| dc.description.abstract | Lithium Germanophosphate Glasses (xLi O - yGeO - zP O ): Corrected Spelling and Format
Lithium germanophosphate glasses (xLi O - yGeO - zP O ) have been synthesised over a wide range of compositions, and their thermal and spectroscopic behaviour has been investigated.
IR and Raman spectroscopies indicate that, except in 10Li O-50GeO -40P O glass, there is no evidence for the formation of [GeO6/2]2 [ \mathrm{GeO}_6/2 ]^{2-}[GeO6 /2]2 units in any of the glasses, and therefore no indication of the germanate anomaly in the presence of P O . In CPL1, however, there is plausible thermodynamic justification for the formation of [GeO6/2]2 [ \mathrm{GeO}_6/2 ]^{2-}[GeO6 /2]2 units. In the IR spectrum of CPL1, the feature at 802 cm ¹ is attributed to the stretching mode of [GeO6/2]2 [ \mathrm{GeO}_6/2 ]^{2-}[GeO6 /2]2 units. Similarly, in the Raman spectrum of CPL1, the unique band around 643 cm ¹ is attributed to the bending modes of Ge-O-P.
³¹P MAS NMR studies suggest that the formation of ultraphosphate, metaphosphate, and pyrophosphate species is consistent with modification of only the P O component of the glass. Therefore, in glasses containing a strong former (GeO ) and a fragile former (P O ), modification predominantly affects the fragile component. A structural model has been developed which is consistent with the observed properties. The presence of [GeO6/2]2 [ \mathrm{GeO}_6/2 ]^{2-}[GeO6 /2]2 units in CPL1 is shown to be a consequence of relative thermodynamic stability.
The properties have been interpreted by considering the glasses as pseudo-binaries of yGeO and xLi O-zP O . The molar volumes of the glasses are higher than the corresponding ideal additive volumes, but lower than the simple sum of the molar volumes of GeO and Li O-P O component glasses. Some interpenetration of the GeO and phosphate networks appears evident.
The glass transition temperatures TgT_gTg of these ternary glasses show a trend different from that of the individual binary systems and exhibit a decreasing trend with increasing NBO/BO ratio. The TgT_gTg values are also higher than those of either of the binary components up to ~20 mol% GeO , which may have technological significance. In strong-fragile mixed glass former systems, the glass transition is predominantly determined by the strong component.
The kinetic fragilities F1/2F_{1/2}F1/2 of these glasses have been calculated, and it is shown that:
Fm=0.98(3 Z)3+DF_m = 0.98 \frac{(3 - Z)}{3 + D}Fm =0.983+D(3 Z)
where D is the mole fraction weighted structural dimensionality.
Transport Properties
The transport properties of the glasses in the Li O-GeO -P O system have been studied using both d.c. and a.c. measurements.
The d.c. conductivity exhibits two different activation energies in two temperature regimes, attributed to cluster-tissue texture in these glasses.
The a.c. conductivity fits a single power law.
Modulus formalism has been used to analyse dielectric loss. Both the power law exponent s and the modulus parameter are temperature independent.
The observations support a transport mechanism involving two steps:
NBO-BO switching, followed by
Li ion jump to the new NBO coordinated site.
Ion Transport in Trialkali Metaphosphate Glasses
Ion transport behaviour in trialkali metaphosphate glasses has also been investigated.
Results from multiple experimental techniques show that the conventional ion transport model is inadequate. It is postulated that charge transport occurs via:
BO-NBO switching as the primary step
alkali ion migration as the following step
Trialkali glasses behave as composites of two subsystems, although no phase separation is observed. As a consequence, they exhibit two BO-NBO subsystems and show conductivity and relaxation characteristics consistent with the proposed transport mechanism.
Dielectric relaxation in trialkali glasses appears as two peaks, which merge at higher temperatures close to the glass transition. | |
