Synthesis Cubic BaSnF 4 was synthesized via a ballmilling process, using a planetary mill (Fritsch Pulverisette 6). Precursors (SnF 2 , Sigma-Aldrich, 99 %; BaF 2 , 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 glove box, 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 hours, divided into 24 cycles. Each cycle consisted of 15 minutes of milling and 15 minutes of pause, which prevented overheating.