Structure, Synthesis and Properties of Antimony Oxychlorides: A Brief Review

Abstract

Antimony oxychloride compounds represent a distinct class of inorganic materials that combine structural features characteristic of both oxides and halides. Their compositional flexibility and anisotropic properties make them promising candidates for use in photocatalytic systems, optoelectronic devices, flame-retardant coatings, and energy storage technologies. This review provides an overview of the structural characteristics and synthesis strategies associated with key members of the Sb_xO_yCl_z family, including SbOCl, Sb₄O₅Cl₂, and Sb₈O₁₁Cl₂. Emphasis is placed on how synthesis parameters—such as temperature, pH, and precursor composition—govern phase formation, morphology, and resulting properties. Recent advances in composite engineering, controlled doping, and surface modification are discussed as effective routes to overcome limitations such as low conductivity and chemical instability. The broader significance of antimony as a strategic element is also addressed in the context of global resource availability and its role in sustainable technologies. Overall, these materials provide a versatile platform for the design of multifunctional systems tailored to meet future demands in materials science and applied engineering.

1. Introduction

In recent decades, nanomaterials have attracted considerable attention due to the pronounced dependence of their functional properties on morphology, particle size, shape, and microstructural features. Among these, antimony-based compounds—particularly oxychlorides—are of special interest, as they occupy a transitional position between oxides and halides [1,2,3]. These materials exhibit a unique set of physicochemical characteristics [4], such as high photocatalytic activity [5], excellent flame retardancy [6], and notable performance in energy storage systems and gas sensors [7] (Figure 1).

Despite its simple chemical formula, antimony oxychloride compound (Sb_xO_yCl_z) demonstrates complex chemical behavior. It can function as an intermediate in hydrolysis reactions, participate in thermal decomposition processes, and display intriguing crystallographic features [8]. Nevertheless, this compound remains relatively underexplored in the scientific literature, especially when compared to more extensively studied antimony oxides such as Sb₂O₃ and Sb₂O₅.

Antimony oxychlorides, including SbOCl, Sb₄O₅Cl₂, Sb₄O₂Cl₈, Sb₃O₄Cl, and Sb₈O₁₁Cl₂, are characterized by polymeric chain-like architectures and layered morphologies that facilitate directional charge transport and enable fine-tuning of electronic structures [9]. The incorporation of halogen atoms into the crystal lattice induces local structural distortions, enhancing anisotropic polarizability and contributing to their distinctive optical behavior [10]. With a balanced combination of band gap width and birefringence, these compounds are particularly promising for applications in infrared optoelectronics [11], photocatalysis [12], and ionic conductivity [13].

Despite their potential, studies focused on the synthesis of nanostructured antimony oxychlorides remain limited. This underscores the need for developing simple, cost-effective, and scalable synthetic approaches, as well as conducting systematic investigations into their structural and functional properties. Moreover, the ability of these compounds to form hybrid structures with organic ligands and dopants opens new avenues for the design of multifunctional materials tailored for next-generation photocatalytic systems and optoelectronic devices.

This review consolidates current insights into the crystallographic characteristics, synthetic approaches, and functional potential of antimony oxychlorides. Based on an analysis of published studies, these compounds are highlighted as versatile platforms for the development of next-generation multifunctional materials.

2. Antimony: Elemental Properties, Historical Use, and Strategic Significance

Antimony (Sb) is a group V element of the periodic table, classified as a metalloid [14]. Its physicochemical characteristics lie at the intersection of metallic and non-metallic behavior [15], resulting in distinctive reactivity across various chemical environments. With an atomic number of 51 and an electron configuration of [Kr]4d¹⁰5s²5p³ [16], antimony possesses five valence electrons capable of forming a wide range of compounds. Chemically, it exhibits similarities to arsenic and bismuth, forming oxides, halides, hydrides, hydroxides, and more complex structures that undergo further transformations [17].

In its elemental form, antimony is a brittle, silver-white crystalline solid with a metallic luster, low thermal conductivity, high corrosion resistance, and a density of 6.697 g/cm³ [18].

Table 1. Basic physical and chemical properties of antimony.

Property	Significance	Ref
Density	6.697 g/cm3 (at 20 °C)	[18]
Melting point	630.6 °C (903.8 K)
Specific heat capacity	≈ 0.207 J/(g·K)
Gas thermal conductivity	≈ 18 W/(m·K)
Standard enthalpy of formation	0 kJ/mol (elemental Sb, standard state)
Melting heat	19.79 kJ/mol

summarizes its fundamental physical and chemical properties. Industrially, antimony is available in the form of ingots, granules, powders, shots, and single crystals. One of its most critical applications lies in enhancing the mechanical strength of lead, making it indispensable in the manufacture of lead acid batteries [19].

In nature, antimony occurs predominantly as sulfide minerals, most notably stibnite (Sb₂S₃), and less commonly as oxides. It is considered a chalcophile element, typically associated with sulfur, lead, copper, and silver, and is found in over one hundred identified mineral species [20]. Its average concentration in the Earth’s crust is estimated at 0.2–0.5 ppm [21]. The use of natural antimony sulfide dates to antiquity, where it served medicinal and cosmetic purposes.

The historical use of antimony dates to around 3100 BCE, when ancient Egyptians employed its sulfide form, stibnite, for cosmetic purposes—most notably as a key ingredient in black eye paint known as kohl. This early application reflects a practical understanding of antimony-bearing minerals long before their formal chemical characterization.

