Ab initio calculation of structural phase transitions in AlN crystal

Density functional theory and the quasiharmonic approximation have been used to study the phase diagram of aluminum nitride. AlN was exposed to pressures ranging from $-20\text{to}20\mathrm{GPa}$. The phonon dispersion relations and phonon density of states have been calculated for the wurtzite, zinc blende, and rocksalt phases of AlN. The pressure-temperature phase diagram of AlN was constructed and compared with the available experimental data. It shows that at ambient temperature and pressure the most stable phase of AlN is the hexagonal wurtzite structure, while at high pressures and temperatures the rocksalt phase becomes more stable.

Figure 1

Calculated ground state energies vs crystal volume for wurzite, zinc blende, and rocksalt phases of AlN.

Figure 2

Calculated phonon dispersion curves together with PDOS for the wurtzite structure of AlN. Phonon dispersion curves are compared with the experimental points measured by inelastic x-ray scattering (Ref. 17).

Figure 3

Calculated phonon dispersion curves together with PDOS for the zinc blende structure of AlN.

Figure 4

Calculated phonon dispersion curves together with PDOS for the rocksalt structure of AlN.

Figure 5

Comparison of phonon dispersion curves of wurtzite structure under pressure of $12\mathrm{GPa}$ (solid circles) and $-12\mathrm{GPa}$ (open circles).

Figure 6

Dependence of the free energy $F(T,V)$ on crystal volume for a number of selected temperatures for wurtzite structure of AlN.

Figure 7

Calculated Gibbs free energy vs temperature for all structures of AlN and for pressure $P=16\mathrm{GPa}$. The arrow indicates the first-order phase transition temperature between the wurtzite and the rocksalt structures.

Figure 8

Calculated volume thermal expansion at $P=0$ for wurzite, zinc blende, and rocksalt phases of AlN.

Figure 9

Calculated $P\text{−}T$ phase diagram for aluminum nitride compared with selected experimental points. A, visual observations (Refs. 7, 11), quench (Ref. 31); B, shock Hugoniot (Ref. 32); C, x-ray diffraction (Ref. 7).