Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE-GGA

K-point grid: 7x5x7

Level of relativity: Non-relativistic

Basis set definition: Plane-wave with cutoff 900 eV

Numerical accuracy: Tolerance for energy minimization 10e-9 eV/atom. interatomic forces after relaxation were below 0.005 eV/Å, and the stresses were below 0.05 GPa

Comment: Local structure optimization: Unit cell + atomic positions [with symmetry constraints], Geometry input source: From Expt ID 19

• Rayan C Duke University

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE-GGA

K-point grid: 2x5x7

Level of relativity: Non-relativistic

Basis set definition: Plane-wave with cutoff 900 eV

Numerical accuracy: Tolerance: 10e-9 eV

Comment: Local structure optimization: Unit cell + atomic positions [with symmetry constraints], Geometry input source: From Expt ID 19

Starting materials: CD3ND2DCl (Sigma-Aldrich, 98 atom % D), DI, D2O, PbI2 (Acros Organics)

Product: Black d6-MAPbI3 powder

Description: Dissolve 1.0 g of CD3ND2DCl in 15 g of 16% DI in D2O (made by dissolving 5 g DI gas in 25 g D2O) and pump to dryness to yield CD3ND2DI. Add material to 6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent. Expect to yield 7.95 g of d6-MAPbI3 (97% yield).

Comment: Deuterated sample MAPbI3 (d6-CD3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The sample size was ~5.5 g for the d6-MAPbI. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Refer to SI: Supplementary Table 1 and Additional dataset; CCDC.

Comment: Deuterated sample MAPbI3 (d6-CD3ND3PbI3)

• Rayan C Duke University

Starting materials: CD3ND2DCl (Sigma-Aldrich, 98 atom % D), DI, D2O, PbI2 (Acros Organics)

Product: Black d6-MAPbI3 powder

Description: Dissolve 1.0 g of CD3ND2DCl in 15 g of 16% DI in D2O (made by dissolving 5 g DI gas in 25 g D2O) and pump to dryness to yield CD3ND2DI. Add material to 6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent. Expect to yield 7.95 g of d6-MAPbI3 (97% yield).

Comment: Deuterated sample: MAPbI3 (d6-CD3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The sample size was ~5.5 g for the d6-MAPbI. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Refer to SI: Supplementary Table 4 and Additional dataset; CCDC.

Comment: Deuterated sample: MAPbI3 (d6-CD3ND3PbI3)

• Rayan C Duke University

Starting materials: CD3ND2DCl (Sigma-Aldrich, 98 atom % D), DI, D2O, PbI2 (Acros Organics)

Product: Black d6-MAPbI3 powder

Description: Dissolve 1.0 g of CD3ND2DCl in 15 g of 16% DI in D2O (made by dissolving 5 g DI gas in 25 g D2O) and pump to dryness to yield CD3ND2DI. Add material to 6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent. Expect to yield 7.95 g of d6-MAPbI3 (97% yield).

Comment: Deuterated sample: MAPbI3 (d6-CD3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The sample size was ~5.5 g for the d6-MAPbI. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Refer to SI: Supplementary Table 5.

Comment: Deuterated sample: MAPbI3 (d6-CD3ND3PbI3)

• Rayan C Duke University

Starting materials: CD3NH2 (Sigma-Aldrich, 99 atom % D)

Product: CD3NH3PbI3

Description: The CD3NH3PbI3 was prepared using CD3NH2 that was 99 atom % D. Synthesis perovskite using a similar method as 4-a.

Comment: Partially deuterated sample: MAPbI3 (d3-CD3NH3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Refer to Additional dataset; CCDC.

Comment: Partially deuterated sample: MAPbI3 (d3-CD3NH3PbI3)

• Rayan C Duke University

Starting materials: CH3NH2, HI, D2O

Product: CH3ND3PbI3

Description: Preparation of CH3ND3I: React methylamine gas with HI to yield methyl ammonium iodide. Exchange the two H atoms attached to the nitrogen atoms with D by dissolving the salt in 10 ml D2O (99 atom % D), drying under vacuum, and then repeating two more times. The resulting CH3ND3I was estimated to be better than 98 atom% D on the ammonium group. Synthesis perovskite using a similar method as 4-a.

