Quasiparticle self-consistent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>G</mml:mi><mml:mi>W</mml:mi></mml:mrow></mml:math> band structures and high-pressure phase transitions of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Li</mml:mi><mml:mi>Ga</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Na…

Quasiparticle self-consistent $GW$ calculations are presented for the band structures of $\mathrm{Li}\mathrm{Ga}{\mathrm{O}}_{2}$ and $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$ in orthorhombic $Pna{2}_{1}$ tetrahedrally coordinated crystal structures, which closely related to wurtzite structure ZnO. Symmetry labeling bands near gap is carried out, effective-mass tensors extracted conduction-band minimum crystal-field split valence-band maxima (VBM) at $\mathrm{\ensuremath{\Gamma}}$. The found be direct $\mathrm{\ensuremath{\Gamma}}$ 5.81 eV 5.46 $\mathrm{Na}\mathrm{Ga}{\mathrm{O}}_{2}$. Electron-phonon coupling zero-point normalization estimated lower these gaps by about $0.2\ifmmode\pm\else\textpm\fi{}0.1$ eV. Optical response functions calculated within independent-particle long-wavelength limit, they show expected anisotropy absorption onsets due splitting VBM. results that both materials promising candidates as ultrawide-gap semiconductors with wurtzite-based bonded structures. Direct transitions from lowest conduction higher bands, relevant $n$-type doped material transparent applications, start only above 3.9 allowed one polarization, several forbidden symmetry. Alternative such $R\overline{3}m$ a rocksalt-type phase tetragonally distorted $P4/mmm$ spacegroup, octahedral coordination cations, also investigated. They have energy but 20% smaller volume per formula unit. transition pressures phases determined, good agreement experimental studies. has comparably high slightly indirect gap, while considerably 3.1 1.0