Mixed-anion compounds

It is our great pleasure to welcome the readers of Dalton Transactions to this spotlight collection devoted to “Mixed-anion compounds”. Mixed-anion or hetero-anionic compounds are single-phase materials containing more than one type of anion in combination with one or more cations. Normally, one or more of the cations is coordinated by a mixture of anions, and if the anions are sufficiently different in radius, charge or chemistry, they will exhibit long-range crystallographic order. In oxides, which have dominated the area of functional ceramics, property control can sometimes be exerted by controlling the anion (i.e. oxide) content, but often the control of the structure, electronic and other properties is dominated by the choice of cations “cation chemistry”. Research on mixed-anion compounds is much more dominated by the “anion chemistry”, where the structural, electronic, optical, thermal, electrochemical, and catalytic properties of complex solid-state compounds can be effectively tuned by controlling the anion composition.

While some mixed-anion compounds, such as oxynitrides and oxyfluorides, have been known for decades, this entire family has recently expanded and diversified, and is attracting increasing attention as a platform for tunable functional properties. In terms of practical applications, mixed-anion compounds span a wide range—from electronics and optics to catalysis, energy conversion, and beyond. For various applications and research targets, mixed-anion materials are synthesized in powder, single-crystal, and thin-film forms via diverse synthetic approaches, often supported by advanced theoretical calculations.

Given this active research landscape, it was deemed timely to launch a themed collection in Dalton Transactions showcasing recent advances in the field of mixed-anion compounds. The collection features studies on representative systems, including oxychlorides (oxide/chloride), oxyfluorides (oxide/fluoride), oxynitrides (oxide/nitride), and oxyhydrides (oxide/hydride). It also includes inorganic compounds incorporating molecular anions such as [SCN]⁻, [NCN]²⁻, [BH₄]⁻ and [S₂]²⁻.

This themed collection brings together 34 articles that cover a broad spectrum of advances in chemistry and physics of mixed-anion compounds. The collection consists of one Perspective and two Frontier articles, two Communications, and 29 Research papers.

Mixed-anion compounds typically present difficulties in synthesis due to the differing volatilities of their precursors (e.g., oxides, chalcogenides, halides, hydrides). In addition, these compounds pose challenges in accurately determining the compositional ratios and distributions of two or more anions. Considerable research efforts have been devoted to the synthesis and characterization of new mixed-anion compounds. The Research paper by Määttänen and Karttunen presents a systematic workflow for predicting the structures of oxyfluorides, oxynitrides, and nitrofluorides with a perovskite-type framework, thereby enabling precise descriptions of their local coordination geometries (https://doi.org/10.1039/D5DT01139F). The Research paper by Geßner et al. reports the synthesis of a highly fluorinated oxyfluoride Ruddlesden–Popper phase, La₄Ni₃O_8.4F_3.5, obtained via topochemical fluorination of La₄Ni₃O₁₀, and investigates its crystal structure and magnetic properties (https://doi.org/10.1039/D5DT01138H). The Research paper by Kawanishi et al. presents experimental and theoretical investigations of the structural relationships among three polymorphs of lanthanide sulfide iodides, LnSI (Ln = lanthanides), offering fundamental insights into the structural control of mixed-anion compounds (https://doi.org/10.1039/D5DT00753D). In the Research paper by Panigrahi et al., the authors report the preparation of single crystals and polycrystalline powders of rare-earth chalcogenide iodides (La₃Si₂Se_1.21S_6.79I, Ce₃Si₂Se_1.39S_6.61I, Pr₃Si₂Se_1.22S_6.78I, and Nd₃Si₂Se_1.18S_6.82I) and demonstrate their physical properties (https://doi.org/10.1039/D4DT03506B).

The Research paper by Harada et al. investigates the growth of single crystals of the perovskite-type oxynitride SrTaO₂N using an NH₃-assisted SrCl₂ flux method, yielding oxynitride crystals with lengths of approximately 300 μm (https://doi.org/10.1039/D3DT01383A). The Research paper by Tang et al. presents the synthesis and structure–property relation analysis of two metal chalcohalides, Cs₂InPS₄X₂ with X = Cl and Br, and characterizes their crystallographic, optical, and thermal properties (https://doi.org/10.1039/D2DT00078D). The Research paper by Fan et al. reports the synthesis and crystallographic characterization of the heavy-metal oxychloride, RbPb₈O₄Cl₉, revealing a rich structural diversity arising from distorted [PbO₃Cl₃] and [PbOCl₅] anion groups (https://doi.org/10.1039/D1DT02665H). The Research paper by Masubuchi et al. establishes a low-temperature synthesis route for Ba₃Si₆O₁₂N₂ and Ba₃Si₆O₉N₄ using a mixture of BaCN₂ and SiO₂. The reaction temperature is several hundred degrees lower than that required for conventional solid-state reactions employing BaCO₃, SiO₂, and Si₃N₄ (https://doi.org/10.1039/D1DT00824B). In the Research paper by Ie et al., the authors report the successful low-temperature synthesis of novel chalcogenide halides, K₂BaTaS₄Cl, Rb₂BaTaS₄Cl, Cs₂BaTaS₄Cl, and K₂BaNbS₄Cl. The crystal structures of these compounds were refined by single-crystal X-ray diffraction, and their optical properties were investigated (https://doi.org/10.1039/D5DT01676B).

