Solubility studies, thermodynamics and electrical conductivity in the Th1−xSrxO2 system

A polymeric gel combustion method was employed for the lower temperature synthesis of 1, 2, 3, 5, 7.5, 10 and 15 mol.% SrO doped ThO2 solid solutions. After final sintering of these samples at 1573 K in Ar, the solubility limit was found to be 1 mol.%, giving rise to a composition-independent value of (559.84±0.02) pm (at 300 K) for the CaF2-type unit cell. AC impedance measurements carried out on discs of 1, 2, 3 and 5 mol.% SrO in ThO2 discs in flowing Ar, yielded values of 116, 115, 128 and 96 kJ/mol, respectively as the activation energies for the bulk ionic conductivity. The thermodynamic activities of SrO in the 1, 2 and 3 mol.% solid solutions were measured using [SrO]ThO2,SrF2,O2,Pt as the test electrodes and SrZrO3,ZrO2,SrF2,O2,Pt as the reference with sintered SrF2 as the electrolyte over the range of approximately 650–800 K. The activity values differed marginally from each other, but were indicative of the combined (and not free) state of SrO. Hence, only a limiting value of aSrO for the just saturated (mole fraction of SrO=0.01) thoria solid solution could be derived as log aSrO=0.36−4602/T (K).