Materials with a perovskite structure in form of solid solutions with general formula: A.sub.zZr.sub.1-xB.sub.xO.sub.3 Where A is Ba or a rare earth element, B is Pt, Ir, Rh or Ce z is 1 when A is Ba and is 2/3 when A is a rare earth, x is in the range 0.01 and 0.8.