Anionogenic Mixed Valency in K_xBa_1–xO_2−δ

Abstract

We have synthesized members of an isostructural solid solution series K x Ba 1−x O 2−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 K x Ba 1−x O 2−δ adopts the tetragonal rocksalt-derived structure of the end members KO 2 and BaO 2 , 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 KO 2 and BaO 2 which both contain just a single mode. We use density functional theory calculations to show that the stretching modes in K x Ba 1−x O 2−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.

An Ionic Liquid State Composed of Superoxide Radical Anions and Crownether-Coordinated Potassium Cations

“…[11][12][13][14][15] These counter cations have small ionic radii and high charge densities, i.e. they were "hard acids" to give strong cation-anion interactions and melting points much higher than RT.…”

Molecular Oxygen-Induced Ferromagnetism and Half-Metallicity in α-BaNaO4: A First-Principles Study

“…A weak FM was reported in Ba1-xKxO2 for x=0.269 and a spin-glass behavior for other K doping levels, which were ascribed to the coexistence and the competition between AFM and FM. 14,15 Partial oxygen deficiency at x=0.28 in RbO2-x was found to result in a transition from AFM to spin glass, and presumably there are short-range FM ordering in clusters at ~50 K. 16 Theoretically, the electrons in the π * orbitals of O2 dimers can be better treated as a strong correlated system. 17,18 Naghavi et al predicted that exertion of pressure would make Rb4O6 transform from AFM insulator to FM insulator then to FM half-metal, which is a consequence of Mott transition.…”

2p -insulator heterointerfaces: Creation of half-metallicity and anionogenic ferromagnetism via double exchange

“…Compared to the mixed-valent sesquioxides with insulating spin-glass character [18], the rough KO 2 /BaO 2 (001) interface has several advantages. First, the strength of double exchange at the interface can be tuned by the concentration of defects, which might be controllable during interface fabrication.…”

“…Dioxygenyl salts are Mott insulators [3], metal superoxides are correlated materials serving as suitable model systems for studying intrinsic 2p magnetism, and metal peroxides are nonmagnetic due to the pairing of electrons in the antibonding π * orbital [4]. In the past decade, much progress on this class of insulating materials has been achieved, including studies on correlation effect [5][6][7][8], 2p orbital ordering [9][10][11][12], lowdimensional magnetism [13][14][15], short-range ferromagnetism [16], frustrated magnetism [6,[17][18][19], interplay of spin, charge, orbital and lattice degrees of freedom [12,[20][21][22][23][24], and halfmetallicity at high pressure [25]. However, the realization of metallicity and long-range ferromagnetism at ambient pressure are still lacking, and they remain central topics in this field.…”

“…Consequently, we might expect that the simultaneous hopping (with special spin selection due to Pauli exclusion principle) of conducting electrons from O 2− 2 to O − 2 can also render the exchange between anions ferromagnetic in the mixed-valent alkali (A) metal sesquioxides A 2 O 3 . However, experimental studies of these compounds have thus far found only spin-glass states with insulating characters [5,[17][18][19].…”