Mixed conducting ion transport membrane comprising a multi-component metallic oxide compound represented by the formula Ln.sub.xA'.sub.x'A''.sub.x''B.sub.yB'.sub.y'O.sub.3-z wherein (a) Ln is an element selected from the f block lanthanides, A' is selected from Group 2, A'' is selected from Groups 1, 2 and 3 and the f block lanthanides, and B and B' are independently selected from the d block transition metals, excluding titanium and chromium, wherein 0.ltoreq.x<1, 0<x'.ltoreq.1, 0.ltoreq.x''<1, 0<y<1.1, 0.ltoreq.y'<1.1, x+x'+x''=1.0, 1.1>y+y'.gtoreq.1.0 and z is a number which renders the compound charge neutral, and (b) the average grain size of the multicomponent metallic oxide is in the range of about 4 .mu.m to about 20 .mu.m.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with government support under Cooperative Agreement No. DE-FC26-97FT96052 between Air Products and Chemicals, Inc. and the U.S. Department of Energy. The Government may have certain rights to this invention.