Synthesis and thermal decomposition of tetragonal RbClO₄ and volume of fluid O₂ from 2 to 9 GPa

Orthorhombic (O) RbClO₄ transforms to a new tetragonal (T) form at pressures above 20 kbar. Thermal decomposition proceeds through the reversible equilibrium reaction T RbClO₄ = B2 RbCl + 2O₂ at modestly increasing temperatures centered ∼550°C over the pressure range investigated from 20 to 90 kbar. Cell volumes for T RbClO₄ and B2 RbCl were determined by high-pressure, high-temperature, in situ X-ray diffraction on station 16.4 of the Daresbury Synchrotron Radiation Source, from which the partial molar volume of O₂ along the decomposition curve was determined. This volume (∼10 cm³/mol) is in close agreement with volumes determined from studies of the thermal decomposition of KClO₃ and KClO₄ but is difficult to reconcile with fluid O₂ volumes determined using shock wave techniques. If these new O₂ partial molar volumes are relevant to molten iron solutions, then storage of substantial amounts of O₂ in the core becomes feasible.