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+2O(2) at modestly increasing temperatures centered similar to 550 degreesC 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 O-2 along the decomposition curve was determined. This volume (similar to 10 cm(3)/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 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.
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