Recent technical advances have significantly added to the utility of single crystal X-ray diffraction experiments at high pressures [1]. New ways of supporting diamond anvils, like Boehler Almax anvils [2], have considerably increased the volume of accessible reciprocal space. Use of Helium or Neon as pressure transmitting medium extends substantially the practicable pressure range. Fast electronic area detectors have noticeably decreased the data collection time and increased the accuracy.

We have studied several molecular systems with single crystal X-ray diffraction. Most data were collected on the new and improved ID15B beamline of the ESRF. It replaces ID09A which was a state of the art high pressure diffraction beamline carrying out monochromatic diffraction experiments with large area detectors.

One system extensively studied is arsenolite (As₄O₆) and its interaction with Helium [3]. At moderate pressure Helium can enter the molecular arsenolite crystal forming an intercalation compound with the stoichiometry As₄O₆ + 2He. Infiltration of the He pressure transmitting medium can be avoided by increasing the pressure rapidly to over 10 GPa permitting us to study the structural properties of arsenolite under quasi hydrostatic conditions to very high compressions. Other systems studied are realgar (As₄S₄) and molecular sulphur (S₈). Intra- and intermolecular structural changes were determined with unprecedented accuracy. In particular no structural transitions or signs of amorphisation were observed in arsenolite and realgar for pressures approaching 40 GPa (V/V₀ ~ 0.5). Above 40 GPa realgar becomes amorphous (see Figure), while aresenolite remains crystalline to at least 45 GPa.

In Sulphur a series of symmetry lowering structural phase transitions starts at 16 GPa. No amorphisation is observed till 38 GPa, were the molecular S₈ phase transforms into the tetragonal spiral chain SII structure [4].

Single crystal diffraction permits an exhaustive characterization of the structural changes under pressure. Molecules in certain molecular solids compressed with a quasihydrostatic pressure transmitting medium as shown here for arsenolite, realgar and sulphur can be surprisingly stable up to exceptionally high compressions.

Amorphous realgar at 42 GPa and at ambient pressure after decompression.

Acknowledgments: Work done in collaboration with H. Müller. Arsenolite single crystals from P.A. Gunka.

1. M. Merlini, M. Hanfland, High Pressure Research 33 (2013) 511
2. R. Boehler, K. DeHantsetters, High Pressure Research 24 (2004).
3. P. A. Gunka, M. Hapka, M. Hanfland, et al., ChemPhysChem 19 (2018).
4. H. Fujihisa et al., PRB 70, 134106 (2004).