Active Lone Pair Research Articles

Sn2P2S6: A lead-free reversible thermochromic ferroelectric with high near-infrared reflectance

Multifunctional thermochromic materials integrated optical, ferroelectric, and/or magnetic properties have been long envisioned and played an invaluable role in signal processing and non-contact optical information storage in smart windows. However, poor materials stability, together with high discoloration temperature, absence of ferroelectric, or magnetic behaviors, impedes most thermochromic materials for practical applications. Here, the thermochromic behavior of all-inorganic lead-free ferroelectric Sn2P2S6 (SPS) is extensively studied, which exhibits a reversible color change between light orange and red around the ferroelectric-paraelectric phase transition temperature (∼ 67 °C). In situ powder X-ray diffraction (PXRD) and Raman spectroscopy indicate the change of P-S and Sn-S bond distance, and thus result in the modification of (P2S6) octahedral geometry and the coordination environment of Sn2+ ions along with the evolution of thermochromism (TCM). Additionally, the first-principles calculations reveal that, the Sn 5s2 stereochemically active lone pair electrons evoke negative lattice expansion comparable to that of the known ZrW2O8 at warming and is responsible for the reversible TCM of SPS. SPS also possesses high near-infrared reflectance (R > 80 %, 1100-2500 nm). The integration of ferroelectricity, high near-infrared reflectance, and TCM below 100 °C renders potential applications of the titled compound in multifunctional optical smart windows.

Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites.

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns2 lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA)2(A)Sn2I7, where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA)2(A)Ge2I7, where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation's lone-pair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier-phonon interactions and nonlinear optical properties. We find that the strength of carrier-phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA)2(A)Ge2I7 exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V-1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA)2(A)Sn2I7 to be ∼10 cm MW-1, which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices.

Synthesis of a Hexachloro Sulfate(IV) Dianion Enabled by Polychloride Chemistry.

The preparation and structural characterization of [NEt3 Me]2 [SCl6 ] is described, which is the first example of a [SCl6 ]2- dianion and of a halosulfate anion of the type [Sx Xy ]z- in general. This dianion belongs to the group of 14-valence electron AB6 E systems and forms an octahedral structure in the solid-state. Interestingly, co-crystallization with CH2 Cl2 affords [NEt3 Me]2 [SCl6 ]⋅4 CH2 Cl2 containing [SCl6 ]2- dianions with C4v symmetry. As suggested by quantum-chemical calculations, the distortion of the structure is not caused by a stereochemically active lone pair but by enhanced hydrogen bonding interactions with CH2 Cl2 . At elevated temperatures, [NEt3 Me]2 [SCl6 ] decomposes to various sulfur chlorine compounds as shown by Raman spectroscopy. Cooling back to room temperature results in the selective formation of [NEt3 Me]2 [SCl6 ] which is comparable to the well-studied SCl4 .

Solution-Grown Ternary Semiconductors: Nanostructuring and Stereoelectronic Lone Pair Distortions in I–V–VI2 Materials

Alkali pnictogen dichalcogenides─I–V–VI2 or APnCh2─have been identified as promising semiconducting materials for energy conversion devices. However, the controlled nanoscale synthesis and our understanding of the effects of cation ordering and stereochemically active lone pairs on the structures of these ternary compounds remain underdeveloped. Here, we use solution-phase chemistry to synthesize a family of APnCh2 materials, including LiSbSe2, NaSbS2, NaSbSe2, NaBiS2, and NaBiSe2. Our approach utilizes alkali metal hydrides (AH) or carboxylates, A(O2CR), PnPh3, and elemental chalcogens as synthetic precursors and oleylamine or 1-octadecene as solvents. Synthetic manipulation via fine-tuning of reaction temperature enables control over the degree of ordering caused by the Sb 5s2 lone pair-induced distortions in NaSbS2. Pair distribution function analysis demonstrates that the structure of the Sb-containing phases deviates much more from a disordered rock salt structure than that of the Bi-containing phases. This local distortion, induced by the Sb lone pair, leads to a previously unreported noncentrosymmetric NaSbS2 crystal structure, which is additionally supported by second-harmonic generation measurements. Infrared and multinuclear solid-state NMR spectroscopies show that oleylamine or chelating carboxylates and, in some cases, unreacted precursors (LiH and PnPh3) remain bound to the nanocrystalline surfaces. A deeper understanding of the local atomic environment, long-range ordering, surface chemistry, and optoelectronic properties of these materials may speed up their fundamental study and application.

