Structural Diversity of Rare-Earth Oxychalcogenides.

Mixed-anion systems have garnered much attention in the past decade with attractive properties for diverse applications such as energy conversion, electronics, and catalysis. The discovery of new materials through mixed-cation and single-anion systems proved highly successful in the previous century, but solid-state chemists are now embracing an exciting design opportunity by incorporating multiple anions in compounds such as oxychalcogenides. Materials containing rare-earth ions are arguably a cornerstone of modern technology, and herein, we review recent advances in rare-earth oxychalcogenides. We discuss ternary rare-earth oxychalcogenides whose layered structures illustrate the characters and bonding preferences of oxide and chalcogenide anions. We then review quaternary compounds which combine anionic and cationic design strategies toward materials discovery and describe their structural diversity. Finally, we emphasize the progression from layered two-dimensional compounds to three-dimensional networks and the unique synthetic approaches which enable this advancement.

Introduction

The design and synthesis of new functional materials has long been dominated by work on single-anion compounds such as metal oxides, chalcogenides, and pnictides where cation composition is used to tune properties.[1] Mixed-anion systems, on the other hand, are comparatively underexplored. Mixed-anion systems contain multiple negatively charged anions, such as oxysulfides, with both oxide (O2–) and sulfide (S2–) ions. These are distinct from materials containing polyatomic anions such as sulfate (SO42–) or sulfite (SO32–) anions, in which the sulfur species has a positive formal oxidation state (+6 and +4 in sulfate and sulfite ions, respectively).

Rare-earth oxychalcogenides have been of interest since the implementation of Ln2O2S as phosphors for cathode ray tubes in the middle of the twentieth century. The recent resurgence of interest in oxychalcogenides is perhaps a result of modern computationally driven materials screening techniques, for which several recent publications point out the promise of oxychalcogenides in various applications, such as p-type transparent semiconductors,[2,3] thermoelectrics,[4,5] and solid-state electrolytes.[6] Advancements in synthetic strategies to control chemical and physical properties by mixing anions has been recently reviewed.[7,8]

Oxychalcogenides, containing O2– and Q2– (where Q = S, Se, or Te), are rarely simply anion-substituted analogues of the single-anion oxide or chalcogenide materials. The diverse relationships between the structures of mixed-anion compounds and structures of single-anion compounds were first pointed out 70 years ago.[9] Many mixed-anion materials adopt distinct structure types, local coordination environments, and dimensionality.[10]

Herein, we discuss critical structural features of oxychalcogenides. We emphasize that understanding the crystal structures of oxychalcogenides with an appreciation of chemical concepts, e.g., ionic radii, electronegativity, and polarizability, and hard–soft acid base theory, will lead toward a better grasp of synthetic challenges and a better rationalization of their electronic structures and physical behavior. The number of metal oxychalcogenides is huge, so we use this mini-review to focus on ternary rare-earth oxychalcogenides as a foundation for identifying key structural building blocks that, if exploited creatively, will advance materials discovery of oxychalcogenide materials. We then progress toward a discussion on recent discoveries of quaternary rare-earth oxychalcogenides that expand our understanding of crystal structures of mixed-anion materials. Rare-earth oxychalcogenides present an opportunity to combine metals of varying hardness (or softness) that yield new structures and interesting physical properties, e.g., magnetism and photoluminescence. The focus of this mini-review is the developments of rare-earth oxychalcogenides over the past ∼10 years. Information on this topic prior to the last 10 years can be found in references (10−13).

Ternary Oxychalcogenides

Ternary rare-earth oxychalcogenides can be found in a variety of compositional ratios. Most oxygen-rich materials reported from conventional solid-state routes belong to four structure types: La2O2S (including La2O2Se), anti-ThCr2Si2 (La2O2Te), La4O4Se3, and La2O2S2.[14−16] These structures, illustrated in Figure , provide an excellent starting point from which to explore the structural chemistry and physical properties of oxychalcogenides. In all four structure types, oxygen and chalcogen species are separated into two distinct layers—one layer of edge-sharing [Ln2O2]2+ units and one chalcogenide layer. This separation of anions is not surprising given the relative softness of S, Se, and Te compared to that of O.

