First-Principles Analysis of Phase Stability and Transformation Suppression for Hydrogen-Doped Alumina

Abstract

Thermally grown oxide (TGO) layers—primarily alumina (Al₂O₃)—provide oxidation resistance and high-temperature protection for thermal barrier coatings. However, during their service in humid and hot environments, water vapor accelerates TGO degradation by stabilizing metastable alumina phases (e.g., θ-Al₂O₃) and inhibiting their transformation to the thermodynamically stable α-Al₂O₃, a phenomenon which has been shown in numerous experimental studies. However, the microscopic mechanisms by which water vapor affects the phase stability and transformation of alumina remain unresolved. This study employs first-principles calculations to investigate hydrogen’s role in altering vacancy formation, aggregation, and atomic migration in θ- and α-Al₂O₃. The results reveal that hydrogen incorporation reduces the formation energies for aluminum and oxygen vacancies by up to 40%, promoting defect generation and clustering; increases aluminum migration barriers by 25–30% while lowering oxygen migration barriers by 15–20%, creating asymmetric diffusion kinetics; and stabilizes oxygen-deficient sublattices, disrupting the structural reorganization required for θ- to α-Al₂O₃ transitions. These effects collectively sustain metastable θ-Al₂O₃ growth and delay phase stabilization. By linking hydrogen-induced defect dynamics to macroscopic coating degradation, this work provides atomic-scale insights for designing moisture-resistant thermal barrier coatings through the targeted inhibition of vacancy-mediated pathways.

1. Introduction

Thermal barrier coatings (TBCs) are critical for protecting hot-section components of aero-engines, including turbine blades, combustion chambers, and nozzles, due to their exceptional thermal insulation, oxidation resistance, and corrosion protection [1,2,3,4,5]. A typical TBC system comprises three layers: a ceramic topcoat, a metallic bond coat (typically MCrAlY), and a superalloy substrate [1,2,3,4,5]. During service, oxygen permeates the top coat layer and reacts with metallic elements in the bond coat, forming thermally grown oxide (TGO) primarily composed of alumina (Al₂O₃) [1,2,3].

The ideal TGO consists of a dense, fast-growing α-Al₂O₃ phase with high thermodynamic stability, which effectively inhibits oxygen diffusion and enhances oxidation resistance [4,5]. However, under operational conditions involving thermal cycling and oxidizing environments, metastable alumina phases (e.g., γ-, η-, δ-, and θ-Al₂O₃) often nucleate prior to α-Al₂O₃ formation due to gradients in oxygen partial pressure, temperature, and stress [6,7,8,9,10]. These metastable polymorphs exhibit distinct physical properties, with γ- and θ-Al₂O₃ growing up to two orders of magnitude faster than α-Al₂O₃ [11,12,13,14,15]. Consequently, the phase stability of α-Al₂O₃ governs the oxidation kinetics, stress evolution, and long-term performance of TBC systems.

Extensive research over the past two decades has elucidated the oxidation behavior of bond coat layers. At temperatures below 1200 °C, metastable γ- or θ-Al₂O₃ typically forms initially due to kinetic favorability, subsequently transforming to α-Al₂O₃ through prolonged oxidation [4,5,6,7,8,9,10]. This phase transition is accompanied by volumetric shrinkage and defect generation, which accelerate ionic diffusion, induce growth stresses, and promote interfacial microcracking [16,17,18]. The metastable-to-stable phase evolution thus critically influences TGO morphology, mechanical integrity, and TBC lifespan. Recent studies highlight the role of metastable alumina in interfacial degradation. For instance, Zhang et al. [19] investigated γ-Al₂O₃/Al interfacial properties using first-principles calculations, while Sakakibara et al. [20] observed transient η- and θ-Al₂O₃ nanoparticles via transmission electron microscopy prior to α-Al₂O₃ crystallization. Despite these advances, the energy landscape governing θ- to α-Al₂O₃ transitions remains poorly characterized.

