Novel high-pressure phases of AlN: A first-principles study

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Abstract

Four novel AlN phases with low density and high hardness are predicted by using a evolutionary methodology on structural search. All four phases are thermodynamically more favourable than the rock salt structure of AlN at ambient pressure and will be transformed to rock salt AlN at certain pressure. All these phases are mechanically and dynamically stable by checking the independent elastic constants and phonon dispersion spectra. The calculated mechanical properties reveal that the proposed AlN phases are hard, and the Vickers hardness of the AlN phases ranges from 13.2 GPa to 15.2 GPa. The calculated band structures of the novel AlN phases reveal that these AlN phases possess semiconductive properties with wide direct band gaps at G points, and the gaps range from 3.627 eV to 3.927 eV.

Introduction

Aluminium nitride (AlN) has been attracted extensive attention on theoretical and experimental studies due to its importance of scientific research and industrial application. At ambient conditions, AlN exists as a closely packed hexagonal wurtzite structure (wz-AlN), which has a widely applications, including cutting and machining tools, semiconductor devices, and field-emission devices because of its excellent chemical and physical properties, such as high thermal conductivity [1], [2], high melting point [3], large bulk modulus [4], [5], [6], good hardness [7], [8], [9], and corrosion resistance [10], large band gap [11].

The materials’ chemical and physical properties are related to their structures. Scientists have been searching for novel allotropes of AlN, which may have other important applications. A zinc blende structure AlN (zb-AlN) has been synthesised by a solid-state reaction [12]. Scientists have acquired a rock salt structure AlN (rs-AlN) transformed from wz-AlN by utilising high pressure technology [13], [14], [15], [16]. This phase transition has been reported to begin at 14–16.6 GPa [13], [14], [15], [16]. Theoretical studies on AlN structures are +underway. In 1973, van Vechten predicted a transition to a β-Sn structure (for the diatomic equivalent of the β-Sn structure of binary compound; this structure was also named d-β-Sn and its space group is I4¯m2 [17], [18]) at 90 GPa [19], whereas no phase transitions to β-Sn were observed [15], [20]. The d-β-Sn structure has been proven to be absent in several III–V/II–VI semiconductors [21], and it is not favoured at any pressure [22]. In addition, Christensen predicted a phase transition for the rs  NiAs structure between 30 and 40 GPa [20]. However, no NiAs structure was found within at least 132 GPa by Uehara et al. [23]. The theoretical study by Serrano illustrated that the NiAs structure cannot correspond to a stable high-pressure phase [22]. Christensen [20] stated that anti-NiAs structure cannot be the high-pressure phase of AlN, which agrees with Serrano’s research [22]. Thus, the typical phases of AlN (wz, zb and rs) were adopted for comparison with the novel proposed phases.

In the present work, we have extensively searched the potential structures of AlN. Four novel high-pressure orthorhombic AlN crystal structures have been predicted. The stabilities of these structures are determined by independent elastic constants and phonon dispersion spectra. We show the results of a total energy study of several phases of AlN. The transition behaviours of various AlN phases under hydrostatic pressure are studied. Further calculations are performed to study the mechanical, thermodynamic and electronic properties of four novel AlN phases.

Section snippets

Computational methods

The crystal structure prediction software CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization code) is widely used [24], [25] and great achievements in structure prediction have been made with this software [26], [27], [28], [29]. The evolutionary simulation method by CALYPSO was performed with none experimental information as guideline. The calculations of geometry optimisations, enthalpies, elastic constants, phonons and band structures were implemented in the CASTEP code [30].

Optimisation of crystal structures

A large selection of candidate structures composed of AlN is studied. In addition to wz-AlN, zb-AlN, rs-AlN and the structure predecessors that have been previously studied, four novel structures of AlN have been predicted. Their space groups are Pmn21, Pbam, Pbca and Cmcm, and the phases are denoted as Pmn21-AlN, Pbam-AlN, Pbca-AlN and Cmcm-AlN, respectively. The four novel structures of AlN all belong to the orthorhombic crystal system. Pmn21-AlN, Pbam-AlN and Pbca-AlN are all

Conclusion

In this work, four novel orthorhombic AlN phases are proposed after using an evolutionary methodology. Based on first-principles calculations, the enthalpy of all four phases (Pmn21-AlN, Pbam-AlN, Pbca-AlN and Cmcm-AlN) will be lower than that of rs-AlN at ambient pressure. The calculated elastic constants and phonon dispersion spectra certify the mechanically and dynamically stabilities of the four novel phases. The calculated Hv is based on an empirical relation; the resulting values suggest

Acknowledgements

This work was supported by the National Science Foundation of China (Grant Nos. 51421091 and 51332005) and the Postgraduate Innovation Project of Hebei Province of China (Grant No. 00302-6370007).

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