Synthesis and thermal decomposition of tetragonal RbClO 4 and volume of fluid O 2 from 2 to 9 GPa

Orthorhombic (O) RbClO 4 transforms to a new tetragonal (T) form at pressures above 20 kbar. Thermal decomposition proceeds through the reversible equilibrium reaction T RbClO 4 = B2 RbCl + 2O 2 at modestly increasing temperatures centered ∼550°C over the pressure range investigated from 20 to 90 kbar. Cell volumes for T RbClO 4 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 O 2 along the decomposition curve was determined. This volume (∼10 cm 3 /mol) is in close agreement with volumes determined from studies of the thermal decomposition of KClO 3 and KClO 4 but is difficult to reconcile with fluid O 2 volumes determined using shock wave techniques. If these new O 2 partial molar volumes are relevant to molten iron solutions, then storage of substantial amounts of O 2 in the core becomes feasible.