Incommensurate phase of the stuffed tridymite derivative BaSrFe₄O₈

We demonstrate a new modulation for a high-temperature polymorph of BaSrFe₄O₈ via synchrotron powder X-ray diffraction and transmission electron microscopy (TEM) experiments. Although the powder X-ray diffraction pattern was successfully analyzed using a structure model of space group $\mathrm{Pnna}$, an unusually large isotropic atomic displacement parameter ($U_{\mathrm{iso}}$) was observed for the O1 atom. A split atom model that was applied to the O1 site and the adjacent Ba/Sr site, both of which are on the two-fold axis along the c-axis, indicated that positional disorder occurred in these atoms along the c-axis. The electron diffraction pattern and high-resolution TEM observations combined with the structure refinements revealed an incommensurate structure with a modulation vector $q=0.4270\left(1\right)a^{*}$ and a displacement vector perpendicular to the a-axis, which gave rise to the large $U_{\mathrm{iso}}$ value of the O1 atom. The modulation observed in this study may be a signature of structural instability, which has commonly been discussed in the stuffed tridymite-type oxides.