Bonding Properties of Aluminum Nitride at High Pressure - Inorganic

Jun 16, 2017 - In addition to the known Fm3̅m phase of AlN, a notable monoclinic phase with N66– anion polymeric nitrogen chains for AlN3 in the pr...
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Bonding Properties of Aluminum Nitride at High Pressure Zhao Liu, Da Li,* Shuli Wei, Wenjie Wang, Fubo Tian, Kuo Bao, Defang Duan, Hongyu Yu, Bingbing Liu, and Tian Cui* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China ABSTRACT: Exploring the bonding properties and polymerization mechanism of stable polymeric nitrogen phases is the main goal of our high-pressure study. The pressure versus composition phase diagram of the Al−N system is established. In addition to the known Fm3̅m phase of AlN, a notable monoclinic phase with N66− anion polymeric nitrogen chains for AlN3 in the pressure range from 43 to 85 GPa is predicted. Its energy density is up to 2.75 kJ·g−1, and the weight ratio of nitrogen is nearly 61%, which make it potentially interesting for the industrial applications as a high energy density material. The high-pressure studies of atomic and electronic structures in this predicted phase reveal that the formation of N66− anion is driven by the sp2 hybridization of nitrogen atoms. The resonance effect between alternating π-bonds and σ-bonds in polymeric nitrogen chains are all responsible for the structural stability. Because of the electrons transfer from aluminums to polymeric nitrogen chains, there is a pseudogap in the electronic structures of AlN3. The N_p electrons form π-type chemical bonds with the neighboring atoms, resulting in the delocalization of π electrons and charge transfer in polymeric nitrogen chains. Furthermore, disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal reason for the metallicity.

1. INTRODUCTION Looking for a promising high-energy material in our society has become an important manner to deal with the problem of energy scarcity. It is well-known that solid polymeric nitrogen phases containing single N−N bonds (bond energy contains 160 kJ/mol) and double NN bonds (bond energy contains 419 kJ/mol) are of significant interest as high energy-density materials (HEDMs).1 With the increase of paired electrons between the two nitrogen atoms, the binding energy of two neighboring nitrogen atoms becomes stronger. The decomposition of polymeric nitrogen chains will release a lot of energy and preferentially produce nitrogen, so the most stable nitrogen gas is generated, and a large amount of energy released. Moreover, the nitrogen gas does not pollute the atmosphere. Therefore, the exploration of new available high-energy polymeric nitrides is one of the means to alleviate the energy shortage. Over the past decade, a lot of efforts about exploring inorganic and organic nitrides have been done to synthesize polymeric nitrogen in experiments.2−4 The nonmolecular nitrogen phase under high pressure was first proposed theoretically by McMahan et al.5 Afterward the single-bond polymeric nitrogen cg-N was successfully synthesized by Eremets et al.6,7 at the high pressure and high temperature (110 GPa, 2000 K) conditions. Meanwhile, the cg-N was predicted to have more than three times higher energy storage capacity than the traditional powerful energetic materials such as trinitrotoluene (TNT), HMX,8,9 and so on. Subsequently, more stable N5+SbF6− and N5+Sb2F11− as HEDMs have been synthesized.10 More interestingly, the synthesis of the pentazolate anion cyclo-N5− in (N5)6(H3O)3(NH4)4Cl became a reality by Hu et al.11 The predicted N2H phases as one © XXXX American Chemical Society