Metals and metalloids form a critical foundation of the modern economy [22] and play a pivotal role in the global transition toward low-carbon and resource-efficient systems [23,24]. Their widespread application spans renewable energy technologies, energy storage systems, electric vehicles, and digital infrastructure. Moreover, these elements are essential for advancing the United Nations Sustainable Development Goals (SDGs) [25,26], particularly in areas related to clean energy, sustainable industry, and climate resilience.

According to data published by the United States Geological Survey (USGS) [27,28], global antimony reserves in 2024 were estimated at approximately 2 million metric tons. With an annual production volume of around 100,000 metric tons, these resources are projected to sustain current extraction levels for roughly two decades. The largest confirmed reserves are in China (Xikuangshan), accounting for 30% of the global total, followed by Russia (Sarylakh and Sentachan) (16%), Bolivia (Kharma) (14%), and Kyrgyzstan (12%), underscoring the strategic importance of these countries in the international antimony supply chain.

For several producing nations, including Iran and Kazakhstan, official reserve data remain unavailable, complicating comprehensive assessments of global resource potential. Figure 2 illustrates the distribution of antimony reserves by country, while

Table 2. Antimony production volumes and estimated reserves for 2023 and 2024 [28].

Country	Mine Production		Reserves
	2023	2024
China	62,300	60,000	670,000
Russia	13,000	13,000	350,000
Tajikistan	17,000	17,000	50,000
Australia	1860	2000	140,000
United States	-	-	860,000
Canada	-	-	78,000
Bolivia	3700	3700	310,000
Turkey	1600	1600	99,000
Vietnam	300	300	54,000
Mexico	800	800	18,000
	106,000	100,000	>2,000,000

presents production volumes and estimated reserves for 2023 and 2024.

The table includes extraction data (in metric tons) for key producing countries, along with the mineral resource estimates available within their territories. All figures are based on reports from national geological agencies and international analytical sources, including USGS (2025) [28].

3. Structural Foundations of Antimony Oxychlorides

During the medieval period of alchemical practice, antimony began to acquire notable significance in early medicinal formulations. Among the compounds actively employed was antimony oxychloride, recognized for its pronounced emetic properties. In alchemical texts, this compound was referred to as the “Mercury of Life” and “Pulvis Algaroth,” names that symbolized its perceived potency and transformative capabilities [29]. These initial applications of antimony-based substances laid a conceptual and empirical foundation for subsequent investigations into their chemical behavior, pharmacological activity, and materials science potential [30].

The emergence of the Sb_xO_yCl_z compound family marked a pivotal development in the study of antimony-based oxychlorides and oxyhalides characterized by the simultaneous presence of oxygen and chloride ligands coordinated to antimony centers [31]. This class of materials encompasses a series of non-stoichiometric compounds, including Sb₄O₅Cl₂, Sb₈O₁₁Cl₂, Sb₄O₂Cl₈, Sb₃O₄Cl, Sb₂OCl₄, SbOCl, and SbOCl₃, as illustrated in Figure 3.

Two stable antimony chlorides are known in nature: antimony trichloride (SbCl₃) and antimony pentachloride (SbCl₅), corresponding to oxidation states +3 and +5, respectively [32,33]. Among them, SbCl₃ is the most encountered compound. It is typically synthesized via the reaction shown below.

$$2\mathbf{S}\mathbf{b}+{5\mathbf{C}\mathbf{l}}_2\to {2\mathbf{S}\mathbf{b}\mathbf{C}\mathbf{l}}_5$$

$$2\mathbf{S}\mathbf{b}+\mathbf{c}\mathbf{o}\mathbf{n}\mathbf{c}.6\mathbf{H}\mathbf{C}\mathbf{l}\to {2\mathbf{S}\mathbf{b}\mathbf{C}\mathbf{l}}_3+{3\mathbf{H}}_2$$

SbCl₃ is a white or colorless crystalline substance with high hygroscopicity and readily undergoes hydrolysis upon contact with water. Its molecular structure adopts a trigonal pyramidal geometry, in which the antimony atom is bonded to three chlorine atoms and retains a lone electron pair [34]. This compound serves as a key precursor for the synthesis of antimony oxychloride, as controlled hydrolysis of SbCl₃ leads directly to the formation of this oxyhalide [35]. Additionally, SbCl₃ is used as a source of antimony in organic synthesis, participating in ligand exchange and complex reactions.

Antimony pentachloride (SbCl₅), in contrast, is a colorless to pale yellow liquid with high reactivity. Its molecular structure is trigonal bipyramidal, corresponding to a coordination number of five. SbCl₅ reacts vigorously with water, forming SbOCl₃ and hydrochloric acid at low temperatures, and under more extensive hydrolysis conditions, it yields antimony pentoxide (Sb₂O₅) [36]. These properties make SbCl₅ a valuable compound for studying oxidation and hydrolysis mechanisms, as well as for synthesizing complex species such as hexachloroantimonates [37].

Both chlorides exhibit a tendency to form coordination compounds in nonpolar solvents such as carbon tetrachloride (CCl₄), where they can interact with alkali metal chlorides to produce salts of the type Na[SbCl₆] [38].

Antimony oxychloride, or antimonyl(III) oxychloride, is one of the classical antimony oxohalides and holds a distinctive position among inorganic compounds due to its historical significance and practical relevance. SbOCl crystallizes in the monoclinic system, space group P2₁/c, with unit cell parameters refined through modern structural studies: a = 7.908(9) Å, b = 10.732(14) Å, c = 9.527(10) Å, β = 103.65°, and Z = 12 formula units per unit cell [39].

