TEMPERATURE-DEPENDENCE OF THE LATTICE-CONSTANT IN METALLIC SODIUM

The temperature dependence of the lattice constant for the parent phase in metallic sodium, which has a body-centred cubic (b.c.c.) structure, has been measured by an X-ray single-crystal diffraction method over a range of 20 to 293 K. An abnormal change of the lattice constant at around 40 K is shown, which is attributed to small regions of the low-temperature phase. On the other hand, the value of the full width at half-maximum (FWHM) for the (110)(b.c.c.). Bragg reflection increased gradually with decreasing temperature and then changes drastically around 40 K. This result was inconsistent with the neutron diffraction one: the value was constant down to the phase-transition temperature and changed drastically at that temperature. The FWHM for the X-ray experiment is very sensitive to the size of the specimen and sample mount because the external strain originating from contraction of the sample holder is often introduced into the specimen.