ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
The line profiles of X-ray reflexions from different MnxFe3 − xO4 samples were investigated. Sample I (x = 1.66) is cubic with no anomalies; small paracrystalline distortions, g 〈 0.1%, cannot be excluded and are calculated to be of this magnitude from considerations of a simple volume effect of the larger Mn3+ ions. Samples II–IV (x = 1.88) show interesting effects which are closely connected with the development of the tetragonal structure, observed when x 〉 1.80. Sample II (quenched) shows paracrystalline distortions which can be quantitatively explained on the basis of the Jahn–Teller effect: the tragonally deformed single Mn3+O2−6 octahedra are statistically oriented and statistically distributed over B sites and have a mean tetragonality {\bar \epsilon} = 0.014. In sample III (cooling rate 20°C.min−1) about 26% of the volume consists of microdomains which result from a correlation between the orientations and positions of Jahn–Teller octahedra; quantitive agreement with the theory can be obtained by introducing a correlation factor γ = 2. In sample IV (cooling rate 7°C.min−1) this correlation proceded in such a way that the crystal consisted of 74% tetragonal matrix which had already attained a value of c/a = 1.056, and about 22% of tetragonal microdomains oriented in two other directions with a value c/a 〈 1.056. In addition about 3 to 5% of a cubic phase is present. If the mean tetragonality of the microdomains is taken to be {\bar \epsilon} = 0.020 the experiments correspond quantitatively with the theory. The detailed study of sample III proves H¼ ¼ ¼ are the paracrystalline coordination statistics with a fluctuation α = 27 XU in the direction [111] and β = 38 XU perpendicular to it. This is explained by a model of the spinel structure having two kinds of blocks: a tetrahedral A-structure and Jahn–Teller affected octahedral subcubes at B sites; the separation distance is ¼ √3a = 3.7 Å.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S0567739470000669
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