In water vapor-enriched gas turbine environments, hydrogen (H) accelerates TBC degradation by stabilizing metastable alumina and inhibiting α-Al₂O₃ formation [21,22]. The proposed mechanisms include enhanced cation diffusion [23], altered Al/O ion mobility [24], and hydroxyl-induced vacancy formation [25,26,27]. For example, Yan et al. [25] suggested that dissociated protons from H₂O disrupt oxide electroneutrality, increasing cation vacancy concentrations and suppressing θ- to α-Al₂O₃ transitions. Similarly, Zhu et al. [26] and Cheng et al. [27] attributed prolonged metastable phase growth to water-vapor-promoted defect density and atomic diffusion. However, the atomistic mechanisms underlying these phenomena remain unresolved.

The θ- to α-Al₂O₃ transition is hypothesized to involve either diffusional restructuring [28] or cooperative oxygen sublattice shear combined with Al³⁺ migration [29]. Bagwell et al. [28] demonstrated that α-Al₂O₃ nucleation proceeds via diffusion rather than shear-driven reconfiguration. In contrast, Huang et al. [29] proposed a simultaneous shear–migration mechanism based on first-principles calculations. Despite these insights, the influence of water vapor on phase transition energetics and kinetics lacks a unified explanation.

Numerous studies have shown that water vapor accelerates TGO degradation, yet the microscopic mechanisms by which it affects the phase stability and transformation of alumina remain unclear. This study aims to elucidate the microscopic mechanisms by which water vapor influences the phase stability and transformation of alumina. To achieve these objectives, we employed first-principles calculations to investigate the impact of hydrogen on vacancy formation, aggregation, and atomic migration in θ- and α-Al₂O₃. These findings enhance the fundamental understanding of oxidation mechanisms and provide theoretical guidance for the design of more durable TBC systems.

2. Materials and Methods of Research

The α-Al₂O₃ crystal (space group R-3c; trigonal system) adopts a close-packed configuration with lattice constants a = b = 0.4759 nm and c = 1.291 nm, containing 30 atoms per unit cell [30,31,32]. As shown in Figure 1a, a (3 × 3 × 1) supercell expansion of the α-Al₂O₃ protocell yielded a 270-atom model with a hydrogen concentration of 0.37 at%. In α-Al₂O₃, O and Al atoms occupy single crystallographic sites, necessitating the consideration of only two vacancy configurations—octahedral Al vacancies (V_Al-oct) and O vacancies (V_O). To construct these defects, one Al atom was removed from both pristine and hydrogen-doped α-Al₂O₃ to form V_Al-oct, while one O atom was removed to generate V_O. Structural optimization was performed to stabilize the models. In addition, θ-Al₂O₃ exhibits monoclinic symmetry (space group C2/m) with lattice parameters a = 11.9 Å, b = 2.9 Å, and c = 5.56 Å, comprising 20 atoms per unit cell (four formula units) with all atoms occupying 4i Wyckoff positions [33]. A (1 × 4 × 2) supercell expansion (Figure 1b) produced a 160-atom model with 0.625 at% hydrogen. In θ-Al₂O₃, O atoms occupy a single equivalent site, requiring only one V_O configuration. Al atoms reside in tetrahedral and octahedral interstitial positions, leading to two distinct Al vacancy models—tetrahedral (V_Al-tet) and octahedral (V_Al-oct). These defects were modeled by removing one tetrahedral or octahedral Al atom from pristine and hydrogen-doped θ-Al₂O₃, respectively, followed by structural optimization.

