described as below. First, the EDS analysis result indicates that K, Na, Eu, In and S are the compositional elements in crystals of 1, and all these elements have nonnegligible contents according to the EDS analysis of more than 30 single-crystals. Second, all the B, S, and K sites are fully occupied with reasonable anisotropic displacement factors (ADPs). Third, two of the three In sites are fully occupied, while the third one (In(1)) is partially occupied, and the occupancy was freely refined to 0.80. Fourth, the occupancy of Eu(1) was freely refined to 0.27. Compared with the refinement results for the model with a full occupation of In(1) and Eu(1) atoms, the R1 value reduced from 0.1153 to 0.0492. Fifth, the Na(1) site was split over two neighboring sites, namely, Na(1A) on Wyckoff site m and Na(1B) on Wyckoff site 2/m. The occupancies for these two sites were refined to 0.56 for Na(1A) and 0.44 for Na(1B). The incomplete occupations of Eu and In atoms in other works were also discovered (Table S5†). Combining the above considerations together, the chemical formula of 1 is given as KNa_0.78Eu_0.27In_3.80B₁₂S₁₂.

2.3. UV–vis–NIR diffuse reflectance spectroscopy

The UV-vis-NIR diffuse reflection spectrum was recorded at room temperature using a powder BaSO₄ sample as a standard on a computer-controlled Cary 5000 UV–vis–NIR spectrometer equipped with a diffuse reflectance accessory at 200–2500 nm. The spectrum was calculated from the reflection spectrum by the Kubelka–Munk function:⁴⁴

F(R) = (1 − R)²/(2R) = K/S

where R is the reflectance, K is the absorption, and S is the scattering. In the (K/S) versus E plot, extrapolating the linear portion of the rising curve to zero gives rise to the onset of absorption.

2.4. Thermal analysis

Differential scanning calorimetry (DSC) analysis on 1 was performed by using a Mettler Toledo thermal analyzer under N₂ atmosphere. The powder sample of 1 was put into a crucible and heated to 1000 °C and cooled back to room temperature at a rate of 10 °C min⁻¹.

2.5. Calculation details

The electronic structure calculation, including band structure and density of states (DOS), was performed using the CASTEP module in Material Studio.⁴⁵ A generalized gradient approximation (GGA) by PBE was adopted to describe the exchange–correlation energy. The electronic configurations for In, B, S, Na, K, and Eu were 5s and 5p, 2s and 2p, 3s and 3p, 3s, 4s, 5s/6s and 4f, respectively, and a cutoff energy of 700 eV was set. The numerical integration of the Brillouin zone was implemented by employing 2 × 2 × 4 Monkhorst–Pack k-point sampling. The Fermi level at 0 eV was chosen as the reference.

3. Results and discussion

3.1. Crystal structure

1 adopts a new monoclinic structure type with the space group C2/m (Pearson code mC119.4). There are three In, eight S, eight B, one K, one Na and one Eu sites in the crystallographically independent unit in the structure of 1 (Fig. 2a). Eight kinds of B atoms comprise two types of B₁₂ icosahedra. For the sake of clarity, each B₁₂ icosahedron is dummied into one sphere. Two B(2), two B(3), four B(4) and four B(7) link together to form a B₁₂ icosahedron (type-I) with the B–B distances in the range 1.768(9)–1.806(8) Å. Meanwhile, two B(1), two B(5), four B(6) and four B(8) build a B₁₂ (type-II) icosahedron, in which the B–B bond lengths are 1.764(8)–1.805(8) Å. In both cases, there are nine types of B–B distances, showing the B–B bond distance dispersity.

Fig. 2 Crystal structure of 1: (a) Coordination geometry; (b) {[In_3.80S₁₂]^12.6−}_∞ polyanionic ribbons along the b axis; (C) 3D view of the crystal structure along the b direction.

