The present invention relates to a method of preparation of a garnet-type inorganic material. It also relates to the garnet-type inorganic material itself. The process comprises the following steps: (1) bringing an aqueous solution S comprising (i) a salt of zirconium, (ii) a salt of lanthanum and (iii) a salt of the element A or a precursor of an oxide of element A into contact with an aqueous solution of a basic compound, as a result of which a precipitate suspended in the reaction medium is obtained; (2) stirring the reaction medium obtained at the end of step (1) for at least 30 min; (3) bringing the precipitate obtained at the end of step (2) into contact with an additive selected in the group consisting of: anionic surfactants; nonionic surfactants; polyethylene glycols; carboxylic acids and their salts; and surfactants of the carboxymethylated fatty alcohol ethoxylate type; (4) calcining in air the precipitate recovered at the end of the previous step at a temperature which is at least 400° C.; (5) bringing into contact the product obtained at the end of step (4) with a salt of lithium; (6) calcining in air the product obtained at the end of step (5) at a temperature between 700° C. and 1100° C.; 20 the inorganic compound M comprising or consisting essentially of a garnet oxide or garnet-type oxide containing, as constituent elements, the elements Li, La, Zr and at least one element A selected in the group consisting of Al, Ga, Nb, Fe, W, Ta, or a mixture thereof.