Sodium phosphate glasses modified by MoO3 andWO3
Glasses from the Na2O–MoO3–P2O5 and Na2O–WO3–P2O5 systems were studied for the compositional series 40Na2O–xMoO3–(60–x)P2O5 and 40Na2O–yWO3–(60–y)P2O5, with x=0–50 mol% MoO 3 and y=0–50 mol% WO3. The density of glasses increases more steeply for WO3-glasses than for MoO3-glasses with the transition metal oxides concentration, but the molar volume of glasses in both series decreases by similar amounts due to the similar radii of both transition metal ions. EPR measurements of the Mo5+ and W5+ species in the glasses showed that both ion concentrations decrease with increasing MoO3 and WO3 contents, respectively, consistent with the increasing basicity in both glass series, as described by the theory of J.A. Duffy. Glass transition temperature increases both with MoO3 and WO3 content revealing a maximum at 30 mol% MoO3 and 40 mol% WO3. We assume that these values correspond to an optimum connectivity of the glass network. The index of refraction for glasses from both series also increases with increasing MoO3 (WO3) concentrations, from 1·49 to 1·75. The evolution of 31P MAS NMR spectra is similar for glasses over the range of 0–30 mol% MoO3 (WO3), where the number of Q2 units sharply decreases. At 30 mol% MoO3 (WO3), Q1 units dominate the NMR spectra, whereas within the range of 30–40 mol% Q1, units are replaced by Q0 units. Raman spectra reveal a strong doublet of vibrational bands in the range of 850–1050 cm–1 for both glass series, ascribed to Mo–O and W–O vibrations, respectively. The observed shift of the maximum of the dominant vibrational band from 951 to 922 cm–1 for MoO3-doped glasses and from 956 to 934 cm–1 for WO3-doped glasses is ascribed to changes in the coordination number of both molybdenum and tungsten. Crystallization experiments showed that the glasses with the highest MoO3 (WO3) content crystallize with the formation of crystalline compounds Na2Mo2O7 and Na2W2O7, respectively, which contain both octahedral MO6 and tetrahedral MO4 units.
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Document Type: Research Article
Publication date: October 1, 2018