An unprecedented pentanary chalcohalide with Mn atoms in two chemical environments: unique bonding characteristics and magnetic properties

Abstract

Mixed-anion chalcohalides have attracted significant attention lately, attributable to their unique structure compositions and captivating physicochemical properties. Herein, an unprecedented pentanary chalcohalide, Cs₂[Mn₂Ga₃S₇Cl] (1), was discovered by solid-state reaction at 1223 K. It is constructed by alternately stacked layers, each of which is made by a 2D [Ga₃S₉]⁹⁻ ribbon embedded with 1D [Mn₂S₈Cl]¹³⁻ chains. The coexistence of two Mn-coordinated polyhedra ([MnS₆] octahedra and hetero-ligand [MnS₃Cl] tetrahedra) in one material is surprisingly observed for the first time in the Mn-containing inorganic chalcogenides or chalcohalides. More interestingly, it exhibits ferrimagnetic (FIM) behaviour, which could be correlated to the magnetic sub-lattice sites with different coordination geometries. This work suggests a new route for designing and searching for unique functional chalcohalides with different chemical environments.

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In recent years, low-dimensional chalcogenides with high mobility and rich electronic band structures have received considerable interest because of their fascinating physical properties, especially magnetism and superconductivity.¹ In particular, mixed-anion oxychalcogenides or chalcohalides constructed by alternating layers are of great importance. In such layered structures, different electronegative (EN) elements are usually involved where: the more EN elements (e.g., O, F and Cl) prefer to adopt strong ionic-bonds via bonding with highly electropositive (EP) elements (e.g., rare-earth, alkaline-earth and alkali metals), whereas the less EN elements (e.g., S, Se and Te) attempt to form steady covalent-bonds with a less EP element (e.g., p-block elements and transition metals). Such interesting bonding characteristics are important in terms of understanding the formation of compounds and their potential applications.^2–5 For instance, iron-lanthanum-oxide-sulfide and -selenide phases La₂O₃Fe₂S₂ and La₂O₃Fe₂Se₂ feature distinctive 2D layers stacked along the ab-plane in the order Fe₂O/Se/La/O/La/Se/Fe₂O and show interesting Van-Vleck paramagnetism.² Oxychalcogenide (Li_0.8Fe_0.2)OHFeSe contains both conductive layers [FeSe] and spacer layers [(Li_0.8Fe_0.2)OH], thereby showing superconductivity at 42 K and antiferromagnetic (AFM) ordering at 8.6 K.³ 2D [(BaF)₂][Fe_2−xQ₃] (Q = S, Se) is formed by alternating [(BaF)₂]²⁺ and [Fe_2−xQ₃]²⁻ layers, and this compound exhibits switchable ferromagnetically ordered moments that are dictated mainly by the [Fe_2−xQ₃]²⁻ layers.⁴ [Ba₄F₄][CrIn₂S₆], which consists of alternate stacking of cationic [Ba₂F₂]²⁺ layers and anionic [Cr_0.5InS₃]²⁻ blocks, exhibits AFM ordering at around 8 K.⁵ Therefore, the synthesis and discovery of new layered mixed-anion compounds present an enticing prospect for exploring unique physicochemical properties and corresponding applications.

Up to now, group 13 metal chalcohalides containing alkali metals are less extensively investigated; to our knowledge, the existing examples include only 0D-K₅(InTe₄)(KCl),⁶ 0D-(Cs₆Cl)₂Cs₅[Ga₁₅Ge₉Se₄₈],⁷ 2D-(Cs₆Cl)₆Cs₃[Ga₅₃Se₉₆],⁸ 2D-[A₃X][Ga₃PS₈] (A = K, Rb; X = Cl, Br),⁹ 3D-(AX₃)[MB₁₂(MS₄)₃] (A = K, Cs; X = Cl, Br, I; M = Ga, In, Sm, and Gd),¹⁰ 3D-ABa₃Ga₅Se₁₀Cl₂ (A = K, Rb, and Cs),¹¹ and 3D-Cs₂Ba₆In_xGa_10−xSe₂₀Cl₄.¹² As yet, except for the aforementioned compounds, materials containing magnetic elements are completely unknown in such systems. The aim of this study is to expand the structure and functional diversity of the above-mentioned chalcohalides by introducing magnetic elements. It is proposed that the introduction of magnetic elements is propitious to the formation of low-dimensional magnetic domains in the parent structures and will engender attractive magnetic properties. In this communication, we discover an unprecedented pentanary chalcohalide, Cs₂[Mn₂Ga₃S₇Cl] (1),‡ which possesses a unique 2D layered structure and interesting ferrimagnetic (FIM) behaviour.