Figure 1

Structure types of La2O2S(Se), anti-ThCr2Si2 (La2O2Te), La4O4Q3, and La2O2S2, with La, O, and Q ions shown in blue, pink, and yellow, respectively. The fundamental [Ln2O2]2+ layer consists of edge-linked OLn4 tetrahedra.

[Ln2O2]2+ Layer in Ternary Oxychalcogenides

The OLn4 building unit (O-centered tetrahedron, Ln = lanthanide cation) is a common structural feature in oxychalcogenides. Two-dimensional [Ln2O2]2+ layers composed of fluorite-like edge-linked OLn4 tetrahedra are quite robust; they are observed in all four structures with only slight distortions or variations in their symmetry with respect to the next [Ln2O2]2+ layer. For example, OLn4 units form trigonal [Ln2O2]2+ layers which are simply translated along the c-axis in La2O2S, whereas the tetragonal [Ln2O2]2+ layers are related by an ab-mirror plane in La2O2Te and by a glide plane in La4O4Se3. Replacing Ln3+ with Bi3+ gives the closely related Bi2O2Q (Q = S, Se, Te) series, with the oxyselenide and oxytelluride isostructural with La2O2Te (Figure ),[5,17] whereas the oxysulfide Bi2O2S adopts a similar structure with a slight orthorhombic distortion.[18] Interestingly, the relative softness of the Bi3+ cation also allows it to occupy sites occupied by softer Q, leading to Bi–O–Q phases containing both [Bi2O2]2+ and Bi–Q layers and giving structures more akin to those adopted by quaternary systems (see below).[19]

Chalcogenide Layers in Ternary Oxychalcogenides

Monoatomic Anion Only

It is the chalcogen layer that exhibits the greater structural diversity. Within these ternary materials, the chalcogen species can be found as Q2– ions and (Q2)2– dimers. In La2O2S and anti-ThCr2Si2 (La2O2Te) structure types, the chalcogen species occur as chalcogenide Q2– ions and are separated from each other by ∼4 Å in the [Ln2O2]2+ layers.

The expected oxidation state of S2– in Ln2O2S compounds can deviate by including mixed-valent Ce3+/4+. Upon exposure to air, S2– can also be oxidized to S4+ and S6+ species in Gd2(1–Ce2O2S nanoparticles.[20] This helps to illustrate the instability of many oxychalcogenides.

Monoatomic Anion and Dimers

α-La4O4Se3 contains both chalcogenide Se2– ions as well as Se species separated by only ∼2.45 Å;[15] this falls within the 2.3–2.5 Å range observed for (Se2)2– dimers in binary metal polyselenides.[21]

The structural chemistry of the chalcogen layers is not independent from that of the oxide layers. In the β-polymorph of the La4O4Se3 structure adopted by Eu4O4Se3, the staggered arrangement of (Se2)2– and Se2– ions, as shown in Figure , lowers the symmetry of the Eu sites with implications for its electronic structure.[22] Calculations of the partial density of states show that the Eu 4f, O 2p, and Se 4p orbitals dominate the valence band while the (Se2)2– and Eu 5d orbitals make up the conduction band. The separation of the (Se2)2– and Se2– p-orbitals raises the question of how structural diversity within the chalcogenide layers influence semiconducting behavior. Increased complexity is observed in the γ and δ structures, formed by Ln = Gd, Tb and Ln = Dy, Ho, Er, Yb, Y, respectively. The Se layer in both forms has been described as multiple chains of ordered (Se2)2– and Se2– that are arranged in a disordered zigzag wave of Se (Figure ).

