Redetermination of the cation distribution of spinel (MgAl2O4) by means of neutron diffraction

Abstract

2014 The cation distribution of synthetic spinel powder of grain size ~ 50 03BC was redetermined by means of neutron diffraction experiments. The oxygen parameter found by Bacon (1952) has been confirmed, but, in contradiction to Bacon, the degree of inversion amounts to about 10 to 15 %, and depends upon the thermal history of the sample. LE JOURNAL DE PHYSIQUE TOME ‘~5, MAI 196~, Magnesium aluminium oxyde crystallizes in the space group Fd3n?. The unit cell can be described by a nearly close packed cubic oxygen arrangement with built-in cations, which are either tetrahedrally or almost octahedrally surrounded by the oxygen atoms. The lattice parameter of the unit cell amounts to 8.0832 A, and there are eight molecules per cell. The spinel structure is characterized by the fact that, relative to the center of symmetry, the cations are located at fixed positions, while the oxygen positions depend upon a parameter u. The spinel structure is called " normal " if the Mg atoms occupy the tetrahedral A-sites, only. However, on the space average, any fraction of Mg can be replaced by Al atoms. This fraction of replacement is called " degree of inversion " and denoted by i. The knowledge of the degree of inversion, and thus of the cation distribution, is of special interest in connection with possible changes of the structure due to the thermal history of the substance. Thus far, two basically different methods have been employed for the problem in question. One is to use nuclear resonance techniques, as was done by Brun and coworkers [1]. They found that synthetically produced Mg-Al spinels are considerably inverse, while naturally grown Mg-Al spinels exhibit approximately the normal spinel structure, when not annealed beyond about 800 °C. However, once annealed at temperatures higher than 800 OC, natural spinel will be inverse to a large degree. A second, and more reliable method to determine the degree of inversion, and thus the cation distribution, is provided by neutron diffraction. From the equation of the structure factor where and = iiinctions of u, i .--degree of inversion and = inn-lear scattering amplitude, it follows that L can be obtained with accuracy, since the b-values of the two cation constituents differ considerably. Bacon [2] was the first to use the above relationship in order to determine the degree of inversion in synthetic spinel powder samples. He found, in contradiction to Brun and coworkers, a nearly vanishing i-value, i. e. a normal spinel structure with at most a slight tendency towards inversion. Because of this discrepancy, it appeared desirable to repeat the neutron diffraction experiments, and to investigate the effect of annealing the sample on its structure. Since Bacon could, in 1952, perform his diffraction work with low resolution only, the aim in our experiments was to improve the resolution. The investigations were performed with the 20 MW reactor " Diorit " of the Swiss Federal Institute for Reactor Research. Synthetic and stoichiometric spinel powder (# 50 tl) was used and both the AI(ill) and Al(311) reflections were employed for monochromatizing the neutron beam. Figures 1 and 2 represent the observed angular distribution of the diffracted intensity. As can be seen by comparison, the Al (311) monochromator produces a much better resolution, especially at high angles, than does Al (111), in good agreement with Caglioti’s [3] theory. The evaluation of our diagrams confirm the oxygen parameter found by Bacon : u = 0,387 -~0,001. However, our spinel sample cannot be described by the normal spinel structure. The experimentally observed intensities agree best with the calculated ones, if one assumes a degree of inversion of roughly between 10 and 15 %. In addition, it was found that this value depends to a certain extent upon the thermal treatment of the sample. In order to obtain more detailed information about this influence, two annealing procedures were made with the same sample : in a first experiment, the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505044700