Comment: Partially deuterated: MAPbI3 (d3-CH3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Refer to Additional dataset; CCDC.

Comment: Partially deuterated: MAPbI3 (d3-CH3ND3PbI3)

• Rayan C Duke University

Starting materials: CD3ND2DCl (Sigma-Aldrich, 98 atom % D), DI, D2O, PbI2 (Acros Organics)

Product: Black d6-MAPbI3 Powder

Description: Dissolve 1.0 g of CD3ND2DCl in 15 g of 16% DI in D2O (made by dissolving 5 g DI gas in 25 g D2O) and pump to dryness to yield CD3ND2DI. Add material to 6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent. Expect to yield 7.95 g of d6-MAPbI3 (97% yield).

Comment: MAPbI3 (d6-CD3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove- box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Heating and cooling rates were 1K/min. Refer to Page 10 Table 2.

• Rayan C Duke University

Starting materials: CD3ND2DCl (Sigma-Aldrich, 98 atom % D), DI, D2O, PbI2 (Acros Organics)

Product: Black d6-MAPbI3 Powder

Description: Dissolve 1.0 g of CD3ND2DCl in 15 g of 16% DI in D2O (made by dissolving 5 g DI gas in 25 g D2O) and pump to dryness to yield CD3ND2DI. Add material to 6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent. Expect to yield 7.95 g of d6-MAPbI3 (97% yield).

Comment: MAPbI3 (d6-CD3ND3PbI3)

Method: Synchrotron X-ray powder diffraction

Description: The X-ray powder diffraction measurements were performed on the bending magnet station at DND-CAT sector 5 of the Advanced Photon Source. The X-ray wavelength used was 0.40012(2) Å, selected to reduce X-ray absorption by the sample to an acceptable level. Temperatures were equilibrated for ~10 min after 1 K changes. Refer to Page 10 Table 2.

• Rayan C Duke University

Starting materials: CH3ND3I

Product: Black CH3ND3PbI3 Powder

Description: Preparation of CH3ND3I: React methylamine gas with HI to yield methyl ammonium iodide. Exchange the two H atoms attached to the nitrogen atoms with D by dissolving the salt in 10 ml D2O (99 atom % D), drying under vacuum, and then repeating two more times. The resulting CH3ND3I was estimated to be better than 98 atom% D on the ammonium group. Add material to ~6 g of lead iodide and mix in ~25mL of DMF. Upon stirring, obtain pale yellow clear solution. Warm resulting solution and stir overnight in a N2 glove box. Evaporate solution to dryness under vacuum and wash the resulting black solution with dichloromethane and n-propanol. Isolate using suction drying. Anneal solid in nitrogen in the glove box at 140 °C for 1hr to remove residual solvent.

Comment: MAPbI3 (d3-CH3ND3PbI3)

Method: Neutron diffraction

Description: Samples were loaded into 8mm diameter vanadium cans in a helium glove-box for analysis on the POWGEN diffractometer situated at the Spallation Neutron Source, Oak Ridge National Laboratory. The analyses of the POWGEN data were carried out using the TOPAS refinement package for Rietveld. Heating and cooling rates were 1K/min. Refer to Page 10 Table 2.

• Rayan C Duke University

Product: Black MAPbI3 Powder

Description: Not explicitly stated in article. But assume close to synthesis methods of the deuterated compounds. Refer to the related data sets.

Method: Synchrotron X-ray powder diffraction

Description: The X-ray powder diffraction measurements were performed on the bending magnet station at DND-CAT sector 5 of the Advanced Photon Source. The X-ray wavelength used was 0.40012(2) Å, selected to reduce X-ray absorption by the sample to an acceptable level. Temperatures were equilibrated for ~10 min after 1 K changes. Refer to Page 10 Table 2.

• Rayan C Duke University