The Subchalcogenides Ir₂In₈Q (Q = S, Se, Te): Dirac Semimetal Candidates with Re-entrant Structural Modulation

Abstract

Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors, and can have unexpected structures and properties because of the exotic nature of their chemical bonding, as they contain both metal-metal and metal-main group (e.g. halide, chalcogenide) interactions.Finding new members of this class of materials presents synthetic challenges, as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (mm scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir 2 In 8 Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir 2 In 8 S analogue.Ir 2 In 8 Se and Ir 2 In 8 Te crystallize in the P4 2 /mnm space group and are isostructural to Ir 2 In 8 S but also have commensurately modulated (with q-vectors q = 1/6a* + 1/6b* and q= 1/10a* + 1/10b* for Ir 2 In 8 Se and Ir 2 In 8 Te, respectively) low temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level, and exhibit high electron mobilities (~1500 cm 2 V -1 s -1 at 1.8 K) and moderate carrier concentrations (~10 20 cm -3 ) from charge transport measurements. This work highlights metal flux as a powerful synthetic route to high quality single crystals of novel intermetallic subchalcogenides.

Chemical bonds in topological materials

“…Examples of quasiparticles are holes, phonons, and excitons. [28][29][30][31][32][33][34][35][36][37][38][39]. As Roald Hoffmann stated in his review article 'How chemistry and physics meet in the solid state', chemists must 'learn the language of solid-state physics' to understand the rich properties of extended structures, and we refer readers to this article for greater understanding of linking crystal and electronic structure [13].…”

A Class of Magnetic Topological Material Candidates with Hypervalent Bi Chains

“…Therefore, it is crucial to provide direction for finding new materials by focusing on chemical features and heuristics in which previously overlooked topological candidates can be investigated. 1,4,[31][32][33][34][35][36][37] Hypervalent chemical bonds have been identified as one key feature for discovering new topological materials. 1,4 They are electron-rich, multicenter bonds where charge density is delocalized along a molecule or structural motif.…”

High pressure structural and magneto-transport studies on type-II Dirac semimetal candidate Ir2In8S: Emergence of superconductivity upon decompression

“…Subchalcogenide compounds with diverse structural motifs have recently been found to be promising topological semimetal candidates. 2,[42][43][44] The sub-valent metallic character in related compounds also leads to competing electronic phases like charge density wave (CDW) and superconductivity. 45,46 Recently, type-II Dirac semimetal candidate subchalcogenides Ir 2 In 8 Q (Q=S, Se, Te) have been grown successfully and are also found to exhibit sig-nificantly large and anisotropic MR (qualifying the ultrahigh mobility criteria).…”

“…45,46 Recently, type-II Dirac semimetal candidate subchalcogenides Ir 2 In 8 Q (Q=S, Se, Te) have been grown successfully and are also found to exhibit sig-nificantly large and anisotropic MR (qualifying the ultrahigh mobility criteria). 43,44 Ir 2 In 8 Q compounds crystallize in tetragonal P 4 2 /mnm space group with 3D framework of IrIn 8 polyhedra with chalcogen atoms in the channels along the c-axis. No superconductivity has been reported in this series of compounds.…”

“…This also reduces the low temperature magnetoresistance. 44 As the effect of pressure is expected to be similar to that of decreasing atomic radii, high pressure study on these compounds may reveal competing electronic phases, including superconductivity. Also the band structure calculations show that the two sets of Dirac points in Ir 2 In 8 S are situated ∼25 and ∼40 meV above the Fermi level.…”