Photon Factory Activity Report 2004 #22 Part B (2005)
High Pressure Science

13A, 18C/2003G194

Equation of State of the High-pressure phase of ZnOYoshihisa MORI1, Naoto NIIYA1, Takafumi MIZUNO1 and Tadashi NISHII1
1
Department of Applied Science, Okayama University of Science
1-1 Ridai, Okayama 700-0005, Japan
Introduction
ZnO is a wide band gap semiconductor and the crystal
structure at ambient pressure is the fourfold-coordinatedwurzite structure (B4). In this material, a theoreticalreport described that the B4 phase transforms to thesixfold-coordinated NaCl structure (B1) and the B1 phasetransforms to the eightfold-coordinated CsCl structure(B2) and the transition-pressure of the second phasetransition was calculated to be at 256 GPa [1]. On theother hand, the high pressure synchrotron X-ray
diffraction study was performed up to 56 GPa byDesgreniers[2], and he reported that the transitionpressure from the B4 to the B1 phase was 9.1 GPa and thebulk moduli of the B4 and B1 phases were 142.6 and202.5 GPa, respectively and the B1 phase was stable up tothis pressure. In this paper, we carried out X-ray study ofZnO to obtain an accurate equation of state of the B1phase and to find the second phase transition to the B2phase.
Experimental
The high-pressure synchrotron X-ray diffraction
studies were carried out using the Angle Dispersive X-ray
diffraction at BL13A and BL18C beam lines at Photon
Factory. The energies of the X-ray source at BL13A andBL18C were 30 and 20 KeV, respectively. Pressure wasgenerated by using a diamond anvil cell (DAC) andbevelled diamonds with 100, 150 µm culet were used. ARe-W alloy was used as a gasket. The typical size of thesample chamber was 60 µm in diameter and the X-ray
source collimated to 40 µm in diameter. The pressure
was determined by the ruby fluorescence method A
16:3:1 mixture of ethanol and methanol and water wasused as a pressure-transmitting medium. The X-ray
diffraction data was detected on the imaging plate andanalyzed by the Rietveld refinement (RIETAN-2000)[3].
Results and discussions
With increasing pressure, the B4 structure transformed
to the B1 phase around 9 GPa, which is good agreementwith the previous experimental reports [2]. The lattice
parameters of a and c for the B3 structure decreasedsmoothly with increasing pressure. The c/a ratio also
decreased with increasing pressure and the pressure-4-1
coefficient was -5.61 × 10 GPa which is almost thesame value as previously reported. The bulk modulus of
the B4 phase was determined by the Birch-Murnaghan'sequation (B0’=4.0) to be 139.9 ± 2.2 GPa which is similar
to the experimental values of 142.6 GPa by Desgreniers[2] and the theoretical values of 133.7 GPa (thegeneralized gradient approximation:GGA) and smaller

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Users' ReportFig. 1. Pressure dependence of volume through thephase transition. The phase transition occurred withthe volume reduction of -16.8 % in the unit cell.
than 162.3 GPa (the local-density approximation:LDA)
by Jaffe [1]. The pressure dependence of the volume
during the phase transition from the B4 to the B1 phase isshown in Fig.1 with the previous experimental data. Thevolume reduction with the phase transition is –16.8% in
the unit cell. The evaluation of the bulk modulus of theB1 high-pressure phase is sensitive to the value of theinitial volume, however it is not easy to determine thevalue with accuracy. The initial volume was estimated by3interpolation to be 0.0196 nm . The bulk modulus and its
pressure derivative of the B1 were determined by theBirch-Murnaghan's equation to be 199.2 ± 6.0 GPa and4.08 ± 0.13, respectively. The value of bulk modulus aregood agreement with the experimental result (202.5 GPa)by Desgreniers and the LDA calculation (205.7 GPa) andlarger than GGA calculation (172.7 GPa) by Jaffe.
Summary
An X-ray diffraction study of ZnO was carried out.
ZnO transformed from the B4 structure to the B1structure around 9 GPa and the B1 phase was stable up to100 GPa and there was no phase transition up to thispressure. The bulk moduli for both phases were 139.9 ±2.2 and 199.2 ± 6.0 GPa, respectively.
References[1] J. E. Jaffe, J. A. Synder, Z. Lin, A. C. Hess, Phys. Rev.
B, 62, 1660 (2000).
[2] S.Desgreniers, Phys Rev B 58, 14102 (1998).
[3] F. Izumi, in The Rietveld Method, edited by R. A.
Young(Oxford University Press,Oxford, 1995), chap. 13* mori@das.ous.ac.jp