Germanium nanocrystals show shifted phase transitions
Germanium nanocrystals enclosed in glass only become liquid at around 200 degrees above the melting point. The second surprise for the investigating researchers was that the molten element only crystallizes out again at around 200 degrees below the usual freezing point.
Scientists led by Eugene Haller from the Lawrence Berkeley National Laboratory in California used an electron microscope to examine a sample of the glass measuring around 300 nanometers in which they had embedded germanium crystals measuring around 2.5 nanometers. They shot a beam of electrons at the material and observed its diffraction at the crystal planes. While the small crystals were solid, the diffraction pattern clearly revealed rings, but these dissolved as the embedments liquified and the crystal structure disintegrated. Conversely, they observed a return of the rings as the germanium resolidified.
So they found out that the embedded element only begins to liquefy well above the actual melting point and only solidifies again well below the freezing point. The researchers were aware of a similar phenomenon from nanocrystals that were enclosed in crystalline materials, since in these cases the strong bonds delayed the melting process. Surrounded by amorphous silicon glass, they had wrongly assumed, however, the melting behavior would be similar to that in air.
The rule of thumb also usually applies: the smaller the body, the lower the melting temperature. Many nanocrystals made up of a few hundred or thousand atoms already begin to liquefy at more than 300 degrees below the melting point of the solid. The background is that as the size decreases, more participating atoms are on the surface, where they have more room to move when heated, which facilitates the melting process itself and also the penetration into the tiny crystal.