Sharp Enhancement of Birefringence in Antimony Oxalates Achieved by the Cation-Anion Synergetic Interaction Strategy.

Birefringent materials with large birefringence play an important role in in laser science and technology owing to their ability to modulate polarized light. However, the lack of systematic and effective synthesis strategies severely hinders the development of novel superior birefringent materials. Herein, the cation-anion synergetic interaction strategy was proposed to successfully synthesize two excellent UV birefringent materials, RbSb(C2O4)F2·H2O and [C(NH2)3]Sb(C2O4)F2·H2O. Both compounds feature unprecedented [Sb(C2O4)F2]∞- anionic chains composed of planar π-conjugated [C2O4]2- units and a distorted SbO4F2 complex with stereochemically active lone pairs, which induce a large optical anisotropy. Remarkably, further enhancement of birefringence in [C(NH2)3]Sb(C2O4)F2·H2O was achieved via cation-anion synergetic interactions between the [C(NH2)3]+ cationic groups and [Sb(C2O4)F2]∞- anionic chains. It exhibited a giant birefringence of 0.323@546 nm, twice larger than that of its analogue RbSb(C2O4)F2·H2O (0.162@546 nm). A detailed structural analysis and theoretical calculations revealed that the cation-anion synergetic interaction strategy is an effective strategy for the efficient exploration of superior birefringent materials, which will guide the further exploration of new structure-driven functional materials.

Understanding Electron-Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs.

The electron-phonon (e-ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e-ph interaction in LHPs. Herein, we advance a fundamental understanding of the e-ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure-property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e-ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e-ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

Regulating the Singlet and Triplet Emission of Sb3+ Ions to Achieve Single-Component White-Light Emitter with Record High Color-Rendering Index and Stability.

The rapid development of solid-state lighting technology has attracted much attention for searching efficient and stable luminescent materials, especially the single-component white-light emitter. Here, we adopt a facile ion-doping technology to synthesize vacancy-ordered double perovskite Cs2ZrCl6:Sb. The introduction of Sb3+ ions with a 5s2 active lone pair into Cs2ZrCl6 host stimulates the singlet (blue) and triplet (orange) states emission of Sb3+ ions, and their relative emission intensity can be tuned through the energy transfer from singlet to triplet states. Benefiting from the dual-band emission as a pair of perfect complementary colors, the optimum Cs2ZrCl6:1.5%Sb exhibits a high-quality white emission with a color-rendering index of 96. By employing Cs2ZrCl6:1.5%Sb as the down-conversion phosphor, stable single-component white light-emitting diodes with a record half-lifetime of 2003 h were further fabricated. This study puts forward an effective ion-doping strategy to design single-component white-light emitter, making practical applications of them in lighting technologies a real possibility.

Zero-dimensional antimony(III) halides templated by ruthenium complexes: photoluminescence, thermochromism and photo/electrical performances

Zero dimensional antimony(III) halides have captured more and more attention as broadband emitters. Herein, Using photochemically active ruthenium complexes as templates, two new antimony(III) halide based hybrids have been prepared and structurally determined, i.e., [Ru(bipy)3](Sb2Cl6O) (1) and [Ru(phen)3](SbI5) (2). Both the (Sb2Cl6O)2- and (SbI5)2-are rare examples of antimony halides, in which Sb centers are in square pyramidal environments with stereochemically active lone pairs. The (Sb2Cl6O)2- ion bearing secondary Sb···Cl interactions are further stabilized by C-H···Cl hydrogen bonds to give a quasi-3D network. The (SbI5)2- anions are anchored in the 1D[Ru(phen)3]n 2n+ chains based on strong π···π stacking interactions among phenanthroline ligands, which are the reason for the formation of its square pyramid geometry.1 exhibits bright red luminescence but 2produces weak blue emission. This trend might be assigned to the larger conjugated system of phenanthroline in 2, which is dominant by ruling out the contribution of antimony(III) halide. Besides, reversible thermochromism can be found in 1. Furthermore, good electrical chemical and photocurrent performance can be detected. The photoluminescence modulation by controlling the conjugated degree of organic ligands is an efficient strategy for the precise preparation of new luminescent materials.

First‐Principles Investigation About Different Sequence of Stereochemical Activity and Birefringence in Antimony Halides

Birefringent compounds play an indispensable role in modern optoelectronics and information communication. In this article, the electronic structures and birefringence of binary and ternary antimony halides are investigated using the first‐principles method. The results show that the stereochemical activity of antimony cations in these compounds gradually decreases from Cl to I, and the birefringence of these compounds gradually increases from Cl to I. The degree of stereochemical activity of lone pairs is determined by the energy difference between the s‐state of the cation and the p‐state of the halogen, implying the revised model about stereochemical activity of lone‐pairs in metal oxides is also appropriate for metal halides. The real‐space atomic cutting and Born effective charges show that the antimony cations and halogen closer to Fermi level give main contribution to birefringence. And the occupied p‐states of antimony and halogen atoms play an important role in determining the birefringence.

