Solid solutions and plastic phases based on HClO4·51/2H2O in the system perchloric acid-water

1.	The system HClO4-H2O has been studied by direct observation of its behavior on heating and cooling and by optical-crystallographic methods. The existence of an individual hydrate of perchloric acid with the formula HClO4·5 1/2H2O has been discovered.
2.	Crystals of HClO4-5 1/2H2O are dimorphic; the a form exists over a very short range of solid solution compositions because of the very slow rate at which equilibrium is established at low temperatures.
3.	Crystals of -HClO4-5 1/2H2O and of the solid solutions in crystals of the form belong to the cubic crystal class. Crystals of - and -HClO4·5 1/2H2O, as well as crystals of the solid solutions based on the -form, are plastic crystals.

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 12–17, January, 1988.