8/3/2019 M. Eremets et al- Structural Transformation of Molecular Nitrogen to a Single-bonded Atomic State at High Pressures

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Structural Transformation ofMolecular Nitrogen to a Single-bonded Atomic State at

High Pressures

M. Eremets1, A. G. Gavriliuk1,2,3, N. R. Serebryanaya4, I. A. Trojan1,3, D. Dzivenko1, R. Boehler1, H. K. Mao5, R. J. Hemley5

1Max Planck Institute fr Chemie, Mainz, Germany, 2A.V. Shubnikov Institute of Crystallography, Russia, 3Institute for

High Pressure Physics, Russia,4

Technological Institute for Superhard and Novel Carbon Materials, Russia,5

GeophysicalLaboratory, Carnegie Institution of Washington.

IntroductionNitrogen exhibits characteristics of special interest as

an archetypical molecular system. Diatomic molecular nitrogenhas the greatest binding energy (after CO) and the shortest bondlength, a unique elemental diatomic molecule with triplecovalent bonds. First-principles theory [1] has predicted thatunder high compression, molecules dissociate so that each of thenitrogen atoms has three single covalent bonds to form a three-dimensional covalent solid. Moreover, Mailhiot et al. [2]predicted that the phase should crystallize in the cubic gauche

(cg-N) structure where all nitrogen atoms are three-foldcoordinated and the bond-lengths are the same for all pairs ofbonded atoms. Due to its similarity to the diamond structure(which is four-bonded, however), cg-N could be called as anitrogen diamond.

Key properties of the polymeric form of nitrogen to bedetermined are its lattice parameters, compressibility,transformation pressure from the molecular state, metastability,and enthalpy [2-4]. The last (energy content) has been of specialinterest, as there is a large difference in average energy betweenthe nitrogen single bond (0.83 eV/atom) and triple bond (4.94eV/atom). Therefore, a very large energy should be released atthe transformation from polymerized nitrogen to diatomicmolecular nitrogen. Thus, nitrogen may form a high energydensity material with energy content higher than that of any

known non-nuclear material.Experimentally, a transition to an opaquenonmolecular phase of nitrogen was identifying above 180 GPaat 80 K [5] and then at room and elevated temperatures [6-8].Physical properties measured to date for the phase are close tothose predicted for polymeric nitrogen. These properties includethe value of equilibrium transformation pressure (about 100GPa) and the large hysteresis indicating recoverability atambient pressure and low temperatures [5]. Optical dataindicated that the material formed on compression wasdisordered or amorphous [5-7]. The single-bonded nitrogen in apurely crystalline form of a cubic gauche structure (space groupI213) has been recently synthesized at high pressures and hightemperatures [9].Detailed information on the crystal structure of molecular

nitrogen is available only below approximately 50 GPa wherethe diatomic solid exhibits a rich phase diagram [10-11]. Athigher pressures of60 GPa at room temperature, Raman andinfrared absorption data indicate a transformation from the - N2(rhombohedral (R3c) [12-13] to a -N2 [11,14,15]. This phasepersists in the molecular state up to further transformation to thenonmolecular state at 150- 180 GPa. The crystal structure of -N2 has not yet been determined. It was proposed asR3c [15-16],but later it was found that this structure does not consist withRaman and IR data performed at low temperatures [11-14]. Alow symmetry (orthorhombic or monoclinic) structure with twosites for atoms was proposed [14] In their x-ray studies of

nitrogen up to 65 GPa at room temperature, Jephcoat et al [17]observed a transition at ~60 GPa, but the signal was weak andthe pressure was not high enough to separate the new phase anddetermine its structure. New molecular phases have also beensynthesized at high pressures and temperatures (70-90 GPa and600-1000 K); although x-ray diffraction data are sufficient toidentify them as new phases, the crystal structures have not yetbeen determined [8]. Here we present x-ray diffractionmeasurements of molecular nitrogen up to 170 GPa and identifythe structure of -N2 phase. The main results have beenpublished in Ref. [18].

