Modelos para la medición de la calidad del servicio

7.6.3.1

Lithium borogermanate glasses and lithium borosilicate model

Fig. 7.26 (a) compares the nBO for the lithium borogermanate glasses (open) to the lithium

borosilicate model [1] (dashed lines). Figs. 7.26 (b) to (g) compare nBO for each K value.

Figure 7.26: Comparison of the nBO for the LBG glasses to the lithium borosilicate model [1]

For data points with K > 8, nBO is compared with the highest K value available from the LBS

153 [1] are included. For K values of 2.7, 3, 5, and 6, nBO obtained is slightly higher than, but

within error of, the model, whilst for other K values (K = 4, and 7), the nBO is as predicted by

the model. An alternative view is that, for samples with R < 1, corresponding to K = 4 and 7, nBO values fit the model well and for R > 1 nBO is slightly higher (but within error) from the

model. The difference is subtle yet it is tempting to suggest that for R < 1, Li+ _{enters the LBG}

structure as described by the LBS model whilst for the higher R values, more tetrahedral [BO4] units are being created and stabilised in the tetrahedral germanate network. However,

with limited data points, such an inference is tentative and the main conclusion is that B in LBG may undergo similar change in environment as a function of Li2O content as in LBS

glasses and that, initially, all Li+ _{goes into the borate network until a certain R value (R} c).

Beyond this Rc value Li+ may be distributed to both B and Ge sites, and NBO units

may be formed which are associated with [BO3] units, but not [GeO4+] units. On the B sites,

Li+ _{is shared by the [BO}

4] and [BO3] (with NBO) units, with linearly increasing fraction of Li+

associated with the latter as a function of K. However, taking the analogy with LBS further would suggest that the Ge environment should be similar to that of Si, with no change in the nGeO values. The actual changes in the nGeO values (and the anomalous behaviour of Ge in

LBG glasses) will be discussed later in the neutron diffraction section.

7.6.3.2

Potassium borogermanate glasses and sodium borosilicate

model

There is no potassium borosilicate model but potassium borogermanate (KBG) glasses, can be successfully compared with the sodium borosilicate (NBS) model [2]. Figs. 7.27 (b) to (g) show this comparison for each K value and, for the data points with K > 8, nBOis compared

with the highest K value available from both the NBS and LBS models (Fig. 7.26 (h)). The data points (open triangles) from the Zhong et al. [1] and Dell et al. [2] study are included. The nBO

for the potassium borogermanate glasses are shown as closed circles, the sodium borosilicate model [2] as solid lines. The nBO for KBG follow the NBS model more closely for

all K values, except K = 7 where nBO is slightly lower than the model. With the limited data

points, it is not possible to discuss the trend of nBO(R) for each K value in the KBG glasses,

however, the local environment change of B in KBG and NBS are closely similar. The first conclusion from the comparison is that, in KBG, for R < 0.5, all K+ _{go to the borate network}

and change nBO as in the binary alkali borate. At R > 0.5, addition of K+ creates more [BO4]

154 NaBS system, this is based on the stabilisation of 1 [BO4] unit in 3 [SiO2] units in a

reedmergnerite-like unit.

Figure 7.27: Comparison of the nBO for the KBG glasses to the sodium borosilicate model [2]

In KBG however, to our knowledge, no crystal phase containing an equivalent group has been reported. Ge on the other hand could change its coordination number with the formation of charged [GeO5] or [GeO6] units and may decrease the number of [GeO4] sites

155 available for stabilising the [BO4] units, due to the charge avoidance principle. The NBS

model is also characterised by the constant nBO region where alkali ions become associated

with the silicate/borosilicate network. If the same trend is followed by K+ _{in KBG glasses and}

there is a region where K+ _{are associated with the germanate network, the n}

BO would stay

constant as well. This may not be a problem for high K value where statistically, B and Ge can be separated further apart, but for low K value, the formation of charged [GeO4+] units

may have an effect on the borate network. The only conclusion from the comparison is that B in KBG behave similarly to the NBS model, with the assumption that Ge behaves like Si. The environment of Ge can only be probed by neutron diffraction, as discussed later in this chapter.

7.6.3.3

Boron in lithium and potassium borogermanate glasses

Figure 7.28: 11_{B NMR spectra for M}

2O–BO1.5–GeO2 (M=Li (dashed blue, 14.15 T) and K (solid pink, 14.1

156 Figure 7.28 shows the high field (14 T) 11_{B MAS NMR spectra for lithium and potassium}

borogermanate glasses. The spectra were normalised to the integrated area. Figures in the top row (Figs (a), (b), and (c)) compare the spectra for both R = Li and K whilst figures in the second (a1 to c1) and third row (a2 to c2) compare the spectra in detail for the region of 4 and 3-coordinated B, B3, and B4 respectively. For the 4-coordinated boron environment, in

each BO1.5 series, as M2O is added to the GeO2 network, it is seen that there is a systematic

change in the shift downfield (to a more positive value), denoting change in the next-nearest neighbours of [BO4] i.e. B or Ge. In borosilicate glasses, [BO4] connected to 4 Si atoms,

[B(4Si)] has the lowest NMR shift value, and this increases on going to [B(B,3Si)] and [B(2B,2Si) [54]. A similar trend may occur for borogermanate glasses.

For low M2O content (hence high GeO2 content, for fixed BO1.5 content), the [BO4]

units produced are likely to be associated with the GeO2 network. The magnitude of change

in shift however is different, with KBG being higher than LBG, especially in high BO1.5 content

glasses. This simply means that in KBG glasses, as K2O is added (and GeO2 content

decreases), the number of the neighbouring [GeO4] units around the created [BO4] will

decrease and therefore the downfield shift. Relative to the KBG, the network intermixing is however less in LBG glasses evidenced from the smaller change in shift, suggesting little change in the number of neighbouring [GeO4] to [BO4] and possible presence of phase

separation. For the [BO3] peak, in general the shift is seen to be unchanged. Comparison of

the peak is however difficult due to the overlapping and complexity of the peaks due to the quadrupole interaction.

7.7

Neutron diffraction