In the crystal structure, the coordination environment of the antimony ion (Sb³⁺) adopts a highly distorted trigonal bipyramidal or pyramidal geometry, where the central antimony atom is surrounded by both oxygen and chlorine atoms. The Sb-O bond lengths range from approximately 1.947 to 2.265 Å, while Sb-Cl distances fall between 2.368 and 2.377 Å [40]. The bond angles, such as O-Sb-O and O-Sb-Cl, vary widely from 69° to 140°, reflecting significant angular distortion within the coordination sphere [41].

Unlike other antimony oxychlorides such as Sb₄O₅Cl₂ or Sb₃O₄Cl, the structure of SbOCl is non-layered. In this compound, chlorine atoms form covalent bonds directly with antimony rather than occupying interlayer positions in an ionic fashion. The crystal lattice features discrete pyramidal [SbO₂Cl] units, where chlorine functions as a true ligand rather than merely serving as a charge-balancing species. This structural motif highlights the covalent nature of the Sb-Cl interaction and distinguishes Sb_xO_yX_z from more ionically organized oxyhalide phases.

The crystal chemistry of antimony oxyhalides exhibits a wide range of structural diversity, reflecting variations in oxidation states, coordination environments, and bonding motifs among oxygen, halogen, and antimony atoms. One of the most thoroughly studied representatives of this class is antimony oxychloride Sb₄O₅Cl₂, whose crystal structure was first described by M. Edstrand [42]. This compound crystallizes in the monoclinic system (space group P2₁/c) with unit cell parameters: a = 6.229 Å, b = 5.107 Å, c = 13.5 Å, and β = 97.27°. The structure features antimony atoms coordinated by both oxygen and chlorine ligands, forming distorted pyramidal units.

In comparison, the structure of Sb₃O₄Cl, reported by Schwarz [43], adopts a layered architecture in which chloride ions are positioned between oxide layers, acting primarily as ionic species. The coordination environment around antimony is less symmetric, and the Sb-Cl interactions are weaker and more ionic in nature than those observed in Sb₄O₅Cl₂.

Another notable phase is Sb₈O₁₁Cl₂, investigated by Menchetti et al. [44], which crystallizes in the monoclinic system with space group symmetry C2/m. The unit cell parameters are as follows: a = 19.047(35) Å, b = 4.0530(3) Å, c = 10.318(3) Å, and β = 110.25(4)°, yielding a unit cell volume of 747(1) ų with Z = 2 formula units per cell. The basic units are ladderlike chains built by Sb³⁺ and O²⁻ ions; all antimony atoms have distorted trigonal bipyramidal [SbO₄E] coordination with the lone pair in the equatorial plane.

The crystal lattice characteristics of antimony oxychlorides, layer type, coordination geometry, and band gap, play a crucial role in determining their electronic and optical behavior.

Table 3. Crystal structure and band-gap overview of Sb_xO_yCl_z phases.

Compound	Space Group Name	Band Gap (eV)	Ref
SbOCl·DMSO	Pca21	3.74	[45]
Sb4O5Cl2	P21/a	Between 2.45 and 2.83	[46,47]
Sb4O5Cl2	P21/c	2.583	[48,49]
Sb8O11Cl2	C2/m	3.59	[48,50]
Sb2Cl7O	P21/c	3.38, 3.05 (DFT)	[51]
Sb3O4Cl	P2/c	2.737	[52]

summarizes the main structural features of representative Sb_xO_yCl_z phases and relates them to the measured or calculated bandgap values and corresponding functional properties. In general, layered or pseudo-layered architectures with mixed [SbO₄E] and [SbO₃E] coordination tend to exhibit narrower band gaps and stronger optical anisotropy due to enhanced covalent interactions between Sb 5s/5p and O 2p orbitals.

Conversely, more ionic and three-dimensional frameworks, such as Sb₈O₁₁Cl₂, demonstrate wider band gaps and higher thermal stability.

For example, monoclinic Sb₄O₅Cl₂ (P2₁/a, P2₁/c) with a layered structure shows an indirect band gap in the range of 2.45–2.83 eV, which is consistent with its high photocatalytic activity and efficient charge separation within the layers. Orthorhombic Sb₈O₁₁Cl₂ (C2/m) exhibits a wider band gap of ≈3.6 eV and pronounced structural robustness, attributed to the highly distorted [SbO₄E] coordination and strong Sb-O-Cl ionic bonding.

In contrast, SbOCl and Sb₃O₄Cl with layered or intercalated frameworks possess direct band gaps of 2.7–3.5 eV and display strong optical birefringence and dielectric anisotropy. The mixed anion environment in Sb₂Cl₇O yields an intermediate band gap depending on the computational or experimental method, reflecting partial delocalization of electrons through Sb-Cl-O linkages.

These observations confirm a consistent structure–property relationship within the Sb_xO_yCl_z system: the evolution from chloride-rich to oxygen-rich compositions leads to a gradual narrowing of the band gap and a shift toward more covalent bonding character.

Such relationships provide an essential foundation for tailoring the electronic, optical, and photocatalytic performance of Sb_xO_yCl_z materials through controlled synthesis, compositional tuning, and morphology engineering.

