Diffuse X-ray Scattering and Short-Range Order in BaFe₂Al₉

Unlike most iron-based intermetallics, BaFe₂Al₉ does not show any magnetic ordering. However, it can host significant charge ordering, leading to a strongly first-order structural transition at 90 K so distortive that large single crystals typically shatter upon going through it. This low-temperature phase has broad, incommensurate superstructure peaks indicating a possible charge density wave. Substituting Sr for Ba increases the transition temperature and shifts its character to second-order, and larger single crystals can withstand this transition.

Diffuse x-ray scattering measurements were performed on single crystals of BaFe₂Al₉ and (Sr,Ba)Fe₂Al₉ to investigate the superstructure peaks and possible short-range order as a function of temperature. While the chemical heterogeneity has little direct effect on the structure, the Sr-substituted samples show enhanced short-range strain fields near the transition, suggesting that a mild disruption of the CDW phase helps stabilize the structure. The CDW phase itself is very similar for all samples, showing an interplay between Al tetrahedra and Ba/Sr columns with a slightly frustrated character.