Figure 2

Polymorph structures of Ln4O4Se3, with Ln, O, and Se ions shown in blue, pink, and yellow, respectively. View of Ln and Q relation along [001] shown below each polymorph structure.

Dimers Only

Chalcogenide dimer units are also observed in La2O2S2, whose chalcogen layer is composed solely of (S2)2– dimers. These dimers have a bond length of ∼2.1 Å, typical of S–S single bond.[23] The dimers in La2O2S2 can be exploited by topochemical reactions to give new materials (Figure ). Upon reaction of La2O2S2 with Rb metal, the (S2)2– dimers can be reduced, thereby deintercalating sulfur from the parent La2O2S2 compound.[24] The product of the deintercalation, coined oA-La2O2S (where oA refers to the orthorhombic Amm2 space group), has a distinct structure from La2O2S discussed previously. In oA-La2O2S, every other [La2O2]2+ layer is shifted along the 1/2(b + c) direction while the tetrahedral OLa4 units themselves are maintained. The formation of oA-La2O2S cannot be achieved by high-temperature methods because it is a metastable product with a relative energy of 72 meV/atom higher than that of the ground-state La2O2S (prepared by high-temperature routes).

Figure 3

Topochemical reactions of La2O2S2 yield oA-La2O2S and LaCuOS, with La, Cu, O, and S ions shown in blue, green, pink, and yellow, respectively.

Sulfur can be (re)intercalated into the structure by heating oA-La2O2S with elemental sulfur. Partial deintercalation of the (S2)2– dimers in La2O2S2 leads to La2O2S1.5, suggesting that topochemical routes may inspire the discovery of new oxychalcogenide phases.[24] Sulfur bonding proved to be a critical factor in optical behavior of the oxychalcogenides: the absorption edge (2.56 eV) arises from the π* → σ* electronic transition of sulfur pairs. Deintercalation of sulfur—removing the sulfur pairs—increases the absorption edge to 3.88 eV in oA-La2O2S. This is lower than the absorption onset of 4.13 eV observed for the thermodynamically stable hexagonal La2O2S, which arises from the S 3p → La 6s/5d transition associated with monatomic S2– in La2O2S.[24]

Quaternary Oxychalcogenides

OLn4 and OLn3M Tetrahedra in Quaternary Oxychalcogenides

The ternary Ln–O–Q oxychalcogenides discussed above contain edge-linked OLn4 tetrahedra to form 2D sheets, giving the layered crystal structures shown in Figure . The OLn4 building unit (Ln = lanthanide or Bi3+ cation) is also common in quaternary lanthanide oxychalcogenides. The addition of a second metal M (M = transition metal or p block cation) in quaternary oxychalcogenides adds diversity to the packing of O-centered tetrahedral units. Depending on the hardness (or softness) of the second metal, the two-dimensional [Ln2O2]+ sheet may be maintained, or discrete 2D [Ln2O2]2+ fragments (consisting of only three or four OLn4 units) or 1D ribbons of OLn4 tetrahedral units may be observed instead. The connectivity of these OLn4 units (and the possibility of including oxide ions in the coordination environment of the M cation giving OLn3M units) generates a huge diversity of structure types. This is perhaps best illustrated by the La–V–O–Se family of materials[25] (Figure ) in which the OLn4 units form 2D sheets of edge-connected tetrahedra as in La2O2S, 2D fragments (truncated by OLn3V tetrahedra) as in La7VO7Se5, and 1D ribbons of OLn4 and OLn3V tetrahedra as in La13V7O15Se16.

Figure 4

Structural units composed of tetrahedral OLa4 units among ternary oxychalcogenides and quaternary rare-earth vanadium oxyselenides, with La, V, O and Se ions shown in blue, cyan, pink, and yellow, respectively.

