Structural changes induced by cation ordering in ferrotapiolite

Structural modifications as a function of the degree of order (Q) in FeTa2O6ferrotapiolite have been characterized by means of single-crystal X-ray diffraction (SC-XRD). A total of 26 datasets covering the range ofQbetween 0.154 and 1 have been obtained by thermal treatments followed by quenching of natural tapiolite crystals. Ordering of Fe2+at theAsites and of Ta5+at theBsites causes a linear increase in thea/clattice constants ratio, as a consequence of a linear decrease of the c dimension and only slight modifications of theaparameter. Calibration ofa/c vs. Qrepresents a very useful tool for a rapid determination of the degree of order of tapiolite samples. Polyhedral volumes of the two octahedral sites vary linearly withQas a consequence of the different ionic radii of the two species. Both the sites remain almost regular at allQvalues but theBsite shows an increasing off-centre displacement of the cation with increasingQ.Observed structure factors of supercell reflections, characterized byl≠ 3n, increase linearly as a function ofQ,thus representing a further tool for a quick evaluation of the degree of order.