First-principles calculations were performed to investigate vacancy formation, aggregation, and atomic migration, as well as hydrogen doping effects on alumina growth with phase transformation. Large supercells with low defect concentrations (≤0.625 at%) were employed to minimize defect interactions and reflect local structural properties. All calculations utilized the VASP software package with the PAW-PBE pseudopotential and generalized gradient approximation (GGA) for exchange-correlation. The H pseudopotential (PAW_PBE H 15Jun2001) was selected to model neutral hydrogen atoms, which is consistent with prior studies of H diffusion in alumina [34,35]. A 3 × 3 × 3 Monkhorst-Pack k-point grid, a plane-wave cutoff energy of 500 eV, and convergence criteria of 10⁻⁴ eV (electronic) and 10⁻⁵ eV (total energy) were applied. Structural optimizations ensured force convergence below 0.001 eV/Å. This approach accurately describes core–electron interactions and defect-related electronic structures, as well as enabling the precise evaluation of key properties including vacancy formation energies and migration barriers. This ensures both reliable structural predictions and computational efficiency for complex oxide systems such as alumina.

In detail, the formulae assessing the vacancy formation difference ($\delta E$) between H-free and H-doped systems, i.e., $E_{form}$ and $E_{form}^H$ are as follows:

$$E_{form}=E_V+\epsilon -E_{pr}$$

(1)

$$E_{form}^H=E_V^H+\epsilon -E_{pr}^H$$

(2)

and

$$\delta E=E_{form}^H-E_{form}$$

(3)

In Equations (1)–(3), $E_V$ and $E_V^H$ represent the total energy for the defective H-free and H-doped Al₂O₃ systems, respectively. $E_{pr}$ and $E_{pr}^H$ represent the total energy for the pristine H-free and H-doped Al₂O₃ systems, respectively. $\epsilon$ represents the chemical potential of the removed atom (Al or O) in its standard state. Furthermore, the vacancy aggregation energy ($\Delta E$) is defined as the difference between the double vacancy and the two separated vacancy energies, as follows:

$$\Delta E=E_{2V}^D-E_{2V}^S$$

(4)

where $E_{2V}^D$ and $E_{2V}^S$ represent the total energy of the system with two vacancies aggregated as nearest neighbors on the dense row surface and the total energy of the system with two isolated vacancies (separated beyond nearest-neighbor distances), respectively. It is seen that $\Delta E<0$ indicates thermodynamically favorable vacancy aggregation, while $\Delta E>0$ suggests mutual repulsion.

For atomic diffusion studies, adjacent Al/O atom pairs in equivalent lattice planes were selected. A vacancy was created by removing one atom, while the neighboring site remained either pristine or hydrogen-doped. The final state for diffusion was generated by swapping the vacancy position with the retained atom, followed by structural optimization. Diffusion pathways were determined using the climbing image nudged elastic band (CI-NEB) method. Prior to pathway interpolation, the Variational Transition State Theory script was employed to verify the geometric similarity between optimized initial and final states, before validating the interpolation process by ensuring the continuity of atomic trajectories, finally confirming force convergence (−0.03 eV/Å) at all intermediate images.

3. Results and Discussion

3.1. Mechanical Property Validation

The mechanical properties of α-Al₂O₃ and θ-Al₂O₃ base cells (Section 2) were evaluated, including bulk modulus, shear modulus, and Young’s modulus. As summarized in

Table 1. Bulk modulus, shear modulus, and Young’s modulus of crystalline cells.

Phase		Bulk Modulus (GPa)	Shear Modulus (GPa)	Young’s Modulus (GPa)
α-Al2O3	Present model	223.26	165.75	399.56
	Reference	220.63 [36]	162.28 [36]	390.98 [36]
θ-Al2O3	Present model	201.74	142.67	343.35
	Reference	199.04 [36]	143.65 [36]	347.38 [36]

, the calculated values for α-Al₂O₃ are 223.26 GPa, 165.75 GPa, and 399.56 GPa, respectively, which are consistent with prior computational results [36]. For θ-Al₂O₃, the corresponding values (201.74 GPa, 142.67 GPa, and 343.35 GPa) exhibit minimal deviations of 1.35%, 0.68%, and 1.16% from the reference data [36]. These agreements validate the structural accuracy of the α-Al₂O₃ and θ-Al₂O₃ models, providing a reliable foundation for defect and atomic migration studies in Section 3.2, Section 3.3 and Section 3.4.