Tailoring Photophysical Dynamics in a Hybrid Gallium-Bismuth Heterometallic Halide by Transferring from an Indirect to a Direct Band Structure.

Low-dimensional lead-free metal halides have emerged as novel luminous materials for solid-state lighting, remote thermal imaging, X-ray scintillation, and anticounterfeiting labeling applications. However, the influence of band structure on the intriguing optical property has rarely been explored, especially for low-dimensional hybrid heterometallic halides. In this study, we have developed a lead-free zero-dimensional gallium-bismuth hybrid heterometallic halide, A8(GaCl4)4(BiCl6)4 (A = C8H22N2), that is photoluminescence (PL)-inert because of its indirect-band-gap character. Upon rational composition engineering, parity-forbidden transitions associated with the indirect band gap have been broken by replacing partial Ga3+ with Sb3+, which contains an active outer-shell 5s2 lone pair, resulting in a transition from an indirect to a direct band gap. As a result, broadband yellow PL centered at 580 nm with a large Stokes shift over 200 nm is recorded. Such an emission is attributed to the radiative recombination of an allowed direct transition from triplet 3P1 states of Sb3+ based on experimental characterizations and theoretical calculations. This study provides not only important insights into the effect of the band structure on the photophysical properties but a guidance for the design of new hybrid heterometallic halides for optoelectronic applications.

Lead(II) complexes with Kemp’s tricarboxylate: Can lone pair activity be discerned?

Kemp’s triacid (H3kta; cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid) has been used to synthesize three lead(II) complexes under solvo-hydrothermal conditions. [Pb3(kta)Cl3] (1) and [Pb3(H3kta)(kta)2] (2) are both diperiodic coordination polymers displaying hydrophobic surfaces with protruding methyl groups, complex 2 being isomorphous with an SrII complex previously reported. In contrast, [Pb(Hkta)(phen)] (3), where phen is 1,10-phenanthroline, is a monoperiodic, helical coordination polymer. In all cases, PbII cations are in seven- or eight-coordinate environments of predominantly hemidirected nature. The possible effects of the PbII valence shell lone pair are discussed in terms of coordination geometry and Pb⋅⋅⋅H weak interactions as revealed on Hirshfeld surfaces. Only in complex 1 is a short Pb⋅⋅⋅H contact at 2.84 Å possibly indicative of appreciable basicity of the metal ion. All three complexes show similar weak luminescence, apparently independent of the nature of the ligands.

Effects of Bi doping on structural and magnetic properties of cobalt ferrite perovskite oxide LaCo0.5Fe0.5O3

Literature reports on magnetic transition of LaFe0.5Co0.5O3 are highly ambiguous. The present study is an endeavour to address that particular issue of this ferrite perovskite. Here we report the low temperature sol-gel synthesis of pure and Bi-doped La1-xBixFe0.5Co0.5O3 (x = 0, 0.1 & 0.2) perovskites. All the samples were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray (EDX) analysis and magnetization studies. LaFe0.5Co0.5O3 crystallizes in a single phase rhombohedral (R-3c) structure. Substitution of Bi at the A-site results in structural transition. La0.9Bi0.1Fe0.5Co0.5O3 exhibits mixed rhombohedra (R-3c) and orthorhombic (Pbnm) phases, whereas La0.8Bi0.2Fe0.5Co0.5O3 is single phase orthorhombic (Pbnm). Favourable grain growth and good crystallinity observed in Bi doped samples are possibly due to the low melting temperature of Bi containing La1-xBixFe0.5Co0.5O3. EDX analysis confirms the chemical homogeneity of the samples. XPS studies and bond valence sum calculations confirm the nominal cationic oxidation states of the ions. Magnetization measurements reveal the weak ferromagnetic nature of La1-xBixFe0.5Co0.5O3 (x = 0, 0.1 & 0.2) which is associated to the canted antiferromagnetic structure. The magnetically active subsystem of Fe3+ ions are responsible for weak ferromagnetism which is explained by the antisymmetric Dzyaloshinskii–Moriya interaction, whereas the Co3+ ions remain predominantly in the low spin state. The increase in magnetic transition and the lower magnetic moment with an increase in Bi content can be related to the weakening of Dzyaloshinskii–Moriya interaction due to structural modification and local lattice distortion associated to the stereochemically active 6s2 lone pair electrons of the Bi3+ ions. Our results confirm the high structure sensitivity of magnetic properties of cobalt ferrite perovskite.

