X-ray diffraction

From visual inspection, the glasses with x = 10, 12.5, 15, 17.5, 20(a) and 20(b) mol% were judged to be amorphous, based on their clarity. However, HEXRD revealed that the glass samples with x = 10, 20(a) and 25 mol% contain some crystal phase(s), whilst ND showed

100 that sample 20(a) had more crystal content than 20(b) so the latter sample was given greater neutron exposure (Table 6.1). As shown in Fig. 6.3, the X-ray diffraction patterns of these samples contain bright dots denoting the presence of crystals. The crystalline content at x = 10 mol% is negligible. However, at x = 20 and 25 mol%, the content is much higher and this affects the composition of the residual glass.

Figure 6.3: High energy X-ray diffraction patterns for xPbO-(100–x)TeO2 glasses for x = 10, 12.5, 15,

17.5, 20(a) and 25 mol%. Significant crystallisation is seen in 20(a) and 25 mol% PbO samples. Negligible crystallisation is seen in 10 mol% PbO sample (spots ringed in red).

The crystal phase present in the x = 10 mol% glass is probably α-TeO2 (based on the high

TeO2 content) but there is insufficient present to be able to determine the d-spacings. Fig.

6.4 shows 1D plots of the HEXRD spectra from the samples and Fig 6.4(a) gives the d- spacings of the 4 sharp crystal peaks. These are similar to those obtained by Kaur et al. [10] for glasses of x = 19 and 21 mol% prepared via splat quenching between two brass plates. In

101 the same glasses, prepared at a slower cooling rate, the crystal phases formed in the glasses are γ-TeO2, Pb2Te3O8, and PbTeO3. The 4-sharp peaks observed in the splat-quenched glasses

were therefore assigned by those authors to a mixture of γ-TeO2, Pb2Te3O8, and PbTeO3

phases. The PanAnalytical X-Ray Suite database matches the crystalline peaks in the 20 mol% and 25 mol% samples to both PbTe3O7 and Pb2Te3O7 phases. Culea et al. [11] assigned

the crystal phase to Pb2Te3O7 based on the better fit. However, the Pb2Te3O7 stoichiometry

seems unlikely for Pb2+ _{and Te}4+ _{cations. PbTe}

3O7 is stoichiometrically more reasonable and

was first reported by Tromel et al. who referred to it as an “anti glass” phase with fluorite- type structure and defined it as a metastable and non-stoichiometric crystal having irregularly displaced atoms such that the short-range order is undefined [12]. This metastable phase was also reported by Silva et al. [13] and Kabalci et al. [14] in their studies of lead halide tellurite glasses. PbTe3O7 is not reported in Fig. 6.1, the nearest phase being

PbO.4TeO2 (equivalent to PbTe4O9, not reported). PbTe5O11 (16.67 mol% PbO) however, has

the composition which is closest to the PbO–TeO2 glasses of this study and this crystal phase

was obtained in a devitrified 20PbO–80TeO2 glass sample [5].

Figure 6.4: 1D plots of high energy X-ray diffraction spectra for xPbO-(100–x)TeO2 glasses for (a) x = 10

and 25 mol% compared to published data for PbTe3O7 from PanAnalytical software database and (b)

102 Table 6.3 summarises the physical appearance of the glasses, which includes the amorphicity of the glasses, based on diffraction information.

Table 6.3: Physical appearance and diffraction information. *Crystal diffraction spots were revealed in the synchrotron X-ray diffraction experiment.

Nominal x (mol% PbO) Observation State Visual inspection HEXRD Neutron diffraction 10 Clear Glass + Crystal Glass* ~90TeO2 glass + “α-TeO2” 12.5 Clear Glass Glass ~87.5TeO2 glass

15 Clear Glass Glass ~85TeO2 glass 17.5 Clear Glass Glass ~82.5TeO2 glass 20(a) Clear Glass +

Crystal Glass + Crystal >80TeO2 glass + crystals 20(b) Clear – Glass + Crystal >80TeO2 glass + crystals 25 Clear, with cloudy bits Glass + Crystal Glass + Crystal >75TeO2 glass + crystals