3.2. Vacancy Formation and Lattice Distortion

In alumina crystals, vacancies arise when atomic vibrational amplitudes exceed critical thresholds, displacing atoms from their lattice sites and inducing local distortions. Water vapor-derived H protons further influence these distortions by interacting with vacancy defects. Figure 2 and Figure 3 illustrate the optimized supercell models of α-Al₂O₃ and θ-Al₂O₃ doped with H protons. In both phases, H protons localize near aluminum vacancies (V_Al), while occupying oxygen vacancy (V_O) sites. Structural optimization reveals that V_Al and V_O induce distinct lattice distortions, which are modulated by H incorporation.

For α-Al₂O₃ (Figure 2), the intrinsic V_O and octahedral V_Al−oct formation energies are 6.73 eV and 12.01 eV, respectively, matching previous calculations (see

Table 2. Vacancy generation energies for H-doped and -undoped α-Al₂O₃ and θ-Al₂O₃.

Phase	Vacancy	${\mathit{E}}_{\mathit{f}\mathit{o}\mathit{r}\mathit{m}}$/eV References	${\mathit{E}}_{\mathit{f}\mathit{o}\mathit{r}\mathit{m}}$/eV Present Model	${\mathit{E}}_{\mathit{f}\mathit{o}\mathit{r}\mathit{m}}^{\mathit{H}}$/eV Present Model	δE/eV
α-Al2O3	VO	6.80 [37,38]	6.73	4.56	−2.17
	VAl-oct	12.13 [37,38]	12.01	6.75	−5.26
θ-Al2O3	VO	6.20 [39]	6.11	3.29	−2.82
	VAl-oct	-	12.51	5.91	−6.60
	VAl-tet	-	12.09	5.14	−6.95

, Refs [37,38]). Upon H doping, these energies decrease to 4.56 eV (V_O) and 6.75 eV (V_Al−oct), corresponding to relative defect formation energies of −2.17 eV and −5.26 eV. Figure 2 demonstrates that intrinsic V_Al−oct causes the outward displacement of octahedral O atoms, whereas H doping mitigates inward shifts of the nearest-neighbor O atoms. Similarly, intrinsic V_O promotes the inward aggregation of diagonally coordinated Al atoms, while H incorporation suppresses this effect. These results indicate that H protons reduce the lattice distortions associated with V_Al−oct and V_O, thereby lowering their formation energies.

In θ-Al₂O₃ (Figure 3), intrinsic V_O, V_Al−oct, and tetrahedral V_Al−tet exhibit formation energies of 6.11 eV, 12.51 eV, and 12.09 eV, respectively, aligning with reported V_O values (6.20 eV) [39]. H doping reduces these energies to 3.29 eV (V_O), 5.91 eV (V_Al−oct), and 5.14 eV (V_Al−tet), with relative defect formation energies of −2.82 eV, −6.60 eV, and −6.95 eV. Figure 3 shows that intrinsic V_Al−tet and V_Al−oct induce outward O displacements, while H incorporation alters the displacement directions of the adjacent O atoms. For V_O, intrinsic vacancies cause the inward aggregation of tetrahedral Al atoms and outward shifts of octahedral Al atoms; H doping counteracts the former and enhances the latter. These structural changes weaken Al-O bonding constraints near V_Al−tet, V_Al−oct, and V_O, mirroring the H-induced reduction in vacancy formation energies observed in α-Al₂O₃.

