Anisotropic stress-sensitive temperature dependence of the lattice constant of silicon

The previous experiment of high-angle double-crystal X-ray diffraction (HADOX) performed on silicon at low temperatures showed a very anomalous behaviour of a(T), the lattice constant as a function of temperature. An extensive study has since been made under various conditions of specimen crystals. The results are in contradiction with the common belief that the silicon crystal with the diamond structure is rigid and stable: It is found that the anomaly is rather sensitive to external stresses and that the behaviour of a(T) can therefore be very complicated. It is also found that the anomaly is anisotropic, namely largest in (111) direction and smallest in (110) direction. The apparent irreproducible behaviour of a(T) is attributed to an atomic rearrangement with a long relaxation time.