Single-crystal XRD diffraction analysis indicates that 1 crystallizes in the orthorhombic system with a = 6.2467(6) Å, b = 7.3879(8) Å, c = 33.600(3) Å, V = 1550.6(4) ų and Z = 4 [space group: Pnma (no. 62); Pearson symbol: oP60; idealized Wyckoff sequence: d³c⁹]. The asymmetric unit includes 2 crystallographically unique Cs atoms, 2 Mn atoms, 2 Ga atoms, 5 S atoms and 1 Cl atom. Among them, the Ga2, S4 and S5 atoms are at general sites 8d, while other atoms are at the Wyckoff positions 4c with m symmetry (for details see Fig. S1 and Table S1 in ESI†). Because there are no S–S and metal–metal bonds in 1, the formal oxidation states of Cs/Mn/Ga/S/Cl can be assigned as 1+/2+/3+/2−/1−, respectively.

The remarkably large c-axis, about 5 times larger than the other two axes, adopts a unique stacking of 2D [Mn₂Ga₃S₇Cl]²⁻ anionic layers along the c axis with the counter Cs⁺ cations locating in between (see Fig. 1a). There are two such layers in the unit cell and these layers are stacked in abab mode along the c-direction (A has a glide to B). Each of the layers consists of Ga–S ribbons (Fig. 1c) embedded with chains A (Fig. 1b) via sharing S1, S2, and S5 atoms. Uniquely, the chain A is built up from the [Mn1S₆] octahedra and the hetero-ligand [Mn2S₃Cl] tetrahedra. Mn-containing chalcogenides (or chalcohalides) with different coordination environments around Mn have never been reported previously to the best of our knowledge. The shortest distances of Mn1–Mn2, Mn1–Mn1 and Mn2–Mn2 are 4.343 Å, 6.247 Å and 6.247 Å, respectively (Fig. S2, ESI†). Within a ribbon B, the GaS₄ tetrahedron serves as the primary building unit, and a Ga₃S₉ trimer is formed by one Ga1S₄ and two Ga2S₄ tetrahedra. Each Ga₃S₉ secondary building unit (SBU) is connected with another 2 neighboring SBUs into a ribbon B (Fig. 1d). Furthermore, such ribbon Bs are interconnected to each other to form a 2D [Ga₃S₉]⁹⁻ layer along the a-axis via vertex-sharing (S3 and S4 atoms), as shown in Fig. 1c.

Fig. 1 Crystal structure of 1: (a) ball-and-stick representation of the crystal structure viewed along the a-axis in abab mode, the remarkably large c-axis, unit cell and atom numbers are marked; (b) the intra-chain polyhedron connection of the chain A with the atom numbers marked; (c) single Ga–S layer with the ribbon B outlined (black dotted line frame); (d) the intra-linking details of the ribbon B with the secondary building unit [Ga₃S₉] trimer outlined and the atom numbers marked. Color legends: black, Cs; blue, Mn; pink, Ga; yellow, S and green, Cl.

Normally in Mn-containing chalcogenides, the Mn atom is either 4-fold or 6-fold coordinated. Examples are in BaLa₂MnS₅,¹³ K₁₀Mn₄Sn₄S₁₇,¹⁴ MnPb₄Sb₆S₁₄,¹⁵ and Ba₄F₄MnIn₂S₆.⁵ In contrast, Mn atoms in oxides usually adopt different coordination numbers (CNs). For instance, the strong antiferromagnetic spin-exchange interactions observed in KMnO₄ at the T > 15 K region are attributed to the Mn1 (CN = 6), Mn2 (CN = 6) and Mn3 (CN = 4) sites;¹⁶ [NH₄]₄[Mn₉B₂(OH)₂(HPO₄)₄(PO₄)₆] shows canted anti-ferromagnetism below 8.0 K, which originates from the Mn/O layers with Mn atoms of CN = 5, 6.¹⁷

