Abstract: Nitric acid is added to an aqueous solution of organic acid salts of metals to be included in a resulting complex oxide powder, into which an aqueous solution of an inorganic salt of other metal component has been incorporated according to need, and the resulting solution is heated until the organic acid salts are decomposed and the decomposition product is heated, whereby a intended complex powder which is very fine and highly bulky and has a large specific surface area is obtained.

Abstract: A sensor element for detecting smoke and/or reducing gases, which comprises a shaped body formed from a material consisting essentially of:A. a complex metal oxide having a perovskite type crystal structure and represented by the general formula ABO.sub.3-.delta. wherein A is at least one element selected from the group consisting of yttrium, a rare earth element having an atomic number of from 57 to 71 and an alkaline earth metal, B is at least one element selected from the group consisting of transition metals having atomic numbers from 21 to 30, O is oxygen and .delta. is a nonstoichiometric parameter which may vary from 0 to 0.25 andB. at least one metal oxide selected from the group consisting of CdO, In.sub.2 O.sub.3, SnO, Tl.sub.2 O.sub.3 and PbO.

Abstract: A gas-sensor element for detecting oxidizable gases and vapors such as alcohols for carbon monoxide, which is characterized by comprising a complex metal oxide having the K.sub.2 MgF.sub.4 -type crystal structure and represented by the general formula A.sub.2-x A'.sub.x BO.sub.4-.sub..delta., wherein A is at least one element selected from the group consisting of rare earth elements of the atomic numbers from 57 to 71, yttrium, and hafnium, A' is at least one element selected from the group consisting of alkaline earth metals and lithium, B is at least one element selected from the group consisting of transition elements of the atomic numbers from 21 to 30, 0 is oxygen, x is in the range of 0.ltoreq.x.ltoreq.2, and .delta. is a nonstoichiometric parameter.

Abstract: A gas-sensor element for detecting reducing gases and vapors such as alcohols or carbon monoxide, or for determining oxygen concentration, which is characterized by comprising a complex metal oxide having a perovskite-type crystal structure and represented by the general formula A.sub.1.sub.-x A'.sub.x BO.sub.3.sub.- .sub..delta., wherein A is at least one element selected from the group consisting of rare earth elements of the atomic numbers from 57 to 71, yttrium, and hafnium, A' is at least one element selected from the group consisting of alkaline earth metals and lithium, B is at least one element selected from the group consisting of transition metals of the atomic numbers from 21 to 30, O is oxygen, x is in the range of 0 .ltoreq. x .ltoreq. 1, and .delta. is a nonstoichiometric parameter.