First principles lattice dynamics and thermodynamic properties of four Al2O3 and solid-state temperature and pressure phase diagrams

Author : Jazmyn

Update time : 2024-05-16 10:21:22

Al2O3 has excellent physical and chemical inertness, good mechanical properties, high hardness, high thermal stability, insulation, wear resistance, and corrosion resistance. Therefore, it has extensive applications in engineering fields such as electronics, catalysis, and coatings. In addition to the most stable α-Al2O3 (corundum, sapphire), there are various so-called transitional Al2O3 phases, η, κ, δ, θ, and γ, which are formed during dehydration before the final form of corundum at 1100 °C The following is very stable. Among the various transition phases, γAl2O3 (γ-Al2O3) is particularly important in the petroleum industry because it can be used as both an active catalyst and a catalyst support. γ-Al2O3 is a common high-temperature Al2O3 phase in the oxidation process of aluminum-based alloys. First-principles predictions of the thermodynamic properties of γ-Al2O3 have been challenging due to partial occupancy of aluminum sites leading to partial ordering and defects in the spinel structure.

Scholars from Chongqing University and Pennsylvania State University, College Park, USA, used the first-principles quasi-harmonic method to obtain the finite-temperature thermodynamic properties of γ-Al2O3, including entropy, chemical potential, heat capacity, thermal expansion coefficient, and elastic constant and reported the results The calculated results are compared with those of the other three Al2O3 phases, namely α-Al2O3, θ-Al2O3 and κ-Al2O3. The calculated lattice constants and predicted relative phase stabilities (α>κ>θ>γ) at 0 K and 0 GPa are consistent with experimental values and existing calculations, respectively. Based on these results, a temperature-pressure phase diagram covering four Al2O3 phases (α, κ, θ, γ) was constructed, providing a basis for the experimental synthesis of different Al2O3 phases. The related work was published in Acta Materialia as a research article titled "First-principles lattice dynamics and thermodynamic properties of α-, θ-, κ- and γ-Al2O3 and solid state temperature-pressure phase diagram".

To date, due to the uncertainty of its crystal structure, the lattice dynamics and finite-temperature thermodynamic properties of γ Al2O3 have not been systematically studied or compared with other Al2O3 phases. In addition, it is understood from the literature that DFT research on the temperature-dependent thermodynamic properties of other important transition phases of Al2O3, such as α-Al2O3, θ-Al2O3, and κ-Al2O3, is relatively scattered, and there is a lack of systematic research. This study first selected three γAl2O3 crystal structures proposed in the literature [24, 27, 30] to study their relative stability and compared γAl2O3 with α-Al2O3 and θ-Al2O3 based on first-principles calculations. And relative phase stability of κ-Al2O3. Then, systematic first-principles quasi-harmonic calculations of the lattice dynamics and finite-temperature thermodynamic properties of γAl2O3 and three other Al2O3 phases (α-Al2O3, θ-Al2O3, and κ-Al2O3) were performed. The calculated results for α-Al2O3 are consistent with experimental measurements and existing calculations and thus serve as a benchmark example to verify the reliability of current calculations for other phases. The predicted thermodynamic properties and pressure-temperature phase diagrams of the four Al2O3 phases show the relative stability of the phases and can guide the experimental fabrication of Al2O3.

Figure 1. (a) α-Al2O3 (space group R3c), (b) θ-Al2O3 (space group C2/m), and (c) κ-Al2O3 (space group Pna21).

Figure 2. Original cell schematic diagram of the bulk γ-Al2O3 models considered: Ouyang tetragonal spinel model, Digne monoclinic non-spinel model (space group P21/m), and Pinto monoclinic spinel model (space group Group C2/m). Solid black lines represent the unit cell boundaries. In the model, aluminum atoms coordinated by tetrahedrons (Al-Tet), octahedrons (Al-Oct), and vacancies (Al-Vac) are represented in green, blue, and black, respectively, and oxygen atoms are represented in red. Here, Al-Vac is used only for spinel structures.

