Photon Factory Activity Report 2004 #22 Part B (2005)
High Pressure Science

14C, NE5C/2003G031

Pressure-induce electronic transition in liquid Rb
Takanori HATTORI*, Yasuhiro KINBARA, Kazuhiko TSUI
Keio Univ., Hyoshi Kohoku-ku, Yokohama 223-8522, Japan
Introduction
Rubidium is considered typical free-electron metals
because of the single s electron in the valence bond.
However, this simple element shows unexpected behavior
when it is subjected under pressure. For example, x-ray
diffraction studies have shown that it undergoes asequence of phase transition leading to low symmetry
instead of close packed structure [1]. The pressureinduced complexity is attributed to the increase role of thed electrons becoming more appreciable with increase inelectron density. Such behavior would occur not only in
crystalline phases but also in liquid states. Actually, themelting curve shows maxima around 2 GPa and thedeviation of the contraction behavior from uniform
contraction model was reported at the higher pressure [2].
However, it is subject of debate because of a lack of thestructural data at pressures where the s-d electronictransition occurs. The lack stems from the experimental
difficulty due to the high reactivity of alkali metals aswell as the highly fugitivity of the sample. To avoid theseproblems, we have developed externally heated diamondanvil cell (DAC), which prevents the reaction and leakageof the sample. We investigate the pressure-inducedchange of liquid Rb up to about 12 GPa using this device
on the basis of the afore-mentioned motivation.
Experimental
The schematic view of the high-pressure cell is shown inFig. 1. The externally heated DAC is equipped with thecoil heater located inside the piston of DAC and the larger
heater surrounding the piston. We put the sample inside
the hole of the Re gasket without any pressure medium orpressure marker to avoid reaction of the sample. We
controlled the pressure from the outside of theexperimental hutch by regulating pressure of the gas
loaded in the bellows which is attached to the body ofDAC. This system enables us to prevent the pressure
decrease on heating as well as the change of the sample
position during compression or decompression.

Fig. 1 Schematic view of the externally heated diamondanvil cell equipped with a gas pressure controller.

Pressure was estimated from the position of the main
peak Q1 in the diffraction pattern on the basis of theresults of the previous study [2]. The pressure in theregion where the investigation has not been conducted
was estimated from the extrapolated curve. The x-ray
diffraction intensity was measured by an energydispersive method using a pure-Ge solid-state detector.
Results and Discussion
The relation between the position of the main peak Q1
and the full width at half maximum in diffraction patterns
at high pressures is shown in Fig. 1. With increase in Q1
up to 1.85 (with increasing pressure up to 4 GPa), theFWHM remains almost constant. However, the value
increases on the further increase in Q1. When the liquid
contracts uniformly the value should become almost
constant because in such case all the profile at highpressures can be scaled by a Q value. This finding
suggests that the local structure change of above 4 GPa.
The high-pressure sequence in crystalline Rb helps us
understand this behavior. When the pressure-induce s-dtransition occurs, the average atomic radium becomes
smaller because of the smaller d-orbital compared sorbital. In the crystalline phase, the contraction of theatomic radius takes place discontinuously. In contrast,
such change occurs gradually in liquid since the local
structure gradually changes over wide pressure region. In
the transient states, two species with different atomic
diameter coexist and therefore, the main peak becomes
broader. The increase in FWHM observed above 4 GPa
implies the occurrence of s-d transition in liquid Rb, andthe s-d transition does not complete at 12 GPa, which-1corresponds to Q1= 2.1 A .

Fig. 2 The relation between Q1 and the FWHM in
diffraction patterns at high-pressures.
References[1] McMahon et al., Phys. Rev. Lett. 87, 055501 (2001).
[2] Tsuji et al., J. Non. Cryst. Solids 205-207, 295 (1996).
* takanori@spring8.or.jp

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