Experimental and theoretical evidence of the temperature-induced wurtzite to rocksalt phase transition in GaN under high pressure

The $p\text{−}T$ conditions of the solid-solid phase transition from the wurtzite to rocksalt structure in GaN are determined both experimentally and by ab initio calculations. Experimental evaluation was based on the x-ray absorption measurements in a laser-heated diamond anvil cell. At 300 K, the transition was observed near 47 GPa. At lower pressures, the wurtzite to rocksalt transition has been induced by high temperature: 1420 K at 42 GPa and about 2100 K at 37 GPa. Thus the slope of the wurtzite-rocksalt borderline could be evaluated as negative and nonlinear. On the part of the theory, the $p\text{−}T$ borderline was determined from a comparative analysis of the temperature dependences of the Gibbs potential of the wurtzite and rocksalt structures for different pressure. The Gibbs potentials were calculated within the quasiharmonic approximation and the self-consistent phonon approach. The results obtained with the self-consistent phonon approach show that the inclusion of the anharmonic phonon effects is indispensable to obtain a very good agreement with the experimental data. Possible consequences of the observed anharmonicity for the still unknown melting behavior of GaN are discussed. In particular, it is suggested that the melting temperature of the rocksalt-GaN, at pressure around 37 GPa, is not much higher than 2100 K.

Figure 1

State of knowledge for phase diagram of GaN. Experimental and theoretical data on GaN solid-solid phase transition (orange polygon): Van Vechten model [4] – green line; experiments at RT: EXAFS study – blue semisolid diamond [7], x-ray diffraction experiments – blue semisolid square [8] and circle [9]; ab initio calculations at 0 K: red semisolid pentagon [10], star and triangle [11], hexagon [12] and inverted triangle [13].

Figure 2

(a) Normalized Ga K edge XANES obtained at RT for several pressures. (b) Normalized Ga K-edge XANES collected at a compression of 42 GPa and heated from RT to 2100 K, where the spectrum obtained at 1420 K is a fingerprint of the WZ and RS crystal mixture at the beginning of the structural transition.

Figure 3

(a) Normalized XANES spectrum from 47 GPa, RT compared with fitted curve that was obtained as a simple algebraic sum of 55% of the WZ (standard – normalized XANES from 39 GPa) and of 45% of the RS (standard – normalized XANES from 55 GPa) experimental curves. (b) Normalized XANES spectrum from 42 GPa, 1630 K compared with fitted curve which was obtained as a simple algebraic sum of 30% of the WZ (standard – normalized XANES from 42 GPa, 1000 K) and of 70% of the RS (standard – normalized XANES from 42 GPa, 2000 K) experimental curves.

Figure 4

(a) Experimental XANES spectra of WZ and RS phases, (b) Simulated XANES spectra of the WZ and RS phases.

Figure 5

A comparison of the components of Gibbs free energies obtained for the WZ and RS structures: (a) static part from Eq. (1), (b) harmonic part – QHA from Eq. (2), and (c) SCPH correction from Eq. (3).

Figure 6

Comparison of the phase boundary for the WZ to RS transition obtained experimentally as well as calculated with the DFT Gibbs potentials with only harmonic phonons (QHA) and including the anharmonic phonon effects (SCPH).

Figure 7

Updated vision of the $p\text{−}T$ phase diagram of gallium nitride: the solid-solid WZ-RS borderline (red, experimental, blue, theoretical) is a contribution of this study.