Neutron scattering studies of the structures and lattice dynamics of the alkali metals

The crystal structures, phonon behavior, and pre-martensitic behavior of Li, Na, K, Rb, and Cs as a function of temperature and pressure (Li, Na, Rb, and Cs) are reviewed. Li has been shown to partially transform to the 9R (Sm-type) structure in the vicinity of 75 K and then partially to fcc on annealing. Its (Tc) increases with applied pressure. Na, on the other hand, partially transforms to a mixture of 9R and nearly hexagonal R-polytypes, which approach hcp on annealing. Its (Tc) is rapidly suppressed by application of modest pressures. No martensitic or pre-martensitic phenomenon, or phonon anomaly, has been observed in K down to 5 K. Also, a search for CDW's in the vicinity of (110) has been inconclusive. Similar negative results have been obtained for Rb.