Compression studies of gibbsite and its high-pressure polymorph |
| |
| Authors: | E Huang J-F Lin J Xu T Huang Y-C Jean H-S Sheu |
| |
| Institution: | (1) Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan, R.O.C., TW;(2) Geophysical Science, University of Chicago, Chicago, IL., U.S.A., US;(3) Synchrotron Radiation Research Center, Hsinchu, Taiwan, R.O.C., TW |
| |
| Abstract: | Various X-ray diffraction methods have been applied to study the compression behavior of gibbsite, Al(OH)3, in diamond cells at room temperature. A phase transformation was found to take place above 3 GPa where gibbsite started
to convert to its high-pressure polymorph. The high-pressure (HP) phase is quenchable and coexists with gibbsite at the ambient
conditions after being unloaded. This HP phase was identified as nordstrandite based on the diffraction patterns obtained
at room pressure by angle dispersive and energy dispersive methods. On the basis of this structural interpretation, the bulk
modulus of the two polymorphs, i.e., gibbsite and nordstrandite, could be determined as 85 ± 5 and 70 ± 5 GPa, respectively,
by fitting a Birch-Murnaghan equation to the compression data, assuming their Ko
′ as 4. Molar volume cross-over occurs at 2 GPa, above which the molar volume of nordstrandite is smaller than that of gibbsite.
The differences in the molar volume and structure between the two polymorphs are not significant, which accounts for the irreversibility
of the phase transition. In gibbsite, the axial compressibility behaves as c/c
o > a/a
o > b/b
o. This is due to the fact that the dioctahedral sheets along the c-axis are held by the relatively weak hydrogen bonding, which results in the greater compressibility along this direction.
In nord- strandite, the axial compressibility is b/b
o > c/c
o > a/a
o, which can also be interpreted as resulting from the the existence of hydrogen bonds along the b-axis.
Received: 28 September 1998 / Revised, accepted: 22 December 1998 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
