Photon Factory Activity Report 2016 #34 (2017)

Structural Phase Transition in SiO2

AR-NE7A/2013G512/2015G501

Shigeaki Ono1,*1
Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-Earth
Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan

2 Experiment
The starting material was SiO2. High-pressure X-ray
diffraction experiments were performed using a multianvil high-pressure apparatus. The cubic anvil assembly
was compressed using a “Max III” high-pressure
apparatus, and was combined with a synchrotron
radiation source located at the KEK in Japan. We also
used experimental data from SPring-8, which is similar
facility of PF-AR. The diffracted X-rays were detectedusing a germanium solid-state detector at an angle of 2θ =6.0°. A cylindrical TiB2 heater was inserted into theoctahedral pressure medium and enclosed within a ZrO2
sleeve for thermal insulation [1,2]. The powdered sampleand gold, which was used as pressure calibrant, wereloaded directly into the TiB2 heater, which also served asa sample capsule. The pressure was determined from theunit cell volume of gold using the equation of state forgold. After reaching the required temperature, weperformed in situ measurements using the synchrotron Xrays. The duration of heating was 1-4 hours.
Determination of the stable phase in each experimental
run was carried out by observing the X-ray diffraction
pattern of the sample. To check the identification of eachphase in the in situ experiments, the recovered samples
were also examined using micro Raman spectroscopy.
3 Results and Discussion
We performed approximately 30 experimental runs toinvestigate the phase boundary between coesite andstishovite [5]. The P-T conditions of the acquired X-ray
diffraction patterns and the stable phases identified areshown in Fig. 1. The gradient of dP/dT of the phaseboundary was positive. The transition boundary shown inFig. 1 is represented by the following linear equation:
P (GPa) = 4.7(3) + 0.0031(2) × T (K).

The boundary determined in this study is in generalagreement with those reported in previous in situ highpressure experiments. However, the dP/dT slope wasmore positive than that in the previous in situ experiments.
The estimated value of the dP/dT slope using thethermodynamic calculation is consistent with that
determined in our study.
The phase transition between coesite and stishovite at~10 GPa, i.e., ~300 km depth, might be related to theobserved seismic X-discontinuity. The transition pressure
determined in our study is in good agreement with that
corresponding to the depth of the X-discontinuity.
1800

CoesiteStishovite

1700
1600

Temperature (K)1 Introduction
Silica, SiO2, is a major mineral in the subductedoceanic plate. Therefore, the physical properties
associated with the phase transitions of silica play animportant role in understanding the composition, structure,and dynamics of the earth’s mantle. Moreover, its
polymorphic transitions from quartz to coesite and fromcoesite to stishovite have been extensively used forpressure calibrations at high pressures and hightemperatures, such as for CaGeO3 [1], Fe2SiO4 [2],MgSiO3 [3], and Mg2SiO4 [4]. Determination of the phase
diagram for silica is therefore of particular interest to thestudy of the high-pressure behaviors of materials.

1500
1400
1300
1200
1100
6789101112Pressure (GPa)Fig. 1: Experimental results and phase boundary of thecoesite-stishovite transition in SiO2.
Acknowledgement
We thank the PF staffs for their helps in carrying out
the experiments.
References[1] Ono et al., Phys. Chem. Minerals 38 735-740 (2011).
[2] Ono et al., Phys. Chem. Minerals 40 811-816 (2013).
[3] Ono et al., Geophys. Res. Lett. 28 835-838 (2001).
[4] Katsura et al., J. Geophys. Res. 109 B02209 (2004).
[5] Ono et al., Phys. Earth Planet. Inter. 264 1-6 (2017).
* sono@jamstec.go.jp