Provided is a complex oxide that has a space group I-43d, has a high hydrogen content, contains almost no impurity phase, exhibits almost no aluminum substitution in the structure thereof, and is suitable for proton conductivity. This complex oxide is represented by a chemical formula Li.sub.7-x-yH.sub.xLa.sub.3Zr.sub.2-yM.sub.yO.sub.12 (M represents Ta and/or Nb, 3.2<x7y, and 0.25<y<2) and is a single phase of a garnet type structure belonging to a cubic system, and the crystal structure thereof is a space group I-43d.

In an embodiment, an adsorbent that absorbs a Group V element compound and contains at least one amino acid selected from basic amino acids, acidic amino acids, and polar uncharged amino acids, or a salt of the amino acid.

A method for extracting the tantalum from an acid aqueous phase A1 comprising at least one step comprising the bringing of the aqueous phase A1 into contact with a phase A2 not miscible with water, and then the separation of the aqueous phase A1 from the phase A2, the phase A2 consisting of one ionic liquid or a mixture of ionic liquids as an extractant. A method for recovering the tantalum that implements this extraction method and to a use of an ionic liquid or of a mixture of ionic liquids as an extractant, in order to extract the tantalum from such an aqueous phase A1.

Provided is a complex oxide that has a space group I-43d, has a high hydrogen content, contains almost no impurity phase, exhibits almost no aluminum substitution in the structure thereof, and is suitable for proton conductivity. This complex oxide is represented by a chemical formula Li.sub.7-x-yH.sub.xLa.sub.3Zr.sub.2-yM.sub.yO.sub.12 (M represents Ta and/or Nb, 3.2<x7-y, and 0.25<y<2) and is a single phase of a garnet type structure belonging to a cubic system, and the crystal structure thereof is a space group I-43d.