Figure 3. Calculated static energy of three γ-Al2O3 bulk models considered in this study, including a model based on Ouyang tetragonal spinel, a Digne monoclinic non-spinel model, and a model based on Pinto monoclinic spinel. Model

Figure 4. Calculated static energy of α-Al2O3, θ-Al2O3, κ-Al2O3 and γ-Al2O3 (based on Ouyang tetragonal spinel model)

Figure 5. (a), (b) partial, complete and generalized phonon density of states (PDOS) calculation results of α-Al2O3; (c), (d) partial, complete and generalized phonon density of state calculations of θ-Al2O3 Results; (e), (f) partial, complete and generalized phonon state density calculation results of κ-Al2O3; (g), (h) partial, complete and generalized results of γ-Al2O3 (based on Ouyang tetragonal spinel model) Calculated phonon density of states; (i), (j) partial, full and generalized phonon density of states calculations for (i), (j) γ-Al2O3 (based on Digne monoclinic non-spinel model); and (k), (l) γ -Partial, total and generalized phonon density of states calculations for Al2O3 (based on the Pinto monoclinic spinel model).

Figure 6. Phonon dispersion calculation results of (a) α-Al2O3, (b) θ-Al2O3, (c) κ-Al2O3, and (d) γ-Al2O3 (Ouyang). The solid line represents the current calculation result. The red open circles in (a) refer to inelastic neutron scattering data from the study of Schober and Strauch.

Figure 7. (a) Calculated entropy (S) and (b) chemical potential as a function of temperature for α-Al2O3, θ-Al2O3, κ-Al2O3, and γ-Al2O3, and the experimental data of Furukawa et al. [77] and Calculated data from Chase et al. [78].

Figure 8. (a) Calculated Debye temperature and (b) thermal expansion coefficient as a function of temperature for α-Al2O3, θ-Al2O3, κ-Al2O3, and γ-Al2O3, and Chung et al. [80], Hoven et al. [81 ] and the computational data of Shang et al. [60] and the experimental data of Halvarsson et al. [82].

Figure 9. (a) Calculated heat capacity of α-Al2O3, θ-Al2O3, κ-Al2O3, and γ-Al2O3 at constant volume and (b) heat capacity at constant pressure as a function of temperature, and Furukawa et al. [ 77] and computational data of Saxena et al. [83], White et al. [84], Chase et al. [78] and Ching et al. [85].

Figure 10. Pressure-temperature phase diagrams of α-Al2O3, θ-Al2O3, κ-Al2O3, and γ-Al2O3 derived from first-principles calculations.

A systematic first-principles study of the lattice dynamics of γ-Al2O3 was performed, and the results were compared with calculated results for the other three Al2O3 phases, namely α-Al2O3, θ-Al2O3 and κ-Al2O3. The effects of temperature and pressure on the phase stability and thermodynamic properties of α-, θ-, κ- and γ-Al2O3 are reported. The results show that α-, θ-, κ- and γ-Al2O3 have dynamic and thermodynamic stability. The relative phase stability is predicted to be α>κ>θ>γ at 0 K and 0 GPa. Among them, the calculated lattice parameters of α-, θ-, κ- and γ-Al2O3 are consistent with the experimental data.

At 0 K and 0 GPa, Ouyang spinel-based γ-Al2O3 is more stable than the Digne monoclinic non-spinel model and the Pinto monoclinic spinel model. The thermodynamic and elastic properties of α-, θ-, κ- and γ-Al2O3 were obtained, including entropy, chemical potential, heat capacity, thermal expansion coefficient, Debye temperature, elastic stiffness coefficient, bulk modulus, shear modulus and Yang Temperature dependence of modulus. The pressure-temperature phase diagram of Al2O3 in the temperature and pressure ranges of 0 K to 2350 K and -10 GPa to 10 GPa was constructed, indicating that γ-Al2O3 and κ-Al2O3 can be stable at relatively high temperatures and negative pressures, And θ-Al2O3 is stable under negative pressure and low temperature.

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