Selected bond lengths are provided in Table S3 in the ESI.† Mn1 is in an octahedron sphere with six Mn–S bond lengths between 2.493 and 2.771 Å, which are close to those observed for Mn²⁺ with a 6-fold coordination, such as 2.464–2.838 Å in Ba₄F₄In₂MnS₆,⁵ and 2.429–2.780 Å in Gd₂Mn₃Sb₄S₁₂.¹⁸ Differently, the Mn2 atom is coordinated by 3 S atoms and 1 Cl atom forming a distorted hetero-ligand [MnS₃Cl] tetrahedron with bond distances of Mn2–S ranging from 2.369 to 2.458 Å, which are comparable to those of 2.372–2.419 Å in K₁₀Mn₄Sn₄S₁₇.¹³ The Mn2–Cl is 2.354 Å, which is similar to those of 2.387–2.962 Å in Mn₄(TeO₃)(SiO₄)Cl₂.¹⁹ Each Ga atom forms a distorted tetrahedron with four Ga–S lengths varying from 2.271 to 2.297 Å, which are consistent with those in the related chalcohalides, such as (KI₃)[SmB₁₂(GaS₄)₃] (2.242–2.342 Å),^10a Ba₃GaS₄Cl (2.232–2.268 Å),²⁰ and [A₃X][Ga₃PS₈] (2.225–2.357 Å).⁹

Light-yellow transparent 1 was obtained by mixing the elements Mn, Ga, and S with CsCl flux using solid-state reaction [for details see the ESI†]. Semi-quantitative EDX analyses displayed the presence of Cs, Mn, Ga, S, and Cl, and the average atomic ratio matches well with the single-crystal XRD data (Fig. S3, ESI†). The title crystals were stable in air for several months. The purity of hand-picked crystals was confirmed by powder XRD analyses and impure phases were not discovered within the measurable limits of the instrument (Fig. 2a). Several peaks in the difference curve indicate some deviation of the diffraction intensity, which may be caused by the crystallite orientation preference. Diffuse reflectance spectroscopy showed an optical band-gap of 3.11 eV for 1 (Fig. 2b), which was in agreement with its crystal color and suggests semiconductor behavior. This value is wider than those of Cs₂Ba₆In_xGa_10−xSe₂₀Cl₄ (2.90–3.01 eV),¹² (Cs₆Cl)₆Cs₃[Ga₅₃Se₉₆] (2.74 eV),⁸ and Ba₃GaS₄Cl (2.14 eV),²⁰ but smaller than that of [A₃X][Ga₃PS₈] (3.50–3.85 eV).⁹ Moreover, 1 exhibits high thermal stability up to 950 K (Fig. S4, ESI†).

Fig. 2 Property measurements on 1: (a) PXRD Rietveld refinement by the program GSAS implemented with EXPGUI; (b) UV-Vis diffuse-reflectance spectrum and a photograph of the as-prepared crystals (inset).

The magnetic susceptibility (χ) of 1 was tested at 1000 Oe in the temperature range from 2 to 300 K. As shown in Fig. 3a, the χ increases with decreasing temperature and a rapid upturn is seen at ∼16 K, indicating the onset of a ferromagnetic (FM) phenomenon. A clear Curie–Weiss behaviour is displayed at the range of 100–300 K, and the calculated Curie constant (C) and Weiss temperature (θ) are 8.6(8) emu mol⁻¹ K⁻¹ and −180.3(4) K, respectively. The negative θ value testifies a dominant AFM interaction among the neighboring Mn²⁺ ions (intrachain interaction). Moreover, the experimental effective magnetic moment (5.8(9) μ_B for per Mn²⁺) is very consistent with the theoretical value (5.916 μ_B) according to the equation of μ_eff² = gS(S + 1). The χT as a function of temperature is shown in Fig. 3b, the value gradually decreases from about 5.38 emu mol⁻¹ K⁻¹ at 300 K to about 1.23 emu mol⁻¹ K⁻¹ at 25 K, and then rapidly increases and reaches the maximum value of 24.15 emu mol⁻¹ K⁻¹ at 16 K, confirming the presence of a ferromagnetic (FM) component, which may lead to the FIM behaviour or spin-canting ferromagnetism in this compound. To further confirm the magnetic behavior of 1, the FC (field-cooling) and ZFC (zero-field-cooling) magnetization data were recorded (Fig. 3c). The disagreement below 16 K suggests the onset of the magnetic phase transition. Furthermore, the strong field-dependent magnetic behaviour can be clearly seen in the FC curves (Fig. S5, ESI†), which testifies to the presence of spontaneous magnetization induced by FIM or spin canting.

