Provided is a intricate oxide that has a high hydrogen content, comprises almost no impurity stage, and is acceptable for proton conductivity. The intricate oxide is represented with a compound formula Li.sub.7-xH. Sub.xLa.sub.3M.sub.2O.sub.12 (M signifies Zr and/or Hf, and 3.2





As a proton-conductive complicated oxide to be used in a gas cell, Li.sub.1.7H.sub.5.05La.sub.3Nb.sub.0.25Zr.sub.1.75O.sub.12 has been understood (Non-Patent Document 1). Li.sub.1.7H.sub.5.05La.sub.3Nb.sub.0.25Zr.sub.1.75O.sub.12 is prepared by washingLi.sub.6.75La.sub.3Nb.sub.0.25Zr.sub.1.75O.sub.12 having great lithium conductivity using water, subjecting the complex oxide to vacuum drying, also subjecting the complex oxide into an annealing treatment at 150. degree. C.

However, regarding the crystal structure of Li.sub.1.7H.sub.5.05La.sub.3Nb.sub.0.25Zr.sub.1.75O.sub.12, just the lattice constant was reported and also a correlation between a hydrogen amount and proton conductivity hasn’t been clarified. Inaddition, Non-Patent Document 2 discloses that lithium can be substituted by hydrogen by washing Li.sub.6.75La.sub.3Ta.sub.0.25Zr.sub.1.75O.sub.12 with water at room temperature. However, the amount of substitution to hydrogen is as little as 21% of theamount of lithium.

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