Hydrates of Gases and Clathrates

Under high pressure, usually in the range of several megapascals, and low temperatures, small gas molecules intercallate into the crystal lattice of ice or heavy ice to form nonstoichiomet-ric intercallation compounds called gas hydrates deuterohydrates) or clathrates. The formation of gas hydrates must meet the four following conditions  

There exist three types of crystal structures for gas hydrates structure 1,11 and H respectively (see Table 19.20), the basic building block of all the three structures being a cage of water molecules with twelve pentagonal faces, and denoted 5 , but for filling the entire space another building unit made with two hexagonal face and denoted 6 must be used. 

Space group (Hermann-Mauguin) Pm3n Fd3m P6/mmm 

Cavity type, size and number 5 small (395 pm) 5 6 large (433 pm) 5 , small (391 pm) 

Guest gases Noble gases, methane, ethane, C02 Propane, isobutane Methane with superior alkanes 

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