Fig. 3 Magnetic property measurements of 1: (a) the curves of χ–T (black) and χ⁻¹–T (red) at 0.1 T; (b) the χT–T curve; (c) magnetic susceptibilities measured with FC and ZFC regimes; (d) the curve of M–H at 2 K.

The isothermal magnetization was recorded at 2 K (Fig. 3d). The magnetization increases linearly below 1.5 T, and then the magnetization increases slowly and exhibits a plateau above 3 T. The saturation value of 0.49 μ_B at 8 T, which is far below the saturation value of 5 μ_B for a high spin Mn²⁺ ion was obtained. This phenomenon indicates the presence of FIM rather than the spin-canting in 1, as the spin-canting usually results in a linear increase of magnetization with the increase of the magnetic field and the unsaturation magnetization behaviour at higher field. However, the field-induced magnetic phase transition from the FIM state to FM state, as seen in CoV₂O₆^21,22 and Co₃(BPO₄)₂(PO₄)(OH)₃,²³ was not observed under our experimental conditions, and a higher field (>8 T) should be required to observe such a magnetic phase transition.

Structurally, two sub-lattice sites are observed in 1, i.e., octahedral site [Mn1S₆] and tetrahedral site [Mn2S₃Cl]. On the other hand, the crystal field has significant influence on the magnetization – that is, Mn1 and Mn2 atoms may contain un-equivalent magnetization. And thus, the magnetization of Mn1 and Mn2 will not set off against each other when the magnetic interaction between Mn1 and Mn2 is AF type, and as a result the FIM behaviour will occur. In other words, the different magnetic sub-lattice sites with different coordination geometries ([MnS₆] octahedron and hetero-ligand [MnS₃Cl] tetrahedron) should be responsible for the FIM behaviour.

In summary, a novel Mn-based chalcohalide with a distinctive structure type, Cs₂[Mn₂Ga₃S₇Cl], is discovered and the single crystal structure and magnetic properties are investigated for the first time. The featured 2D [Mn₂Ga₃S₇Cl]²⁻ anionic layers in this structure are constructed from the [Ga₃S₉]⁹⁻ ribbons embedded with infinite chains of [Mn₂S₈Cl]¹³⁻. Specially, the Mn atoms in this compound adopt two types of chemical environments: [MnS₆] octahedra and hetero-ligand [MnS₃Cl] tetrahedra. This compound exhibits FIM behaviour that is correlated to the two magnetic sub-lattice sites with different coordination geometries. Such unique bonding characteristics and fascinating magnetic properties of Cs₂[Mn₂Ga₃S₇Cl] may shed useful light on further exploration. In addition, this work will facilitate the design or prediction of 2D functionalized chalcohalides.

This research was supported by the National Natural Science Foundation of China (21771179, 21571020 and 21301175), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB20010200), the support from “Chunmiao Projects” of Haixi Institute of Chinese Academy of Sciences, and the One Thousand Young Talents Program under the Recruitment Program of Global Youth Experts. We thank Prof. Zhang-Zhen He at FJIRSM for helpful discussions.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, together with additional figures and tables. CCDC 1848728. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc08380k

‡ Crystal data for Cs₂[Mn₂Ga₃S₇Cl] (1) at 293(2) K: orthorhombic; Pnma; Z = 4; a = 6.2467(6) Å, b = 7.3879(8) Å, c = 33.600(3) Å, V = 1550.6(3) ų. Collection and refinement data: d_calc = 3.62 g cm⁻³; μ = 12.43 mm⁻¹; 13 245 total reflections; 2117 unique reflections [F_o² > 2σ(F_o²)]; GOF = 1.092; R₁ = 3.81% and wR₂ = 9.44% for I > 2σ(I). CCDC: 1848728.†

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