Ab initio thermodynamics of body-centred cubic and face-centred cubic Cs

The equation of state of solid caesium (body-centred cubic (bcc) and face-centred cubic (fcc) structures) is examined theoretically by means of ab initio calculations. The Helmholtz free energies are calculated for pressures (P) up to 5 GPa and temperatures (T) in the range 0 - > 300 K. The electronic contributions are calculated within density-functional theory (local density approximation (LDA) and generalized gradient approximation (GGA)), whereas vibrational contributions to energy and entropy are calculated within the quasi- harmonic approximation. The thermal expansion coefficients ( alpha ) of bcc as well as fcc Cs are calculated as functions of P and T. Both phases are predicted to have (P, T) regimes where alpha is negative. For the fcc phase, alpha goes negative for P above 3.5 GPa (and up to the end of the stability range of the fcc phase) for all T. The value alpha = 3.0 mult 10 exp -4 K exp -1 found for Cs at ambient conditions agrees with experiments, and so does the Debye temperature, 39.5 K. The calculation shows that the bcc - > fcc transition occurs at P approx = 2.2 GPa and low T, and at a higher pressure, approx =3 GPa, at room temperature. The fcc phase becomes unstable around 4.3 GPa, where a transverse phonon mode with q along (110) becomes soft. The calculations do not indicate that this transition is isostructural (fcc - > fcc), a result which is at variance with earlier theoretical and experimental work.