The Role of the Bi3+ Lone Pair Effect in Bi(H3O)(SO4)2, Bi(HSO4)3, and Bi2(SO4)3.

Three new members in the Bi2O3-SO3-H2O system are identified by single crystal X-ray diffraction and Rietveld refinement after a fundamental examination of this phase space. Bi(H3O)(SO4)2 crystallizes in space group P21/c (no. 14, a = 1203.5(4), b = 682.9(2), c = 821.2(2) pm, β = 102.99(1)°, 861 independent reflections, 88 refined parameters, wR2 = 0.14) homeotypic with Nd(H3O)(SO4)2 featuring edge-sharing BiO9 polyhedra. Bi(HSO4)3 crystallizes in a new structure type in space group P1 (no. 2, a = 492.04(7), b = 910.8(1), c = 1040.8(2) pm, α = 85.443(5)°, β = 86.897(5)°, γ = 74.542(4)°, 3227 independent reflections, 154 refined parameters, wR2 = 0.05) comprising dimers of edge-sharing BiO8 polyhedra. For Bi2(SO4)3, a new modification crystallizing in space group P21/n (no. 14, a = 1308.03(7), b = 473.25(3), c = 1452.61(8) pm, β = 100.886(2)°, 3189 independent reflections, 155 refined parameters, wR2 = 0.03) isotypic to Sb2(SO4)3 with noncondensed BiO7 polyhedra is presented. The role of the Bi3+ lone pair effect as elucidated by density functional theory (DFT) calculations is discussed for all three compounds with respect to their structural and optical properties. Additionally, the Bi3+ lone pair activity is compared to the recently reported borosulfates Bi(H3O)[B(SO4)2]4 and Bi2[B2(SO4)6]. Geometrical calculations based on structural data are correlated with electron localization function (ELF) calculations to establish the origin of the direction and strength of the lone pair stereoactivity of Bi3+ in oxidic compounds. Finally, the thermal properties of the three compounds are reported.

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids.

Understanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat ) in crystalline materials. Herein, we demonstrate local cationic off-centring within the global rock-salt structure of AgSbSe2 by using synchrotron X-ray pair distribution function analysis and unravel the origin of its ultralow κlat ≈0.4 W mK-1 at 300 K. The cations are locally off-centered along the crystallographic direction by about ≈0.2 Å, which averages out as the rock-salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off-centering distortions within an anharmonic double-well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next-generation crystalline materials with tailored thermal properties.

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids

AbstractUnderstanding the correlations of both the local and global structures with lattice dynamics is critical for achieving low lattice thermal conductivity (κlat) in crystalline materials. Herein, we demonstrate local cationic off‐centring within the global rock‐salt structure of AgSbSe2 by using synchrotron X‐ray pair distribution function analysis and unravel the origin of its ultralow κlat≈0.4 W mK−1 at 300 K. The cations are locally off‐centered along the crystallographic direction by about ≈0.2 Å, which averages out as the rock‐salt structure on the global scale. Phonon dispersion obtained by density functional theory (DFT) shows weak instabilities that cause local off‐centering distortions within an anharmonic double‐well potential. The local structural distortion arises from the stereochemically active 5s2 lone pairs of Sb. Our findings open an avenue for understanding how the local structure influences the phonon transport and facilitates the design of next‐generation crystalline materials with tailored thermal properties.

The controlled in-situ growth of silver-halloysite nanostructure via interaction bonds to reinforce a novel polybenzoxazine composite resin and improve its antifouling and anticorrosion properties

Halloysite nanotube (H) is a renewable aluminosilicate material that can act as a nano-container to carry metal nanoparticles. In the present work, the external sheet of the halloysite nanotube was grafted with 3-aminopropyltriethoxysilane (APTES) and 2-furoyl chloride, which effectively enabled the interaction between Ag+ ions and the nitrogen/oxygen atoms of the grafted chemical units via their active lone pair electrons and empty orbits. Ag nanoparticles were precisely controlled in-situ to grow on the H external sheet and inner lumen. The strong Ag + -N/O interaction allowed the Ag+ cations dissolved from the Ag nanoparticles to be partially stored on the nanotubes for slow release. The interaction was experimentally verified by antibacterial measurements and computationally illustrated by density functional theory (DFT). The Ag+-functionalized halloysite nanotubes were in-situ introduced into a novel coating and bonded in the composite polybenzoxazine framework. Such a high density structure coating exhibited outstanding antifouling performance, such as bacterial killing and anti-attachment. It was also extraordinarily corrosion-resistant in immersion and salt-spray environment for 126 days, much better than most of the coating systems reported so far. The study could extend practical applications of the eco-friendly and multifunctional composite coating in the industry.

