2Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, ... 4School of Physics and Astronomy and Centre for Science at Extreme Conditions,. University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, United ...
Metallization and molecular dissociation of dense fluid nitrogen Shuqing Jiang1,2, Nicholas Holtgrewe2,3,†, Sergey S. Lobanov2,‡, Fuhai Su1,2, Mohammad F. Mahmood3, R. Stewart McWilliams2,4*, Alexander F. Goncharov1,2* 1
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China. 2 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA. 3 Department of Mathematics, Howard University, 2400 Sixth Street NW, Washington D.C. 20059, USA. 4 School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Peter Guthrie Tait Road, Edinburgh, United Kingdom EH9 3FD. †Current address: Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA. ‡Current address: Department of Geosciences, Stony Brook University, Stony Brook, NY 11790, USA. Abstract: Diatomic nitrogen is an archetypal molecular system known for its exceptional stability, abundance in natural settings, including planetary envelopes, and rich solid polymorphism at high pressures. Finding a pathway from diatomic molecular insulators to monatomic metals under pressure is fundamental for reaching energetic states of matter and for understanding planetary interiors and atmospheres. Applying dynamic laser heating in diamond anvil cells and fast optical spectroscopy, we determined a boundary with a negative pressuretemperature slope between insulating (molecular) and conducting (presumably polymeric) fluid states of nitrogen, and established the conditions of metallization, which occur above 125 GPa at elevated temperatures above 2500 K. This study of the interplay between molecular dissociation, melting, and metallization paves the way to understanding such phenomena in other simple molecular systems, including hydrogen isotopes and infers a possible presence of metallic nitrogen in the interiors of giant planets and the Earth’s core. The behavior of molecular nitrogen under pressure (P) and temperature (T) is in many aspects parallel to that of hydrogen, with each exhibiting melting temperature maxima 1,2, a fluid insulator-metal, molecular-nonmolecular transformation that is predicted to be continuous at high temperatures and first-order at low temperatures 3-5, and progressive molecular breakdown in the solid state 1,6-8. On the other hand, the evolution of elemental nitrogen at the high pressures and temperatures of the planetary interiors - in particular, the stability of its molecular state in related to this formation of conducting states - may be critical for understanding the interiors of giant planets, where nitrogen may be present due to ammonia dissociation 9 and Earth’s core, where it can partition with liquid Fe 10. Thus, exploring the properties of nitrogen at extreme P-T conditions can yield fundamental knowledge about the physics and chemistry of simple molecular systems that is central to our understanding of planets and energy materials. This is especially the case given the challenges of reliably predicting 11 and measuring 5,12-15 the properties of hydrogen itself. Nitrogen differs from hydrogen in that it forms a variety of metastable polynitrogen molecules with a reduced bond order 16 and compounds with many elements due to its ability to adopt different bonding types (single to triple).
Solid nitrogen shows a rich variety of stable and metastable diatomic (triple-bonded) molecular phases at high pressures, explored theoretically 17 and experimentally 18-21 (Fig. 1). Application of pressure favors the stability of single and double bonds thus promoting molecular dissociation and the formation of energetic polyatomic and polymeric structures (e.g. Ref. 22), including amorphous solid 1,7,19,23,24 and single-bonded cubic-gauche cg-N 8,25 and layered LPN 26 structures, with the onset of dissociation shifting to lower P at higher T 24. Probing fluid nitrogen in single-shock experiments revealed anomalous densification, cooling, and conductivity increase at 30-60 GPa and 7000-12000 K 27, interpreted as signatures of molecular dissociation, while dynamic multiple-compression experiments at ~7000 K reported similar cooling effects at ~90 GPa and a transition to a metallic state above 120 GPa 27,28. First principles theoretical calculations 3,29-31 predict a first-order transformation from molecular insulating fluid to polymeric conducting fluid ending in a critical point near 75 GPa and 4500 K with a continuous dissociation into a semiconducting atomic liquid at higher T and lower P. The onset of dissociation with increasing P and T has also been linked to an increase of optical absorption 7,27,32,33 , manifesting a connection between dissociation and electronic transformation at optical wavelengths.
8000
Temperature (K)
Double shock
Metal
6000 Onset of absorption
4000
Polymeric fluid Neptune
Hugoniot Molecular fluid
Theory
2000
Geotherm
0 0
Onset of reflectivity
50
cg-N LP-N
'
100
150
Pressure (GPa) Fig. 1. Phase diagram of nitrogen at extreme thermobaric conditions. Current measurements of the onset of absorptive states in fluid nitrogen (optical depth ≲10m) are presented by red circles, while the reflective states are shown by blue squares; solid lines are guides to the eye. Also shown are the shockwave Hugoniot of nitrogen without dissociation (dashed purple line) and that observed experimentally (solid line) 27,34, metallization in reverberating shock experiments (black square) 28, and fluid-fluid boundaries deduced from double-shock experiments (dotted-dashed line) 35 and theoretical calculations (solid and dot-dot-dashed lines, indicating regions of first- and second-order transformation, respectively) 29 . The solid-state boundaries are from Refs. 2,19; a thin dashed line demarcates the proposed stability 2
limits of cg-N, while dotted black line shows the conditions of its formation in experiments. The gray solid and dashed bands are the temperature in Earth’s mantle (
