Intracrystalline disorder in spinels: sensitivity of cell parameter a and Mossbauer data

In spinels, accurate determination of cell edge a is crucial, if X-ray powd er diffraction methods are used to derive the variations in cation distribu tions due to thermal treatment. The reproducibility of this parameter was t ested by carrying out several a measurements on two spinel samples (VASA an d CR5A), the first a near-spinel end-member and the second a chromium-rich sample, both untreated and quenched from high temperatures. Further, Mossba uer spectra were performed on the same powders with the aim of understandin g the intracrystalline distributions responsible for the detected variation s in a. Experiments on quenched samples showed an a decrease greater in VAS A than in CR5A. Moreover, untreated VASA showed greater reproducibility in unit cell determination than CR5A even when an internal standard was used i n the latter sample. Repeated experiments quite satisfactorily substituted an internal standard when such a standard could not be used for experimenta l reasons (i.e. heat treatment of the same sample). Mossbauer spectroscopy of CR5A indicated that heat treatment produced a lim ited exchange of cations with the same electric charge between the tetrahed ral and octahedral sites, and thus a small decrease in a. Instead, in VASA, the greater changes in a which occurred after heat treatment may have been due to considerable substitution of cations with different charges (i.e. M g2+ and Al3+) between the two non-equivalent sites of the structure, as evi denced by the dramatic changes in the spectra.