Polytype structures of lithium at low temperatures

After cooling below 80 K a considerable amount of diffuse scattering is observed in Li, in addition to 9R and fcc Bragg reflections. The diffuse scattering localized on the [101] line of the 9R reciprocal lattice is modulated along this line indicating a short-range order in the stacking sequence of the close-packed planes. The results show that, in addition to the long-range ordered fcc and 9R structures, Li forms at low temperatures a disordered polytype where short-range ordering tendencies of the hcp, fcc, and 9R types are simultaneously present. These observations may be explained in terms of competing cubic and hexagonal ordering tendencies in the stacking sequence of close-packed planes.