Ab initiosimulations in liquid caesium at high pressure and temperature

Abstract

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.

Dynamical Crossover at the Liquid-Liquid Transformation of a Compressed Molten Alkali Metal

“…Remarkably, the bcc-fccliquid Rb triple point is roughly located in the P-T region 0.93-10.2 GPa and 490-520 K [18], and above 12 GPa solid Rb undergoes a further phase transition to Rb-III associated with a large volume change [19]. The structural transformation that we observe occurs in the proximity of this portion of the solid phase diagram, at striking similarity with liquid Cs under pressure [12], where an analogous reduction of the NNN from 13 to 7-8 was observed at 493 K, in the pressure region of the bcc-to-fcc transition. We remark that the observed structural deviation from a simple liquid behavior in Rb at high pressure coincides with the non monotonic trend of its electronic counterpart, i.e.…”

“…Jointly with experimental observations in solid Li [10] these discoveries set a stage of unusual properties and new structural transformations in liquid metals at high pressures and temperatures. AIMD simulations were also successfully applied in highpressure studies of structural features of other liquid alkali metals, such as Cs [11,12] and Rb [13], and in liquid Si [14].…”

Lattice Dynamics of Incommensurate Composite Rb-IV and a Realization of the Monatomic Linear Chain Model

“…Above 16.7 GPa, both host and guest components exhibit long-range order, and the interchain longrange order is presumably mediated via minute distortions of the host lattice. Below 16.7 GPa the interchain correlation length decreases rapidly and is only 40 A at 16.1 GPa [14,15]. The breakdown of interchain correlations below 16.7 GPa suggests that the host-guest interactions in Rb-IV are particularly weak.…”

Evidence for Liquid-Liquid Phase Transitions in the Transition Metals

“…Since, with few exceptions, transition metals do not undergo solid-solid phase changes along the melting curve, a sudden decrease in the melting slope implies an increase in the liquid density and an increase of the communal entropy, which may be attributed to an LLPT. In the case of Cs, which has 5d-electron character, and in a broad sense may be considered as a very early transition metal, Falconi et al [23,24] have interpreted structural changes and density increases from x-ray diffraction patterns above 3.9 GPa that mark the change from a simple to a locally complex liquid. Their analysis "strongly suggests the existence of dsp 3 electronic hybridization", a P/JT distortion.…”

“…In the case of Cs, which has 5d-electron character, and in a broad sense may be considered as a very early transition metal, Falconi et al [23,24] have interpreted structural changes and density increases from x-ray diffraction patterns above 3.9 GPa that mark the change from a simple to a locally complex liquid. Their analysis "strongly suggests the existence of dsp 3 electronic hybridization", a P/JT distortion.…”