Band gap, electronic structure, and surface electron accumulation of cubic and rhombohedral<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>In</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mtext>O</mml:mtext><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

The bulk and surface electronic structure of ${\text{In}}_{2}{\text{O}}_{3}$ has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline ${\text{In}}_{2}{\text{O}}_{3}$ is determined as $2.93\ifmmode\pm\else\textpm\fi{}0.15$ and $3.02\ifmmode\pm\else\textpm\fi{}0.15\text{ }\text{eV}$ for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigated from x-ray photoemission spectroscopy measurements and density-functional theory calculations. These show excellent agreement, supporting the absence of any significant indirect nature of the ${\text{In}}_{2}{\text{O}}_{3}$ band gap. Clear experimental evidence for an $s\text{\ensuremath{-}}d$ coupling between $\text{In}\text{ }4d$ and $\text{O}\text{ }2s$ derived states is also observed. Electron accumulation, recently reported at the (001) surface of bixbyite material, is also shown to be present at the bixbyite (111) surface and the (0001) surface of rhombohedral ${\text{In}}_{2}{\text{O}}_{3}$.