TITLE:
Structural Studies on Ag2O-P2O5 Glasses
AUTHORS:
Gomaa El-Damrawi, Abdelmajued Hassan, Hamdy Doweidar, Ahamed Shaboub
KEYWORDS:
Nuclear Magnetic Resonance, Chemical Shift, Phosphate Species, Orthophosphates
JOURNAL NAME:
New Journal of Glass and Ceramics,
Vol.7 No.3,
July
7,
2017
ABSTRACT: Silver phosphate glasses of general formula xAg2O·(100
- x)P2O5 have been investigated over compositional
range from x = 40 to 62.5 mol%. The local structure around
phosphorus atom has been studied via 31P nuclear magnetic
resonance. The distribution of [PO4]Qn species
as a function of composition has been shown to slightly deviate from the simple
binary alkali phosphate model. An anomalous behavior has been recorded and
interpreted in terms of mixed ring-chain effect in metaphosphate composition.
The splitting of NMR spectra into sub resonances is assigned to different
binding sites characterizing Q1 ring and Q1 chain
structure. Higher Ag2O concentration (≥50 mol%) leads to formation
of phosphate groups with specific resonance peaks which are mainly related to
pyro and orthophosphate species. The rate of change of the chemical shift of
the 31P NMR depends on the bond type, which in turn reflects
the extent of double bonding between phosphorus and oxygen atoms. Increasing
concentration of Q0 with increasing Ag2O content leads to decreasing quantities of
bridging and double bonds. As a consequence, specific symmetric resonance peak
of higher intensity and chemical shift (Q0) is a feature of silver
rich glasses (orthophosphate). The latter species is therefore proposed to
compose of separated membered rings,which cause deshielding of phosphate units.XRD and EDP studies have shown
that, amorphous phosphate network is the dominant structure of glasses
containing ≤ 55 mol% Ag2O. Some ordered and well crystallized phases
are formed at higher Ag2O concentration. Increasing non-bridging
oxygen atoms is shown to have the main effect on crystallization behavior. Orthophosphate
composition is the most crystalline one among the other compositions (ultra,
meta- and pyrophosphate). Presence of orthophosphate species which typically
contains highest concentration from isolated Q0 units is the
main reason for building up crystalline Ag3PO4phosphate
phase.