The mixed-anion family has attracted increasing attention in the field of catalysis. This spotlight collection includes several articles that address this topic. The Frontier article by Aschauer and Kabbour presents the emerging field of spintro-catalysis and discusses how magnetic ordering influences catalytic performance, including prospects for mixed-anion chemistry to further enhance the tunability of electronic and magnetic structures (https://doi.org/10.1039/D5DT01128K). The Communication by Raabe et al. reports on the successful synthesis of Al- and Si-substituted Keggin-type polyoxometalates, which correspond to inorganic polyanionic cluster compounds and are potential catalytic materials (https://doi.org/10.1039/D5DT00894H). The Research paper by Sato et al. demonstrates that novel perovskite-type oxyhydrides BaLnO₂H (Ln = Tb–Lu), synthesized via a mechanochemical method, effectively promote the activity of Ru catalysts for low-temperature ammonia synthesis (https://doi.org/10.1039/D5DT01036E). The Research paper by Okazaki et al. reports on photocatalytic CO₂ reduction activity of Zn-based mixed-(hetero)anion compounds Zn(OH)(NO₃) and Zn₃(OH)₄(NO₃)₂, which can convert CO₂ into HCOOH with >80% selectivity under visible light (https://doi.org/10.1039/D5DT01157D). The Research paper by Hojamberdiev et al. establishes conditions for the syntheses of tantalum oxynitride and nitride, including β-TaON, a potential photocatalyst candidate for solar water splitting (https://doi.org/10.1039/D5DT01193K). The Research paper by Kundu et al. demonstrates the high oxygen evolution reaction (OER) electrocatalytic activity of Mn-doped Bi₃O₄Br, which involves the in situ formation of Mn-doped Bi₂O₃ as the active species during the OER process (https://doi.org/10.1039/D4DT00633J).

Several articles in this spotlight collection discuss the role of anion mixing in controlling the optical properties, including the non-linear optic (NLO) characteristics. In their seminal Perspective, Kitagawa et al. (https://doi.org/10.1039/D4DT00191E) discuss the effects of mixed-anion coordination on the optical properties of Eu³⁺ ions, using the oxynitride YSiO₂N and oxyhalide YOX (X = Cl or Br) systems as examples; the article also presents material design guidelines for new functional Eu³⁺-doped phosphors and outlines their future application areas. The Communication by Maimaiti et al. (https://doi.org/10.1039/D4DT03212H) introduces a salt-inclusion strategy to enrich the structural diversity of lead oxyhalides for new mid-IR functional crystals. The Research paper by Kim and Ok reports the successful synthesis of organic–inorganic hybrid materials comprising 4-(2-ammonioethyl)pyridinium organic cations and cadmium Cl/Br mixed-halides and demonstrates their tunable structural dimensionality and photoluminescent performance (https://doi.org/10.1039/D5DT00908A). The Research paper by Gabilondo and Halasyamani presents the synthesis of new mixed-anionic sodium methanesulfonates Na₅(SO₃CH₃)₄(X) with X = BF₄⁻, ClO₄⁻, PF₆⁻, and I⁻, and discusses anionic substitution effects on their optical properties (https://doi.org/10.1039/D5DT00223K).

The Frontier article by Shi et al. (https://doi.org/10.1039/D1DT00222H) discusses the emerging family of non-centrosymmetric oxychalcogenides as promising candidates for IR-NLO materials; these materials can combine the impressive second-harmonic generation (SHG) responses of chalcogenides with the wide energy gaps of oxides. The Research paper by Tang et al. (https://doi.org/10.1039/D5DT01124H) reports the rational design and synthesis of ZnHg₂P₂S₈ for a novel diamond-like IR-NLO material to simultaneously host an enhanced band gap and strong SHG response. The Research paper by Yan et al. (https://doi.org/10.1039/D5DT01100K) describes the single crystal growth of the non-centrosymmetric oxysulfides Ln₃Ga₃Ge₂S₃O₁₀ (Ln = La, Ce, Pr, Nd) in which the Ga atoms are tetrahedrally coordinated with two O and two S atoms; related polycrystalline samples were studied by neutron powder diffraction.

