Unraveling the transformation pathway of the 𝛽 to 𝛾 phase transition in Ga₂⁢O₃ from atomistic simulations

Journal Article · Thu Jan 16 00:00:00 EST 2025 · Physical Review Materials

DOI:https://doi.org/10.1103/PhysRevMaterials.9.014601· OSTI ID:2585347

Lee, Channyung ^[1]; Scarpulla, Michael A. ^[2]; Varley, Joel B. ^[3]; Ertekin, Elif ^[1]

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1. University of Illinois at Urbana-Champaign, IL (United States)
2. University of Utah, Salt Lake City, UT (United States)
3. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

Defect spinel 𝛾−Ga₂⁢O₃ is the least stable polymorph of Ga₂⁢O₃, so its frequent appearance as a structural defect within or on the surface of monoclinic 𝛽−Ga₂⁢O₃ remains a mystery. Through first-principles calculations, we explore potential pathways for the phase transition from 𝛽−Ga_2⁢O₃ to 𝛾−Ga₂⁢O₃, and examine two key driving forces: tensile strain and Ga deficiency. When configurational entropy contributions to phase energies are included, the 𝛾 phase becomes energetically competitive with the 𝛽 phase, with the free energy difference between these phases diminishing even further under Ga-deficient conditions. Notably, a stability crossover occurs at room temperature at high vacancy concentrations ([V$$^{3−}_{Ga}$$]>3%) . A simple model 𝛽 → 𝛾 transformation pathway is identified, comprising two primary reactions, that enables the formation of the 𝛾 phase via simultaneous migration of Ga atoms from tetrahedral lattice sites to octahedral interstitial positions. The transformation barriers are prohibitively large in pristine Ga₂⁢O₃, but can be substantially reduced by: (1) the presence of Ga vacancies, (2) elongational strains along the crystallographic 𝑎-axis, and (3) when volumetric relaxations are possible during transformation. These results elucidate prior experimental observations, where 𝛾−Ga_2⁢O₃ is seen on damaged surfaces or in highly 𝑛-type 𝛽−Ga_2⁢O₃ environments, which support Ga deficiency and mechanical strain. The insights into the driving forces and mechanisms of 𝛾−Ga_2⁢O₃ formation enhance understanding of how localized strain and nonequilibrium defect concentrations may facilitate its formation from the 𝛽 phase.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: Air Force Office of Scientific Research (AFOSR); National Science Foundation (NSF); USDOE
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 2585347
• Report Number(s): LLNL--JRNL-2005476
• Journal Information: Physical Review Materials, Journal Name: Physical Review Materials Journal Issue: 1 Vol. 9; ISSN 2475-9953
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

Defects
Density functional theory
First-principles calculations
Phase transitions
Transparent conducting oxides
Wide band gap systems