hydronitrogen with rich nitrogen-containing materials have been investigated both by theoretical study and experimental exploring. Its energy density exhibited up to 4.40 kJ·g−1, and the forms of polynitrogen chains have been verified by Wang et al.12,13 For the exploration of polymeric nitrogen, organic azides have also made a great progress. Not only azide−tetrazole transformation and azide−alkyne cycloaddition but also the 4toluenesulfonyl azide (C7H7N3O2S, 4-TsN3) have been investigated under pressures up to 15.3 and 15.6 GPa. The high-pressure bonding behavior of azide moieties has also been explored.14,15 Polymeric nitrogen in metal nitrides have attracted much attention in high-pressure science because of their interesting mechanical properties and widespread applications in HEDMs.16,17 As a means of exploring high-energy density materials, high pressure has given us many important insights to explore polymeric nitrogen in metal nitrides. As we all know, high pressure can change the properties of atoms and bonding patterns, directing the experimental synthesis of novel compounds with fantastic properties. During the past few years, the forms of polymeric nitrogen in metal nitrides had been explored. High pressure induced a lot of notable structures, such as N24−, N22−, N3−, N42+, N44−, N5−, N6+, N6−, N62+, N84−, N∞n−, and other various polymeric nitrogen forms.3,18−22 Most of those notable structures almost found in the first or second main families of periodic table of chemical element. Especially, a novel pseudobenzene “N6” molecule at 34.7 GPa was successfully predicted in LiN3 by Zhang et al.23 Received: April 17, 2017

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DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Another two high-pressure phases of KN3 with “N6” rings were uncovered above 20 and 41 GPa by Zhang et al., respectively.24 The stable N5− anions were first predicted at high pressures in LiN5 crystalline states.2 An extending N84− unit was predicted in CsN2 by Peng et al.19 Significantly, the high-pressure synthesis of pentazolate salts (CsN5) has become a reality by Brad et al.25 Those striking metal nitrides have a common characteristicnitrogen-rich content. And all those may become potential high-energy density materials. The III−VI elements almost form semiconductor compounds. Massive researches of structural properties, electronic properties, polymerization mechanism, elastic stiffness, and Debye temperatures in Al−N compounds and clusters have received extensive attention.26−28 Aluminum nitrides are considered to be significant and promising wide-band gap semiconductors for the development of short-wavelength laser, optoelectronic, and microelectronic devices under external conditions.26 First, Zheng group obtained some small AlN cluster ions when the mixtures of aluminum and sodium azide were used in the laser ablation experiment.29 Then the process of Al3N cluster is studied. Two isomers of Al3N without imaginary frequencies in the phonon dispersion are found.29 However, only a few stable isomers are achieved for the AlN3 and Al3N cluster systems. It is particularly important to study whether other ratios of Al−N compounds could exist under the external conditions. The AlN with hexagonal wurtzite structure has been subjected to extensive research.28 There is a band gap in the electronic structure of AlN, indicating the semiconducting properties. Besides, the effective mass of the electron is found to be nearly isotropic for wurtzite AlN.30 On the basis of the Raman spectra, it is found that the wurtzite phase of AlN transform to the Fm3̅m phase at 20.73 GPa.27 The electronic energy band structure of Fm3̅m-AlN above 20 GPa shows a large band gap at the Fermi level, exhibiting insulator characteristics. However, most studies are confined to AlN. The high-pressure study of novel crystal structures has not been well-established. Many fundamental aspects of aluminum nitrides, particularly their high-pressure behaviors, should be well-understood. In the present study, N66− anion as uniting of nonmolecular nitrogen polymeric chains in P21/c phase of AlN3 crystal (P21/ c-AlN3) at 43−85 GPa conditions was predicted. The N66− anion in the lattice seems to compose zigzag distorted chains, which stretched in three-dimensional space. Aluminum was specifically adopted to ionize the N6 molecule, because a lot of electrons transport from aluminum atoms to nitrogen atoms. Electronic band structure and the partial density of states suggest the metallic properties of P21/c-AlN3 phase. This points to a huge difference in the electronic properties comparing with other Al−N semiconductor compounds. The distributions of band-decomposed charge density confirm the presence of πtype chemical bonds between neighboring nitrogen atoms. The π-type chemical bonds result in the delocalization of πelectrons, causing charge transfer.