Methods and MaterialsX-ray diffraction measurements of nitrogen at

megabar (>100 GPa) pressures are challenging because it is alight element and a weak scatterer. We used diamond anvil cellswith beveled anvils and flats 50-80 m. An important featurewas that a gasket was prepared from powder of cubic boronnitride mixed with epoxy. This gasket produced only few weak,well-defined diffraction peaks and provided a sample that is1.52 times thicker than with typical hard metallic gaskets. Thegasket hole was filled with fluid nitrogen at pressure about 0.2-0.3 GPa and then clamped for further pressurizing. A majorityof the diffraction patterns of the molecular phases werecollected at the Advanced Photon Source (APS, HPCAT at

Sector 16), while the cg-N-phase and some patterns ofmolecular nitrogen were collected at the European SynchrotronRadiation Facility (ESRF, beamlines ID-9 and ID30). In allcases, we used an x-ray beam focused down to ~5x5 m andangle dispersive diffraction techniques [9]. Pressure wasdetermined from the ruby scale [19] and from the equation ofstate of c-BN [20].

ResultsDiffraction patterns of nitrogen in the 60-150 GPa pressurerange are shown in Fig. 1. At 60 GPa, several diffraction peaksof the previous -phase widen. At 69 GPa, these peaks are splitbut new peaks do not appear, which suggests that the -N2-N2 transformation is not accompanied by a large lattice

distortion. Above 80 GPa, diffraction patterns demonstrate onlyone phase which does not change up to 150 GPa, with theexception of the appearance of an amorphous halo above 100GPa (Fig.1) and some redistribution of the intensity.

We begin with an analysis of the diffraction pattern ofthe -phase obtained at 80 GPa. First, we tried the hexagonalindexing to verify theR3c structure. The structure was found notto be rhombohedral structure; only hexagonal indices fit thedata, verifying that the -phase does not have theR3c structure.To identify this low-symmetry structure, we took into accountadditional considerations. We assumed that the structure of the-phase retains some properties of the lower pressure -phase,consistent with the diffraction and earlier spectroscopic data. It

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2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

h-BNgraphite

-N cg-Nat 2000K

cg-N

-N2

P

(GPa)

Vat

(3)

-N2

T = 300 K

theory [3]

Nitrogen

diamond

c-BN

Figure 2. Pressure-volume equation of state (EOS) of nitrogen.Our experimental data for -N2 are shown with open circles.Solid circles are data from Ref. [13]. Both sets of data werefitted with a third-order Birch-Murnahgan (BM) equation ofstates (EOS) with parameters determined at 16.3 GPa: V=11.00 3, B= 64.78 GPa, B= 5.445. The volume drop at ~60GPa identifies the transition to the -N2 phase. The B-M EOSfit for this phase is described by parameters determined at 69GPa: B= 310 GPa, B= 4.0, V= 7.5 3. On room temperaturecompression, the -phase remains stable at pressures up to~150 GPa, whereupon it transitions to a nonmolecular phasewith an amorphous-like structure. -N2 can be directlytransformed to the cubic gauche structure (cg-N) with laserheating above 2000 K at ~110 GPa [9]. The EOS of this phasehas been measured with increasing pressure up to 134 GPa,and then on releasing pressure down to 42 GPa where thesample escaped the cell. The experimental points were fittedwith B-M EOS with B42= 460.72 GPa, B42= 4.0, V42= 5.8783 taken at 42 GPa. Extrapolation of this EOS to zero pressuregives a volume of cg-N structure about 6.6 3 (in excellentagreement with theoretical predictions of 6.67 3 [2]), B0= 298GPa, B= 4.0. The zero-pressure bulk modulus B0 calculatedusing other EOS forms was found to lie in the range ~300-340GPa. This result is also in a good agreement with theoreticalpredictions ~340 GPa [2]. The EOS for carbon (graphite [29]and diamond [30]) and BN (hBN and cBN [31]) are alsopresented to show the proximity of these covalent bonded

materials with nitrogen.

9 10 11 12 13 14 15

170 GPa

150 GPa

024300122211104113202 -N2 phase-N

2phase

60 GPa

138 GPa

132 GPa

120 GPa

110 GPa

100 GPa

80 GPa

75 GPa

Intensity(a

.u.)

