| Abstract
| - The structures of sodium aluminophosphate glasses prepared by both sol−gel as well as melt-cooling routeshave been extensively characterized by high-resolution solid-state 23Na, 27Al, and 31P single and double-resonance NMR techniques, including quantitative connectivity studies by 27Al ↔ 31P and 23Na ↔ 31P rotationalecho double-resonance (REDOR) methods. Studies along four compositional lines, I: (AlPO4)x−(NaPO3)1-x,II: (Na2O)x−(AlPO4)1-x, III: (NaAlO2)x−(NaPO3)1-x, and IV: (Al2O3)x(NaPO3)1-x, reveal that the networkstructures of those glasses that are accessible by either preparation method are essentially identical. However,the significantly extended glass-forming ranges available by the sol−gel route facilitate exploration of thestructure/composition relationships in more detail, revealing a number of interesting universal featuresthroughout the whole glass system. Both short- and medium-range order appear to be controlled strongly bythe O/P ratio of the glasses studied: Up to an O/P ratio of 3.5 (pyrophosphate composition), aluminum ispredominantly six-coordinated and fully connected to phosphorus (Al(OP)6 sites). In the region 3.5 ≤ O/P ≤4.0, a dramatic structural transformation takes place, leading to the appearance of additional four- and five-coordinated aluminum species whose second coordination spheres are also entirely dominated by phosphorus.The structure of glasses with an O/P ratio of precisely 4.0 (orthophosphate) is dominated by Al(OP)4 units.As the O/P ratio increases beyond 4.0, the average extent of Al−O−P connectivity is decreased significantly.Here, new types of five- and six-coordinated aluminum units, which are only weakly connected to phosphorus,are formed, while the network modifier is attracted mainly by the phosphate units.
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