Structure, elastic moduli, and thermodynamics of sodium and potassium at ultrahigh pressures

The equations of state at room temperature as well as the energies of crystal structures up to pressures exceeding 100 GPa are calculated for Na and Ii. It is shown that allowance for generalized gradient corrections in the density functional method provides a precise description of the equation of state for Na, which can be used for the calibration of a pressure scale. It is established that the close-packed structures and bcc structure are not energetically advantageous at high enough compressions. Sharply nonmonotonic pressure dependences of elastic moduli for Na and Ii are predicted, and melting temperatures at high pressures are estimated from various melting criteria. The phase diagram of Ii is calculated and found to be in good agreement with experiment.