How Cr3+ and Fe3+ affect Mg-Al order-disorder transformation at high temperature in natural spinels

Three natural Mg(Al2-yCry)O-4 spinels (y similar to0.07-0.16), highly ordered in terms of Mg-Al, and one Mg(Al2-yFey3+)O-4 spinel (ysimilar to0.08), highly ordered also in terms of Fe3+, were studied by means of X-ray single-crystal diffraction. All samples were heated in situ from 25 to 1000 degreesC in order to follow both thermal expansion and evolution of the structural state of spinel with temperature. Thermal expansion was monitored by means of the variation of cell edge a with temperature, and found to be well represented throughout the temperature range by a regression line a = a(0) (1 + alphaDeltaT), slightly different at lower and higher temperatures. Thermal expansion coefficient alpha(1), referring to the lower temperature range (i.e. during pure thermal expansion), was slightly lower than alpha(2), calculated only over the highest temperatures. The trend showed different slopes for individual crystals. Structural evolution with temperature was studied by means of the variation of oxygen positional parameter u, which is strongly influenced by intersite cation exchange and thus closely correlated with inversion parameter x. In particular, in the three Cr samples, in which Cr resides only in the octahedral site, u parameter variations and hence the order disorder process, started at about 700 degreesC. Instead, in the Fe3+ sample, this process was triggered at lower temperatures, starting at 550 degreesC with Fe3+-Mg exchange followed at higher temperatures by that of Mg-Al. Cr contents in the Cr samples affected the occupancy of Al in the tetrahedral site at the highest temperatures. In both Mg-Al-Cr and Mg-Al-Fe3+ compositions, if Cr-Fe3+, parameter u reached the same value only when the Mg-Al exchange was dominant, i.e. at the highest temperatures, but not before. Cation distribution at each temperature was obtained by the bond-length model, applying thermal expansion to pure bond lengths. This method is applied here to complex compositions for the first time.