STRUCTURAL MODELS FOR COVALENT NONOXIDIC GLASSES - ATOMIC DISTRIBUTION AND LOCAL ORDER IN GLASSY CDGEP2 STUDIED BY P-31 AND CD-113 MAS AND SPIN-ECHO AND P-31-CD-113 SPIN-ECHO DOUBLE-RESONANCE NMR-SPECTROSCOPY

The structure of glassy CdGep2 is discussed on the basis of complementary solid-state NMR experiments, including P-31 and Cd-113 magic-angle spinning (MAS) and spin-echo techniques, as well as P-31-Cd-113 spin-echo double resonance (SEDOR) NMR. Modeling of the dipolar interactions in conjunction with experimental studies on the crystalline model compounds CdP2 and CdGeP2 reveals the short-range order present in crystalline CdGep2 is not preserved upon vitrification. In contrast to the crystalline analogue, glassy CdGeP2 contains a substantial fraction of phosphorus-phosphorus bonds, which can be quantitated by means of P-31 spin-echo decay data. The analysis reveals that the number of P-P bonds amounts to 55 +/- 5% of that expected for a completely random distribution of atoms. The Cd-113-P-31 SEDOR results are qualitatively consistent with this conclusion but suggest a more complete randomization of the Cd atoms. As a consequence of this randomization of atomic occupancies, the number of Cd-P bonds is significantly reduced in the glassy state. The study provides important experimental data for the development of realistic atomic distribution models for covalent non-oxidic glasses.