Possible existence of a higher-pressure liquid state in SnI4
Kazuhiro Fuchizaki1 , Nozomu Hamaya2 , Takaki Hase3 , and Takumi Kikegawa4
1
Department of Physics, Ehime University, Matsuyama 790-8577, Japan2
Department of Physics, Ochanomizu University, Tokyo 112-8610, Japan3
Altekna Co. Ltd, Nagoya 451-6004, Japan
4
Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan
We have performed in situ synchrotron x-ray diﬀraction measurements on tin tetraiodide, which consists of SnI4 tetrahedral molecules at ambient pressure, and establishedthat the liquid forms existing above and below 1.5 GPa, referred to as the break pressure,
where the slope of the melting curve of the crystalline phase changes abruptly, have different structures [1]. Our ﬁnding that a high-temperature region (up to ∼ 1300 K) below
the break pressure is occupied by the molecular liquid phase suggests a possible locationof the second critical point close to the break point [2]. This anticipation prompted us
to explain the whole aspect of polyamorphic transitions, including those between the two
amorphous states [3], hitherto observed in SnI4 according to a critical-point scenario [4].
This was indeed accomplished within mean-ﬁeld treatments of the scenario [1,2].
With the aid of molecular dynamics simulations, we showed that once the crystalline
state, in which the nearest molecules are facing each other, melts down, and the molecules
can rotate, they frequently take the vertex-to-face orientation that makes the intermolecular I – I distance shorter than the intramolecular distance [1]. That is, the melting
helps to undergo polymerization. If the molecules are in the face-to-face orientation as in
the crystalline state, much pressure is needed for them to come close within the former
distance. This is why a solid-state amorphization to a denser state occurs at ∼ 15 GPa,
almost ten times as large as the pressure that can induce a transition to the denser liquid
phase.
The high-density amorphous state was shown to be a transient state towards thehighest-density crystalline state [3,5]. This is brought about by molecular dissociation,and the transition completes at 61 GPa [3]. That the liquid can take the higher-pressure
structure in advance can be expected to happen also in a higher-pressure region; a newliquid phase attained by molecular dissociation will realize under much lower pressures.
Indeed, our recent in situ synchrotron x-ray diﬀraction measurements showed that thestructure factor of the liquids under 4 through ∼ 6 GPa has a tendency to resemble thatof the high-density amorphous state undergoing molecular dissociation [5].
We conclude from a series of our ﬁndings regarding polyamorphs of SnI4 that theliquid “foresees” the higher-pressure structure which is realized in the solid state undermuch higher pressures.
[1] K. Fuchizaki et al., J. Chem. Phys. 130, 121101 (2009).
[2] K. Fuchizaki, T. Hase, N. Hamaya, and Y. Katayama, “Possible scenario for the liquid- liquid phase transition in SnI4 ”, in preparation.
[3] N. Hamaya et al., Phys. Rev. Lett. 79, 4597 (1997).
[4] K. Stokely et al., Proc. Natl. Acad Sci. USA 107, 1301 (2010).
[5] A. Ohmura et al., Phys. Rev. B 80, 054201 (2009).