M–Q Bonding Motifs in Quaternary Oxychalcogenides

The relative softness of the M cation (often a transition metal) compared to a rare-earth metal is appropriate for the introduction of M–Q bonding in quaternary oxychalcogenides. This can be understood in terms of allowing ordering of the harder O2– anions (coordinated predominantly by hard Ln3+ cations), whereas the softer S2–/Se2– anions are often coordinated by Ln3+ and the softer second cation M.[26,27] The M–Q structures in these materials are very diverse, ranging from MQ4 tetrahedra to MOQ6– octahedra. The quaternary oxychalcogenides discussed in this mini-review are focused on M = transition metal; quaternary oxychalcogenides with M = p-block cation (e.g., Ga, Ge, As, In, Sn, Sb, Bi) exhibit interesting structures with separate O–Ln and M–Q sheets.[11,12]

Combined with the various structural motifs of OLn4 units, the structures of quaternary oxychalcogenides become quite varied and complex. This categorization of quaternary lanthanide oxychalcogenides in terms of the connectivity of OLn4 or OLn3M units (2D sheets, 2D fragments, and 1D ribbons) is helpful to illustrate structural diversity as well as the factors that influence the structure(s) adopted by a given composition.

2D Sheets in Quaternary Oxychalcogenides

Continuous two-dimensional [Ln2O2]2+ sheets of edge-shared OLn4 tetrahedra as observed in ternary oxychalcogenides (Figures and 2) are extended to quaternary oxychalcogenides including those of La2O2M2OS2,[28] ZrCuSiAs,[29] and cation-ordered ZrCuSiAs-related structural families. In these materials, [Ln2O2]2+ sheets are separated by chalcogenide-rich layers (of net negative charge), and the layered structures of these materials (Figure ) can have a significant role in determining physical properties.[30]

Figure 5

Quaternary Ln–M–O–Q oxychalcogenides with structures containing 2D [Ln2O2]2+ layers including (a) ZrCuSiAs structure for LnMOQ phases, (b) Ln2O2M2OQ2 phases, and (c) MQ2 layers (from above) in cation-ordered ZrCuSiAs-related phases Ln2O2MQ2 containing M2+ ions, with Ln, M, O, and Q ions shown in blue, cyan, pink, and yellow, respectively.

ZrCuSiAs structured materials are composed of alternating fluorite-like [Ln2O2]2+ layers and anti-fluorite-like [M2Q2]2– (M = transition metal) layers of edge-linked M+Q4 tetrahedra (e.g., LnCuOQ (Ln = Bi, lanthanide).[29] While most of these materials have been prepared by classical solid-state methods, it is possible to exploit (Q2)2– dimers in ternary oxychalcogenides to prepare quaternary materials. If empty σ* orbitals of (Q2)2– dimers become occupied, the Q–Q bond can be cleaved by reduction (Q2)2– + 2e– → 2Q2–. Subsequently, vacancies become available for the intercalation of metal atoms.

Upon heating with elemental copper, La2O2S2 undergoes a topochemical reaction to achieve LaCuOS.[31] [La2O2]2+ units remain intact, and the [La2O2]2+ layers are shifted by 1/2b. In addition, the S–S bonds within the (S2)2– dimers undergo rotation and tilt, thereby opening vacant tetrahedral sites. Cu(I) occupies these vacant sites, forming [Cu2S2]2– layers reminiscent of Cu–S layers in LaCuOS. The concomitant Cu insertion and breaking of (S2)2– dimers increases the band gap from 2.5 eV for La2O2S2 to 3.1 eV in LaCuOS.[18] An important aspect of this synthetic route is that the topochemical reaction occurs at 340 °C, while heating binary metal oxides, metal sulfides, and Cu together did not successfully yield the quaternary LaCuOS. This further emphasizes the need to exploit bonding of chalcogens to propel materials discovery.

