Hydronium perchlorate
Hydronium perchlorate is the chemical compound with the formula . It is an unusual salt due to it being a solid and stable hydronium salt. Production
Hydronium perchlorate is produced by the reaction of anhydrous perchloric acid and water in a 1:1 molar ratio:

A more analytically reliable method was reported using the macrocyclic Schiff base of sodium 2,6-diformyl-4-methylphenolate and 2,6-diformyl-4-methylphenol as a chelating agent to sequester Cu(II): transmetallation of the macrocycle with copper(II) perchlorate yielded colorless crystals of hydronium perchlorate. It can also be produced by the reaction of anhydrous nitric acid and perchloric acid. References

Oxonium compounds
Perchlorates