Ceramics of the composition: Ln.sub.1-xSr.sub.yCa.sub.x-yMO.sub.3-.delta. where Ln is an element selected from the f block lanthanides and yttrium or mixtures thereof; M is an element selected from the d block transition metals or mixtures thereof; 0.1.ltoreq.x.ltoreq.0.4; 0.01.ltoreq.y.ltoreq.0.25 and .delta. is a number that varies to maintain charge neutrality are provided along with methods of their use. These ceramics are useful in making and using gas-impermeable ceramic membranes that exhibit high electronic and ionic conductivity, low coefficients of expansion and high chemical and thermal stability under catalytic membrane reactor conditions. The ceramics provided are particularly useful for promotion of oxidation-reduction reactions and for separating molecular oxygen from oxygen containing gases.