Abstract:

Phosphate glass is widely used in optical applications; however, its generally low chemical stability and poor thermal mechanical properties hinder the application of phosphate glass to the rapidly evolving laser industry. The addition of a small amount of silicon can form a six-coordinate Si (Si⁽⁶⁾) network and improve the above-mentioned poor properties of phosphate glass. Therefore, it is important to characterize and understand the structural details of phosphosilicate glasses. It is difficult to investigate the glass structure because of its complicated and disordered characteristics. However, solid-state nuclear magnetic resonance (NMR) spectroscopy can provide detailed local structural information, regardless of the presence of its long-range order. To study the effect of alkaline earth metals on Si⁽⁶⁾ species formation, we prepared phosphosilicate glasses (2MO-3P₂O₅)_(1−x)·(SiO₂)_x (M = Ca, Sr, Ba) by conventional melt-quenching, and the glass structure was investigated by solid-state NMR and Raman spectroscopy. The ³¹P and ²⁹Si NMR spectra indicated that the glass networks consisted of P⁽²⁾ and P⁽³⁾ tetrahedrons linked via four- and six-fold coordinated silicon units (Si⁽⁴⁾ and Si⁽⁶⁾). The fraction of six-coordinated silicon Si⁽⁶⁾ decreased with increasing SiO₂ content. Similarly, the Raman spectra showed that the vibration band of the P＝O stretching mode in P⁽³⁾ linked with Si⁽⁶⁾ neighbors reduced as the silica content increased. The connectivities between various phosphorus species were probed by ³¹P one- and two-dimensional refocused INADEQUATE experiments. This experimental technique is based on homonuclear J-coupling and yields correlation peaks between nuclei engaged in P―O―P linkages (P⁽²⁾ and P⁽³⁾ units). The signals from isolated ³¹P nuclei are suppressed because of the absence of J-coupling, which precludes the formation of double quantum coherences. The results indicated the segregation of P⁽²⁾ and P⁽³⁾ units in the prepared glass, which were also compared with those in the previously reported Na₂O-P₂O₅-SiO₂ glasses. They differed from alkali phosphosilicate glasses, where each P⁽³⁾ unit exhibited a maximum average of one Si⁽⁶⁾―O―P⁽³⁾ linkage, and in the alkaline earth phosphosilicate glasses, the average was approximately 0.4–0.7. When the content of Si⁽⁶⁾ units reached its maximum, further increase in the SiO₂ content did not increase the Si⁽⁶⁾ content, and the surplus Si were present as Si⁽⁴⁾. Alkaline earth metal ions exhibit weaker stabilizing effects for Si⁽⁶⁾ species. Based on the results presented herein, we constructed sketches to illustrate the local structural organization of the glass. The relationships between the compositions and structures are important for glass composition and property design. It is important to improve the performance of phosphate glass by changing its composition, particularly for large laser device applications.

Key words: Solid state NMR, Glass structure, Six-coordination silica, Akaline-earth, Phosphosilicate glasses