STRUCTURAL PROPERTIES OF SILICON AT LOW TEMPERATURES. A. Okazaki, Y. Soejima, Department of Physics, Kyushu University, Fukuoka 812-81, Japan

A review is given of recent high-precision X-ray diffraction experiments on perfect or nearly perfect crystals of silicon in a temperature range 20-300 K, having been carried out by the group in the Department. The technique applied is the high-angle double-crystal X-ray diffractometry (HADOX), the Bond method coupled with a quadruple- crystal monochromator, and the Pendellösung fringes method. It is found that d(440)(T), the lattice spacing of (440) as a function of temperature, is reproducible and coincides with the thermal expansivity data in a higher accuracy; on the other hand, d(444)(T) is not a one-to-one function of temperature and depends on specimen and its thermal history. A variety of d(444)(T) curves can be specified by the temperature where d shows a minimum, or the minimum d value. This means that the irreproducibility of d(T) or the instability of the lattice spacing is directly related to the negative thermal expansion that is commonly observed in sp³-bonded crystals except diamond but not theoretically explained yet. The present experiment suggests that silicon in an ideal situation shows a structural transition around 60 K accompanied with a discontinuous change in the lattice constant a smearing of the transition over a temperature range of several ten K can bring about a temperature region of apparent negative thermal expansion. Moreover, the results of d(444)(T) experiment under uniaxial external stresses show that the stress-strain response is very slow, the relaxation time being several ten minutes. This viscoelastic character may be connected with the history dependent behaviour of d(444)(T), and with the apparent negative thermal expansion. Another feature of silicon that is observed in all the experiments mentioned above is the aging effect at the lowest temperature. As a mechanism of rearranging atoms, a phonon-mediated long-range interaction between defects in the silicon lattice is mentioned.