Figure 4a–d show the representative crystal structures of four typical antimony oxychlorides. SbOCl consists of interconnected (SbO₄)^5- and (SbO₂Cl)^2- polyhedra forming a three-dimensional channel framework, where Cl^- ions occupy the channels to maintain charge balance (a). Sb₂OCl₄ is composed of (SbOCl₃)^2- and (SbO₂Cl₂)^3- units linked through [Sb₄O₂Cl₈] clusters, resulting in a three-dimensional framework (b). Sb₃O₄Cl forms a two-dimensional layered [Sb₃O₄]∞ network built from (SbO₄)^5- and (SbO₃)^3- polyhedra, with Cl^- ions located between the layers (c). Sb₈O₁₁Cl₂ exhibits a similar two-dimensional [Sb₈O₁₁]∞ layered structure consisting of (SbO₄)^5- and (SbO₃)^3- groups, where interlayer chlorine ions ensure charge neutrality (d).

Overall, the structural evolution from SbOCl to Sb₈O₁₁Cl₂ demonstrates a gradual transition from three-dimensional channel frameworks to two-dimensional layered architectures, reflecting variations in the coordination environment of Sb and the arrangement of chlorine ions.

The structure of SbOCl is non-layered and composed of isolated pyramidal [SbO₂Cl] fragments. In this compound, chlorine is covalently bonded to antimony, and no interlayer ionic species are present, distinguishing it from more ionically organized oxyhalides [53].

Sb₄O₅Cl₂ thus occupies an intermediate position between layered and discrete oxyhalide structures. Unlike Sb₃O₄Cl, where chlorine primarily serves an ionic role, and SbOCl, where chlorine is fully integrated into the coordination sphere, Sb₄O₅Cl₂ exhibits partially covalent Sb-Cl interactions.

Such comparative structural analysis underscores the importance of detailed coordination and bonding studies in antimony oxyhalides, particularly in relation to their synthetic accessibility, phase stability, and functional properties.

4. Fabrication Methods of Antimony Oxychlorides

Up to now, a diverse array of antimony oxychloride nanostructures exhibiting distinct morphologies has been synthesized via both solid-state and wet chemical methodologies.

The Sb₈O₁₁Cl₂ microrod samples were synthesized via a simple solvothermal method [50]. Specifically, 0.001 mol of 2,6-pyridinedicarboxylic acid and 0.001 mol of SbCl₃ at a 1:1 molar ratio were dissolved in a mixture of 25 mL of deionized water and 5 mL of ethanol and stirred for 30 min. This solution was then transferred into a 50 mL teflon-lined stainless steel autoclave and heated at 180 °C for 24 h. The resulting product was collected, washed several times with ethanol, and dried under vacuum at 70 °C for 12 h. As a result of the described synthesis method, Sb₈O₁₁Cl₂ was obtained in the form of microrods with an average diameter of approximately 100 nm and varying lengths. These microstructures exhibit a pure monoclinic phase and possess a distinctive rod-like morphology. A lattice spacing of approximately 0.893 nm was identified, corresponding to the (200) crystallographic plane of Sb₈O₁₁Cl₂. The synthesized material demonstrated excellent electrochemical performance as an anode for sodium-ion batteries, delivering high specific capacity and good cycling stability. Moreover, antimony-based compounds are considered promising anode materials for aqueous trivalent metal batteries due to their high specific capacity (up to 660 mA h g⁻¹), natural abundance, and low cost, making them attractive for use in high-energy-density and environmentally friendly energy storage devices.

In this study [54], Sb₈O₁₁Cl₂ serves as a key semiconductor component in the fabrication of heterostructures with TiO₂, designed to enhance photocatalytic activity. The Sb₈O₁₁Cl₂ nanoparticles exhibit a stable, leaf-like morphology with an average size of approximately 5 nm. When integrated with TiO₂ nanosheets featuring selectively exposed crystal facets, these nanostructures markedly influence charge carrier separation and transport, thereby improving overall photocatalytic efficiency.

In the work of B. Li et al., needle-shaped microcrystals Sb₈O₁₁Cl₂(H₂O)₆ with an orthorhombic crystal structure were successfully synthesized using the method of low-power ultrasonic mixing in the presence of an aqueous solution of the surfactant Triton X-100 at room temperature. The resulting microcrystals exceed 20 microns in length with an average diameter of about 2 microns. The mechanism of formation of these needle-like microcrystals is explained by the synergistic effect of low-power ultrasound, which activates the liquid–solid boundary layer and enhances mass transfer, as well as the self-assembly of surfactant molecules into rod-shaped micelles. Sb₈O₁₁Cl₂(H₂O)₆ is also known for its potential as an effective flame-retardant additive in polymer composites. Upon thermal decomposition, it releases water and Sb₈O₁₁Cl₂, increasing the fire resistance of the base polymer. In addition, the one-dimensional needle morphology and micrometer dimensions of these microcrystals contribute to improving mechanical properties when embedded in polymer matrices, expanding the scope of their application in modern functional materials. Needle microcrystals Sb₈O₁₁Cl₂(H₂O)₆ represent promising candidates for the development of flame-retardant composites and other high-performance functional 1D [T1-T6] inorganic materials [55].

