Negative thermal expansion coefficient and isostructural transition in fcc cesium

Christensen, N. E.; Boers, Dave; Velsen, J. L. van; Novikov, D. L.

doi:10.1103/physrevb.61.r3764

Cited by 27 publications

(24 citation statements)

References 21 publications

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“…[5,6]. The frequencies obtained for b.c.c.-Cs at the experimental equilibrium volume, V 0 , agree well with other theoretical results [34 to 36] and experiments [37,38] in Table 1.…”

supporting

confidence: 81%

“…The softening of the lattice, also reflected by the rapid decrease of Q D in Fig. 5 slightly larger volumes, does lead to a negative overall Gru È neisen parameter, but the effect does not produce a Van der Waals loop in the isoterms [5,6]. The softening, on the other hand, causes the thermal expansion coefficient to be negative above 3.5 GPa.…”

mentioning

confidence: 94%

mentioning

confidence: 99%

“…by diagonalizing a dynamical matrix as in usual harmonic theory, but with volume dependent force constants. The force constants are calculated from first principles [5,6]. Having calculated FV; T, we derive the PV; T, and other physical quantities like the thermal expansion coefficient, a, can be calculated as function of temperature for fixed pressure or for varying P with chosen values of T. Also, the Gibbs free energy is calculated, and the temperature variation of the coexistence pressure for two phases can be predicted.…”

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confidence: 99%

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Elemental Metals under Pressure

Christensen

2000

phys. stat. sol. (b)

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Parameter‐free calculations based on the density‐functional theory are used to examine high‐pressure phases of solids. For the elemental semiconductors particular attention is paid to the orthorhombic (Cmca) structure (Si‐VI). The same structure, even with very nearly the same relative atomic coordinates, is found for Cs in the high‐pressure phase Cs‐V. In the Cmca structures the atoms tend to form dimers. Ge and Rb also have high‐pressure phases with the same Cmca structure. The thermodynamic properties of the low‐pressure phases of cesium, Cs‐I (b.c.c.) and Cs‐II (f.c.c.), are examined, and the equation of state is calculated for P up to 4.5 GPa and temperatures from 0 to 300 K. The contributions to energy and entropy from the phonons are calculated within the quasi‐harmonic approximation. The thermal expansion coefficient of f.c.c.‐Cs is predicted to be negative for P above 3.5 GPa for all T. Cs‐II becomes dynamically unstable when P exceeds 4.3 GPa, where a transverse phonon mode with wavevector along (110) becomes soft. As a consequence, a Van der Waals loop does not develop in the isotherms, and an isostructural (f.c.c. → f.c.c.) transition cannot occur. In that case Cs‐III must have a structure that is not f.c.c.

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“…[5,6]. The frequencies obtained for b.c.c.-Cs at the experimental equilibrium volume, V 0 , agree well with other theoretical results [34 to 36] and experiments [37,38] in Table 1.…”

supporting

confidence: 81%

mentioning

confidence: 94%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Elemental Metals under Pressure

Christensen

2000

phys. stat. sol. (b)

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“…5 In the case of caesium a similar densification and change in the coordination number has been observed experimentally in the solid phase in the same pressure range. 6,7 Several numerical studies have been also performed even if none of them have shown the appropriate details 8 and/or the right phase transition sequence 9-11 until recently. [12][13][14][15] The reported isostructural phase transition in caesium 9 occurs in the same pressure range as the liquid structure anomaly.…”

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confidence: 99%

Ab initiosimulations in liquid caesium at high pressure and temperature

Falconi

Ackland

2006

Phys. Rev. B

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The structure of liquid caesium undergoes a sharp change at around 4 GPa from a simple liquid to a low-coordination complex structure. This mirrors a similar change in the crystal structure at this pressure. Here, we show that both changes are accurately described in good agreement with experiment by density functional theory calculations, which shed light on the nature of the liquid structure and its electronic origins. Analysis of the wave function character shows s-d hybridization, but not s-d transfer, in both solid and liquid at the pressure of the complex Cs-III phase. This implies that a nearly free electron picture is more appropriate than one based on the atomic orbitals. The similarity of s-d hybridization in crystal and liquid phases indicates that hybridization is a general consequence of densification, rather than being induced by a particular crystal structure. The free-electron picture predicts that stable structures will have diffraction peaks associated with the Fermi vector, and this is borne out by comparison with experiment.

show abstract

Elemental Metals under Pressure

Christensen

2000

phys. stat. sol. (b)

View full text Add to dashboard Cite

Parameter-free calculations based on the density-functional theory are used to examine high-pressure phases of solids. For the elemental semiconductors particular attention is paid to the orthorhombic (Cmca) structure (Si-VI). The same structure, even with very nearly the same relative atomic coordinates, is found for Cs in the high-pressure phase Cs-V. In the Cmca structures the atoms tend to form dimers. Ge and Rb also have high-pressure phases with the same Cmca structure. The thermodynamic properties of the low-pressure phases of cesium, Cs-I (b.c.c.) and Cs-II (f.c.c.), are examined, and the equation of state is calculated for P up to 4.5 GPa and temperatures from 0 to 300 K. The contributions to energy and entropy from the phonons are calculated within the quasi-harmonic approximation. The thermal expansion coefficient of f.c.c.-Cs is predicted to be negative for P above 3.5 GPa for all T. Cs-II becomes dynamically unstable when P exceeds 4.3 GPa, where a transverse phonon mode with wavevector along (110) becomes soft. As a consequence, a Van der Waals loop does not develop in the isotherms, and an isostructural (f.c.c. 3 f.c.c.) transition cannot occur. In that case Cs-III must have a structure that is not f.c.c.

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Negative thermal expansion coefficient and isostructural transition in fcc cesium

Cited by 27 publications

References 21 publications

Elemental Metals under Pressure

Elemental Metals under Pressure

Ab initiosimulations in liquid caesium at high pressure and temperature

Elemental Metals under Pressure

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