Na, K, and Eu metal elements are surrounded with five, eight, and six S atoms to form a NaS₅ trigonal bipyramid, KS₈ cuboctahedron, and EuS₆ octahedron, respectively. Each B₁₂ icosahedron has twelve nearest S atoms to form the B₁₂S₁₂ cluster, in which each type-I B₁₂ icosahedron is surrounded by two InS₄ tetrahedra and four InS₅ trigonal bipyramids by sharing edges. Whereas each type-II B₁₂ icosahedron is surrounded by four InS₆ octahedra through sharing faces, two InS₄ tetrahedra and four InS₅ trigonal bipyramids by sharing edges. Both types of B₁₂S₁₂ cluster are isolated, and they only connect with InS_x polyhedra. The In(1)S₆ octahedra and In(3)S₅ trigonal bipyramids build two types of zig-zag chains and then connect with In(2)S₄ tetrahedra to form {[In_3.80S₁₂]^12.6−}_∞ polyanionic ribbons extending along the b direction (Fig. 2b), which further connect with two types of B₁₂ to build a 3D structure (Fig. 2c).

The simplified 3D structure graph of 1 viewing along the b and c directions is depicted in Fig. 3, where K, Na and Eu atoms are omitted for the sake of clarity, and the B₁₂ icosahedron, In(1)S₆ octahedra, In(2)S₄ tetrahedra and In(3)S₅ trigonal bipyramid are dummied to one atom, namely I-B₁₂, O-In(1)S₆, T-In(2)S₄, TP-In(3)S₅. The connections between the diverse polyhedra can be clearly observed. There are two-types of connections around I-B₁₂, one with four O-In(1)S₆, four TP-In(3)S₅ and two T-In(2)S₄, and the other with four TP-In(3)S₅ and two T-In(2)S₄. The I-B₁₂ are isolated from each other, and they are connected by InS_x polyhedra. Each O-In(1)S₆ is surrounded by two I-B₁₂, two T-In(2)S₄, and two TP-In(3)S₅, while each T-In(2)S₄ is surrounded by two I-B₁₂, TP-In(3)S₅, and two O-In(1)S₆; and each TP-In(3)S₅ is surrounded by two I-B₁₂, one T-In(2)S₄, and one O-In(1)S₆.

Fig. 3 Simplified open-framework of 1 viewed along the b (a) and c (b) directions.

Specifically, the coordination diversity of In atoms is fully exhibited in this structure. In(1), In(2) and In(3) atoms coordinate with six, four and five S atoms to form octahedral In(1)S₆, tetrahedral In(2)S₄ and trigonal bipyramidal In(3)S₅ units. The distances from In to the three equatorial and one of the apical S atoms are short (2.4596(14)–2.5778(9) Å); while the distance to the other apical S atom is considerably longer (2.9409(8) Å) (Fig. S2†). This longer In–S distance is comparable with those in NaIn₃S₅, Ba₁₈F₁₈In₈S₂₁, Fe_1.5Pb_5.5In₁₀S₂₂, Ca_0.76In_2.84S₅ and La₄Ag₂In₄S₁₃ (Table S6†). In other In-containing chalcogenides, tetrahedral and octahedral coordination geometries are normally found for the In atom; in contrast, examples of trigonal bipyramidal geometry are very few, including but not limited to γ-In₂Se₃,⁴⁶ Ga_xIn_2−xSe₃,⁴⁷ BaRE₂In₂Q₇ (Q = S, Se)⁴⁸ and K₅In₃P₆Se₁₉.⁴⁹ In general, a structure containing three In coordination geometries simultaneously is very rare. At present, it's only discovered in K₅In₃P₆Se₁₉.⁴⁹ Hence, 1 is the first chalcogenoborate and the second chalcogenide containing three InS_x coordination geometries.