3.3. Vacancy Aggregation Behavior

The aggregation of vacancies in alumina crystals drives vacancy migration and redistribution. This study investigates how H proton incorporation alters the vacancy aggregation tendencies in α-Al₂O₃ and θ-Al₂O₃ by analyzing aggregation energy variations (Figure 2, Figure 3, Figure 4 and Figure 5). Due to structural differences, α-Al₂O₃ contains one type each of Al (V_Al-oct) and O (V_O) vacancies, while θ-Al₂O₃ exhibits two Al vacancies (V_Al-oct and V_Al-tet) and one O vacancy (V_O).

In the absence of H protons, the vacancy aggregation energies for α-Al₂O₃ are 0.71 eV (V_Al-oct) and −0.21 eV (V_O), whereas θ-Al₂O₃ shows values of 1.72 eV (V_Al-oct), 0.34 eV (V_Al-tet), and −0.07 eV (V_O) (Figure 4 and Figure 5). These results indicate mutual repulsion between V_Al vacancies in both phases under natural conditions. The aggregation of Al vacancies in the intrinsic state is thermodynamically unfavorable, which limits Al diffusion via vacancy-mediated mechanisms. Notably, the V_Al-oct aggregation energy in θ-Al₂O₃ is significantly higher than that of V_Al-tet in θ-Al₂O₃ or V_Al-oct in α-Al₂O₃, suggesting that tetrahedral Al vacancies (V_Al-tet) are more prone to aggregation. This will provide a low-energy migration pathway for Al atoms from tetrahedral to octahedral sites, thereby facilitating their diffusion. Conversely, V_O vacancies exhibit mutual attraction, with θ-Al₂O₃ displaying a slightly lower aggregation energy (−0.07 eV) than α-Al₂O₃ (−0.21 eV).

H proton incorporation markedly reduces vacancy aggregation energies. For α-Al₂O₃, V_Al-oct and V_O energies decrease by 0.52 eV (to 0.19 eV) and 2.03 eV (to −2.24 eV), respectively. In θ-Al₂O₃, reductions of 0.15 eV (V_Al-tet), 1.51 eV (V_Al-oct), and 1.19 eV (V_O) are observed (Figure 4 and Figure 5). These reductions facilitate vacancy defect generation, particularly for O vacancies, which exhibit greater energy sensitivity than Al vacancies. Upon the introduction of hydrogen protons, the aggregation behavior of oxygen vacancies is significantly enhanced. The aggregation of multiple oxygen vacancies will promote the formation of larger defect clusters, which may further evolve into nanoscale voids within the lattice.

The observed vacancy interactions arise not from classical forces but from charge imbalance effects and electron trapping induced by vacancy formation [38,40]. H protons amplify these interactions, enhancing O vacancy aggregation while weakening Al vacancy repulsion. In α-Al₂O₃ and θ-Al₂O₃, O atoms carry negative charges. O vacancy creation leaves two positive charge centers, promoting attraction to electrons or negatively charged species. H protons increase the positive charge of O vacancies, further driving aggregation. Conversely, Al vacancies generate three negative charge centers due to missing positively charged Al atoms. While these vacancies initially attract positive charges, dominant repulsion from neighboring O ions suppresses aggregation. H proton incorporation mitigates the negative charge of Al vacancies, reducing this repulsion.

3.4. Atomic Migration in Alumina Phases

Extensive research has demonstrated that water vapor critically influences the oxidation dynamics at TBC interfaces, particularly by prolonging the metastable θ-Al₂O₃ phase formation and altering its transition to the stable α-Al₂O₃ phase. This study investigates the mechanistic role of H doping in modulating the migration of Al and O atoms within α-Al₂O₃ and θ-Al₂O₃ supercells. The analysis accounts for two key factors, as follows: (i) during Al migration between octahedral sites, atoms traverse tetrahedral interstitial positions; (ii) oxygen vacancies are introduced during O migration, as H protons preferentially occupy these lattice vacancies.

