Structural models for covalent non-oxidic glasses: Atomic distribution and local order in CdGeAs2-xPx glasses studied by use of 31P and 113Cd spin-echo and 31P-113Cd spin-echo double-resonance NMR spectroscopy.

Abstract

The structure of glasses in the system ${\mathrm{CdGeAs}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ is discussed on the basis of complementary solid-state NMR experiments, including $^{31}\mathrm{P}$ and $^{113}\mathrm{Cd}$ magic-angle spinning (MAS) and spin-echo techniques, as well as $^{31}\mathrm{\ensuremath{-}}^{113}$Cd spin-echo double resonance (SEDOR) NMR. Computer simulations of atomic distribution models and experimental studies on crystalline model systems are used to quantify the results. The analysis reveals striking differences in the short-range order between the glassy and the stoichiometrically analogous crystalline materials. The structure of glasses in the system ${\mathrm{CdGeAs}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ is characterized by the presence of a substantial fraction of homopolar pnictogen-pnictogen bonds and by a distribution of cadmium relative to phosphorus that is close to random. These results lend credence to the bond-switching model invoked for the structural description of amorphous tetrahedral semiconductors.