All samples were prepared using BaTiO 3 (500 nm particle size, 99.9%, ABCR GmbH), which was dried in an oven at 200 for 14 h. The steps involving sample preparation for synthesis were performed in an Ar-filled glovebox. The oxyhydride sample containing hydrogen was obtained by mixing BaTiO 3 (BTO) with CaH 2 in molar proportions of 1:0.6. Approximately 10 g of the mixture was pressed into pellets and placed inside a sealed stainless steel ampule and heated in an evacuated silica jacket at a temperature of 600 for 48 h. The deuterated samples (BTOD NAB and BTOD CA ) were synthesized following a similar procedure, but instead using NaBD 4 ( 98%, Sigma) and CaD 2 . In both synthesis the precursors were mixed at the same BTO to reducing agent proportion as before (1:0.6). CaD 2 was prepared as described previously. The third sample (BTOD EXCH ) was obtained by first reducing BTO with CaH 2 in molar proportion of 1:2.25 under identical conditions as given above. Subsequently, hydrogen to deuterium exchange was performed by subjecting the sample to D 2 (99.9%, AGA) in a stainless-steel autoclave pressurized to 30 bar at 600 for 12h. Afterwards, the products were washed 3-4 times with 0.1 M NH4Cl/Methanol solution (BTOH, BTOD CA and BTOD EXCH ) or 0.1 M hydrochloric acid (BTOD NAB ). Finally, all samples were dried under vacuum at 120 C for ⇠ 24 h.