This invention relates to a new group of complex ceramic oxides and a process for their production. This group of novel complex ceramic oxides are of the general formula Mg2MM'06+X, where M = Rare Earth metal ion sor Yttrium or Lanthanum and M' = Sn, Sb, Zr, Hf, Ta, and Nb; and where - 0.5x+0.5; and have a defective pyrochlore structure. Minor variations in the compositions up to 8% in any of the metal ions may be tolerated for the formation of the required phase. The process for the preparation of the novel class of compounds comprises: (a) Mixing the compounds of magnesium, M and M' (where M may be Yttrium, Lanthanum or any of the Rare Earth metals; and M'= Antimony, Tin, Zirconium or tantalum) in such a way as to get the molar ratio of Mg:M:M' as 2:1:1 required for the formation of the compounds of the general formula Mg2MM'06+X. Minor variations in the compositions up to 8% in one metal ions may be tolerated for the formation of the required phase. (b) The compounds employed in step (i) may be selected from their oxides, carbonates, chlorides, alkoxides, nitrates, sulfates etc.; preferably the salts used are selected from oxides or carbonates. (c) The mixture obtained in step(i) may be ball milled or mixed with a wetting medium. The wetting medium may be water, alcohol, acetone or any other organic solvent. The resultant ball milled slurry may be dried to remove the liquid and obtain dry powder, (d) Heating the resultant mixture to a temperature in the range 1000-1600°C, as required for different duration ranging from 3 hours to 50 hours, either in a single step or by taking out the reactant after few hours of heating, checking for phase formation and heating it again after grinding, if necessary. Up to a maximum of 5 intermediate grindings may be required.
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