When referring to the different In–S polyhedra, the connection between In and S atoms can be described as a highly distorted In(3)S₅ trigonal bipyramid, slightly distorted In(2)S₄ tetrahedron and normal In(1)S₆ octahedron with gradually reduced calculated dipole moments of 18.8529, 9.2955, and 3.9815 D, respectively. They connect with each other to form {[In_3.80S₁₂]^12.6−}_∞ polyanionic ribbons (Fig. 2b), which represent a new In–S slab. Differently, the three InSe_x coordination geometries in K₅In₃P₆Se₁₉ are connected with each other by Se(1) atoms to form the [In₃Se₁₃]¹⁷⁻ anion (Fig. S2†).⁴⁹

Differently from the arrangement of other B₁₂Q₁₂ compounds,^25,34 the B₁₂S₁₂ clusters in 1 are arranged in a cubic closest-packed manner with the center to center distance within 8.987 to 11.857 Å (Fig. S3†).⁵⁰ The cluster anions exhibit their centers of gravity at the Wyckoff positions 2a (for type-I B₁₂) and 2d (for type-II B₁₂), with distances of almost 1.7 Å to the twelve boron atoms, providing them an inner diameter of 3.4 Å, which is close to that of other inorganic compounds containing B₁₂ clusters (Table 2) and indicates the high stability of B₁₂ clusters.

Table 2 Different B₁₂ clusters in known chalcogenoborates

Compound	d (B–B)/Å	d (C–B)/Å	ID	The CE of B12
Note: d (C–B), ID and CE represent the average values of the B12 gravity's center to B atoms, inner diameter of B12, and chemical environments of B12, respectively.
K3Cl[InB12(InSe4)3] ^36	1.79	1.70	3.40	6[InSe4], 4[InSe6]
Ag2B12Cl12 ^50	1.78	1.70	3.40	6[AgCl6]
Sn4B12Se15 ^31	1.79	1.70	3.40	6[SnSe4], 2[SnSe6]
Cs8B12(BSe3)6 ^25	1.78	1.70	3.39	6[BSe3]
1 (type-I B12)	1.79	1.71	3.41	4[InS5], 2[InS4]
1 (type-II B12)	1.79	1.70	3.40	4[InS6], 2[InS4], 4[InS5]

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As far as we know, there are two-types of connection around B₁₂ icosahedra: one is surrounded by six BS₃ units to form [B₁₂(BS₃)₆]⁸⁻ polyanion (Fig. 4a), while the other one is connected with two InS₆ and six InS₄ units to construct the [In₈B₁₂S₃₀]²⁶⁻ polyanion (Fig. 4b).^26,35 Different from these two styles, the B₁₂ icosahedron in this work adopts a new connection mode with diverse InS_x units to form the [In_7.1B₁₂S₂₉]^26.7− anion in 1 (Fig. 4c). Among the two known structure types, a family of multinary (A₃X)[MB₁₂(M′Q₄)₃] (A = alkali metal; X = halogen element; M = Ga, In, rare-earth metal; M′ = Ga, In) compounds have the maximum relevance to the structure of 1.^33–36 (K₃I)[InB₁₂(InS₄)₃] crystalizes in the NCS hexagonal space group P6₃22 featuring a 3D honeycomb-like open-framework, providing channels occupied by face-sharing IK₆ octahedral chains.³⁵ Accordingly, 1 features {[In_3.80S₁₂]^12.6−}_∞ polyanionic ribbons constructed by diverse InS_x units, which further connect with two types of B₁₂ icosahedra to form the 3D structure (Fig. 4c). Unlike 1 with two types of B₁₂ icosahedra, (K₃I)[InB₁₂(InS₄)₃] has only one type of B₁₂ icosahedron with two unique B atoms, and each B₁₂ icosahedron is surrounded by two In(1)S₆ octahedra via sharing faces and six In(2)S₄ tetrahedra via sharing edges (Fig. 4b and c).

Fig. 4 The chemical environment around one B₁₂ icosahedron: [B₁₂(BS₃)₆]⁸⁻ (a); [In₈B₁₂S₃₀]²⁶⁻ (b); [In_7.1B₁₂S₂₉]^26.7− (c).

