High-resolution X-ray diffraction of silicon at low temperatures

he temperature dependence of the lattice spacing of silicon (440) and (444) has been measured by means of X-ray diffraction according to the Bond method coupled with a quadruple-crystal monochromator; measurements were repeated over several thermal cycles in a range 300-20 K, when the specimen crystal was kept in the state of a perfect crystal. It is found that the spacing of (440) shows normal behaviour, while that of (444) is not a unique function of temperature, depending on specimens and thermal treatments at low temperatures. Simultaneous measurements of the four {444} spacings show that the lattice symmetry, in the temperature region of negative thermal expansion, slowly varies to trigonal through an unstable stage. The results together with those of related X-ray diffraction experiments such as that carried out with external uniaxial stresses suggest an occurrence of phonon-mediated atomic rearrangement at the low temperature.