title: Mixing unmixables: Unexpected formation of Li-Cs alloys at low pressure.
abstract: Contrary to the empirical Miedema and Hume-Rothery rules and a recent theoretical prediction, we report experimental evidence on the formation of Li-Cs alloys at very low pressure (>0.1 GPa). We also succeeded in synthesizing a pure nonstoichiometric and ordered crystalline phase from an approximately equimolar mixture and resolved its structure using the maximum entropy method. The new alloy has a primitive cubic cell with the Li atom situated in the center and the Cs at the corners. This structure is stable to at least 10 GPa and has an anomalously high coefficient of thermal expansion at low pressure. Analysis of the valence charge density shows that electrons are donated from Cs to the Li "p"-orbitals, resulting in a rare formal oxidation state of -1 for Li. The observation indicates the diversity in the bonding of the seeming simple group I Li element.
authors.affiliation: Laboratoire de physique des solides denses, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Serge
authors.initials: S
authors.lastname: Desgreniers
authors.affiliation: Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5B2, Canada.
authors.forename: John S
authors.initials: JS
authors.lastname: Tse
authors.affiliation: SPring-8/JASRI, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan. ; Department of Materials Science and Technology, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Matsuoka
authors.affiliation: SPring-8/JASRI, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohishi
authors.affiliation: Robarts Research Institute, Western University, London, Ontario N6A 5B7, Canada.
authors.forename: Justin J
authors.initials: JJ
authors.lastname: Tse
keywords: chemical bonding; crystallography; electron density; high pressure; maximum entropy method; metallic alloy