The In(1)–S bond distances in the InS₆ unit are 2.680(6) Å and the In(2)–S bond distances in the InS₄ unit are 2.399(6)–2.492(7) Å in (K₃I)[InB₁₂(InS₄)₃], indicating that In(1)S₆ octahedra are normal and centrosymmetric (CS). In(1)S₆ octahedra arrange in a parallel manner and In(2)S₄ tetrahedra settle orderly at a certain angle. Although all the coordination polyhedra in 1 are distorted, it crystallizes in the CS structure, since all the InS_x units in its structure stack in a back-to-back style, which cancels out their hyperpolarizabilities.^51,52 Their NCS/CS structures can also be explained by analyzing their structural symmetry. Considering their unit-cell centers, the In atom coordinates with four S atoms to form a highly distorted InS₄ tetrahedron in (K₃I)[InB₁₂(InS₄)₃] with side lengths of 3.854–4.181 Å, whereas in the symmetric center of 1, Na and S atoms connect with each other to form two parallelograms with Na–S distances of 3.261(6) and 3.007(5) Å (Fig. 5).

Fig. 5 The center of the unit cell: (a) parallelograms formed by Na and S in 1; (b) highly distorted InS₄ tetrahedron in (K₃I)[InB₁₂(InS₄)₃].

3.2. Optical properties

The optical band gap of 1 was determined to be approximately 2.26 eV from the result of UV-vis-NIR diffuse-reflectance spectroscopy (Fig. 6), which is consistent with its dark-red color.

Fig. 6 UV–vis–NIR diffuse reflectance spectrum of 1.

3.3. Thermal behavior

The differential scanning calorimetry (DSC) result shows that it undergoes melting upon heating and crystallization upon cooling events (Fig. 7).^53,54 The inherently high kinetic stability of the B₁₂ icosahedron makes it resistant to decomposition reactions at elevated temperatures, and it is thermally stable up to 739 °C.

Fig. 7 DSC curve for 1 measured from room temperature to 1000 °C under a N₂ atmosphere.

3.4. Theoretical calculations

To better understand its structure, an electronic structure calculation for 1 was performed. The calculated band gap of 1 is 1.08 eV (Fig. 8a). Usually, the calculated band gap is underestimated compared with its experimental one. The density of states (DOS) calculation was carried out to observe the orbitals’ contribution near the Fermi level (Fig. 8b). Total and Partial DOS show that the top of the valence band is mainly composed of S-3p and minor B-2p orbitals, while the bottom of the conduction band is primarily constituted of In-5s and S-3p orbitals, suggesting that charge transfer between the frontier orbitals is primarily from S 3p to In 5s orbitals. Recalling the case for (K₃I)[InB₁₂(InS₄)₃],³⁴ similar calculation results are available and also verify that these calculations are reliable.

Fig. 8 Calculated electronic structure, including band structure (a) and DOS (b). 0 eV is selected as the Fermi level.

4. Conclusion

In summary, a novel hexanary thioborate KNa_0.78Eu_0.27In_3.80B₁₂S₁₂ (1) was obtained and systematically studied. Its structure features an unprecedented {[In_3.80S₁₂]^12.6−}_∞ polyanionic ribbon, two types of B₁₂ icosahedra, and diverse InS_x units. 1 is the first thioborate and the second chalcogenide to include InS₄, InS₅ and InS₆ polyhedra in one structure. This work not only provides a novel type of thioborate with a special structure, but also enriches both B and In chemistry. Next, it is proposed to explore its potential structural tolerance for diverse derivatives and physical properties. We hope this work can evoke increasing interest on chalcogenoborates, which really represents an amazing while insufficiently explored material system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge the financial support by the national natural science foundation of China (21771159, 22071212), and the Qinglan project from Yangzhou University.

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Footnote

† Electronic supplementary information (ESI) available: Crystal data tables, crystal photographs, EDS and other tables. CCDC 2086848. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi00520d

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