In order to explain a perfect crystal, I think you should at least mention the attributes that cause a crystal to be imperfect -- [Chris Markides]
Theoretically perfect crystals can only exist at temperatures of Absolute zero or slightly above this. I'm not entirely sure about this though. Logically, since htermal energy causes atoms to vibrate in their atomic positions and eventually move from them (leaving behind a vacancy) this should be the case for metals (and alloys). As for stoichiometric salts, I'm not sure... -- [Chris Markides]