If, on the other hand, classical solid-state synthetic routes (using La2O3, La2S3, Cu, and S) are used, LaCuOS and La5Cu6O4S7 can be achieved. The structure of La5Cu6O4S7 is related to the ZrCuSiAs-type. In La5Cu6O4S7, however, one of every five oxygen atoms in the [010] direction of the [La2O2]2+ layer is replaced with a sulfur atom, leading to fluorite-like [La5O4S]2+ layers (instead of [La2O2]2+).[32] This creates a quasi-1D chain of sulfur (Figure ). The intriguing consequence here is that the sulfur chains are characterized by split sites, forming (S2)2– dimers. The dimers serve an important role in the material’s intrinsic transparency and electrical conductivity.[33]

Figure 6

Comparison of LaCuOS and La5Cu6O4S7 crystal structures. Sulfur replaces every fifth oxygen in the [La2O2]2+ layer. La = blue, O = red, Ch = orange, Cu = green.

The LnCuOQ structure is conducive to electronic applications. The insulating [Ln2O2]2+ layer can be electronically doped, acting as charge-reservoir layers to the conductivity (or superconductivity) of the intervening chalcogenide layers,[34] and the layered nature of the material can lower the bandwidth of the chalcogenide conduction band.[30] It was recently shown that thin films of NdCuOS containing Cu deficiencies demonstrated an exceptional p-type conductivity (6.4 S·cm–1) and a transparency of ∼50%.[35]

The [Ln2O2]2+ layers, often containing fairly heavy cations such as Bi3+, can help reduce the thermal conductivity of these layered materials, enhancing their thermoelectric figure of merit ZT.[34] In the three-anion homologous series, Bi2+2O2+2Cu2−δSe2+Xδ (X = Cl, Br), the increasing number of n Bi2O2Se blocks results in lower band gaps, changes carrier type (from holes to electrons), and reduces thermal conductivity. Cu vacancies are stabilized by halide substitution of the Se atom.[36,37]

Cation-ordered ZrCuSiAs-related materials are like the ZrCuSiAs-structured materials above. When the M site is occupied by M2+, M sites are half-occupied in an ordered checkerboard (e.g., La2O2CdSe2 in ref (22)), stripe fashion (oI polymorphs of Ln2O2MnSe2 and Ln2O2FeSe2 (Ln = La, Ce)[38,39]) or intermediate ordering patterns (Ln2O2MSe2 (Ln = La, Ce; M = Mn, Fe, Zn)[40−42]) (Figure c).

Materials that possess a structure like La2O2Fe2OQ2 comprised [Ln2O2]2+ layers and [Fe2O]2+ layers, separated by Q2– ions. The Fe2+ cations are coordinated by both O2– and Q2– anions (forming face-linked FeO2Q4 octahedra), and the presence of O2– in the Fe2+ coordination environment contributes to the band narrowing (and Mott insulating nature) of these materials.[43] The magnetic structure of La2O2Fe2OQ2 is stabilized by antiferromagnetic Fe–O–Fe stripes, which are coupled by ferromagnetic Fe–Se–Fe interactions.[44,45] Short-range orthorhombic distortions associated with Fe and O were found in La2O2Fe2OSe2 with neutron pair distribution function analysis. Such short-range distortions may play an important role in Fe-based superconductivity.[46]

2D Fragments in Quaternary Oxychalcogenides

Rather than continuous 2D [Ln2O2]2+ layers, discrete 2D [Ln2O2]2+ fragments may form, often only three or four OLn4 units long. In contrast to the 2D phases (in which O2– anions only coordinate Ln3+ cations), in these fragment phases, the harder O2– anion also forms part of the coordination sphere of the second cation (Figure ), giving both OLn4 and OLn3M tetrahedra. This might be expected for systems in which the second cation is of intermediate hardness. The Gd4O4TiSe4 family of materials[40,47] illustrates this, with small and highly charged Ti4+ cations coordinated by both Se2– and O2– anions forming TiO2Se4 octahedra. The octahedra break up the [Gd2O2]2+ units into bands of OGd4 or OGd3Ti units (four units wide) that extend along the [010] direction.

