Solid salts of hydronium ion Are there any solids with hydronium ion in its crystal structure? 

If so, what are they?

If not, why not? I understand there are hydroxide-containing solids and gels such as aluminium hydroxide but why not hydronium ions? Does this have to do with the positive charge of hydronium ion? I could see the positive charge of hydronium ion excluding the possibility of forming any salts/solids with the group 1 and 2 metals.
There is a section in this Wikipedia link entitled, "Solid hydronium salts."  It says that any acid with an ionization constant of 10^9 or higher may form stable hydronium ion salts.  Examples they give include hydronium perchlorate and carborane-based hydronium ion salts.