JCP-LiN3

Polymerization of nitrogen in lithium azideXiaoli Wang, Jianfu Li, Jorge Botana, Meiguang Zhang, Hongyang Zhu et al.Citation: J. Chem. Phys. 139, 164710 (2013); doi: 10.1063/1.4826636View online: /10.1063/1.4826636View Table of Contents: /resource/1/JCPSA6/v139/i16 Published by the AIP Publishing LLC.Additional information on J. Chem. Phys.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsTHE JOURNAL OF CHEMICAL PHYSICS 139,164710(2013)Polymerization of nitrogen in lithium azideXiaoli Wang,1,2,3,a)Jianfu Li,1,4Jorge Botana,2Meiguang Zhang,5Hongyang Zhu,3Li Chen,1Hongmei Liu,1Tian Cui,3and Maosheng Miao 2,6,a)1Institute of Condensed Matter Physics,Linyi University,Linyi 276005,People’s Republic of China 2Beijing Computational Science Research Center,Beijing 100084,People’s Republic of China 3State Key Laboratory of Superhard Materials,College of Physics,Jilin University,Changchun 130012,People’s Republic of China 4Department of Physics and Materials Science,City University of Hong Kong,Hong Kong SAR,People’s Republic of China 5Department of Physics and Information Technology,Baoji University of Arts and Sciences,Baoji 721016,People’s Republic of China 6Materials Research Laboratory,University of California,Santa Barbara,California 93110,USA(Received 4July 2013;accepted 10October 2013;published online 28October 2013)Additional electrons can drastically change the bonding trend of light elements.For example,N atoms in alkali metal azides form the linear N 3−anions instead of N 2molecules with the introduc-tion of additional electrons.The effect of the additional electrons on the polymerization of N under pressure is important and thus far ing first principles density functional methods and the particle swarm optimization structure search algorithm,we systematically study the evolution of LiN 3structures under pressures up to 600GPa.A stable structure featuring polymerized N under pressures higher than 375GPa is identified for the first time.It consists of zig-zag N polymer chains that are formed by N 5−five-member rings sharing N–N pairs.Throughout the stable pressure range,the structure is insulating and consists of N atoms in sp paring with the atomic and electronic structures of previous phases,our study completes the structural evolution of LiN 3under pressure and reveals the structural changes which are accompanied and driven by the change of atomic orbital hybridization,first from sp to sp 2and then from sp 2to sp 3.©2013AIP Publishing LLC .[/10.1063/1.4826636]I.INTRODUCTIONThe structural changes of light elements under pressure,especially their tendency of forming extended solid (poly-merization)have attracted intensive attention due to their im-portance in understanding the evolution of chemical bonds in light elements and making new generations of high en-ergy density materials (HEDM).Many previous works show that the light elements except B and C are quite resistive to polymerize and can only become extended solid under very high pressure.However,several recent studies suggest that the mixing of small amount of hydrogen could greatly en-hance the propensity of polymerization on N.1However,the underlying mechanism is not clear and the additional charge transfer from H to N can be one important factor.Alkali metal azides was used as an example system to study the effect on charge transfer to the polymerization of N.2Un-der ambient condition,they are stable in solid state,there-fore the issue of mixing and loading of the electron donors and the nitrogen are greatly reduced.These materials them-selves have been widely used as nitrogen sources,initial explosives,and photographic materials.3Extensive experi-mental and theoretical studies have been undertaken on the structural,electronic,and optical properties of alkali metal azides.The high-pressure behaviors of LiN 3,4NaN 3,KN 3,a)Authors to whom correspondence should be addressed.Electronicaddresses:use126126@ and miaoms@.and CsN 3have been intensively studied by using Raman spec-troscopy and X-ray diffraction techniques.More recently,the pressure-induced structural transition sequence of CsN 3up to 54.4GPa was determined by Hou et al.5Ji et al.6,7predicted a structural phase transition of KN 3at a pressure of 15.5GPa by in situ synchrotron powder X-ray diffraction.The N 3−anions in NaN 3were found to transform to a non-molecular nitrogen state with an amorphous-like structure when compressed to 120–160GPa.8Among all the alkali metals,Li has the smallest radius;therefore Li azides may possess the strongest N–N interac-tion.Medvedev et al.4reported that LiN 3is stable in C 2/m structure up to 60GPa at room temperature,significantly different to NaN 3,KN 3,and CsN 3.This resistivity of LiN 3to structural transition below 60GPa are also observed un-der elevated temperature (100–300K)as studied by x-ray diffraction 9and Raman spectroscopy.10,11A density func-tional (DFT)study also suggested that the C 2/m structure