Figure 7

Quaternary Ln–M–O–Q oxychalcogenides with structures containing 2D [Ln2O2]2+ fragments including (a) Gd4O4TiSe4 structure, (b) β-Ln2O2MSe2, (c) La4O4MnSe3, and (d) La6O6MnSe4 with Ln, M, O, and Q ions shown in blue, cyan, pink, and yellow, respectively.

La4O4MnSe3 is closely related to the Gd4O4TiSe4 structure but contains fewer Se2– anions as Ti4+ is replaced with Mn2+. For Gd4O4TiSe4, the Mn–Se layers are separated by bands of OLa4 or OLa3Mn units (four units wide).[25] La6O6MnSe4 is an extension of the La4O4MnSe3 structure but with wider six-unit wide OLa4/OLa3Mn bands separating the Mn–Se layers.[25] In β-La2O2MSe2 (oP-La2O2MSe2) (M = Mn, Fe)[38,48] two divalent cations (Fe2+ or Mn2+) replace Ti4+, forming a tetrahedral MSe4 site in addition to the octahedral MSe4O2 site in the M–Se layers. These M–Se layers are separated by [La2O2]2+ bands four units wide (as in Gd4O4TiSe4). Although the compositions of these phases are the same as the cation-ordered ZrCuSiAs-related phases, their structures are quite different. The polymorphism is influenced by composition and reaction temperature.[38] La7O7VSe5 (Figure ) is composed of V3+ cations in VSe4O2 octahedra, which break the [Ln2O2]2+ building units into bands of OLa4 and OLa3V units (seven units long) which extend along [100].[25]

Considering the connectivity of the [Ln2O2]2+ units gave us a way to explore and understand the structural chemistry of the quaternary oxychalcogenides, but often the physical properties of the material rely on the coordination of the M cation and the connectivity of its sublattice. Long-range magnetic order results from (often indirect) exchange interactions between magnetic ions, and so the [Ln2O2]2+ units can influence the magnetic ordering by breaking up magnetic exchange pathways and by separating magnetic layers/chains. Perhaps this is best illustrated by considering β-La2O2MnSe2 and the La2O2MnSe series. The n = 0 member, β-La2O2MnSe2 (Figure b), is built from Mn–Se magnetic layers separated by [Ln2O2]2+ fragments (separation ∼8.8 Å) and orders antiferromagnetically below TN = 27 K.[48] The analogous Mn layers in the n = 1 member of the series, La4O4MnSe3, are separated by ∼7 Å, but the Mn cations are separated into pseudo-1D chains (rather than the 2D layers of β-La2O2MnSe2).[49] The decreased exchange interactions within the magnetic layers are likely to contribute to the reduced TN = 15 K. With increasing n (i.e., from n = 0 mC-La2O2MnSe2, n = 1 La4O4MnSe3, and n = 2 La6O6MnSe4), the distance between the layers containing the magnetic chains increases. This increased separation might explain the increasingly broad magnetic phase transitions in this series.[25]

1D Ribbons in Quaternary Oxychalcogenides

As the hardness of a second M cation in a quaternary oxychalcogenide increases, its bonding preferences become more like those of the Ln3+ cation. The segregation of O2– and Q2– anions in the structure is reduced, and OLn4 units are typically less extensive. Several quaternary oxychalcogenides containing +3 cations (e.g., Cr3+, V3+) contain bands of only two OLn3M units wide, forming one-dimensional “ribbons” that separate M cations (Figure ).

Figure 8

Quaternary Ln–M–O–Q oxychalcogenides with structures containing 1D [LnO]+ ribbons including (a) CeCrOQ2, (b) LaCrOS2, and (c) Ln2O2CrSe2 with Ln, M, O, and Q ions shown in blue, cyan, pink, and yellow, respectively.

LnCrOSe2 (Ln = Pr, Nd; Q = S, Se)[50] comprises corner-linked chains of edge-shared CrS6 and edge-sharing CrO2S4 octahedra separated by ribbons of two OLn3Cr tetrahedra. LnCrOQ2 (Ln = Ce–Nd; Q = S, Se) are isostructural to LaVOSe2[25] (Figure ).

