Photon Factory Activity Report 2017 #35 (2018)

AR-NE1A/2015G004

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

1 Introduction
The III-V compound semiconductors are well-known
for their use in optoelectronic devices. Many of these
semiconductors crystallize in the cubic zinc-blende
structure. As pressure increases, this cubic phasetransforms to a tetragonal metallic phase, which has theβ-tin structure. The transition of GaP fromsemiconducting to the metallic phase around 22 GPa has
been confirmed repeatedly by previous studies. Although
the physical properties of GaP at high pressures have beeninvestigated repeatedly, most studies have been
performed at ambient temperature. It is known that thesemiconducting-metallic transition induces a significant
change in the electrical resistivity, which is easy toobserve at extreme conditions. Therefore, thesemiconducting-metallic transition has been used as apressure calibration point at high-pressure experiments.
Thus, the precise phase boundary of GaP needs to bedetermined in high-pressure science.

pressure was estimated by the equation of state for B1type NaCl. The stable phase could be identified using theX-ray diffraction data. The diffraction peaks of thesemiconducting or the metallic phase can be indexed as
the cubic or the orthorhombic symmetry. After theidentification of the stable phase, the temperature waschanged to the desired value. The P-T conditions of theacquired X-ray diffraction data and the stable phases
identified are shown in Fig. 1. The transition pressure atroom temperature was 22.2(±2) GPa. The transition
boundary shown in Fig. 1 is represented by the linear
equation:
P (GPa) = 22.6(2) - 0.0014(5) x T (K).
1000
Semiconductor

Metal

900

3 Results and Discussion
Several experimental runs were carried out at pressures
between 13 and 27 GPa. In each run, the sample wascompressed to roughly the desired pressure, which wasconfirmed by the pressure scale of Raman spectra fromthe diamond. After the desired pressure, the sample wasthen heated, and the X-ray diffraction data of the sample
was acquired to identify the stable phase of GaP at highpressure and temperature. As the pressure scale of theRaman spectra from the diamond could not be usedduring the X-ray diffraction measurements, the sample

Temperature (K)800

2 Experiment
The high-pressure and high-temperature X-ray
diffraction experiments were performed using ahydrothermal diamond anvil cell (HDAC) high-pressure
apparatus. The temperature was controlled by adjusting
the power supply. After reaching the required temperature,
we performed in situ measurements using the synchrotron
X-rays and the temperature was maintained for 15–30
min. The GaP reagent (purity = 99.999%) was used as astarting material, which was ground in an agate mortar tograin sizes less than a few μm. The powdered material
was loaded into a 100-150 μm diameter hole drilled into arhenium gasket, which was pre-indented to a thickness of~50 μm. The sample was sandwiched between pellets ofNaCl powder. The sample was probed using angledispersive X-ray diffraction, employing the AR-NE1A
synchrotron beamline at KEK, Japan. The angledispersive X-ray diffraction patterns were obtained on animage plate system (Rigaku R-AXIS, Japan).

700600500400300
121416182022242628Pressure (GPa)Fig. 1: Experimental results and the boundary of thesemiconducting-metallic transition in GaP. Blue and red
circles denote the stability conditions of thesemiconducting and the metallic phases, respectively. The
dashed line is the inferred phase boundary.
Acknowledgement
We thank the PF staffs for their helps in carrying out
the experiments.
References[1] S. Ono, T. Kikegawa Determination of the phaseboundary of GaP using in situ high-pressure and hightemperature X-ray diffraction, High Press. Res., 37,28-35 (2017)* sono@jamstec.go.jp