α-glycine is studied up to 50 GPa using synchrotron angle-dispersive X-ray powder diffraction (XRD), Raman spectroscopy, and quantum chemistry calculations performed at multiples levels of theory. Results from both XRD and Raman experiments reveal an extended pressure stability of the α phase up to 50 GPa and the room temperature (RT) equation of state (EOS) was determined up to this pressure. This extended stability is corroborated by density functional theory (DFT) based calculations using the USPEX evolutionary structural search algorithm. Two calculated EOSs, as determined by DFT at T = 0 K and semiempirical density functional tight-binding (DFTB) at RT, and the calculated Raman modes frequencies show a good agreement with the corresponding experimental results. Our work provides a definitive phase diagram and EOS for α-glycine up to 50 GPa, which informs prebiotic synthesis scenarios that can involve pressures well in excess of 10 GPa.
The employment of high-pressure gases as a pressure-transmitting medium, sample, or reactant for diamond anvil cell experiments is widespread. As a pressure transmitter, high-pressure gases are crucial to forming quasi-hydrostatic compression atmospheres for samples inside the uniaxially driven cell. We describe an optical design for forming high-resolution images of the gasket and sample chamber of the diamond anvil cell under high gas pressures in a gas loading apparatus. Our design is simple, is of low-cost, and may be easily adapted to suit gas loading apparatus of any design, as well as other common hard-to-reach environments in diamond anvil cell experiments, i.e., those with large stand-off distances, such as cryostats
The pyrite mineral was recently shown to undergo a giant pressure-induced volume collapse at 12 GPa, via a disordered intermediate phase. The high pressure arsenopyrite phase is stabilised by metal-metal bonding, a mechanism now shown to be ubiquitous for chalcogenides. Here we report a spectroscopic investigation of this transition up to pressures of 22 GPa. Using XANES we show that the transition does not involve a change in oxidation state, consistent with the arsenopyrite crystal structure proposed at high pressure. Notably, the XANES spectrum is almost identical in the pressure-induced disordered phase, and after crystallisation induced by laser-heating. The former is therefore a ‘valence bond glass’, and is likely disordered due to kinetic hindrance of the phase transition. We also detect electronic changes in the compressed pyrite phase, and this is confirmed by Raman scattering which shows that the disulphide vibrations in the pyrite phase saturate before the volume collapse. Together with detailed DFT calculations, these results indicate that electronic changes precede valence bond formation between the cations.