using the projected augmented wave (PAW)35 potentials, which were performed in the framework of density functional theory (DFT). The Perdew−Burke−Ernzerhof (PBE) functional with exchange-correlation treated within the generalized gradient approximation (GGA) was implemented in the VASP code,36,37 while the electron configurations of 3s23p1 and 2s22p3 for Al and N are adopted as valence states, respectively. The tested cutoff energy (700 eV) for the expansion of the wave function into plane waves38 and Monkhorst−Pack k meshes spacing of 2π × 0.03 Å−1 are used to make sure that all the enthalpy calculations are well-converged. All the Mayer Bond Order results are performed in the DMol3 program software package, which uses DFT to calculate the bonding properties of crystalline solids. Crystal orbital Hamilton population (COHP)39,40 analyses are also performed for Al−N compounds to elucidate the bonding information. The calculation is based on the PW method and performed by reextracting atom-resolved information from the delocalized PW basis sets. Phonon calculations are performed by using the direct supercell method as implemented in the PHONOPY code.41 The crystal structure graphics are produced by using VESTA software.

3. RESULTS AND DISCUSSION 3.1. Phase Stability of Al−N Compounds at High Pressure. We mainly focus on the aluminum nitrides with relatively nitrogen-rich content,42,43 which are more favorable for obtaining single- or double-bond polymeric nitrogen. Variable-composition structural searches were performed at a variety of AlxNy compositions (x = 1−3, y = 1−5). The equation of ΔHf = [H(AlxNy) − xH(Al) − yH(N)]/(x + y) were adopted to calculate the average atom formation enthalpy of the compositions.44 The most stable structures of solid aluminum (Fm3m ̅ phase) and solid nitrogen (α, Pbcn, P2/c, and cg-N phases)45 are chosen as reference structures in their corresponding pressure ranges. Most favorable structures with the lowest enthalpies that obtained in the structure searches at the pressures of 50, 60, 80 GPa are presented in Figure 1. Solid spheres, connected by solid lines, represent energetically stable phases, whereas the unstable or metastable phases are shown as open spheres above the convex hull. They are unstable and all will decompose into other binary stoichiometry and/or element unless the kinetic barrier is sufficiently high.

2. COMPUTATION DETAILS The structural searching for the Al−N system was based on the global minimization of free-energy surfaces using ab initio total energy calculations and an automatic structure search based on a particle swarm optimization algorithm implemented in CALYPSO code.31−33 The candidate structure was predicted at 0, 20, 50, 100, and 150 GPa using the simulation cell consisting of 2−4 formula units (fu).34 The local structural optimization at different pressures was performed by

Figure 1. (a) Relative enthalpies of formation of Al−N phases with respect to elemental aluminum and nitrogen solids. The convex hulls connecting stable phases (●) are shown by solid lines. (○) Unstable/ metastable phases. (b) Predicted pressure−composition phase diagram of Al−N crystal phases. B

DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The energetically stable phases at different pressures and stoichiometries are shown in Figure 1b. At ambient pressure, the P63mc phase (wurtzite structure) of AlN is the only stable compound in the experiments.25 At pressures above 14 GPa, the P63mc phase transforms to the Fm3m ̅ phase (rock salt structure), which is preserved up to 85 GPa. At higher pressures, we only find a novel P21/c-AlN3 phase at 43−85 GPa as shown in the convex hull graph of Al−N system, which hopefully will be synthesized in future experiments. The phonon dispersion relation in Figure 5a,b confirmed the mechanical and dynamical stability of these structures. 3.2. Ground-State Structures. The predicted highpressure phases of the Al−N system are shown in Figure 2.

Table 1. Unit-Cell Parameters and Atomic Positions of the P63mc-AlN, Fm3m ̅ -AlN, and P21/c-AlN3 Phases at 0, 20, and 50 GPa, Respectively structures P63mc P = 0 GPa

Fm3m ̅ P = 20 GPa

P21/c P = 50 GPa

lattice parameters (Å) a = 3.04 b = 3.04 c = 3.75 α = 90.00 β = 90.00 γ = 120.00 a = 4.02 b = 4.02 c = 4.02 α = 60.00 β = 60.00 γ = 60.00 a = 11.12 b = 3.57 c = 10.84 α = 90.00 β = 166.43 γ = 90.00

atomic coordinates (fractional)

sites

N1 (0.333,0.667,0.750)

2d

Al1 (0.333,0.667,0.250)

2c

N1 (0.000,0.500,0.500)

4a

Al (0.500,0.500,0.500)

4b

N1 (0.262,0.767,1.373) N5 (0.103,0.666,1.103) N9 (0.733,0.967,1.043)

4e 4e 4e 4e

Al1 (0.239,0.874,1.085)

Figure 2. Structure of predicted stable Al−N crystals. (a) P63mc-AlN phase at ambient condition. (b) Fm3m ̅ -AlN phase at 20 GPa. (c) P21/ c-AlN3 phase at 50 GPa; large and small spheres denote aluminum and nitrogen atoms, respectively.