2 (deg , =0.4218 A)

69 GPa

NitrogenT = 300 K

201200

111102

110011002

c-BN 111

Figure 1. X-ray diffraction patterns of nitrogen taken atroom temperature. (a) Evolution of diffraction patternswith pressure. Nitrogen at 60 GPa was identified as the -N2 phase. The onset of-phase occurs at 69 GPa. Verticalbars indicated Bragg angle positions, and indices ofdiffraction reflections are shown for the -phase at 60GPa and for the -N2 phase at 69 GPa and higher. Theywere calculated for each pressure using a Rietveldrefinement.

is also possible that it exhibits features of the densernonmolecular phase (cg-N). Accordingly, we began byassuming that the rhombohedral structure of -N2 transformsinto a rectangular structure with screw axes and without aninversion center as in the cg-N cubic space group I213. Indeed,the best indexing was found with an orthorhombic unit cell.Systematic extinctions showed the best correlation with aprimitive space group without an inversion center and with onlyeven indices for axis reflections. There are three such spacegroups: P2221, P21212, and P212121. The volume for one atom

of nitrogen in the -phase should be between those in the -phase (8 3 at 69 GPa) and those in the cg-phase (5.15 3 at 115GPa [9]). We also examined correlations in cell parameters ofthe -, and - and cg-phases. It is possible to represent theunitcell parameters of the orthorhombic -phase as a derivationof-N2and the cubic phase: c (5.039 ) is almost equal to a(5.08 ) and taking into account that acgN= 3.454 [9], acub. ~c/2=3.81 , acub. ~ b2=3.91 . There are commondirections in these structures and the transformations may berepresented as a result of diagonal displacements of atoms fromthreefold axes of the rhombohedral -phase. The screw axes ofthe space group P2221 are in the same crystallographic directionas those in space group I213, which leads to a choice of P2221for the space group of the -phase structure. The atom positions

4e (x y z)are general for both atoms of nitrogen. The volumechange of the transition is ~6% at 69 GPa. The Rietveld

profile analysis of the diffraction pattern obtained at 80 GPa was

carried out with an Rfactor (Bragg angle) = 0.08, and RF(structural factor) = 0.05 The unitcelldimensions and atomicpositions were refined.

In the -phase, the shortest distance between nitrogenatoms in the molecule N1N1=0.982 (pressure 80 GPa), andthe shortest intermolecular distance N1N2=1.93 indicatesthat the -phase retains a diatomic structure. However, theintramolecular and intermolecular distances significantly change

in comparison with the -phase, where the shortestintermolecular distance is equal to 2.38 (at 69 GPa). Between60 and 138 GPa, the intermolecular distances further shortenfrom 2.38 to 1.73 . Importantly, intramolecular distancesextend from 0.9684 to 1.002 in this pressure range. Thisexpansion indicates a weakening of the (intramolecular) bondsand can explain the softening of the vibron frequencies [21],which starts at the same pressure as the transformation to - N2.These changes in intra- and intermolecular distances arefavorable for the later transformation of- N2 to the cg-N phase,

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where distances between atoms (bond lengths) are the same forall pairs of bonded atoms.

DiscussionIt is interesting to compare transitions between

differently bonded phases in nitrogen and carbon (and itsanalogue BN). There is a striking similarity in both volume drop(Fig. 2) and metastability. The volume drop at the -N2 to cg-Ntransition at 110 GPa is 22%, comparison to 24% for hBN cBN and 27 % for the graphite diamond transition at 10 GPa.At zero pressure, the energy difference between cubic diamondand hexagonal graphite is nearly zero [24]. The energetic barrierbetween these degenerate phases is E = 0.33 eV/atom and 0.38eV/pair for BN [24-25]. For nitrogen, a larger barrier of ~0.86eV 2 is calculated for the cg-N and -O2molecular phases. Thisimplies metastability of cg-N even at zero pressure [2,26].Experimentally, the diffraction experiment showed that thephase can be stabilized down to at least 25 GPa [9]. Thedisordered nonmolecular phase created on cold compression canbe recovered to near ambient pressure at low temperatures(