Ge-based clinopyroxene series: First principles and experimental local probe study

The electronic properties of the Ca/Sr and Mn site substitution of ${\mathrm{CaMnGe}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{SrMnGe}}_{2}{\mathrm{O}}_{6}$ clinopyroxene systems have been investigated by ab initio calculations within the density functional theory (DFT) framework, using on-site Hubbard $U$ to describe the highly correlated Mn $3d$ states and a hybrid exchange-correlation functional to obtain the energy band-gap values. Compositions such as ${(\mathrm{Ca},\mathrm{Sr})}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{MnGe}}_{2}{\mathrm{O}}_{6}$ and $(\mathrm{Ca},\mathrm{Sr}){\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}{\mathrm{Ge}}_{2}{\mathrm{O}}_{6}$ (where $x=0.125,0.25$) are predicted to be stable. Also, we proved that implanted Cd impurity could indeed replace either the Ca/Sr or the Mn sites in the crystalline structures. These findings were obtained by combining first principles electric field gradient calculations, using a supercell scheme, with experimental time-differential perturbed angular correlation results. Additionally, DFT calculations showed that Cd substitution is expected to lead to a reduction in the band-gap width. Cd-doped systems were successfully synthesized and experimental results evidencing opportunities for potential band-gap engineering are reported.