Sb₄O₅Cl₂ nanostructures were successfully synthesized via a straightforward wet-chemical approach utilizing a single-source precursor system, without the need for non-aqueous solvents. This environmentally benign and facile method enables morphological control through simple modulation of reaction parameters, yielding either one-dimensional (1D) nanorods or two-dimensional (2D) nanosheets. Structural analysis confirms that both morphologies crystallize in a single-phase monoclinic structure with space group P2₁/c. The nanorods exhibit an average diameter of approximately 90 nm and a length of ~2 μm, whereas the nanosheets possess a thickness ranging from 50 to 150 nm and tend to assemble into layered configurations. Optoelectronic characterization reveals that Sb₄O₅Cl₂ exhibits semiconducting behavior, with a band gap that increases upon transitioning from bulk to nanoscale forms, attributed to quantum confinement effects. Specifically, the band gap values were determined to be 3.25 eV for the bulk material, 3.31 eV for the nanorods, and 3.34 eV for the nanosheets. The principal advantage of this synthesis route lies in its simplicity and the elimination of complex, time-intensive procedures typically involving additional solvents or matrix components. The ability to tailor morphology, reduce crystallite size, and modulate electronic properties positions Sb₄O₅Cl₂ nanostructures as promising candidates for applications in energy storage, flame retardancy, and photocatalytic systems [11].

This study [56] explores the incorporation of Ni²⁺ ions into the Sb₄O₅Cl₂ lattice as a strategy to enhance its electrochemical performance. Given the comparable ionic radii of Ni²⁺ and Sb³⁺, Ni²⁺ can substitute into the crystal lattice, inducing local distortions that facilitate charge transport. This substitution improves electrical conductivity, reduces charge-transfer resistance, and reinforces structural integrity. To further augment ion mobility and electronic conductivity, graphene was introduced as a conductive matrix, leveraging its two-dimensional architecture to create efficient transport pathways. A series of samples with varying Ni doping concentrations (0%, 1% 3%, and 5%) were synthesized via a hydrothermal method using SbCl₃ and nickel(II) acetate in an acidic aqueous medium, followed by vacuum drying. Electrochemical evaluation demonstrated that the 3% Ni-doped Sb₄O₅Cl₂/Ag electrode delivered a specific capacity of 74.19 mAh g⁻¹ at a current density of 0.3 A g⁻¹, retaining 85% of its capacity after 200 cycles and 79.8% after 1000 cycles. Enhanced rate capability was also observed, with the electrode maintaining 34.1 mAh g⁻¹ at 2 A g⁻¹. Overall, Ni doping significantly improves the electrical conductivity, structural robustness, and chloride-ion storage capacity of Sb₄O₅Cl₂-based electrodes in aqueous environments. These findings underscore the potential of Ni-doped Sb₄O₅Cl₂/graphene composites as stable, multifunctional electrode materials for applications in energy storage and water desalination technologies.

In another study [57], Sb₄O₅Cl₂ was synthesized via a mild hydrothermal method using an optimized water-to-ethanol volume ratio of 3:1. The resulting product exhibited a three-dimensional flower-like morphology composed of interconnected nanosheets with moderate thickness. This architecture enhances electrolyte wettability and provides robust structural integrity during electrochemical cycling. X-ray diffraction analysis confirmed a monoclinic crystal structure with a prominent (120) plane and an interplanar spacing of 0.25 nm. Nitrogen adsorption–desorption measurements revealed a specific surface area of approximately 8.34 m²/g, with dominant pore sizes ranging from 15 to 30 nm, facilitating efficient electrolyte infiltration and rapid ion transport. The unique morphology shortens ion diffusion pathways and improves electron transfer kinetics, thereby enabling fast K⁺ ion transport. Electrochemical measurements demonstrated that Sb₄O₅Cl₂ operates predominantly below 1.5 V versus K/K⁺, making it a promising anode material for potassium-ion batteries. Density functional theory (DFT) calculations and kinetic analyses confirmed a high diffusion coefficient for K⁺ ions and revealed a hybrid storage mechanism involving both conversion reactions and intercalation/deintercalation processes. These features contribute to the material’s high reversibility, delivering a specific capacity of up to 530 mAh/g at a current density of 50 mA/g, along with excellent cycling stability.

In [58], a novel heterojunction photocatalyst, denoted as SCL-CX, was developed by integrating g-C₃N₄, Sb₂S₃, and Sb₄O₅Cl₂. In this composite, Sb₄O₅Cl₂ serves as a critical component due to its distinctive electrochemical and optical properties, which enable the formation of efficient hybrid structures for visible-light-driven photocatalysis. Although the synthesis of Sb₄O₅Cl₂ presents certain challenges, its incorporation into the heterojunction significantly contributes to the overall functionality of the system. The introduction of g-C₃N₄ into the composite enhances visible light absorption and induces a red shift in the absorption edge, thereby promoting the generation of electron-hole pairs. Photoluminescence spectroscopy reveals a marked quenching of emission intensity in the SCL-C2 sample compared to pure Sb₄O₅Cl₂ and the binary SCL system. This quenching effect indicates efficient separation of photogenerated charge carriers and a reduction in recombination rates, which are essential for improved photocatalytic activity.

The synthesis parameters, particularly pH, temperature, and precursor composition, play a decisive role in the phase formation and morphology of antimony oxychlorides. Literature reports indicate that the acidity of the reaction medium strongly affects the crystallization pathway and determines which Sb_xO_yCl_z phase is preferentially formed.

Under acidic conditions (pH 1–2), the hydrothermal synthesis of Sb₄O₅Cl₂ typically yields monoclinic nanostructures with a well-defined nanosheet or nanorod morphology, attributed to a high concentration of chloride ions that stabilize layered [Sb₄O₅]²⁺ units.

When the solution pH is increased to around 4, the phase composition gradually shifts toward orthorhombic Sb₈O₁₁Cl₂, reflecting enhanced hydrolysis and partial dechlorination of the precursor species. At alkaline conditions (pH > 8), further substitution of chloride by hydroxide ions favors the formation of Sb₂O₃ (valentinite or senarmontite phases).

