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Site Preference of Cations and Structural Variation in MgAl 2–x Ga x O 4 (0 ≤ x ≤ 2) Spinel Solid Solution
Author(s) -
Ito Tomokazu,
Nakatsuka Akihiko,
Maekawa Hideki,
Yoshiasa Akira,
Yamanaka Takamitsu
Publication year - 2000
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/(sici)1521-3749(200001)626:1<42::aid-zaac42>3.0.co;2-o
Subject(s) - bond length , solid solution , ionic radius , chemistry , crystallography , spinel , lattice constant , ionic bonding , crystal (programming language) , crystal structure , analytical chemistry (journal) , diffraction , materials science , ion , physics , organic chemistry , programming language , computer science , optics , metallurgy
The crystal structures of MgAl 2–x Ga x O 4 (0 ≤ x ≤ 2) spinel solid solutions ( x = 0.00, 0.38, 0.76, 0.96, 1.52, 2.00) were refined using 27 Al MAS NMR measurements and single crystal X‐ray diffraction technique. Site preferences of cations were investigated. The inversion parameter ( i ) of MgAl 2 O 4 ( i = 0.206) is slightly larger than given in previous studies. It is considered that the difference of inversion parameter is caused by not only the difference of heat treatment time but also some influence of melting with a flux. The distribution of Ga 3+ is little affected by a change of the temperature from 1473 K to 973 K. The degree of order‐disorder of Mg 2+ or Al 3+ between the fourfold‐ and sixfold‐coordinated sites is almost constant against Ga 3+ content ( x ) in the solid solution. A compositional variable of the Ga/(Mg + Ga) ratio in the sixfold‐coordinated site has a constant value through the whole compositional range: the ratio is not influenced by the occupancy of Al 3+ . The occupancy of Al 3+ is independent of the occupancy of Ga 3+ , though it depends on the occupancy of Mg 2+ according to thermal history. The local bond lengths were estimated from the refined data of solid solutions. The local bond length between specific cation and oxygen corresponds with that expected from the effective ionic radii except local Al–O bond length in the fourfold‐coordinated site and local Mg–O bond length in the sixfold‐coordinated site. The local Al–O bond length in the fourfold‐coordinated site (1.92 Å) is about 0.15 Å longer than the expected bond length. This difference is induced by a difference in site symmetry of the fourfold‐coordinated site. The nature that Al 3+ in spinel structure occupies mainly the sixfold‐coordinated site arises from the character of Al 3+ itself. The local Mg–O bond length in the sixfold‐coordinated site (2.03 Å) is about 0.07 Å shorter than the expected one. Difference Fourier synthesis for MgGa 2 O 4 shows a residual electron density peak of about 0.17 e/Å 3 in height on the center of (Ga 0.59 Mg 0.41 )–O bond. This peak indicates the covalent bonding nature of Ga–O bond on the sixfold‐coordinated site in the spinel structure.