BCC INSTABILITY OF LITHIUM AT LOW-TEMPERATURES

Static and dynamic precursor phenomena of the lattice instability of bcc lithium were investigated by elastic-diffuse- and inelastic-neutron-scattering techniques. An elastic diffuse scattering around the (200) bcc reciprocal lattice point was found in a deteriorated single crystal. The diffuse-scattering pattern shows intensity streaks along [110] directions and indicates the presence of a static [110][110BAR] shear distortion in the deteriorated crystal. A softening of the transverse-acoustic phonon branch TA1[110] especially at the zone boundary was confirmed. In the presence of inclusions of the low-temperature phase within the bcc matrix, a dip in the phonon-dispersion curve near the reduced q value of 1/3[110] was found. These observations are related to recent findings on the low-temperature phase diagram of Li, indicating the presence of a complex polytype state where different ordering tendencies are simultaneously observed in the stacking sequence of close-packed planes.