Crystal Growth

Many children grow crystals in solution as a science project. How does a crystal grow from a small “seed” to its final size? That is, exactly how do the atoms know where to adhere to the growing structure without fouling up the precise cubic crystal structure development, for example? Do you see the dilemma? And the surprise?

Answer

The speed and precision of the crystal growth depend on many factors, including the temperature, concentration, and purity of the solution. Assuming the ideal solution, each additional atom to be added from the solution must first find a location on the growing surface of the developing crystal. But these atoms in solution are randomly moving about, making random collisions with the crystal at random locations on the surface. How can they build a perfect single crystal?

Their little secret is that some atoms that have been added at marginal locations, say, can escape from these surface locations to allow other atoms from the solution to find a better location nearby, “better” here meaning to be held electrostatically tighter to the crystal. But these better locations do not occur in chronological order because they are determined by the collective influence of numerous atoms already in the crystal, and the best position one microsecond ago may not be the best position for an atom now. Therefore, the addition and subtraction of atoms from the growing crystal surface proceeds almost by trial and error! Consequently, one cannot write down an algorithm for placing atoms from the solution onto the growing crystal.

When the crystal grows slowly, there is plenty of time for the sampling process to proceed to fruition, and the crystals tend to form with fewer dislocations and inclusions. When the crystal grows rapidly, errors in the crystal structure become trapped, and these crystals tend to have many dislocations and inclusions.