Baf 2

Nanostructured La ${}_{0.9}$ Ba ${}_{0.1}$ F ${}_{2.9}$ was mechanosynthesized with the help of a high-energy planetary mill (Fritsch Pulverisette 7 Premium line, Fritsch GmbH, Idar-Oberstein, DE). For this purpose, stoichiometric amounts of the educts viz. LaF ${}_3$ (99.99%, Alfa Aesar, Kandel, DE) and BaF ${}_2$ (99.99%, Sigma Aldrich, Darmstadt, DE) were loaded into a ZrO ${}_2$ milling beaker with a volume of 45 mL. We added 180 balls made of the same material; the diameter of each milling ball was 5 mm. The milling procedure was carried out at a rotation speed of 600 rpm; the mixture was milled for 10 h whereby 15 min milling was followed by a break of 15 min to allow cooling of the mixture and the beaker. Loading as well as unloading of the beakers was strictly carried out under inert atmosphere; we used an Ar-filled glovebox (O ${}_2$ , H ${}_2$ O < 0.5 ppm) to avoid any contamination by water vapor or moisture.

Cubic BaSnF₄ was synthesized via
a ball-milling process using a planetary mill (Fritsch Pulverisette
6). Precursors (SnF₂, Sigma-Aldrich, 99%; BaF₂, Sigma-Aldrich 99.99%) were dried at 150 °C under vacuum
for 3 h and stored under Ar inert atmosphere. The desired amounts
of precursors were weighed and sealed in Zirconia milling jars in
an argon-filled glovebox, with a powder-to-ball ratio of 1:13. The
balls were 10 mm in diameter and made out of zirconia. The precursors
were then milled at 400 rotations/min for 12 h, divided into 24 cycles.
Each cycle consisted of 15 min of milling and 15 min of pause, which
prevented overheating.