title: Anionogenic mixed valency in KxBa1-xO2-δ.
abstract: We have synthesized members of an isostructural solid solution series KxBa1-xO2-δ (x < 0.41, δ < 0.11) containing mixed-valent dioxygen anions. Synthesis in liquid ammonia solution allows a continuous range of compounds to be prepared. X-ray and neutron diffraction show that KxBa1-xO2-δ adopts the tetragonal rocksalt-derived structure of the end members KO2 and BaO2, without any structural phase transition down to 5 K, the lowest temperature studied here. We identify four oxygen-oxygen stretching modes above 750 cm(-1) in the measured Raman spectra, unlike the spectra of KO2 and BaO2 which both contain just a single mode. We use density functional theory calculations to show that the stretching modes in KxBa1-xO2-δ arise from in-phase and anti-phase coupling of the stretching of nearest-neighbor oxygen dimers when the valence state of the dimers lies between -1 and -2 because of mixed cation coordination. This coupling is a direct signature of a novel type of anionogenic mixed valency.
authors: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors: Shivakumara
authors: S
authors: Giriyapura
authors: Baomin
authors: B
authors: Zhang
authors: Robert A
authors: RA
authors: de Groot
authors: Gilles A
authors: GA
authors: de Wijs
authors: Antonio
authors: A
authors: Caretta
authors: Paul H M
authors: PH
authors: van Loosdrecht
authors: Winfried
authors: W
authors: Kockelmann
authors: Thomas T M
authors: TT
authors: Palstra
authors: Graeme R
authors: GR
authors: Blake