From the perspective of crystal structures in Figure 2a,b, we can clearly see that all nitrogen atoms are isolated. In P63mc-AlN phase, those neighboring nitrogen atoms and central aluminum atoms form a tetrahedral structure (fourfold coordinated nitrogen atoms), and the distribution of the layers is formed along the c-axis. In Fm3̅m-AlN phase, each aluminum atom combined with six nitrogen atoms, in a sixfold coordinated mode, forming a close-packed octahedron structure. Crystal structure of the stable P21/c-AlN3 phase at 50 GPa is shown in Figure 2c. Nitrogen atoms form finite chains distribution in the three-dimensional space, and detailed structure information provided in Table 1. The AlN3 crystal adopts a P21/c symmetry structure at 50 GPa; each Al atom has six nearest neighboring N atoms (sixfold coordinated nitrogen atoms) in the P21/c phase. The lattice constants of this predicted phase are a = 11.12 Å, b = 3.57 Å, and c = 10.84 Å. The nonmolecular phase of “polymeric nitrogen” was predicted in AlN3 compound. Six nitrogen atoms adjacent to each other form minimum unit and look like zigzag distorted chain that stretches in the three-dimensional space. Upon exact calculations, we can confirm that valence electrons of Al atoms are depleted. Almost all the valence electrons transfer to nearby N6 molecules, resulting in nominal N66− anions in the crystal. Nevertheless, the connection of the intermediate nitrogen atoms (N5−N5) distorted in the Figure 3b, so the six atoms nitrogen chains are not in the same plane. Additionally, we also calculate the distance between adjacent connected nitrogen atoms in the N66− anion chain. It is found that the lengths from left to right are 1.36, 1.33, 1.30, 1.33, and 1.36 Å, which are longer than that of NN double bond (1.20

Figure 3. Calculated electronic properties of P21/c-AlN3 phase at 50 GPa. (a) The ELF isosurface (value = 0.85). Chemical polymeric nitrogen chain (minimum repeat unit) is discovered in P21/c space group and seems like zigzag distorted shape stretched in threedimensional space. (b) ELF map shown on the (1.4, 0, 1) cross section of the P21/c-AlN3 phase. Numbers adjacent to the N−N bond represent the bond length (black, in Å) and MBO value (red), respectively.

Å) but shorter than that of N−N single bond (1.45 Å). To further explain the bonding features, we calculated the Mayer Bond Order (MBO)46 values for the N66− anion chain in Figure 3b. The MBO values for the N−N bonds alternately are 1.46, 1.23, 1.20, 1.23, and 1.46. Those results confirm that the N−N bond of N6 chain is weaker than the NN (2.0) double bond but stronger than the N−N (1.10) single bond again.19 The alternated MBO values for the N−N bonds correspond well to the resonance effect in the Lewis structure,47,48 which reduces the energy of system and improve structural stability. Complete analysis can get from the details of the electron localization function (ELF) as shown in Figure 3a,b. 3.3. Bonding Properties of P21/c-AlN3 Phase with N66− Anions. The ELF is derived from an earlier idea of LennardJones; it was first reported by A. D. Becke and K. E. Edgecombe.49,50 The ELF of P21/c-AlN3 in Figure 3 was calculated for bonding analysis and could offer a reliable measure of electron pairing and localization. To our best knowledge, ELF is useful in distinguishing the metallic, C

DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry covalent, and ionic bonding characters. When the scale larger than 0.75 corresponds to the perfect localization characteristic of covalent bonds or lone pairs. In the P21/c-AlN3 phase, the zigzag distorted N6 chains indicate that N atoms are in the sp2 hybridization.51 The ELF value between two nearest neighboring nitrogen atoms in Figure 3a is ∼0.85, which is typical covalent bonding feature. The Lewis structure in Figure 3b has five N−N σ bonds with the adjacent nitrogen atoms. N9 atoms on the edge of the chain have two lone electronic pairs and only one lone electronic pairs located in the intermediary nitrogen atoms (N1 and N5). However, comparing with the polymeric nitrogen that has six electrons filling π-bonding state, N66− has extra four electrons. These electrons will partially occupy πantibonding states, leading to a metallicity state.52 All results of overlapping σ-bonds and π-bonds between nitrogen chains confirmed the resonance effect once again. We next analyze the COHP in Figure 4 and integral COHP (ICOHP) to characterize the bonding nature of N−N pairs.53

Figure 5. Phonon dispersion curves for the (a) P21/c-AlN3 phase at pressure of 50 GPa (b) Fm3̅m phase of AlN at 20 GPa.

obvious vibrational band gaps. These “pseudomolecular” modes are mainly due to the vibrational contribution of the polymer nitrogen chains. 3.4. Exotic Electronic Structure and Metallicity. To study the formation mechanism and inner nature of the P21/cAlN3 phase, its electronic band structures and projected density of states (PDOS) were calculated. They reveal that the P21/c structure is metallic with small density of states at the Fermi level, which are contributed by N_2p electrons. In the energy range from −10 to 8 eV, the band broadening is very smooth, indicating the strong electronic localization. It is found that the majority of occupied states below the Fermi level are the N_p and N_s states. However, the contributions of Al_s and Al_p to the occupied states are quite small. Those phenomena indicate that all aluminum atoms are the electron donor and that nitrogen chains are electron acceptor. The PDOS of nitric s and p orbits are very similar in the whole energy range, reflecting the strong orbital hybridization between N_s and N_p. Furthermore, the variation of DOS for P21/c-AlN3 phase is graceful, which indicates the layered distribution of nitrogen chains and aluminum atoms.54 The pseudogap near Fermi level served as the borderline between the bonding and antibonding states. In addition, total numbers of electrons at the Fermi level decreased, which was accompanied by the energy level of occupied bonding states decreasing and the energy level of antibonding states increasing. Its main function is to improve the stability of AlN3 structure.55,56 It is interesting to note that the pseudogap is found not only in transition-metal compounds,57 quasicrystal,58 amorphous alloy59 but also in metal nitrides. Generally, there are two main mechanisms for the formation of the pseudogap.54 The first one is the electron transfer due to the difference of the electronegativity between the two different elements. Although the difference of electronegativity between metal aluminum and nitrogen is

Figure 4. Plot of COHP for P21/c-AlN3 at 50 GPa. The negative and positive COHP values denote bonding and antibonding interactions, respectively. (a) N9−N1 pairs separated by 1.36 Å. (b) N5−N5 pairs separated by 1.30 Å. (c) N1−N5 pairs separated by 1.33 Å.

Figure 4 shows that nitrogen atom pairs with different distances exist in the form of covalent bonds. The COHP plot of the N5−N5 pairs and N1−N5 pairs are shown in Figure 4b,c, respectively. It is clear to see that bonding states are fully occupied and that antibonding states are partially occupied, leading to covalent bonding between the two adjacent nitrogen atoms. The theoretical value of ICOHP can express the bonding strength based on counting the energy-weighted population of wave functions between two atomic orbitals. Values of ICOHP corresponding to the Figure 4a−c are listed in the order 0.18, −7.68, and −7.59. It is shown that the interaction between N5−N5 nitrogen atoms is stronger than that of N1−N5 atoms and that the interaction between N1− N9 nitrogen atoms is weakest. Those calculation results are consistent with the results of MBO in Figure 3b. Additionally, an important means to judge the dynamic stability of materials is whether the phonon spectrum appears imaginary frequencies. We use the supercell method to calculate the phonon spectra as shown in Figure 5a. No imaginary frequency was found in the entire Brillouin zone, indicating that the P21/c phase of AlN3 is dynamically stable. It is found that all the acoustic vibration modes are mainly concentrated at 0−11 THz. Remaining optical branch vibrational modes at frequencies between 24 and 45 THz have D

DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the zigzag distorted polymeric nitrogen chains, in P21/c-AlN3 phase determine a lot of novel chemical and physical characteristics. The charges transferred from Al to N make the nonmolecular combination of nitrogen with kinetic stability. Nitrogen atoms with sp2 hybridization, the resonance effect between alternating π-bonds and σ-bonds in polymeric nitrogen chains are all responsible for the structural stability. N_p electrons form π-type chemical bonds between neighboring atoms result in the delocalization of π-electrons, causing charge transfer. And disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal cause for the metallicity. Moreover, we find the existence of pseudogap in the electronic structure of P21/cAlN3 phase. The electrons transferred from aluminum atoms to polymeric nitrogen chains led to the formation of pseudogap. On decomposition, the AlN3 is expected to release an enormously large amount of energy (2.75 kJ·g−1); thus, it may find applications as a high energy-density material and can hopefully encourage experimental efforts in its synthesis.

relatively small, we obtained the relationship between the donor and acceptor by the density of states. It is confirmed that there is charge transfer between metal aluminum and polymeric nitrogen chains. Therefore, the charge transfer is one reason for the pseudogap. The second reason may be the hybridization between aluminum and nitrogen orbits. Obviously, this situation is almost nonexistent, because the density of states of metallic aluminum near the Fermi level is very small. Herein, the electronic transfer from metal aluminum to polymeric nitrogen chains is the reason for the pseudogap. Visualizing the charge distribution at the Fermi level could help us get the bonding situation. The band-decomposed charge-density analysis of the N_p orbital corresponding to the high-symmetry points was studied in Figure 6a,b. Since the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (D.L.) *E-mail: [email protected]. Phone/Fax: +86-431-85168825. (T.C.) ORCID

Tian Cui: 0000-0002-9664-848X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51632002, 11404134, 51572108, 11634004, 11574109, and 11674122), Program for Changjiang Scholars and Innovative Research Team in Univ. (No. IRT_15R23), National Found for Fostering Talents of Basic Science (No. J1103202), and China Postdoctoral Science Foundation (2014M561279 and 2016T90246). Parts of calculations were performed in the High Performance Computing Center (HPCC) of Jilin Univ.

Figure 6. Calculated electronic properties of P21/c-AlN3 at 50 GPa. (a) Electronic band structure and PDOS of the P21/c phase. The minimum of conduction band named EC(B) and the maximum of valence band named EV(Γ). (b) Band decomposed charge density of the EV(Γ) and EC(B) states.

degree of electron distribution in the valence band, for instance, is higher than that in the conduction band, the electron transfer between σ-bond and π-bond maybe exists between nitrogen atoms.47 Furthermore, comparing the redistribution of the projected charge on the conduction band and valence band of polymeric nitrogen chains, we can find that both of them have parallel π-orbits. And the π-orbits are connected together. N_p electrons form π-type chemical bonds between neighboring nitrogen atoms, resulting in the delocalization of π-electrons and causing charge transfer.46 Charges in valence band orbits can easily transfer to the conduction band orbits under the condition of external magnetic field or electric field, because these two orbits have little difference in energy. So, disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal cause for the metallicity of P21/c-AlN3 phase, which are consistent with the ELF calculation.

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DEDICATION Dedicated to Prof. Guangtian Zou on the occasion of his 80th birthday. REFERENCES

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4. CONCLUSIONS As a summary, the crystal structure of the most-stable ground state of aluminum nitride at high pressures up to 100 GPa are extensively explored by using first-principles methods. The N66− anion nitrogen chain of P21/c-AlN3 under high pressure has been studied for the first time. The unique Lewis structures, E

DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b00980 Inorg. Chem. XXXX, XXX, XXX−XXX