Such transformations confirm that the hydrolysis–condensation equilibrium between Sb-Cl and Sb-O bonds governs the crystallization behavior of the Sb_x-O_y-Cl_z system. Adjusting the hydroxide concentration or modifying the solvent polarity allows for fine-tuning of nucleation and growth kinetics, thus directly influencing the phase purity and particle morphology.

Table 4. Methods for fabricating antimony oxychloride compounds.

Synthesis Conditions	Reagents	Crystal Structure/Space Group	Morphology	Material Type	Application	Features	Ref
Hydrothermal, 100 °C, 12 h	SbCl3, ethylenediamine, ethanol	Monoclinic, P21/c	3D flower-like nanosheets	Pure Sb4O5Cl2	Anode material for potassium-ion batteries (PIBs)	3D flower-like morphology, high reversible capacity (≈530 mAh g−1 @ 50 mA g−1), stable structure after cycling	[57], 2022
Hydrothermal, unspecified	SbCl3, thiourea	Monoclinic, P21/a	Mixed-phase composite (Sb2S3, Sb4O5Cl2)	Composite (Sb2S3 Sb4O5Cl2)	Photocatalyst for crystal violet dye degradation	Sb2S3, Sb4O5Cl2 composite with enhanced charge carrier separation	[59], 2022
Hydrolysis/alcoholysis 25 °C, pH ≈ 2	SbCl3+ H2O/EtOH/EG	SbOCl: P21/a Sb4O5Cl2: P21/c Sb3O4Cl: P2/c	Fine precipitates after hydrolysis	Mixed products SbOCl, Sb3O4Cl, Sb4O5Cl2	Selective extraction of Sb from anode slime	Formation of stable SbOCl, Sb3O4Cl, Sb4O5Cl2 depending on pH, confirmed by DFT	[52], 2022
Solid-state reaction, 120 °C, 5 days	CuO, CuCl2, SbCl3	Crystalline Cu-Sb-O-Cl phase (nanocrystalline, 2–8 nm)	Rough and uneven surface; small spherical/cylindrical aggregates (5–50 µm)	Ternary oxide halide (Cu Sb O Cl)	Removal of organic dyes	Cu-Sb oxychloride nanomaterial, high stability	[60], 2019
Sealed evacuated silica tubes, 500 °C, 96 h	Sb2O3 + SbCl3, (11:2 molar ratio)	Triclinic/P-1	Rod-like colorless crystals	Sb8O11Cl2	Mineral form (Onoratoite),	Tubular Sb–O structures with [SbO4E] and [SbO3E]; halide ions between tubes	[61], 2006
Hydrothermal + variation	SbCl3, ethanol, ammonia, PVP, etc.	Monoclinic, P21/c	Nanorods (2 μm), Nanosheets (50–150 nm)	Nanostructured Sb4O5Cl2	Photocatalysis, optics, dielectric materials	Sb4O5Cl2 nanostructures (nanorods, nanosheets), varied morphologies, and high dielectric constants	[11], 2019
On-line gas-phase formation at 230–550 °C, under 0.2–0.5 torr	SbCl3, Ag2O, NaF	C3v symmetry (molecular)	Gas phase species (SbOX3)	SbOCl3	Spectroscopy, IR characterization of molecules	SbOX3 (X=F, Cl), first-time IR band assignment for O-Sb bond	[62], 2000
Electrochemical deposition in concentrated electrolyte, room temperature	SbCl3 + LiCl aqueous solution	XRD-confirmed formation of SbOCl	Uniform spherical deposits (500 nm), dendrite-free	SbOCl (condensed phase)	Aqueous trivalent Sb batteries	High Coulombic efficiency (99.7–99.8%), long lifespan (7300 h), reversible Sb↔SbOCl transformation, stable morphology	[63], 2025
Gas–solid reaction, 523–773 K	Sb2O3 + HCl vapor;	Monoclinic, orthorhombic	Polycrystalline SbOCl (h) and SbOCl (l)	SbOCl (h), SbOCl	Flame retardants in polymers (PE, PP)	Phase transformation affects flame-retardant efficiency;	[64], 1985
Solvothermal, 120 °C, 12 h	SbCl3 + H2O/ethylene glycol (1:1) + TiONT	Sb4O5Cl2 (monoclinic, Sb8O11Cl2 (orthorhombic)	8–11 nm nanoparticles on titanate nanotubes	SbxOyClz/TiONT heterostructures	Photocatalytic degradation of methyl orange under UV/Vis	Band gap ≈3.05 eV; enhanced charge separation; activity strongly dependent on pH; stable up to 300 °C	[65], 2017
Direct addition at 0 °C or in CH2Cl2 solution	SbCl5 + oxygen-donor ligands	Molecular adducts, local C4v symmetry around Sb	Crystalline solids; no defined morphology	SbCl5·L (L = O-donor ligand)	Vibrational spectroscopy studies	C4v-like symmetry, distinct Sb-O and Sb-Cl modes, donor strength affects stretching frequencies	[66], 1976
Hydrothermal, 120 °C, 12 h, pH 1–2, solvent: EG–H2O or EtOH–H2O	SbCl3	Monoclinic (Sb4O5Cl2)	Nanoparticles	Sb4O5Cl2	Flame retardants, optical properties	Synthesized in acidic conditions, first report on nanostructured form	[67], 2008
Hydrothermal, 120 °C, 12 h, solvent: EG–H2O	SbCl3 + NaOH	Monoclinic	Nanobelts and nanowires	Sb8O11Cl2		Rare antimony oxychloride phase, nanowire	[67], 2008
Hydrothermal, 120 °C, 12 h, pH 8–9, solvent: EG–H2O or EtOH–H2O	SbCl3 + NaOH	Cubic (senarmontite) or orthorhombic (valentinite)	Nanocrystals	Sb2O3	Catalysis, flame retardants, optical	Selective phase control via solvent and pH; strong photoluminescence	[67], 2008
Heating mixture of Sb2O3 and organochlorine compounds	Sb2O3 + chlorinated organics	Main phase: Sb4O5Cl2 (monoclinic P21/c)	Fine solid products; XRD-identified Sb4O5Cl2	Antimony oxychloride (mainly Sb4O5Cl2)	Flame-retardant additives for polymers	Sb4O5Cl2 is thermally stable at 400–600 °C; reacts to form volatile SbCl3;	[68], 1990

