TABLE OF CONTENTS The ability of cations to enter into the glass structure is characterised by their field strength, which is defined as
| (17.2) |  |
where Z is the valence and a is the ionic distance ( ) in the oxide. Lower field strength ions (e.g. alkalis) are network modifiers, whereas ions with higher field strength (such as Si, P or B) are network formers. Table 17.3 lists cations according to their function in the glass structure.
| Element | Valence Z | Coordination number (CN) | Field strength F | Bond strength (kJ/mol) | Function in glass |
|---|---|---|---|---|---|
| Si | 4 | 4 | 1.57 | 443 | Network formers: F ~ 1.5 2.0 |
| B | 3 | 3 | 1.63 | 498 | |
| 4 | 4 | 1.34 | 372 | ||
| P | 5 | 4 | 2.1 | 368 464 | |
| Ti | 4 | 4 | 1.25 | 455 | Intermediates: F ~ 0.5 1.0 |
| 4 | 6 | 1.04 | 304 | ||
| Al | 3 | 4 | 0.96 | 335 423 | |
| 3 | 6 | 0.84 | 224 284 | ||
| Fe | 3 | 4 | 0.85 | ||
| 3 | 6 | 0.76 | |||
| Be | 2 | 4 | 0.86 | 263 | |
| Zr | 4 | 6 | 0.84 | 338 | |
| 4 | 8 | 0.77 | 255 | ||
| Mg | 2 | 4 | 0.53 | ||
| 2 | 6 | 0.45 | 155 | ||
| Fe | 2 | 6 | 0.43 | Network modifiers: F ~ 0.1 0.4 | |
| Pb | 2 | 6 | 0.34 | 310 | |
| 2 | 8 | 0.27 | 151 | ||
| Ca | 2 | 8 | 0.33 | 134 | |
| Sr | 2 | 8 | 0.28 | 134 | |
| Li | 1 | 6 | 0.23 | 151 | |
| Na | 1 | 6 | 0.19 | 84 | |
| K | 1 | 8 |
