once normalised. Raman spectra were collected from a minimum of ~200 cm-1, (limited by
the notch filter) to a high wavenumber (~3000 cm-1). A baseline for each spectrum was
subtracted by a (exponential) spline within this region. The spectrum of a glass is characterised by the Boson peak at low wavenumber (to up to 500 cm-1) which arises due to
phonon vibration in the glass as described by Bose-Einstein statistics. The Boson peak is often fitted with a Rayleigh peak and is known as a Rayleigh wing. In the glasses where the region of interest is close to / included within the Boson peak, the spectrum has to be reduced to remove the Boson peak for accurate quantification. The reduction factor used is defined as 1 1 ( ) 1 ( ) 4 ( ) exp 1 reduced experimental I I L k TB 3.7
Where vL is the frequency of the laser, (L – )-4 is the usual correction for the wavelength
dependence of the scattered intensity and n(,T) = [exp( /kBT)-1]-1 is the mean number for
phonon occupation, is the Plank’s constant divided by 2, is the Raman wavenumber, kB
is the Boltzmann constant, and T is the temperature. The description of the low-frequency region treatment regarding the Bose-Einstein statistics is discussed herein [10]. After the spectrum is baseline-corrected and reduced, it is normalised to the total area under the curve. The spectrum is then fitted with Gaussian peaks assigned to certain species present in the glass. The area of each peak was calculated and the average coordination number of ij pair of atoms were calculated based on the ratio of the peaks associated with the different species. Fig. 3.3 illustrates the reduction process.
26
3.6
X-ray powder diffraction
A powder X-ray diffractometer model Bruker D5005 with Cu Kα X-ray wavelength of 0.154
nm at the Department of Physics, the University of Warwick was used to obtain the diffraction patterns of the samples in this study. The diffractometer measures the intensity (I(2θ)) of the diffracted X-ray beam according to Bragg’s law as in Eqn 3.8 and shown in Fig. 3.4.
2d sinθ = nλ 3.8
Where d is the d-spacing between planes, 2θ is the diffraction angle (an angle between the diffracted and transmitted beams), λ is the X-ray wavelength, and n is the order of reflection (usually 1).
Figure 3.4: X-ray diffraction process according to Bragg’s law.
The X-ray beam is reflected at the 2θ angle as a function of the d-spacing and the intensity of the reflected beam is collected. The X-ray diffraction pattern is compared to a the built-in powder diffraction database (PanAnalytical XRD software) and the ICSD crystal database [30]. The glass samples were powdered and compacted in a sample holder of diameter ~2.5 cm. The X-ray diffraction pattern was collected for 2θ angle between 5o and 90o taken with
0.02o steps. The amorphicity of the glass samples was confirmed by the presence of the
broad halo peak, and in the case of partially crystallised samples, the crystal phase present in the glass was determined.
3.7
Energy dispersive X-ray spectroscopy
Elemental analysis of the samples was carried out using a field emission gun scanning electron microscope, FEGSEM (Zeiss SUPRA 55-VP) at 20 kV, courtesy of the Microscopy Group, Department of Physics, the University of Warwick, with help from Mr Steve York. Pulverised glass sample was dabbed onto a sticky carbon pad on a metal holder. This provides sufficient conductivity to avoid surface charging of the sample. Measurements were taken from 6 different spots for 100 s of exposure for each spot. The relative amounts of elements in the sample are quantified by the detection of the characteristic X-rays
27
emitted by elements after ZAF (Z: atomic number, A: X-ray absorption, and F: X-ray fluorescence) correction was performed using the EDAX Genesis software to correct the background-subtracted integrated intensities.3.8
Secondary ion mass spectroscopy SIMS
For SIMS analysis, a depth profiler model Atomika 4500 was used, courtesy of Dr Richard Morris. SIMS analysis was done to verify the isotopic abundances of 6Li : 7Li and 10B : 11B
isotopes in the glass samples. For the Li isotope measurement, a mineral spodumene LiAl(SiO3)2 courtesy of Dr Ian Farnan of the University of Cambridge was used to optimise the
SIMS condition for the measurements. SIMS determines the relative abundance of elements/isotopes in a sample by bombarding its surface with a primary ion beam and separating the ejected secondary ions according to their masses and then measuring them. In depth profiling, the secondary ions are emitted from below the initial surface. To measure the relative abundance of 6Li and 7Li in NatLi
2O-TeO2 and NullLi2O-TeO2, and 10B and 11B in K2O-
B2O3-GeO2, 3 scans per sample using an O2+ primary ion beam at 1 keV and 20 nA were taken
within the suitable mass ranges; 5 to 9 a.m.u for 6Li and 7Li, and 9 to 12 a.m.u for 10B and 11B.
The incident angles and the scan sizes are 30o and 400 microns, and 45o and 200 microns for
the lithium and boron systems respectively. As for the depth profiling analysis, all parameters were kept similar except for the primary beam current and scan size for the lithium system which were changed to 80 nA and 350 microns, respectively. The samples were depth-profiled for about 35 minutes ignoring the signals for the first 10 minutes to avoid any SIMS transient effects. From the profiles obtained, the average count of mass for each isotope was determined to find the relative isotope abundance. The relative abundances of 6Li and 7Li were used to confirm the null average neutron scattering length of
the null samples and, for the 10B and 11B, it was used to improve the neutron diffraction data
processing by correcting the absorption due to the 10B isotope.
3.9
Synchrotron X-ray diffraction [38]
The high energy X-ray total scattering data were collected at the Argonne National Laboratory APS Beamline 6-ID-D by Dr Chris Benmore (Magnetic Materials Group, Advanced Photon Source, Argonne, Chicago USA) and Dr Oliver Alderman (Materials Development Inc Chicago). An X-ray wavelength of 0.14349 Å (86.406 keV), just below the 88keV Pb K-edge, was used on the lead tellurite (PbO–TeO2) glass samples. The synchrotron was in continuous