remains stable up to 60GPa.Vaitheeswaran et al.explored the structural and electronic properties of LiN 3and KN 3at pressure range of 60GPa.12The structure,elastic and dynam-ical properties of KN 3and RbN 3were studied 13.More re-cently,Zhang et al.14predicted a structural phase transition of LiN 3at a pressure of 36GPa,from azide to benzene-like “N 6”six-member rings in a structure belong to P 6/m space group.Although a great amount of experimental and theo-retical studies have been carried out for this compound,no polymerization has ever been observed or predicted before.0021-9606/2013/139(16)/164710/5/$30.00©2013AIP Publishing LLC139,164710-1Under ambient conditions,lithium azide crystallizes in a mon-oclinic lattice with C2/m symmetry,which is isostructural to the low-temperature phase of sodium azide in all respects.To the best of our knowledge,no polymerization of N in LiN3 has been observed or predicted.One major reason might be that the pressure required for the polymerization of N in LiN3 goes beyond the scope of the previous experimental and the-oretical works.It is therefore of importance to investigate the structural evolution of LiN3under pressure and propose the possible electron effect on this polymerization process.In this work,we systematically study the structural change of LiN3under pressures up ing an ef-fective and unbiased automatic structure search method,we thoroughly search the stable structures of LiN3globally in the enthalpy potential surface calculated byfirst principles method.In the following,wefirst introduce the methodol-ogy including both the structure search and the underlying DFT methods in Sec.II.The main results and the discussions are shown in Sec.III.The work is also summarized and con-cluded in Sec.IV.II.THEORETICAL METHOD AND COMPUTATIONAL DETAILSWe have performed extensive structure searches to un-cover the high-pressure structures of LiN3using the Parti-cle Swarm Optimization(PSO)algorithm15as implemented in the Crystal structure Analysis by Particle Swarm Opti-mization(CALYPSO)code.16This method has successfully predicted the ground state structures for various systems in-cluding lithium,17polymeric nitrogen,18HgF2,19and super-hard carbon nitride,20among which the insulating Aba2–40 (or oC40)structure of Li has been confirmed by independent experiments.21The variable-cell structure predictions are per-formed within a pressure range from0to600GPa,with sys-tems containing from one to eight f.u.per simulation cell as implemented in CALYPSO code.The underlying ab initio structural relaxations and elec-tronic band structure calculations are performed in the frame work of density functional theory by generalized gradient ap-proximation Perdew-Burke-Ernzerhof(GGA-PBE),22as im-plemented in the V ASP code.23The projector augmented wave(PAW)24pseudopotentials are used to describe the ionic potentials.The cutoff energy(800eV)for the expansion of the wave function into plane waves and Monkhorst-Pack25 k-meshes0.03are chosen to ensure that all the enthalpy calculations are well converged to better than1meV/atom (10×10×6,8×8×14,6×8×11for the simple C2/m,P6/m,and P21phases,respectively).The phonon cal-culations were carried out by using a supercell approach as implemented in the PHONOPY code.26Raman,infrared spectrum,absorption coefficients,and elastic constants are obtained from evaluations of the stress tensor by the CASTEP code.27The Norm-conserving pseudopotentials are used,the exchange and correlation effects are described by the GGA-PBE.The electron wave functions and the electron density are expanded by the plane-wave basis sets with a cutoff energy of 770eV,with10×10×6,8×8×14,6×8×11Monkhorst-Pack grids for the electronic Brillouin zone(BZ)integrations for the simple C2/m,P6/m,and P21phases,respectively. III.RESULTS AND DISCUSSIONThe enthalpies of selected LiN3structures and the vol-umes of the most stable structure are plotted as functions of pressure in Fig.1.The results show that C2/m remains as the most stable structure up to36GPa.Moreover,P6/m structure is favored beyond36GPa.It agrees well with the previous calculations.13Furthermore,for thefirst time,our calculations also predict a polymerized phase for LiN3which is stable un-der pressure higher even than375GPa.This structure has not been seen in other alkali metal azides system.The struc-ture is monoclinic and low in symmetry(P21space group). There is another polymerized structure with higher symmetry (P212121)that is close in energy to P21structure.Thorough structure searches using CALYPSO did notfind any other structural change up to600GPa.The entirely different crys-tal symmetries in triple structures suggest that the transitions among them arefirst order,which is indeed confirmed by the calculated P–V curves(Fig.1(b)).The reductions of the vol-umes are found to be12.8%and4.6%for the transitions from C2/m to P6/m and from P6/m to P21,respectively.The P21structure consists of zig-zag chains of N5rings, as illustrated in Figure2.The N5rings are slightlydeviated FIG.1.