Recently discovered alongside its sister compounds KV3Sb5 and RbV3Sb5, CsV3Sb5 crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV3Sb5, demonstrating bulk superconductivity in single crystals with a Tc = 2.5 K. The normal state electronic structure is studied via angleresolved photoemission spectroscopy (ARPES) and density functional theory (DFT), which categorize CsV3Sb5 as a Z2 topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (EF ), and signatures of normal state correlation effects are also suggested by a high temperature charge density wave-like instability. The implications for the formation of unconventional superconductivity in this material are discussed.

There has been a great deal of recent interest in extended compounds containing Ru3+ and Ru4+ in light of their range of unusual physical properties. Many of these properties are displayed in compounds with the perovskite and related structures. Here we report an array of structurally diverse hybrid ruthenium halide perovskites and related compounds: MA2RuX6 (X=Cl or Br), MA2MRuX6 (M=Na, K or Ag; X=Cl or Br) and MA3Ru2X9 (X=Br) based upon the use of methylammonium (MA=CH3NH3+) on the perovskite A site. The compounds MA2RuX6 with Ru4+ crystallize in the trigonal space group R3‾m and can be described as vacancy‐ordered double‐perovskites. The ordered compounds MA2MRuX6 with M+ and Ru3+ crystallize in a structure related to BaNiO3 with alternating MX6 and RuX6 face‐shared octahedra forming linear chains in the trigonal P3‾m space group. The compound MA3Ru2Br9 crystallizes in the orthorhombic Cmcm space group and displays pairs of face‐sharing octahedra forming isolated Ru2Br9 moieties with very short Ru–Ru contacts of 2.789 Å. The structural details, including the role of hydrogen bonding and dimensionality, as well as the optical and magnetic properties of these compounds are described. The magnetic behavior of all three classes of compounds is influenced by spin–orbit coupling and their temperature‐dependent behavior has been compared with the predictions of the appropriate Kotani models.

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Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of Ru IV halides are presented; A 2 RuCl 6 and A 2 RuBr 6 , where A is K, NH 4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with J eff = 0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion. Remarkable developments in perovskite-based photovoltaics over the last decade have driven the discovery of a wide range of new halide perovskites and related solids. [1-4] These include 3D perovskites with different divalent metals (i.e., Pb II and Sn II), [5, 6] 3D double perovskites with a combination of univalent and trivalent metals (i.e., K I /Bi III and Ag I / Bi III), [7, 8] as well as low dimensional 2D, [9] and 1D perovskites. [10] Progress in this area has also rekindled interest in K 2 Pt IV Cl 6-type vacancy-ordered double perovskites, comprising isolated metal halide octahedra interspersed with mono-valent cation. For example, the vacancy-ordered halides of Sn IV , [11] Se IV , [12] Te IV , [13, 14] and Ti IV , [15] have shown to be promising photovoltaic materials. The analogous variants of