It was shown that, apart from classical vacancies formed as a result of the thermal fluctuations, a crystal could contain so-called non-classical vacancies of non-fluctuation nature. The latter vacancies appeared when the temperature exceeds a critical value TC. The factor responsible for their formation was a mechanical instability of an ideal crystal. The temperature TC was a second-order phase transition point. The vacancies formed as a result of this phase transition were joined together into small clusters with sizes of the order of several atoms. The above transition made a substantial contribution to the pre-melting process observed experimentally.

Role of Vacancies of a Strongly Strained Crystal in the Melting Process. L.A.Bulavin, O.Y.Aktan, Y.F.Zabashta: Physics of the Solid State, 2010, 52[4], 712-7