Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

NE5C/2004G046

Structure of liquid I-VII compounds under pressure
Kazuhiko TSUJI*, Takuya HIGAKI, Masatoshi TOMOMASA,
Yoshiharu TAGAWA and Ayano CHIBA,
Department of Physics, Keio Univ., 3-14-1, Hiyoshi, Kohoku, Yokohama 223-8522, Japan
Introduction
In order to elucidate the effects of the ionicity inchemical bonding on the structure of covalent liquids, wehave investigated the structure of liquid I-VII compounds
under pressure. Recently we have reported the pressure
dependence of the local structure of liquid III-V and II-VI
compounds [1-6]. The ionicity of I-VII compounds ishigher than that of III-V compounds. The local structureof liquid CuX (X=Cl, Br and I) is known to be different
from that of the typical ionic liquids, in which the paircorrelation functions between cation-cation correlation
function and anion-anion correlation function is same. In
the crystalline phase, CuI has a superionic conductive
phase in which Cu ions distributed in disordered structure.
Presence of ionic bonding and covalent bonding isexpected in both crystalline and liquid phases. It isinteresting to investigate the pressure dependence of thelocal structure of liquid CuX.

X-ray diffraction of liquid CuBr and liquid CuCl wasalso measured under high pressure. Detailed analysis is inprogress.

Experimental
X-ray diffraction patterns were taken by an energydispersive method using the synchrotron radiation.
Pressure was generated by using the multi-anvil highpressure apparatus, MAX80, installed at AR-NE5C.
Results and Discussion
The static structure factor S(Q) of liquid CuI at several
pressures is shown in Fig. 1. With increasing pressure, thepeak position of the first peak shifts towards higher Q. On
the other hand the peak position of the second peak shifts
towards higher Q up to 4.1 GPa and then shifts to lower Q
above 5.1 GPa. If the structure of liquid contracts
uniformly, the shape of S(Q) expands to higher Q withoutchanging its height. Therefore, above change of thesecond peak suggests that the drastic change in the local
structure of liquid CuI occurs between 4.1 GPa and 5.1GPa. The ratio of Q2/Q1 changes from 1.70-1.66 below
4.1 GPa to 1.60 above 5.1 GPa.
The pair distribution functions, g(r) was obtained by
the Fourier transformation of S(Q). The change in thevalue of r1, the first peak position of g(r) of liquid is small
while r2, the second peak position of g(r) decreases andbecomes narrow with increasing pressure. The ratio ofr2/r1 decreases from 1.63 at 0.8 GPa to 1.44 at 6.6 GPa.
The value of r2/r1 is 1.63 for the zincblende structure and1.41 for the α-AgI structure. It suggests that Cu ionslocate in the space of tetrahedral site between I ionsbelow 4.1 GPa and other site above 5.1 GPa such as
tetrahedral site of α-AgI structure.

Fig. 1. S(Q) of liquid CuI at several pressures.
References[1] T. Hattori et al., Phys. Rev. B 68, 224106 (2003).
[2] T. Hattori et al., J. Phys.: Condens. Matter 16, S997
(2004).
[3] T. Hattori et al., Phys. Rev. B72, 064205 (2005).
[4] T. Kinoshita et al., Phys. Rev. B 72, (2005) 060102R1-4.
[5] T. Hattori et al., Phys. Rev. B 73 (2006) 054203-1-9.
[6] T. Narushima et al., Proc. 20th AIRAPT Conference,Karlsruhe, 2005, T2-O80.
*tsuji@phys.keio.ac.jp

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