Ionics is an application area with strong links to the mixed-anion materials. In their Research paper, Kelly et al. (https://doi.org/10.1039/D5DT00159E) report the reversible lithium intercalation into the van der Waals oxytelluride V₂Te₂O, using both chemical and electrochemical methods; this leads to new Li_xV₂Te₂O phases with x approaching 2 in which the intercalated Li ions occupy vacant pseudo-octahedral sites with no change in symmetry. The Research paper by Ma et al. (https://doi.org/10.1039/D5DT00262A) reports topochemical fluorination synthesis of two distinct oxyfluorides from brownmillerite SrCoO_2.5 thin films using MgF₂ powder for the soft-chemical reduction; this material system, involving reversible transitions between randomly distributed F-ions and a F-ordered structure, could provide a pathway to extract high-purity F₂ with a perovskite catalyst. The Research paper by Zimmermanns et al. (https://doi.org/10.1039/D4DT01132E) introduces lithium thiophosphate glasses as promising solid electrolyte materials for all-solid-state batteries, and demonstrates that not only the total oxygen content but also the oxygen distribution within the material, may affect the ion conductivity.

Electromagnetic properties of mixed-anion compounds have also been investigated. The Research paper by Murrell et al. (https://doi.org/10.1039/D5DT00979K) reports the magnetic properties of Ruddlesden–Popper type oxyhydrides, La_xSr_2−xCo_0.5Ir_0.5O_4−yH_y, synthesized through topochemical reaction with LiH; magnetization and μSR data indicate that these materials exhibit strong magnetic frustration, attributed to the high degree of cation and anion disorder. The Research paper by Ohmi et al. reports successful syntheses of thiocyanate-containing organic–inorganic hybrid halides (CH₃NH₃)₂Co(NCS)₄ and (CH₃NH₃)₃CoI₄(NCS) and discusses the effect of anion exchange on their magnetic properties (https://doi.org/10.1039/D5DT00924C). In the Research paper of Oró-Solé et al. (https://doi.org/10.1039/D5DT01093D), a new oxynitride compound Eu₃Ta₂ON₅ with an n = 2 Ruddlesden–Popper structure is shown to possess ordered anion vacancies and partial anion ordering is suggested; magnetic susceptibility measurements indicate low-temperature ferromagnetic order of Eu²⁺ ions. The Research paper by Ohishi et al. investigates the ferroelectric and structural phase transition behaviours of hydrogen-bonded molecular relaxors—protonated 1,4-diazabicyclo[2.2.2]octane (dabco) salts—and their ferroelectric tunability through anion compositional control (https://doi.org/10.1039/D5DT00583C). The Research paper by Wolber et al. investigates the structural and magnetic properties of mixed-metal Aurivillius oxyfluorides, (Bi₂O₂)(Fe_1−xM_xF₄) with M = Mn, Co, and Ni. The mixed Fe/M phases exhibit magnetic ordering with antiferromagnetic colinear spin arrangements, in contrast to mixed-cation solid solutions, which typically display spin-glass-like magnetic states (https://doi.org/10.1039/D5DT01303H). The Research paper by Matsunaga et al. presents a novel synthetic strategy involving a liquid-phase process for obtaining complex solid solutions of perovskite-type oxynitrides, such as Ca_0.2Sr_0.2Ba_0.2La_0.2Pr_0.2Ti_1/3Nb_1/3Ta_1/3(O,N)₃ and Ca_1/3Sr_1/3Ba_1/3Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2(O,N)₃ (https://doi.org/10.1039/D5DT01464F).

The Research paper by Zheng et al. demonstrates a design strategy for advanced photoelectronic functional materials and reports the successful synthesis of a new wide-bandgap selenohalide, Cs₉Si₈Se₂₀Cl, featuring unprecedented [CsSe₇Cl] mixed-anionic units (https://doi.org/10.1039/D5DT01021G). The Research paper by Bråtalien et al. presents the crystal growth and polycrystalline synthesis of a new layered oxychloride Ca₂TlO₃Cl. This oxychloride exhibits n-type semiconducting behavior and is electrochemically activated under solar light (https://doi.org/10.1039/D5DT01111F). In the Research paper by Medina-Jurado et al., the authors present a comprehensive investigation of the optical and magnetic properties of the 2D phosphor Tb₂O₂NCN, focusing on its peculiar thermal quenching behavior, which may be applied to temperature sensing (https://doi.org/10.1039/D5DT01003A).

We sincerely hope that the readers of Dalton Transactions enjoy reading the articles of this spotlight collection which – in our opinion – nicely outlines the current research trends and showcases some highlights of the rapidly growing research field of mixed-anion compounds, spanning from new-material synthesis efforts to characterization of various application-relevant functional properties. We are most grateful to all the authors who have contributed to this collection.

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