An experimental and theoretical study of martensitic phase transitions in Li and Na under pressure

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Abstract
By using the methods of acoustic emission and neutron-structure analysis, the effect of pressure p on low-temperature phase transitions in Li and Na is investigated. In Li, within the investigated interval of p <or approximately=3 GPa, the transition temperature rises with pressure, and the low-temperature phase has a 9R structure. In Na, on the contrary, pressure suppresses the transition, so that it disappears even at p=0.1-0.2 GPa but the 9R phase is the low-temperature one too. The shape of acoustic emission signals suggests the presence of pretransition phenomena and differences in the kinetics of transitions in Li and Na. Calculations performed explain the difference in the influence of pressure on structural stability of Li and Na by the effect of proximity of the Fermi level to peaks in the electronic density of states, which appears under pressure in Li but is absent from Na. The same effects account for a premartensitic softening of the shear constant, observed under compression of Li.