Photon Factory Activity Report 2005 #23 Part B (2006)
Crystallography

10A/2005G010

Single crystal X-ray diffraction study of high pressure phase of gibbsite
1Kazuki KOMATSU*1, Takahiro KURIBAYSHI1, Yasuhiro KUDOH1
Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan

Introduction
The minerals of the alumina–water system, which havethe composition Al(OH)3, are gibbsite [γ-Al(OH)3],bayerite [α-Al(OH)3], nordstrandite [β-Al(OH)3], anddoyleite. Among them, gibbsite has been studiedintensively both from experimental and theoreticalviewpoints. In 1983, two new polymorphs was found in
the alumina-water system [1]. Over the past 10 years, anumber of studies have investigated gibbsite using in situ
high-pressure powder X-ray diffraction analysis [2,3].
However, the crystal structures of the two polymorphs
remain yet to be determined.
In this study, one of the two polymorphs, which istermed as η-Al(OH)3, was investigated by single crystal
X-ray diffraction method to determine its crystal structure.
Experimental details and Results
In this analysis we used a colorless platy single crystal(0.2 x 0.1 x 0.1 mm) of natural gibbsite from
Langesundfjord, Oslo Region, Norway. The sample wasloaded into a modified Merrill–Bassett type DAC thatconsists of a pair of Type Ia diamonds with 600 µm flatculets. As gibbsite has strong (001) cleavage, the (001)plane of the sample was placed parallel to the diamond
culet. An SUS 301 stainless steel plate of 250 µm
thickness and with a 300-µm diameter hole in the center
was used as a gasket. FluorinertTM (FC-40, 3M) was used
as a fluid pressure transmitting medium. No ruby chip for
determining pressure was mounted to avoid extra spots.
The wavelength of the synchrotron radiation wascalibrated to λ = 0.7003(1) Å from lattice constants of aruby standard crystal at ambient conditions. The fourcircle angles of 45 reflections in the 2θ range between 9º
and 31º were centered for determining the correct crystal
system and lattice constants. Lattice constants refinedassuming a triclinic unit cell were a = 8.612(3), b =5.013(2), c = 9.194(5), α = 90.34(7), β = 90.00(6), and γ= 90.00(4). The obtained α angle was significantly
different from 90º; thus η-Al(OH)3 is unequivocallyconfirmed as a monoclinic phase. Lattice constants withconstraints of a monoclinic cell were a = 8.612(3) Å, b =5.013(2) Å, c = 9.194(5) Å, and α = 90.34(6)º. Reflection
conditions derived from the indexed reflections were 0kl:
k=2n and h00: h=2n. According to reflection conditions,the space group of η-Al(OH)3 was determined uniquely as
P21/b11(#14). X-ray diffraction intensities were collectedup to sinθ/λ < 0.603 (maximum 2θ = 50º) by ω scan withthe fixed φ method. Although a relatively large crystal(0.2 x 0.1 x 0.1 mm) and synchrotron radiation were used,
the observed reflections were weak and broad; this meansthat η-Al(OH)3 has low crystallinity. Extinction andabsorption corrections for the crystal were not applied.
Data were corrected for Lorentz and polarization effects.
The model of crystal structure except for H atoms wassolved by a direct method using the Sir2004 program
package [4]. Atomic coordinates and isotropic atomic
displacement parameters except for hydrogen atoms wererefined using SHELXL from the SHELX-97 program
package [5]. We were unable to resolve the H positions ofη-Al(OH)3 in the D-Fourier map. This may be due tofactors such as weak and broad reflections with large
uncertainties, noise from Be backing plates or thestainless steel gasket, or the effect of unobserved 00l
reflections related to the sample setting in DAC.
The solved crystal structure of η-Al(OH)3 consists ofan Al octahedral layer as well as gibbsite [γ-Al(OH)3].
Comparing the crystal structures between gibbsite and ηAl(OH)3 (see Fig. 1), it is expected that the mechanism ofthe transition from gibbsite to η-Al(OH)3 approximately
involves a shift of Al octahedral layers with respect toeach other by the vector a/2 or b/2.
η-Al(OH) 3Gibbsite [γ-Al(OH)3]

bb
aaab

Al1

Al2Fig. 1: Crystal structure of (a) gibbsite [γ-Al(OH)3] and(b) η-Al(OH)3. The solid line denotes the unit cell. To
clarify Al octahedral stacking, H atoms are not describedhere.
References[1] F. Dachille and P. Gigl, High Temp-High Press, 15,657 (1983)[2] E. Huang et al., Phys Chem Minerals, 26, 576 (1999)[3] H. Liu et al., Phys Chem Minerals, 31, 240 (2004)[4] M.C. Burla et al., J. Appl. Cryst. 38, 381 (2005)[5] G.M. Sheldrick: SHELXL97. University of Gottingen,Germany (1997)* komatsu@ganko.tohoku.ac.jp

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