Extreme electron-phonon coupling in magnetic rubidium sesquioxide

Rb$_2$O$_3$ is a black, opaque oxide. Early work suggests that the stability range of the sesquioxide phase in the rubidium-oxygen phase diagram is rather broad. Rb$_2$O$_3$ remains cubic down to the lowest temperature measured (5~K). The oxygens form dumbbells with interatomic distances in between those of peroxide and superoxide anions, and strong athermal motion persists down to low temperatures. [1] Electronic-structure calculations show that the dynamics at low temperature is caused by 6 phonon modes of zero frequency, which induce a very strong electron-phonon interaction. The softness of half of these modes is suppressed by the application of pressure. Calculated using the average oxygen positions, rubidium sesquioxide is a half-metallic ferromagnet. [2] \newline [1] CR CHIM (11-13): 591-594 NOV 1999\newline [2] JACS 127 (46): 16325-16328 NOV 23 2005