X-ray diffraction on laser heated nitrogen above 1 Mbar: molecular to polymeric transition in the fluid and in the solid near melting.

Nitrogen is an archetypical system to understand the essential effects of pressure on simple molecules. Notably, first-principle simulations predict a singular first-order transition from a semiconducting molecular fluid to a metallic polymeric one at high P-T. Before the upgrade of the ESRF and thanks to the developments performed during our long-term project HD-463 we were able to measure the structure factor of laser heated fluid nitrogen in the megabar range. This allowed us to demonstrate that molecular fluid nitrogen is stable up to at least 110 GPa and 2500 K. This proposal aims at pursuing these measurements in order to reach the predicted stability domain of the polymeric fluid.