Ln2O2CrSe2 adopts structures closely related to CeCrOS2 but with CrO2Se4 chains (instead of CrS6 octahedra) separated by ribbons of two OLn3Cr tetrahedra.[51] Hysteretic structural phase transitions are associated with second-order Jahn–Teller distortions of the Cr2+ (d4) ions. Low-temperature neutron diffraction studies show that these materials undergo magnetic transitions associated with both Ln3+ and Cr2+ ions.

In La5V3O7Se6, edge-sharing VSe4O2 octahedra are separated by meandering ribbons of corner and edge-sharing OLa4 and/or OLa3V tetrahedra. Two different vanadium sites, V1 and V2, are assigned formal oxidation states of +4 and +3, respectively. In La13V7O15Se16, very curvy ribbons of corner and edge-sharing OLa4 and/or OLa3V tetrahedra alternate with ribbons like those seen in La5V3O7Se6 along [010]. The ribbons are linked along [010] at their curves by OLa3V tetrahedral and VSe4O2 octahedral units repeating along [100]. Perpendicular to the ribbons are one-dimensional strands of isolated Se atoms along [100]. La13V7O15Se16 is also mixed valent with V3+ and V5+ cations. Magnetic ordering of the vanadium moments is observed at low temperatures in these quaternary lanthanum vanadium oxyselenides.[25]

A–O–M–Q Quaternary Oxychalcogenides (A = Group 1 A+ or Group 2 A2+ Cations)

Whereas the OLn4 structural unit is ubiquitous in lanthanide quaternary oxychalcogenides, the analogous OA4 (A = s-block cation) is less common. Cations from groups 1 and 2 are typically softer than Ln3+ cations with hardness more comparable to that of M cations.[27] As a result, A and M cations may favor more similar coordination environments, making discrete [AO] units less likely and giving a greater diversity in structural chemistry. This extends the trend described above for Ln–M–O–Q systems with decreasing connectivity of [Ln2O2]2+ units as the hardness of M cations increases. This preference for the softer chalcogenide anions to play a greater role in A cation coordination as the A cations get softer is illustrated by considering families of quaternary AMOS phases (A = Sr, Ba) (Figure ).

Figure 9

Quaternary A–M–O–Q oxychalcogenides with structures including (a) AMSO (A = Ca, Sr; M = Fe, Co, Zn) and (b) BaMSO (M = Co, Zn) with A, M, O, and Q ions shown in blue, cyan, pink, and yellow, respectively.

SrMOS and CaMOS (M = Fe, Co, Zn)[52−55] form polar structures composed of layers of edge-linked MOS3 tetrahedra that are coaligned with each other. The A2+ cations are coordinated to O2– anions. On the other hand, BaCoOS[53] containing larger Ba2+ cations (of comparable hardness to the M2+ cations) adopt nonpolar structures built up from corner-linked MO2S2 tetrahedra which allow the Ba2+ cations to be simultaneously coordinated by O2– and S2– anions.

Summary

This mini-review presents the structural diversity that has been recently achieved with ternary and quaternary rare-earth oxysulfides and oxyselenides. The OLn4 tetrahedra are a recurring structural unit in these materials, and variations of these tetrahedra and chalcogen layers diversify structural dimensionality and connectivity. Whether by elemental substitution, modification of dimensionality of OLn4, OLn3M, and Q structural motifs, or rearrangement of structural blocks, the crystal structures of oxychalcogenide materials can be tuned to control electrical and magnetic properties. Continued creative synthetic strategies (outside of conventional solid-state synthesis) and high-throughput screening may help to overcome challenges associated with the synthesis of new ternary, quaternary, and even quinary oxychalcogenides[56] for applications in thermoelectrics, transparent conducting materials, and superconductors.