(a)Enthalpy of formations of selected structures of LiN3as a function of pressure(relative to C2/m structure).(b)The phase diagram of LiN3at high pressure.The symmetries,crystal structures,phase transition pressures,and the volume of the formula unit(f.u.)of LiN3as a function of pressure for the C2/m, P6/m,and P21structures).FIG.2.The projected views of the crystal structures of P21-LiN3,(a)in the perpendicular direction of the polymer chain and N5rings(along a axis);(b)in the direction of the side of the chains and rings(along b direction); and(c)along the chain(in the c direction).The green and grey spheres rep-resent Li and N atoms,respectively.At400GPa(3.585Å3/atom),the lattice parameter are:a=4.070Å,b=3.015Å,and c=2.402Å.Li atoms sit at2a(0.0573,0.7929,0.0219)and N atoms at three inequivalent crystallo-graphic sites2a(0.3182,0.1459,0.3840),2a(0.5921,0.2824,0.2626),and 2a(0.1833,0.4729,0.5706).from the planar configuration.The neighboring rings are con-nected by shared N–N bonds.At400GPa,the length of these shared N–N bonds are1.283Å.The lengths of other three N–N bond are1.260Å,1.303Å,and1.382Å.The neighboring N5rings are positioned with an angle of118◦.Viewing along the direction of the chains[Fig.2(c)],a two-dimensional oblique lattice with an angle of76.2◦can be observed.This is different to cg-N,in which the bound lengths and bound angle are all equal.The Li atoms locate at the interstitial sites between the chains.The closest distance between the Li and neighboring N atoms is1.620Å,indicating no covalent bond-ing between N and Li.The role of Li is only to provide the extra electrons to the N atoms throughout the pressure range.While comparing the structural features of the three sta-ble structures in pressure range from0to600GPa,wefind the transition trend toward polymerization of Li azides un-der pressure.Under atmospheric even low pressures,N atoms cluster exhibits the linear N3−molecular anions,with the bond length of1.166Åfor N=N bond36GPa.In the pressure range from36to375GPa,N atoms form planar Benzene like rings with the N=N bond length of1.321Åat50GPa and 1.217Åat375GPa.The closest N–N distance in neighboring rings is2.066Åat375GPa,excluding covalent bonding be-tween the rings.At the same pressure,the shortest N–N bond length in the polymeric P21phase is1.264Å.Interestingly, although the Li azides tends to form the denser structures fea-turing larger N clusters under higher pressure,the lengths of N–N actually increase during the transitions,which might be ascribed to the change of the atomic orbital hybridization.The dynamic stability of P21structure is examined by calculating the phonon spectra using the supercell method.25 No imaginary phonon frequency is found in the whole Bril-louin zone at the pressure ranged of375–600GPa(Fig.3). The elastic constants of P21structure are calculated to ex-amine the mechanical stability in the pressure range of 0–600GPa.The stability requires the strain energy to be pos-itive while the whole set of elastic constant C ij tosatisfyFIG.3.The phonon-dispersion curves for P21at400GPa.Born-Huang criterion.28,29Our results show that this struc-ture is dynamically and mechanically stability in this pressurerange.The primitive cell of P21structure contains8atoms,giving24phonon branches.The calculated zone-center( )phonon eigenvectors were used to deduce the symmetry la-bels of phonon modes.The group theory analysis shows thatthese24vibrational modes at the zone center have the ir-reducible representations P21=11A I+R+10B I+R,where the Raman active modes are labeled by superscript R,and in-frared active modes are labeled with superscript I.The Ramanand infrared spectra of C2/m and P21phases are presented inFig.4.The dominant Raman peak of azide ion appears at1313cm−1(Fig.4(a)),the peak of polymeric nitrogen at1273cm−1(Fig.4(b)),respectively.The disappearing of azideion peaks and the appearance of N–N peaks are signatures ofthe phase transition and can guide the experimental observa-tion of high pressure structure transition in Li azides.The structure evolution of the Li azide under pressure in-terplays with the change of the electronic properties.To ex-plore that,we calculated the electronic structures and theirdependence on pressure in several aspects,including the bandstructures,the electron localized functions(ELF)andtheFIG.4.Raman and infrared spectra of C2/m(a)and P21(b)at20and400 GPa.The dominant Raman azide ion peak appears at1313cm−1(a),poly-meric nitrogen peaks at1273cm−1(b),respectively.FIG.5.The electron localization functions,band structure,and partial density of states for C2/m((a)and(d)),P6/m((b)and(e))and P21((c)and(f))at 20GPa,100GPa,and400GPa.projected density of states(PDOS).As shown in Fig.5,the calculated results suggest that the C2/m phase is insulator with a large energy gap of3.16eV at20GPa(Fig.5(d)).At ambi-ent pressure,the band gap of