Orthorhombic symmetry and anisotropic properties of β−PbO2

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The structural and electronic properties of the rutile-type oxide $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ (plattnerite) are studied by neutron and synchrotron radiation diffraction and first-principles density functional theory (DFT) calculations. Both diffraction measurements and DFT calculations show that $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ has a $\mathrm{Ca}{\mathrm{Cl}}_{2}$-type orthorhombic structure (space group $Pnnm$) instead of the widely accepted $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ rutile-type tetragonal structure (space group $P{4}_{2}/mnm$). This symmetry lowering in $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ is a robust effect observed at ambient pressure at temperatures between 100 and 400 K. The orthorhombic symmetry rules out the possibility of a semimetallic symmetry-protected state in $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$. Both diffraction measurements and DFT calculations show an anisotropy of thermal expansion, atomic vibrations, and elastic constants of $\ensuremath{\beta}\ensuremath{-}\mathrm{Pb}{\mathrm{O}}_{2}$ along the [100] and [010] directions.