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

La2O2MQ2 phases: stability and synthetic challenges

Oxychalcogenides containing transition metal or p block cations have potential for thermoelectric, photocatalytic and magnetic applications but the synthetic pathways to these quaternary phases are not fully understood. This presents a challenge to the design and preparation of new functional materials. Our combined experimental and computational study of La2O2MQ2 (M = +2 cation; Q = sulfide, selenide anion) systems explores the thermodynamic constraints on synthesis and highlights the subtle balance in stabilities of phases formed via competing reaction pathways. 1) Introduction Designing functional materials to meet the demands of current innovations is challenging and often requires complex combinations of physical properties, such as the high electrical conductivity and low thermal conductivity for thermoelectrics, 4 or specific symmetry and band gap requirements for new photovoltaics 5 and photocatalysts. 6 The opportunities offered by mixed-anion materials have motivated researchers to explore more diverse compositions and the additional degrees of freedom from multiple anions can tune the structure and symmetry, the band gap and therefore physical properties. 7, 8 This makes for exciting opportunities in functional materials design, but also presents difficulties in preparing these materials; the synthetic routes to these, often metastable, phases, are not as well understood as for oxides and other homo-anionic materials. Oxychalcogenides, containing both oxide (O 2-) as well as larger sulfide, selenide or telluride (S 2-, Se 2-or Te 2-) anions are gaining increased attention for their thermoelectric, 3, 9 non-linear optical 10, 11 and photocatalytic properties. 12-14 Recent research has highlighted new synthesis routes 1, 15, 16 prompting us to explore the synthesis of some M 2+ oxychalcogenides in this work. Our initial motivation was to explore oxysuflides containing ns 2 lone pair cations (such as 5s 2 Sn 2+ and Sb 3+ cations), given the promising electronic structure of several Sb 3+ containing phases for possible photocatalytic applications. 17-20 This led us to consider the electronic structure of several Sn 2+ oxysulfides for possible photocatalytic applications (see supporting information) and their synthesis, alongside analogues containing the 3d 10 Zn 2+ cation for comparison. The difference in size and charge between the oxide and sulfide/selenide/telluride anions favours anion-ordered structures for many oxychalcogenides which are often layered. 3, 21 A frequently observed structural motif in this family is the fluorite-like [Ln2O2] 2+ layer, composed of edge-linked OLn4 tetrahedra, 22 as found in the ZrCuSiAs structure, adopted by thermoelectric BiCuOSe 23-25 (and related phases 26, 27 including LaCuOS 28) and the isostructural iron oxyarsenides known for their superconductivity, as illustrated in Figure 1. 29, 30 Oxychalocgenides of general formula Ln2O2MQ2 (Q = S, Se) containing M 2+ cations can adopt ZrCuSiAs-derived structures with half-occupied cation sites within the anti-fluorite-like M-Q layers. These cation sites may be occupied in a disordered fashion (as originally reported for CeOMn0.5Se) 31 , or might order in a checkerboard arrangement (Figure 1b), 32-34 into stripes (Figure 1d), 35, 36 or an intermediate structure containing both checkerboard and stripe fragments (Figure 1c), 37 or more complex ordered arrangements. 38-41 These fluorite-like [Ln2O2] 2+ layers are also observed in ternary oxychalcogenides including the metastable oA polymorph of La2O2S which is formed from the topochemical anion deintercalation reaction of La2O2S2 (Figure 1e). 42 La2O2S2 is interesting in that the [La2O2] 2+ layers are separated by layers containing (S2) 2-dimer anions 43, 44 leading to the possibility of topochemical redox reactions involving either the intercalation of a metal cation, or the deintercalation of sulfur. 2, 45 The structural chemistry of Ln2O2MQ2 (Q = S, Se) materials is rich with several different structure types (and polymorphism for several compositions) reported; 46-48 the possibility of targeting metastable phases and structures by topochemical intercalation reactions (that may not be accessed by hig