**2. High Pressure Experiments**

The H*P* synchrotron single-crystal X-ray diffraction experiments were carried out at ID-15B beamline at ESRF (Grenoble) dedicated to the determination of structural properties of solids at high pressure using angle-dispersive-diffraction with diamond anvil cells. A membrane-type Diamond Anvil Cell with an opening angle of +/−32 degrees, equipped with 600 μm diamond culets was used. Helium was used as a pressure transmitting medium. According to Singh [24] helium is superior in ensuring near to hydrostatic conditions at pressures of 20 GPa or over compared to argon.

Ruby sphere was loaded as a fluorescent *P* calibrant together with the galenobismutite sample (80 <sup>×</sup> 10 <sup>×</sup> 20 <sup>μ</sup>m3) in the 300 <sup>μ</sup>m hole in the center of a pre-indented stainless steel gasket of 80 <sup>μ</sup>m thickness. Pressure was measured before and after each data collection.

The sample-to-detector distance was 279.88 mm and calibrated, along with the wavelength, using Si standard and Fit2D software [25]. The same synthetic sample of galenobismutite used in Olsen et al 2007 was selected to collect the present set of measurements, to avoid differences in compressibility ascribable to different chemical compositions in the samples.

The X-ray beam was monochromatized to a wavelength of 0.41125 Å and focused down to 10 × 10 μm area. Data were collected with a DAC rotating 64◦ around the ω-axis (from −32 to +32◦) with angular step of 0.5◦ and counting time of 1s per step. The scattered radiation was collected by a MAR555 flat panel detector, with 430 × 350 mm (555mm diagonal) active area.

The extraction and correction of the intensity data, merging of reflections, and the refinements of the crystal lattice parameters were done by means of the CrysAlis software (Agilent technologies) [26]. Measurements were performed at different pressures from 0.5 to 20.9 GPa on increasing pressure, and at 16.43, 8.12, and 2.1 GPa on decreasing pressure to evaluate the reversibility and hysteresis phenomena of structural changes. The absorption correction was applied by means of ABSORB-7 software [27].

The structure refinements were carried out with ShelXle [28] on F2. Scattering curves for neutral atoms were used. Table 1 summarizes details of data collection and structure refinements up to 20.9 GPa. Final atomic coordinates and isotropic displacement factors are listed in Table 2. Bond lengths, polyhedral volumes and polyhedral distortion parameters at different pressures, are reported in Table 3. Cif files with the hkl, i.e., the Miller indices of the collected reflections, of individual refinements are in Table S1 (deposited).



 collected during decompression, Observed reflections |F0| > 4σ, Completeness is reported relative to 2θ max.





\* Data collected during

decompression.


**Table 3.** Polyhedral evolution with Pressure. (a) M1, (b) M2, (c) M3 polyhedra. For each polyhedra, single bond lengths (in A), average bond lengths (in Å) and polyhedral volumes (in Å3) are reported. The standard errors are on the last digit. Measurements at 0.0001 GPa come from Olsen et al. [21].
