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The crystal structure of delta -Al(OH) (sub 3) ; neutron diffraction measurements and ab initio calculations

Masanori Matsui, Kazuki Komatsu, Emi Ikeda, Asami Sano-Furukawa, Hirotada Gotou and Takehiko Yagi
The crystal structure of delta -Al(OH) (sub 3) ; neutron diffraction measurements and ab initio calculations
American Mineralogist (May 2011) 96 (5-6): 854-859


delta -Al(OD) (sub 3) powders were synthesized from Al(OD) (sub 3) bayerite at 4 GPa and 523 K using a cubic press apparatus. Neutron powder diffraction analyses of delta -Al(OD) (sub 3) at ambient conditions revealed that the crystals are orthorhombic with space group P2 (sub 1) 2 (sub 1) 2 (sub 1) , not Pnma as reported previously based on X-ray diffraction data. The P2 (sub 1) 2 (sub 1) 2 (sub 1) delta -Al(OH) (sub 3) structure contains seven independent atoms in the asymmetric unit, including one Al, three O, and three H atoms. The initial lattice parameters and the atomic positions of both Al and O were taken from previous X-ray structural analyses of the Pnma structure, while the positions of H were determined in the present study using ab initio calculations to (1) give the least energy among trial structural models for P2 (sub 1) 2 (sub 1) 2 (sub 1) delta -Al(OH) (sub 3) , (2) accurately reproduce the measured lattice parameters of delta -Al(OD) (sub 3) , and (3) show reasonable energetic relations between the Al(OH) (sub 3) polymorphs; namely, gibbsite is stable at ambient pressure, delta -Al(OH) (sub 3) has the lowest enthalpy at pressure greater than 1.1 GPa, and both bayerite and eta -Al(OH) (sub 3) are metastable over the entire pressure range. Furthermore, we found that the structure of delta -Al(OH) (sub 3) obtained from ab initio calculations is in good agreement with that derived from a Rietveld refinement of delta -Al(OD) (sub 3) , based on the present powder neutron diffraction data. The delta -Al(OH) (sub 3) structure possesses one relatively long and two short O-H...O hydrogen bonds. Ab initio calculations also reveal that delta -Al(OH) (sub 3) with space group P2 (sub 1) 2 (sub 1) 2 (sub 1) transforms to another high-pressure polymorph with space group Pnma at around 67 GPa, and that the two short hydrogen bonds in delta -Al(OH) (sub 3) become both symmetric through the P2 (sub 1) 2 (sub 1) 2 (sub 1) to Pnma transformation, in which the protons are located at the midpoints of the O...O hydrogen bonds.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 96
Serial Issue: 5-6
Title: The crystal structure of delta -Al(OH) (sub 3) ; neutron diffraction measurements and ab initio calculations
Affiliation: University of Hyogo, School of Science, Hyogo, Japan
Pages: 854-859
Published: 201105
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 24
Accession Number: 2011-048412
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
Secondary Affiliation: University of Tokyo, JPN, JapanJapan Atomic Energy Agency, JPN, Japan
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201127
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