ARTICLE DOI: 10.1038/s41467-018-05011-z OPEN Metallization and molecular dissociation of dense fluid nitrogen Shuqing Jiang1,2, Nicholas Holtgrewe2,3,6, Sergey S. Lobanov 2,4,7, Fuhai Su1,2, Mohammad F. Mahmood3, R. Stewart McWilliams2,5 & Alexander F. Goncharov 1,2 Diatomic nitrogen is an archetypal molecular system known for its exceptional stability and complex behavior at high pressures and temperatures, including rich solid polymorphism, 1234567890():,; formation of energetic states, and an insulator-to-metal transformation coupled to a change in chemical bonding. However, the thermobaric conditions of the fluid molecular–polymer phase boundary and associated metallization have not been experimentally established. Here, by applying dynamic laser heating of compressed nitrogen and using fast optical spectro- scopy to study electronic properties, we observe a transformation from insulating (mole- cular) to conducting dense fluid nitrogen at temperatures that decrease with pressure and establish that metallization, and presumably fluid polymerization, occurs above 125 GPa at 2500 K. Our observations create a better understanding of the interplay between molecular dissociation, melting, and metallization revealing features that are common in simple mole- cular systems. 1 Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 230031 Hefei, Anhui, China. 2 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA. 3 Department of Mathematics, Howard University, 2400 Sixth Street NW, Washington, DC 20059, USA. 4 Department of Geosciences, Stony Brook University, Stony Brook, NY 11790, USA. 5 School of Physics and Astronomy and Centre for Science at Extreme Conditions, University of Edinburgh, Peter Guthrie Tait Road, Edinburgh EH9 3FD, UK. 6Present address: Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA. 7Present address: GFZ German Research Center for Geosciences, Section 4.3, Telegrafenberg, 14473 Potsdam, Germany. Correspondence and requests for materials should be addressed to R.S.M. (email: [email protected]) or to A.F.G. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:2624 | DOI: 10.1038/s41467-018-05011-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05011-z he physical and chemical properties of pure nitrogen at high pressures (P) and temperatures (T) including the T 8000 Ross stability of the molecular state and related formation of model electrically conducting phases offer fundamental constraints on Metal how simple molecular systems related to energetic materials respond to extremes. Moreover, these properties are central for Dissociation 6000 models understanding how nitrogen, the primary constituent of Earth’s Polymeric fluid atmosphere, behaves in the deep interiors of planets, where it can Onset of appear as a molecule due to ammonia dissociation1, in subduc- absorption 2 Planetary interiors tion zones at oxidizing conditions , or as an impurity in iron-rich 4000 Hugoniot Theory cores at reduced conditions3. Exploring nitrogen under extremes Onset of is especially relevant given the challenges of reliably predicting4 (K) Temperature reflectivity 5–9 – and measuring the high P T properties of another important Molecular fluid cg-N LP-N diatomic element, hydrogen. In fact, condensed molecular 2000 nitrogen under high P and T behaves similar in many respects to Non-molecular 10–12 hydrogen: each exhibits melting temperature maxima and a Diatomic solid η fluid insulator-to-metal, molecular-to-nonmolecular transforma- molecular solid 0 tion that is predicted to be continuous at high temperatures and 050100 150 first order at low temperatures7, 13, 14 and progressive molecular – Pressure (GPa) breakdown in the solid state10, 15 17. While such thermodynamic and electronic characteristics are similar in the limit of molecular Fig. 1 Phase diagram of nitrogen at extreme thermobaric conditions. breakdown into a metallic liquid13, 18, nitrogen differs from Current measurements of the onset of absorptive states in fluid nitrogen hydrogen in its detailed chemical behavior forming a variety of (optical depth ≲10 μm) are presented by magenta circles, while the metastable polynitrogen molecules with a reduced bond order19 reflective states are shown by blue squares; solid lines are guides to the due to its ability to adopt differing bonding types (single to triple). eye. Also shown are the single-shock states (Hugoniot) of nitrogen Solid nitrogen at high pressures shows a rich variety of stable observed experimentally (thin solid black line) and that predicted assuming – 32, 41 and metastable diatomic (triple-bonded) molecular phases20 24. no chemical dissociation (dashed) , metallization in reverberating shock Application of pressure