Figure 6 presents the migration pathways and energy barriers for Al and O atoms in both α-Al₂O₃ and θ-Al₂O₃ phases under H-doped and -undoped conditions. For undoped α-Al₂O₃, the Al and O migration barriers are 1.98 eV (Al₁₀₇O₁₆₂) and 3.69 eV (Al₁₀₈O₁₆₀), respectively. The corresponding values for undoped θ-Al₂O₃ are 1.75 eV (Al₆₃O₉₆) and 4.25 eV (Al₆₄O₉₄). H doping significantly alters these barriers—in α-Al₂O₃, Al migration increases to 3.36 eV (Al₁₀₇O₁₆₂H₁), while O migration decreases to 2.83 eV (Al₁₀₈O₁₆₀H₁). Conversely, θ-Al₂O₃ exhibits a dramatic increase in the Al migration barrier to 5.06 eV (Al₆₃O₉₆H₁) and a reduction in the O migration barrier to 1.76 eV (Al₆₄O₉₄H₁). These correspond to net changes of +1.38 eV (Al) and −0.86 eV (O) in α-Al₂O₃, and of +3.31 eV (Al) and −2.49 eV (O) in θ-Al₂O₃. The results reveal two distinct effects of H doping. Firstly, H doping inhibits Al migration while enhancing O mobility in both phases. This differential behavior arises from proton interactions with charge carriers—H protons increase Coulombic resistance for Al migration but facilitate oxygen ion mobility through vacancy charge redistribution. Secondly, enhanced O migration promotes oxygen vacancy accumulation while suppressing Al vacancy formation. However, excessive H concentrations may saturate oxygen vacancies, paradoxically hindering O migration. The underlying mechanism involves H-induced electronic structure modifications. Proton doping increases the positive charge density at oxygen vacancy sites, attracting negatively charged oxygen ions via Coulombic interactions, thereby reducing their migration barriers. For Al migration, transient H-Al interactions during atomic displacement elevate energy barriers as Al atoms must overcome additional Coulombic forces from localized protons.

3.5. Alumina Growth with H Proton Incorporation

The crystal structures of θ-Al₂O₃ and α-Al₂O₃ exhibit distinct differences in the distribution of Al atoms and the arrangement of O sublattices (Figure 1). In θ-Al₂O₃, Al atoms occupy tetrahedral interstitial sites within a face-centered cubic oxygen sublattice, whereas in α-Al₂O₃, Al atoms reside in octahedral interstitial sites within a densely packed hexagonal oxygen sublattice. Early-stage alumina phases (γ, δ) demonstrate oxygen sublattice configurations and Al distributions that are more compatible with θ-Al₂O₃ than with α-Al₂O₃ [11,12,13,14]. This structural compatibility reduces nucleation energy barriers and favors lattice matching during initial growth, as the θ-phase requires smaller atomic rearrangements compared to the α-phase. Consequently, θ-Al₂O₃ preferentially forms during the early oxidation stages at TBC interfaces, as illustrated by the lattice structure schematic in Figure 7.

Water vapor exposure facilitates H₂O dissociation into H⁺ and OH⁻ on oxide surfaces, with OH⁻ further decomposing to release O²⁻ for oxide growth [41,42,43,44,45,46]. Meanwhile, H⁺ diffuses into the oxide lattice, altering vacancy formation and atomic migration (Figure 7). As shown in Table 2, H⁺ significantly reduces the Al and O vacancy formation energies in both θ- and α-Al₂O₃. However, θ-Al₂O₃ exhibits lower vacancy formation energies compared to α-Al₂O₃ under H⁺ influence, whereby the tetrahedral Al vacancy (V_Al-tet) energy in θ-Al₂O₃ is 0.77 eV lower than its octahedral Al vacancy (V_Al-oct) energy, as well as being 1.61 eV lower than the V_Al-oct energy in α-Al₂O₃. Similarly, oxygen vacancy (V_O) formation energy in θ-Al₂O₃ is 1.27 eV lower than in α-Al₂O₃. These reductions promote the preferential growth of Al and O vacancies in θ-Al₂O₃, providing abundant migration sites for sustained θ-phase growth while suppressing its transformation to the α-phase. This mechanism aligns with experimental observations of θ-Al₂O₃ stabilization under water vapor environments [26,27].