shows a summary of the synthesis parameters, morphological features, and functional applications of antimony oxychloride-based compounds and composites. It highlights how variations in synthesis conditions, such as temperature, pH, precursor type, and reaction environment, directly affect phase formation, particle morphology, and the resulting photocatalytic and electrochemical performance.

These results demonstrate that fine control of synthesis parameters is crucial for obtaining phase-pure, anisotropic Sb_x-O_y-Cl_z materials with optimized structural and functional characteristics.

Table 5. Methods for fabricating antimony oxychloride compounds and their characteristics.

Preparation Method	Influence on Morphology and Structure	Advantages	Limitations
Hydrothermal	Nanoplates and microrods of Sb4O5Cl2 are formed with a well-defined orthorhombic structure; crystallites are oriented along the (001) direction.	High crystallinity; single-phase products; controlled morphology (plates, rods); improved photocatalytic and electrochemical properties.	Requires long heating time (up to 24 h); partial formation of Sb2O3 impurities; pressure control is required. multistage washing and vacuum drying.
Sonochemical	Needle-like microcrystals of Sb8O11Cl2 (1–3 μm), weakly ordered.	Rapid process (≤1 h); no heating or pressure required; easily scalable.	Moderate phase purity; relatively large particles (microscale); difficult morphology control.
CVD	Dense thin films of SbxOyClz and mixed phases are formed; the structure is layered, with crystallites oriented along the (001) direction.	Enables deposition of uniform films with strong adhesion and good compositional reproducibility; allows doping and fabrication of multilayer structures	Requires volatile Sb precursors; high temperatures (350–500 °C) may induce phase transformation to Sb2O3; difficult to control the Cl/O ratio.
Gas-phase method	SbOCl(h) and Sb4O5Cl2 are formed with ordered layered structures; the structure depends on temperature.	The stability of SbOCl at high temperatures is confirmed, along with the formation of stable phases.	High processing temperatures lead to partial sublimation of SbCl3 and the formation of metallic Sb, resulting in an uneven product surface.
Wet–Chemical	Micron-sized Sb4O5Cl2 microplates are formed, assembling into “sand-rose” structures; the crystals are orthorhombic and anisotropic.	Simple and reproducible synthesis procedure; high photocatalytic activity (up to 94% degradation of methylene blue).	Large particle size (microscale); non-uniform thickness.
Sol–gel method	Porous xerogels of Sb4O5Cl2 and Sb8O11Cl2 are formed with nanoplate-like morphology; the porous structure is preserved during drying below 200 °C, while densification occurs at 400 °C.	Low synthesis temperature; possibility of obtaining nanoporous coatings and monolithic structures.	Long gelation stage (12–48 h); difficult to control Cl− content during prolonged drying; possible dehydration leading to Sb2O3 formation

further compares the main fabrication methods employed for antimony oxychloride compounds, focusing on their influence on morphology, structural features, advantages, and limitations. Among the discussed techniques, hydrothermal and solvothermal approaches enable the formation of highly crystalline Sb₄O₅Cl₂ nanostructures with well-defined morphology, whereas vapor-phase and CVD methods facilitate thin-film growth suitable for device applications. Meanwhile, wet–chemical and sol–gel techniques remain promising for scalable synthesis and the development of porous or composite materials.

Together, these comparative data illustrate the diversity of available synthesis strategies and provide a framework for selecting appropriate methods depending on the desired phase composition, morphology, and application field.

5. Challenges and Future Prospects of Antimony Oxychlorides Compounds

Over the past several decades, antimony oxychlorides have attracted considerable attention due to their unique crystal chemistry and promising potential in photocatalysis, optoelectronics, and flame-retardant coatings. Research efforts have primarily focused on synthesis methods, phase control, morphological tuning, and the functional properties of the material. Nevertheless, several fundamental and technological limitations continue to hinder its widespread industrial application.

One of the key challenges in working with antimony oxychlorides remains the complexity of phase control. Antimony oxychlorides can crystallize in multiple structural modifications, including monoclinic and orthorhombic phases, with stability and reproducibility highly dependent on precisely regulated synthesis conditions such as temperature, pH, and gas atmosphere. According to a previous study [39], antimony oxychlorides are characterized by a wide range of morphologies, including sheaf-like, rhombic-plate, oval leaf-like, and quasi-wafer forms. Precise control over the size and shape of these nanocrystals requires careful optimization of synthesis parameters such as hydroxide ion concentration (NaOH) and the presence and concentration of surface stabilizers, notably polyvinylpyrrolidone (PVP). These constraints significantly limit scalability and pose challenges for transitioning from laboratory-scale samples to industrial technologies.