6.4

Raman spectroscopy

Fig 6.5(a) shows the measured Raman spectra of xPbO.(100-x)TeO2 glasses (x = 5, 7.5, 10, 15,

20(a) and 25 mol%) collected at room temperature from 100 cm-1 _{to 1000 cm}-1_{. Glass}

samples of x = 5, 7.5, 20(a), and 25 mol% PbO are partially crystallised. The clear glass bits were selected to be analysed using Raman spectroscopy. The spectra show the typical broad Boson peak to up to 400 cm-1_{, and the Te-O stretching and vibration modes for TeO}

2–

based glasses from 400 to 900 cm-1 _{[15-18]. Fig. 6.5(b) shows the reduced and area-}

normalised Raman spectra, which were obtained by multiplying the experimental intensity with the correction factors for the wavelength-dependent scattered intensity and temperature-dependent mean number for phonon occupation in the glasses as discussed in Section 3.5. The reduction was done so that the main features of the Te–O vibrations in the glass can be highlighted for study. The Raman spectra show a broad continuous peak manifold from approximately 500 to 900 cm-1_{, characteristic of Te–O vibrations as typically}

observed in tellurite glasses encompassing the vibrations in various Te environments, e.g., 3, 3+1, and 4 coordinated Te. As PbO content is increased, a major change was observed in the shape of the manifold, i.e., the peak at 600 cm-1 _{decreased whilst the peak at about 800 cm}-1

increased.

The assignment of peaks to various TeO2 species is discussed in Chapter 5, where the

region from 500 to 900 cm-1 _{is typically resolved into 4 peaks assigned to the vibrational}

103 trigonal pyramidal (tp). The Raman shifts covering the lower wavenumbers in the broad peak (Peak 3 and Peak 4, from 620 to 670 cm-1_{) are assigned to the [TeO}

4] unit whereas the

higher wavenumber region (Peak 1 and Peak 2, from 720 to 780 cm-1_{) is attributed to the}

vibrations of the [TeO3] units.

Figure 6.5: :(a) experimentally obtained Raman spectra (normalised); (b) reduced and area-normalised Raman spectra showing the region of interest encompassing the Te-O vibrations; (c) Raman peak

deconvolution of contributions from [TeO4] and [TeO3] units; (d) area of the peaks in (c) as a function

of x; and (e) average nTeO obtained compared to the values from Sekiya et. al. [17] x = 20 represents

104 The peak at about 490 cm-1 _{is attributed to the bending modes of –Te–O–Te– bridges and is}

not included in determining the average Te-O coordination number, nTeO, in the glass. From

the fits shown in Fig. 6.5(c), the areas of the individual peaks were extracted and the nTeO for

each glass composition was calculated. Similar peak width was applied for the individual peaks in the figure. The sum of the fits gives good agreement with the experimental spectrum. The average Raman shifts of the peaks from the fit are consistent with the expected values from the literature.

In Fig. 6.5(d), the areas for Peak 1 and Peak 2, contributed from the vibrations of Te- O bonds in [TeO3] units [15] increases as a function of PbO content. This simply means that,

as Pb2+ _{is added, there is formation of [TeO}

3] units in the glasses. Peak 3 and Peak 4 are the

contributions from Te-O vibrations in [TeO4] units and the areas for these peaks both

decrease slightly as PbO content is increased. As a function of PbO content, the area of Peak 5 decreases, reflecting the decrease of Te-O-Te bridges because of the formation of non- bridging oxygens, NBO. This implies that Pb2+ _{behaves as a charge compensator (i.e.}

modifier) in the glass.

The average nTeO values for PbO-TeO2 glasses are shown in Fig 6.5(e). The values and

their trend with x are comparable to those in TeO2 glasses modified by divalent cations such

as Ba2+_{, Mg}2+_{, Sr}2+_{, and Zn}2+ _{[17]. This suggests that Pb}2+ _{behaves as a divalent modifier.}

Surprisingly, the average nTeO values are also similar to those of the Li2O-TeO2 glasses,

suggesting that the Te environments in PbO-TeO2 and Li2O-TeO2 are similar. However,

because Raman spectroscopy is semi-quantitative, the average nTeO obtained is less reliable

and associated with greater error as shown in the figure.