Structure, elastic moduli and thermodynamics of sodium and potassium at ultra-high pressures

TL;DR

This paper investigates the behavior of sodium and potassium under ultra-high pressures, calculating their equations of state, elastic properties, and phase diagrams, with implications for pressure calibration and understanding high-pressure phases.

Contribution

It provides a detailed first-principles analysis of Na and K at pressures over 100 GPa, including elastic moduli and phase stability, using GGA density functional methods.

Findings

GGA improves equation of state accuracy for Na

Non-monotonous elastic moduli pressure dependence predicted

K phase diagram aligns well with experimental data

Abstract

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 K . It is shown that the allowance for generalized gradient corrections (GGA) in the density functional method provides a precision description of the equation of state for Na, which can be used for the calibration of pressure scale. It is established that the close-packed structures and BCC structure are not energetically advantageous at high enough compressions. Sharply non-monotonous pressure dependences of elastic moduli for Na and K are predicted and melting temperatures at high pressures are estimated from various melting criteria. The phase diagram of K is calculated and found to be in good agreement with experiment.