In the absence of H⁺, Al vacancies in α-Al₂O₃ and θ-Al₂O₃ exhibit mutual repulsion, while O vacancies tend to aggregate (Figure 4 and Figure 5). H⁺ reduces Al vacancy repulsion and enhances O vacancy aggregation by lowering their respective interaction energies. For Al vacancies, the aggregation energies in θ- and α-Al₂O₃ reduce slightly to 0.21 eV (V_Al-oct), 0.19 eV (V_Al-tet), and 0.19 eV (V_Al-oct) under H⁺ influence; however, they remain repulsive, limiting vacancy redistribution. In contrast, the O vacancy aggregation energies decrease sharply to −2.24 eV (α-Al₂O₃) and −1.26 eV (θ-Al₂O₃), promoting vacancy clustering. This disrupts the hexagonal oxygen sublattice in α-Al₂O₃ and induces lattice misalignment, further hindering phase transition. In addition, H⁺ preferentially occupies O vacancy sites in both phases (Figure 6), impeding oxygen sublattice shear deformation and reducing Al migration pathways. While H⁺ lowers O migration barriers, facilitating O atom mobility and oxide growth, it simultaneously immobilizes near Al vacancies, increasing the Al migration barrier in θ-Al₂O₃ by 1.7 eV compared to α-Al₂O₃. This dual effect stabilizes θ-Al₂O₃ by promoting O-driven growth while restricting the Al rearrangement required for α-phase formation.

4. Concluding Remarks

First-principles calculations were employed to investigate vacancy defect formation, aggregation, and migration in α-Al₂O₃ and θ-Al₂O₃ crystals, with the explicit consideration of hydrogen (H) incorporation from water vapor. Key findings include the following:

• Hydrogen protons significantly reduce the formation energies of aluminum (Al) and oxygen (O) vacancies in both α-Al₂O₃ and θ-Al₂O₃. This promotes the generation of tetrahedral Al vacancies (VAl-tet), octahedral Al vacancies (VAl-oct), and O vacancies (VO). The resulting local lattice disordering stabilizes metastable alumina phases by mimicking their defect-rich structures, enabling the transient growth of metastable θ-Al₂O₃ under operational conditions.
• Hydrogen preferentially occupies O vacancy sites in both α-Al₂O₃ and θ-Al₂O₃, while stabilizing near Al vacancies at low-energy configurations. High H concentrations hinder O migration by saturating VO sites, disrupting the oxygen sublattice shear that is required for the θ- to α-Al₂O₃ transition. This inhibits the structural reorganization from a metastable face-centered cubic oxygen arrangement to the thermodynamically stable hexagonal close-packed α-Al₂O₃ structure.
• Hydrogen enhances the clustering of O vacancies while suppressing repulsion between Al vacancies. Aggregated O vacancies redistribute to form migration pathways for Al and O atoms, sustaining metastable phase growth by expanding oxygen sublattice disorder and defect-mediated Al mobility.
• Hydrogen suppresses Al migration but accelerates O diffusion in both phases. While enhanced O mobility facilitates metastable phase growth through defect propagation, the inhibition of Al migration prevents structural relaxation into the steady-state α-Al₂O₃ configuration. This kinetic imbalance perpetuates a disordered alumina matrix, favoring prolonged metastability.

These results reveal that hydrogen ingress from water vapor stabilizes metastable θ-Al₂O₃ by amplifying defect generation, altering vacancy dynamics, and disrupting atomic migration, which is critical for phase transitions. These findings provide atomic-scale insights into the degradation mechanisms of thermal barrier coatings in humid environments, establishing a theoretical foundation for the design of hydrogen- and oxidation-resistant coatings via defect engineering and hydrogen mitigation strategies.