Photon Factory Activity Report 2018 #36 (2019)

NE5C/2016G186

Pressure-induced local symmetry lowering on liquid–liquid transition of GeI4 and SnI4
Kazuhiro FUCHIZAKI1,* and Takahiro SAKAGAMI2
1Department of Physics, Ehime University Matsuyama, 790 8577, Japan
2
Material Group, Technology Center, AGC Techno Glass Co., Ltd., Shizuoka 421-0302, Japan
1 Introduction
SnI4
undergoespressure-inducedliquid–liquidtransition (LLT) from a low-density liquid (Liq-II) to ahigh-density liquid (Liq-I) phase at ~1.5 GPa [1] whereasan LLT of GeI4 from Liq-II to Liq-I ends up as acrossover, which occurs at around 3 GPa [2]. Our next
task is to identify the microscopic mechanism of theLLTs. To this end, we carried out in situ XAFS and X-ray
diffraction (XRD) measurements for liquid SnI4. We
encountered the quite intriguing phenomenon of pressureinduced molecular symmetry lowering on a reverse
Monte Carlo (RMC) analysis for the XRD data. We
confirmed that the symmetry lowering is universally
recognizable when liquid GeI4 experiences the crossoverfrom the analysis for the previous data. The phenomenon
and the consequence inferred from the fact are reported.
The full reports are now available [4]. The results for
XAFS measurements are saved for another report.
2 Experiment and Analysis Method
We chose the beamline NE5C at KEK-AR, Japanbecause the beamline offers not only a cubic-type
multianvil press, MAX80, for high-pressure and hightemperature measurements but also a superior
environment in which switching between white andmonochromatic mode is easily achieved. We employed
the same sample assembly as that depicted in [5]. XRD
measurements for the sample and a pressure standard
made of NaCl were made with the energy-dispersive
method using white X rays. We used entrance andreceiving slits of cross sections 0.3 × 0.1 mm2 and0.1 × 0.3 mm2, respectively. A collimator with width
0.1mm collimated diffracted X rays. We measured thesample and pressure standard with diffraction angles
fixed at 4° and 8°, respectively. It took 5 min. to obtaindiffraction patterns with a reasonable S/N ratio for boththe sample and pressure standard. The method of EXAFS
measurement is fully described in [6].
The number of molecules employed for the RMC
analysis was 2744 for both SnI4 and GeI4. The molecularconfiguration to be used as input to the RMC analysis
was prepared by conducting the isothermal–isobaric
molecular dynamics simulation [7]. During the RMC run,the movements of the atoms were constrained so as not toapproach unphysically close to each other. The minimum
allowable distances adopted were 3.0, 0.5, and 2.0 Å for
Sn–Sn (Ge–Ge), Sn–I (Ge–I), and I–I distances,respectively, throughout the analysis. For other details,see [4].
3 Results and Discussion

We found that a regular tetrahedral SnI4 molecule withSn–I length of 2.66 Å at ambient pressure is “squashed”
to a tetrahedral molecule with one short (2.25 Å) andthree long (2.86 Å) Sn–I distances at 3.4 GPa. The
curious behavior that the intramolecular Sn–I distance iselongated upon compression was detected in the reduced
radial distribution function [2]. The present RMC analysis
clarified that the shorter length correlation was buried in
artifacts resulting from finite truncation upon the Fourier
inversion.
The most intriguing fact that we found is that the local
symmetry lowering from Td to C3v was also confirmedprior to the LLT (see the upper panel of the figure, inwhich the experimental structure factor is compared with
the intramolecular structure factor derived assuming themolecular symmetry in question).

Fig. 1: Comparison of the experimental structure factor
and the symmetry-adapted structure factors (upper panel)and the appearance of the two-length scales in the local
potential on lowering symmetry from Td to C3v (lower).

Photon Factory Activity Report 2018 #36 (2019)We confirmed that the symmetry lowering from Td toC3v also occurs in liquid GeI4 during the crossover fromLiq-II to Liq-I [4].
Quite interestingly, this local symmetry breaking brings
about a double-minimum character in the intermolecularinteraction. (See a lower panel of the figure, in whichmin𝜔1,𝜔2 ϕ(𝑟12 , 𝜔1 , 𝜔2 )/(16𝜖) is plotted against reduced
intermolecular distance 𝑟12 /σ , the interaction ϕ being
modeled by the 4-center Lennard-Jones potential
characterized by ϵ and σ [4]. The minimum over possible
molecular orientations, ω1 and ω2 , was taken.) The twolength scales implemented in the Jagla potential [8] thus
naturally appear.
4 Concluding Remarks
Pressure-induced LLT in liquid SnI4 and GeI4 isreflected in a shift of the broad peak at 7.5 Å−1 of thestructure factor toward lower wavenumbers. This feature
has been used to judge the occurrence of the LLT. The
structure factor can offer useful information, such as
reduced radial distribution function when converted into
real space. As far as inferred from the aspect of the radial
distribution, the intramolecular Sn–I or Ge–I bond length
seemed to be elongated upon compression. The RMC
method offers an opportunity to address this curious
finding. The elongation results from the lowering ofmolecular symmetry from Td to C3v; four degenerated
lengths are breaks into one long and three shorter lengths.
The radial distribution function has suffered a loss of thepeak corresponding to the shorter-length correlation upon
Fourier transform with a finite wavenumber window.
Quantum chemical calculations for the electronicenergy of a SnI4 molecule could not explain such a typeof symmetry lowering [4], leaving a new issue regarding
the origin for the anisotropic deformation.
Acknowledgment
This work was supported by JSPS KAKENHI (GrantNo. 17K05581). We thank the Geodynamics ResearchCenter, Ehime University, for the use of facilities.
References[1] K. Fuchizaki et al., J. Chem. Phys. 135, 091101(2011).
[2] K. Fuchizaki et al., J. Phys.: Condens. Matter 30,045401 (2018).
[3] https://www.szfki.hu/ nphys/rmc++/opening.html[4] K. Fuchizaki et al., J. Chem. Phys. 150, 114501(2019).
[5] K. Fuchizaki and N. Hamaya, J. Phys. Soc. Jpn. 83,074603 (2014).
[6] H. Naruta, K. Fuchizaki, et al., to be submitted to J.
Phys.: Condens. Matter.
[7] K. Fuchizaki et al., J. Chem. Phys. 112, 10379 (2000).
[8] E. A. Jagla, J. Chem. Phys. 111, 8980 (1999).
* fuchizak@phys.sci.ehime-u.ac.jp