Site occupancy in gamma alumina

1996 ◽  
Vol 54 ◽  
pp. 654-655
Author(s):  
C. A. Bateman ◽  
A.Z. Ringwelski ◽  
R.W. Broach
Keyword(s):  
Spinel Structure ◽  
Site Occupancy ◽  
Unit Cell ◽  
Neutron Diffraction Data ◽  
Gamma Alumina ◽  
Tetrahedral Sites ◽  
Powder Neutron Diffraction ◽  
Cation Sites ◽  
Powder Neutron Diffraction Data ◽  
Insufficient Number

Gamma (γ) alumina is referred to as a defect spinel because it has a tetragonally distorted spinel structure (AB2O4) and an insufficient number of cations to fill all cation sites. In the spinel structure, the oxygen lattice is cubic close packed with A- and B-site cations in tetrahedral and octahedral coordination, respectively. The 2l⅓ Al atoms per unit cell of γ alumina can distribute themselves across 16 octahedral and 8 tetrahedral sites.The literature differs on where the 2⅔ cation vacancies per unit cell are located. Wilson and McConnell proposed that the vacancies in γ alumina, as first formed by calcining boehmite, are predominantly on the tetrahedral lattice but, with further heat treatment, move to occupy random positions on both octahedral and tetrahedral lattices. One study using NMR showed that the vacancies lay exclusively on the tetrahedral lattice, independent of the calcination temperature. A more-recent study using Rietveld refinement of powder neutron diffraction data suggested that both octahedral and tetrahedral lattices were partially occupied.

The crystal structure of Zr2NiD4.5

2006 ◽  
Vol 62 (6) ◽  
pp. 972-978 ◽  
Author(s):  
M. H. Sørby ◽  
A. E. Gunnæs ◽  
O. M. Løvvik ◽  
H. W. Brinks ◽  
H. Fjellvåg ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Neutron Diffraction Data ◽  
X Ray Diffraction ◽  
Functional Theory ◽  
X Ray ◽  
Tetrahedral Sites ◽  
Powder Neutron Diffraction ◽  
Powder Neutron Diffraction Data

The crystal structure of Zr2NiD4.5 has been determined by a combination of synchrotron radiation powder X-ray diffraction, electron diffraction and powder neutron diffraction data. Deuterium ordering results in a triclinic supercell given by a super = 6.81560 (7), b super = 8.85137 (9), c super = 8.88007 (10) Å, αsuper = 79.8337 (8), βsuper = 90.0987 (9), γsuper = 90.3634 (9)°, which relates to the non-super unit cell as a super = −a, b super = −b − c, c super = −b + c. The centrosymmetric and fully ordered deuterium sublattice was determined by simulated annealing and Rietveld refinement. Deuterium was found to occupy three types of tetrahedral sites: two that are coordinated by four Zr atoms and one that is coordinated by three Zr atoms and one Ni atom. All D—D distances are longer than 2 Å. The feasibility of the crystal structure was supported by density functional theory calculations.

Commensurate and Incommensurate Oxide Structures Related to 2H Perovskite

1998 ◽  
Vol 547 ◽  
Author(s):  
P. D. Battle ◽  
G.R. Blake ◽  
J.C. Burley ◽  
E. J. Cussen ◽  
J. Sloan ◽  
...  
Keyword(s):  
Neutron Diffraction ◽  
Diffraction Data ◽  
Nearest Neighbour ◽  
Neutron Diffraction Data ◽  
Magnetic Structures ◽  
Powder Neutron Diffraction ◽  
Powder Neutron Diffraction Data

AbstractThe magnetic structures of Ca3LiRuO6 and Ca3NaRuO6 have been deduced from powder neutron diffraction data. In each case successive Ru5+ cations in the [001] chains of the Sr4PtO6-like structure are antiferromagnetically coupled, and nearest-neighbour Ru5+ cations in adjacent chains are also antiferromagnetically coupled.

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