By using the methods of acoustic emission and neutron-structure analysis, the effect of pressurep on low-temperature phase transitions in Li and Na is investigated. In Li, within the investigated interval ofp S 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 a t 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 pre-transition 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 pre-martensitic softening of the shear constant, observed under compression of Li.

Band-structure andshear-constant (C,,) calculations have been performed for the Li, Na and Ba BCC phase and the Cr, Ca and Sr BCC and FCC phases within a broad range of mmpressions(~fromOto40-55~.Todescribethe'baiidwntributionstoC,.,useismade of calculations based on the linear muffin-tin orbital/atomic sphere approximation (LMTO-ASA) method; the other contributions to C,. are described by using the semiempirical 'electrostatic' model. At U = 0 the calculated C,. values are usually close to the observed ones. We have revealed pronounced effects of softening for the shear constants, up to the loss of stability (C,, < 0). when the Fermi level approaches the maximum points of the density of states n(~), as well as under certain changes of shape of " ( P ) . The compression values, at which this softening takes place, are in good agreement with the position of structural phase transitions under pressure. observed in the metals considered. A number of anomalies in the C'(W and C, ( U) dependences are predicted. in particular a sharp drop of C'(LJ'J near the phase transition points U = U, in BCC Li and Ba and FCCCS. Ca and Sr. as well as a significant decrease of C d U ) with rising U in FCC Cs and BCC Sr at U a 0.5.

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