favors the stability of single and double experiments (thin open black square)33, and fluid–fluid boundaries deduced bonds thus promoting molecular dissociation and the formation from double-shock experiments by Ross and Rodgers (gray dotted line)43 of energetic polyatomic and polymeric structures25, including and theoretical calculations (gray solid and dot-dashed lines, indicating amorphous solid (η)10, 16, 21, 26, 27, cubic-gauche (cg)17, 28, and regions of first- and second-order transformation, respectively)36. The layered (LP)29, 30 structures, with the onset of dissociation shift- melting line (thick black line) and the domain of nonmolecular solids (thick ing to lower T at higher P27, 31. Probing fluid nitrogen in single- black dashed line) are from refs. 11, 21, while a dotted black line shows the shock experiments revealed anomalous decreases in volume, conditions of formation of crystalline cg-N and LP-N17, 20, 29. The results of temperature, and electrical resistivity at 30–60 GPa and experiments that recorded crystallization of cg-N on pressure increase38 7000–12,000 K32, interpreted as signatures of molecular dis- are shown by black yellow filled crosses. The turquoise line indicates the sociation. In contrast, dynamic multiple compression experi- conditions in Earth’s core47 and Neptune’s deep interior48 ments at ~7000 K reported similar cooling effects at ~90 GPa and a transition to a metallic state above 120 GPa32, 33, leaving, experiments show that nitrogen reaches the metallic state at much however, intermediate temperatures of 2000–6000 K unexplored lower temperatures (<2500 vs ~7000 K). The plasma transition is above ~50 GPa. The onset of dissociation with increasing P and T at higher pressures than theoretically predicted posing a challenge has also been linked to an increase of optical for theory. absorption12, 16, 31, 34, 35, manifesting a connection between dis- sociation and electronic transformation at optical wavelengths. Results First principles theoretical calculations12, 13, 36, 37 predict a first- Reaching equilibrium fluid states. The microseconds (μs) long order transformation from molecular insulating fluid to poly- experiments reported here (Supplementary Fig. 1) are of dura- meric conducting liquid ending in a critical point near 75 GPa tions comparable to or greater than shock experiments commonly and 4500 K with a continuous dissociation into a semiconducting considered as reaching thermodynamic equilibrium32 and are atomic liquid at higher T and lower P. sufficient for reaching thermodynamic equilibrium in fluid sam- The previous gap in experimental data probing nitrogen’s ples39. However, we find this timescale sufficiently short to avoid states and properties at higher pressure owed to restrictions on crystallization of the polymeric solids17, 29 (e.g., cg-N) in the dynamic compression pathways to above ~6000 K32, 33 and static quenched samples. The P–T paths along which we probed the compression heating experiments to below ~2500 material properties (by varying the heating laser energy at a given K11, 17, 22, 23, 29, 38 (Fig. 1). Here we access this unexplored nominal pressure) can be considered nearly isobaric, with the domain, critical for understanding the dissociation–metallization maximum added thermal pressure <5 GPa at 3000 K40 or even transformation of nitrogen, by combining static and dynamic smaller given the local nature of the pulsed heating6. techniques in single-pulse laser heating experiments on pre- compressed N2 samples in a diamond anvil cell, with in situ time- domain measurements of optical emission, transmission, and Fluid conducting states. A strong extinction in the transmitted reflectance in the visible spectral range (480–750 nm) using a probe light has been detected above a certain threshold laser streak camera6, 39. We present the experimental observation of a power; representative data at 121 GPa are shown in Fig. 2. This is transition line that delineates insulating and conducting fluid clearly seen as a pronounced, transient decrease in intensity of the nitrogen phases (plasma line) and find the conditions of the probe pulses after arrival of the heating pulse. In this regime, we insulator-to-metal transition that is associated with a long-sought also observe a continuum thermal radiation (Fig. 2a) witnessing fluid polymeric nitrogen state. These conditions are in a broad the corresponding sample temperature. The transient absorption agreement with the previous shock wave experiments albeit our is the strongest at the times shortly after the highest temperatures 2 NATURE COMMUNICATIONS | (2018) 9:2624 | DOI: 10.1038/s41467-018-05011-z | www.nature.com/naturecommunications