Hierarchical Modulation of Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates MII B4 O6 F2 (MII =Be, Mg, Pb, Zn, Cd).

The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d10 electron configuration, and 6s2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB4 O6 F2 (AE=Ca, Sr) is adopted for UV/deep-UV birefringent structures, namely, MII B4 O6 F2 (MII =Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6 % with the cation substitution from Ca, Sr to Be, Mg (route-I), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB4 O6 F2 structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm, and the PbB4 O6 F2 structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.

Lead Tellurite Crystals BaPbTe2O6 and PbVTeO5F with Large Nonlinear-/Linear-Optical Responses due to Active Lone Pairs and Distorted Octahedra.

The exploration of nonlinear-/linear-optical crystal materials with high performance is an extremely difficult research project. Herein, the two new lead tellurite crystals BaPbTe2O6 and PbVTeO5F were successfully obtained through a facile hydrothermal synthesis strategy. BaPbTe2O6 lies in the noncentrosymmetric (NCS) and chiral orthorhombic space group P212121, featuring a unique ∞1[PbTe2O6] chain consisting of the PbO4 and TeO3 building units, while PbVTeO5F belonging to the centrosymmetric (CS) orthorhombic space group Pbca manifests a 2D layer made up of ∞1[PbO4F2] chains and novel [V2Te2O10F2] clusters. Further, a systematic analysis of lead tellurites finds that the coordination geometries of the Pb atom exert a considerable influence on the connection modes of Pb-O and Te-O building units. BaPbTe2O6 shows a great second-harmonic-generation (SHG) effect of ∼5× the benchmark KH2PO4 (KDP) and a large optical birefringence of 0.086 at 590 ± 3 nm. PbVTeO5F demonstrates a remarkably larger birefringence of 0.142 at 590 ± 3 nm, benefiting from the introduction of the VO5F octahedral unit. Theoretical studies reveal that the large SHG and birefringence in BaPbTe2O6 can be attributed to TeO3 and PbO4 polyhedra with active lone pairs, while the remarkably enlarged birefringence in PbVTeO5F is attributable to the highly distorted octahedral VO5F. The functional orientations of active building units may offer a practical insight into the design of the desired optical functional materials.

Lone pair driven anisotropy in antimony chalcogenide semiconductors.

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) have emerged as promising earth-abundant alternatives among thin-film photovoltaic compounds. A distinguishing feature of these materials is their anisotropic crystal structures, which are composed of quasi-one-dimensional (1D) [Sb4X6]n ribbons. The interaction between ribbons has been reported to be van der Waals (vdW) in nature and Sb2X3 are thus commonly classified in the literature as 1D semiconductors. However, based on first-principles calculations, here we show that inter-ribbon interactions are present in Sb2X3 beyond the vdW regime. The origin of the anisotropic structures is related to the stereochemical activity of the Sb 5s lone pair according to electronic structure analysis. The impacts of structural anisotropy on the electronic, dielectric and optical properties relevant to solar cells are further examined, including the presence of higher dimensional Fermi surfaces for charge carrier transport. Our study provides guidelines for optimising the performance of Sb2X3-based photovoltaics via device structuring based on the underlying crystal anisotropy.

LiSrSbS3: parallel configurations of lone pair electrons inducing a large birefringence.

Enhancement of birefringence is significant since the birefringent materials can create and control polarized light and be used extensively in various advanced optical systems. By optimizing the arrangement of [SbS3] units with stereo-chemical active lone pair electrons, a new quaternary thioantimonate LiSrSbS3 with a large birefringence has been successfully synthesized by a high temperature solid-state reaction method. LiSrSbS3 crystallizes in the monoclinic space group of P21/c. In the structure, the isolated infinite [LiS4] chains and zigzag [SrS6] chains are alternately connected with each other to compose a three-dimensional (3D) framework, and the isolated pyramid [SbS3] units are located between them. To analyze the source of large birefringence, the electronic structure and optical properties of LiSrSbS3 were further investigated by the first-principles method, and the results show that the optimized arrangement [SbS3] trigonal pyramid induces a large birefringence.