Fig. 2. X-ray diffraction patterns and their analysis.

a Calculated X-ray diffraction patterns for perfect powders of NH₄F-IIIt (space group P-4m2) and NH₄F-VIII (space group P4/nmm) at 115 GPa (a = 3.9 Å and c = 2.6 Å). Tick marks indicate the positions of the Bragg reflections and Miller indices are also reported. b Representative X-ray diffraction patterns of the sample at the indicated pressures (λ = 0.4101 Å). The symbols * and # indicate a Re (gasket material) and a Au (pressure standard) peak, respectively. The vertical lines indicate two peaks which do not emanate from the sample (their positions do not shift with pressure). c Best fits of the most intense diffraction signal using two Gaussians. d Unit cell volume per number of NH₄F molecules in the cell. e Cell dimensions. f √2c/a ratio. Error bars correspond to one standard deviation. Legend of (d) also applies to (e, f). Symbols represent experimental data, dashed lines represent DFT calculations. Data of ref. ¹ are also reported.