Compression studies of gibbsite and its high-pressure polymorph

Various X-ray diffraction methods have been applied to study the compressio n 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 sta rted to convert to its high-pressure polymorph. The high-pressure (HP) phas e 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 ene rgy dispersive methods. On the basis of this structural interpretation, the bulk modulus of the two polymorphs, i.e., gibbsite and nordstrandite, coul d be determined as 85 +/- 5 and 70 +/- 5 GPa, respectively, by fitting a Bi rch-Murnaghan equation to the compression data, assuming their K-o' as 4. M olar volume cross-over occurs at 2 GPa, above which the molar volume of nor dstrandite is smaller than that of gibbsite. The differences in the molar v olume and structure between the two polymorphs are not significant, which a ccounts for the irreversibility of the phase transition. In gibbsite, the a xial compressibility behaves as c/c(o) > a/a(o) > b/b(o). This is due to th e fact that the dioctahedral sheets along the c-axis are held by the relati vely weak hydrogen bonding, which results in the greater compressibility al ong this direction. In nordstrandite, the axial compressibility is b/b(o) > c/c(o) > a/a(o), which can also be interpreted as resulting from the the e xistence of hydrogen bonds along the b-axis.