Ordnung/Unordnung und Ultrarotabsorption IV. Die Absorption einiger Metalloxyde mit Spinellstruktur

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Abstract
Infrared absorption measurements of a number of metal oxides with spinel structure have been carried out in the range of 10 to 30 μ . All spinels show mainly two chief groups of absorption ( A and B ). By preparing a number of mixed crystal series it was possible to determine the mutual relations of A and B and the dependence of their wave lengths on chemical composition and lattice constants. The force constants of the normal vibrations were calculated according to the approximation of W aldron (1955). While A corresponds approximately to a valency vibration between the tetrahedral cations and oxygen, B approximates to a valency vibration between the octahedral cations and oxygen. The following relation exists between the spinel structure (with given composition and cation distribution) and the absorption spectrum: 1. The half width of the absorption is mainly determined by “near symmetry” (Nahsymmetrie) of the individual cations. Different types of cation in the same lattice points lead to increased half width. 2. The force constants of the cation-anion valency vibrations diminish with increasing values of the Goldschmidt radius of the cation. The influence of covalent bonding can definitely be observed (increase of the force constant in hybrid compounds). 3. The force constants depend upon the distribution between tetrahedral and octahedral sites of the cations present in the structure. (This was examined on CuFe 2 O 4 and MgAl 2 O 4 –Al 2 O 3 .)

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