Ab initiothermodynamics of body-centred cubic and face-centred cubic Cs

作者：N. E. Christensen, Dave Boers, J. L. van Velsen, D. L. Novikov · 发表于：Journal of Physics Condensed Matter · 年份：2000 · DOI：10.1088/0953-8984/12/14/307 · 被引用次数：22 · 研究领域：High-pressure geophysics and materials、Advanced Chemical Physics Studies、Rare-earth and actinide compounds

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 ( ) 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 is negative. For the fcc phase, 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 = 3.0 ×10 -4 K -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 2.2 GPa and low T , and at a higher pressure, 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.