Experimental observations of large changes in electron density distributions in β−Ge

Abstract

The electron density distributions in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ have been experimentally determined at in situ high-pressure conditions using synchrotron diffraction techniques in a diamond anvil cell. Upon decompression, the electron density along the $c$ axis in tetragonal $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ displays a sudden drop at 10--11 GPa close to that of the structural \ensuremath{\alpha}-\ensuremath{\beta} transition, while the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ samples remain as single crystals until transforming to metastable Ge phases around 6.7--8.5 GPa. In contrast to the covalently bonded $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ge}$ that displays only a weak participation of $d$ electrons in the valence band under compression above 7.7 GPa, our experimental results suggest that a large change in $d$-orbital participation can occur in the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ lattice which has mixed covalent and metallic bonding. The $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Ge}$ below 10 GPa may display large fluctuations in electronic properties, which sheds light for exploring novel materials with intriguing electronic and optical properties.