Effects of the substitution of P2O5 with B2O3 on the structure, iron valence, thermal and crystallization behavior of lithium-iron-phosphate glasses

On the base of 30Li2O-20Fe2O3-50P2O5 glass, 5, 8, 12, 15 mol of B2O3 were included to replace P2O5. The glasses were prepared by conventional melt-quenching method in air. The influence of the B2O3 substitution on the structure, properties and especially the thermal and crystallization behavior of the lithium-iron-phosphate glass was investigated. The samples were characterized with DTA, FTIR and Mӧssbauer spectroscopy. The density and chemical stability of the glasses were measured and compared. The crystallization of the heat-treated sample was analyzed by DTA and XRD. The results indicate that when P2O5 was substituted by 5 and 8 mol% of B2O3, the glass network tends to be strengthened due to formation of [BO4], and consequently, large increases in the density, chemical stability, transition temperatures are observed. Both the temperature and the activation energy of crystallization also increase. With further increasing the amount of B2O3 from 8 to 10 and 15%, the density slightly increases, while the chemical stability and the activation energy of crystallization decrease. In addition, the DTA results suggest that the replacement of P2O5 with more than 8% B2O3 results in the formation of a second phase with lower transition and crystallization temperatures. The inclusion of B2O3 does not change the surface crystallization mechanism of the phosphate glasses. However, the B2O3-free glass only shows the crystallization of LiFeP2O7. In contrast, the B2O3 containing glasses have increasing crystallization of LiFe(II)PO4 and Li3Fe2(PO4)3 at the expense of the LiFeP2O7 phase. 57Fe Mӧssbauer spectra measurements reveal that the fraction of Fe(II) increases with the addition of B2O3. In principle, it is revealed that the reduced P/Fe and P/Li ratios and the increased Fe(II) ions in the B2O3 containing glasses favor the crystallization of the LiFe(II)PO4 and Li3Fe2(PO4)3phases.