One of the fundamental challenges limiting the development of functional materials based on antimony oxychlorides and their derivatives is the difficulty in controlling the stereochemical activity of the lone electron pair on the Sb³⁺ ion. This activity directly influences the spatial distribution of electron density and, consequently, affects the optical properties of the crystals, including birefringence and anisotropy [69]. The stereochemical activity of Sb³⁺ is governed by interactions between its 5s and 5p orbitals and the p-orbitals of surrounding anions such as oxygen, chlorine, and other halogens. These orbital interactions generate a locally anisotropic electronic environment around the antimony center, resulting in directional electron density distribution and distortion of the crystal lattice symmetry. Consequently, pronounced optical anisotropy arises, which is highly sensitive to subtle variations in local coordination.

Further complexity is introduced by structural diversity stemming from anion substitution within the crystal lattice. Single-site replacement can lead to the formation of distinct polyhedral configurations around the Sb atom, including octahedral, tetrahedral, or distorted pyramidal geometries, each exerting a unique influence on the magnitude and orientation of birefringence. Controlling these configurations requires precise regulation of synthesis conditions—such as temperature, medium composition, pH, and atmospheric environment—alongside stringent phase purity.

To optimize the optical properties of antimony oxychloride-based materials, a comprehensive understanding of the stereochemistry and electronic interactions underlying crystal structure formation is essential. This encompasses both theoretical modeling of electron density distribution and experimental investigation of phase transitions and local coordination environments. Addressing these aspects will enable the design of materials with tailored optical characteristics suitable for applications in photonics, nonlinear optics, and sensing technologies.

Moreover, the inherently low electronic conductivity of antimony oxychlorides limits its direct use in electrochemical devices such as batteries and sensors. To overcome this constraint, active research is focused on strategies including doping, heterostructure engineering, and compositing with conductive materials such as graphene, carbon nanotubes, or transition metal oxides. For instance, SbOCl/graphene oxide composites have demonstrated enhanced conductivity and improved performance in the photocatalytic degradation of organic pollutants [70].

Another critical factor is the limited chemical stability of antimony oxychlorides in aqueous and acidic environments. The material is prone to hydrolysis and structural transformations, which compromise its durability under harsh operating conditions. To enhance its stability, strategies such as surface modification, encapsulation, and the formation of protective layers have been proposed. In the study by [65], it was demonstrated that encapsulating SbOCl within a TiO₂ matrix significantly improves resistance to acid-induced corrosion while preserving photocatalytic activity over extended periods.

Solving these problems requires the advancement of synthesis techniques, including hydrothermal, sol–gel, and solid-state approaches, as well as a deeper understanding of phase transformation mechanisms and the interactions of antimony oxychlorides with their surrounding environment. In the long term, the development of stable and functional antimony oxychloride-based composites may unlock new opportunities in photocatalytic reactors, thermoelectric converters, and smart coating technologies.

Despite existing limitations, Sb_xO_yCl_z compounds continue to exhibit strong potential as active components in advanced materials. Their structural adaptability, along with distinctive optical and chemical properties, positions them as promising candidates for further research and integration into next-generation technological platforms.

5.1. Prospects for the Development of Antimony Oxychloride-Based Materials

5.1.1. Molecular Engineering and Doping

The incorporation of organic molecules (e.g., DMSO) into the SbOCl framework enables the formation of helical [SbOCl]_∞ chains, which enhance optical anisotropy and polarizability. This approach paves the way for the design of novel nonlinear optical materials with high second-harmonic generation coefficients.

5.1.2. Hybrid Structures and Composites

Integrating SbOCl with graphene, TiO₂, or other nanomaterials can compensate for its low electrical conductivity and improve performance in photocatalytic and electrochemical applications.

5.1.3. Application in Flame-Retardant Systems

Sb_xO_yCl_z can form in situ during the thermal decomposition of antimony-based flame retardants. Its presence in the condensed phase contributes to flame inhibition, particularly in polymers such as polyethylene (PE) and polypropylene (PP).

5.1.4. Development of Vapor-Phase Transport Methods

Sublimation of SbCl₃ in an oxygen-rich atmosphere enables the deposition of pure Sb_xO_yCl_z crystals onto cooled surfaces, offering potential for thin-film technologies and sensor fabrication.

6. Conclusions

In conclusion, antimony oxychloride compounds exhibit significant promise across a spectrum of advanced technological applications. Their structural complexity, morphological tunability, and distinctive electronic configurations, particularly the stereochemically active lone pair on Sb³⁺, enable targeted modulation of functional properties relevant to photocatalysis, flame retardancy, and energy conversion processes. Nonetheless, several critical challenges persist, including limited phase control, susceptibility to chemical degradation, and inherently low electronic conductivity. Addressing these limitations necessitates the development of advanced synthetic methodologies, surface modification techniques, and rational composite architectures.

Recent progress in integrating antimony oxychloride compounds with conductive matrices and dopants has yielded encouraging improvements in both performance and long-term stability. Given the increasing global demand for strategic and multifunctional materials, antimony’s unique chemical behavior and resource significance underscore its relevance in future-oriented research. Continued investigation into the antimony oxychloride system is expected to not only enhance our understanding of its fundamental physicochemical characteristics but also expand its applicability within sustainable, high-performance material platforms tailored for next-generation energy and environmental technologies.

This review is expected to provide valuable guidance for future research on antimony oxychloride-based materials by deepening understanding of their coordination chemistry, phase behavior, and composite design, thereby supporting the development of multifunctional systems for environmental, energy, and flame-retardant applications.