Fig. 2. Superstructure and magnetization of Li_xFe₇Se₈.

(A) Powder x-ray diffraction (XRD) of Li_0.53Fe₇Se₈ and Fe₇Se₈ with both Cu and Co Kα radiations. To see the superstructure peaks clearly, low-angle parts are shown exclusively. They are 8° to 31° and 8° to 36° for Cu and Co targets, respectively. These patterns are collected for long and same time duration. Peaks corresponding to superstructure are marked by red arrows. The number on each peak is its intensity counts (background is subtracted). These superstructure peaks of Fe₇Se₈ can be seen clearly with both Cu and Co Kα radiations, but systematically disappear in Li_0.53Fe₇Se₈. Red dot lines show the shifts of the peaks of fundamental NiAs structure, indicating the enlargement of lattice parameters after doping. (B) Suppression of magnetism of Li_xFe₇Se₈ with the increment of lithium content. Inflections at around 130 to 150 K are caused by residual Fe₇Se₈, which experiences spin reorientation at this temperature (33). The magnetization of Li_0.53Fe₇Se₈ is multiplied by 30 for clarity.