C2/m is3.65eV,which agrees with previous theoretical results of3.7eV14and3.46eV30. The band gap of P21structure is 1.35eV at400GPa (Fig.5(f)).On the other hand,the bands of P6/m structure cross the Fermi level along →A,H→K,M→L directions in the Brillouin zone(Fig.5(e)),indicating a metallic character. Therefore,the two phase-transitions of Li azides are accom-panied by metal-insulator transitions,first from insulating to metallic state at36GPa and then from metallic back to insu-lating state at375GPa.This unusual change of the electronic structure of LiN3is governed by the change of the bonding features in the three phases.In C2/m structure N forms to the linear[N=N=N]−molecular anions,in which N atoms are in sp hybridization.The p x and p y orbitals of the center N forms twoπbonds with the p x and p y orbitals locating at both N atoms at each side.The rest of the p x and p y orbitals at the both ended N atoms arefilled and formed lone pairs. Therefore,crystal orbitals dominated by N p states arefilled whereas those consist of Li2s orbitals are unoccupied,result-ing in a sizable gap in the system.The strong covalent bond-ing between the center N and the side N as well as the lone pairs on the side N atoms are revealed clearly by the ELF.In order to examine the stability of the structures under the radiation of lights,the absorption coefficients of C2m, P6/m,and P21phases have been calculated and are shown in Fig.6.We concentrate on the spectra from infrared to the visible light range,correspondingly from0to about4eV.The ambient pressure phase C2/m and the newly predicted high-pressure phase P21can resist the visible light,since their ab-sorption of light in the region of0–4eV is zero.As for the P6/m phase,there are two adsorption peaks located at0.5and 1.5eV,respectively,which is caused by free electrons,and the adsorption peak located at5eV is broadened down to3eV. Therefore,we predict that the C2/m and P21phases are un-stable and will decompose into Li rich nitrides(most proba-bly Li3N)and nitrogen under the ultraviolet light.However,it is stable under visible light in contrast to the P6/m structure.The Benzene-like N6rings in the P6/m structure indicates that the N atoms are in sp2hybridization.Each N atom forms twoσbonds with two neighboring N atoms.The extra sp2or-bitals are alsofilled and formed the lone pairs.The p orbitals will form a conjugatedπbond circling the ring as in Benzene. However,comparing with Benzene that has6electronsfilling only theπbonding states,N62−has8electrons that not only fill all theπbonding states but also partiallyfill theπanti-bonding states,leading to a metallic state.In contrast,the N atoms in P21structure are all in sp3hybridization.The fourN FIG.6.Absorption coefficients for C2m,P6/m,and P21phase of LiN3at 20GPa,100GPa,and400GPa.atoms that are shared by neighboring rings form three N–Nσbonds with neighboring N atoms and one lone pair.The extra N atoms in the rings form two N–Nσbonds and two lone pairs.All the bonding states and lone pair states arefilled and all the anti-bonding states are unoccupied,which is the cause of an insulating state.IV.CONCLUSIONSIn summary,we studied the evolution of the structures of LiN3under high pressure using an unbiased automatic structure search method based onfirst principles total energy calculations and geometry optimization.We identify a new structure featuring zigzag chains offive member N5−rings connected by a pair of bonded N–N.This result can be ex-tended the previous high-pressure structures LiN3,presenting linear N3−molecular anions and Benzene-like N62−rings. For thefirst time,a stable structure with polymerized N is predicted for LiN3.The analysis of the electronic struc-ture reveals that transition trend of N3−ions from the linear molecules to Benzene-like rings and then to polymer chains is driven by hybridization of N atomic orbitals,which changes from sp to sp2andfinally to sp3under very high pressure. In view of the successful synthesis of the cubic gauche(cg) phase in solid nitrogen after theoretical predictions,it would be of great interest to experimentally synthesize the other polymeric nitrogen materials at very high pressure,such as LiN3that has been in the current work. ACKNOWLEDGMENTSThis work is supported by the National Natural Science Foundation of China(Grant Nos.11147007,11304139, 11274151,11204007,11304111,and11204120),the National Basic Research Program of China(Grant No. 2011CB808200),Program for Changjiang Scholars and Innovative Research Team in University(No.IRT1132), NSAF U1230202,Open Project of State Key Laboratory of Superhard Materials(Jilin University No.201204),Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics/Key Disciplines of Con-densed matter Physics of Linyi University.M.M.was supported by the ConvEne-IGERT Program(NSF-DGE0801627)and MRL(DMR-1121053).1M.I.Eremets,A.G.Gavriliuk,I.A.Trojan,D.A.Dzivenko,and R. 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