129 I and 119 Sn Mössbauer spectroscopy , reversibility window and nanoscale phase separation in binary Ge x Se 1 x glasses
The molecular structure of the prototypical chalcogenide glass system—GexSe1 x in the 0oxo3 range, has received much scrutiny over the years. These glasses have been probed by modulated DSC, Raman scattering, Sn absorption and I emission Mössbauer spectroscopy, and neutron scattering. The I measurements utilize Te parent as a dopant in glasses, and reveal a bimodal (A, B) distribution of sites, with the site intensity ratio, IB/IA (x), tracking changes in glass structure as a function of x. At low x (o0.15) Senchains are stochastically cross-linked by Ge additive, and IB/IA (x) sharply declines with x. But at x40:15, rigid regions nucleate at the expense of floppy ones, and the ratio IB/IA (x) reverses slope to display a global maximum in the 0:20oxo0:25 range. The latter coincides with the reversibility window usually taken as signature of self-organization of these networks. At x40:26, these glasses enter a stressed-rigid elastic phase and in the 0:31oxo1 3 range nanoscale phase separate into Se-rich and Ge-rich regions. The signature of the latter is saturation of IB/IA (x) at a high value of 1.5 at x 1⁄4 13. Sn Mossbauer spectroscopy measurements independently support the picture of broken chemical order of the stoichiometric glass inferred from the I Mössbauer experiments. These observations using local probes are well correlated to Raman scattering, modulated differential scanning calorimetric and diffraction measurements. r 2006 Elsevier B.V. All rights reserved. PACS: 61.40.Fs; 63.50.+x; 78.30.Ly; 76.80.+y
Mössbauer Spectroscopy—A Rewarding Probe of Morphological Structure of Semiconducting Glasses
It has been fashionable to discuss the structure of stoichiometric melt-quenched network glasses in terms of chemically-ordered continuous random networks (CRN) since Zachariasen’s pioneering work on the subject nearly 50 years ago [1]. Glasses of the type AB2, such as SiO2and GeSe2in analogy to a-Ge, for example, have been described as random networks of geometrically well-defined A(B1/2)4 tetrahedral units.
NMR studies of the structure of non-metallic glasses
Abstract Boron-11 NMR spectroscopy has been used for 25 years to determine for borate glasses and crystalline compounds the fractions of borons in BO 2 ( N 4 ) and BO 3 ( N 3 ) configurations, and the fractions of BO 3 units having all bridging ( N 3S ) and one or more non-bridging ( N 3A ) oxygens. Iron lead borate glasses containing up to 15.3 mol% Fe 2 O 3 are found to have the iron ions distributed uniformly throughout the glasses, and the iron oxide influences the borate network only up to 4.64 mol% Fe 2 O 3 . Recent studies of glasses in the systems Li X Li 2 OB 2 O 3 where X = F, Cl reveal that N 4 is determined solely by the oxygen available as Li 2 O in these superionic conducting glasses. The greater sensitivity of boron-10 NMR spectra to structural details has now permitted identification in glasses of structural groupings found in the crystalline compounds of borate glassforming systems, and quantitative determination of the amount of each unit. Spectra for alkali borate glasses have permitted construction of quantitative models for these glasses at all compositions. Oxygen-17 NMR spectra for vitreous B 2 O 3 and SiO 2 yield bonding and structural information confirm models for these glasses. Silicon-29 NMR spectra for glasses in the system K 2 OSiO 2 show agreement with the spectra for the crystalline compounds at compositions for which compounds exist, and differ significantly from compound to compound.