The aim of this work was to investigate by X-ray photoelectron spectroscopy the effect of high pressure on the chemical environments of Si 2p, O 1s and Li 1s in lithium disilicate glass ceramic with stoichiometric composition Li2O·2SiO2 (LS2). A group of samples was processed at 2.5 GPa, 4 GPa and 7.7 GPa at room temperature and a second group was crystallized under high pressure and high temperature. Large shifts of the binding energy toward higher energies were observed in the X-ray photoelectron spectroscopy spectra for samples of the first group after densification at 2.5 and 4 GPa. For samples processed at 7.7 Gpa, the major component of the binding energy for the Si 2p environment remained practically unchanged compared to the pristine sample but new components, with smaller intensities, appeared in the spectra, indicating the existence of distinct Q-species induced by high pressure. This behavior may be related to changes in the number of bridged and non-bridged oxygen atoms in the glass structure. The results for the second group of samples, crystallized under high pressure, showed evidences of three binding energies for the O atoms, one of them related to non-bridged and two of them to bridged O atoms.
Structural analysis of glasses under high pressure is important to understand the effect of densification mechanisms on the glass properties [
Kitamura et al. [
Schmelzer et al. [
The crystallization of LS2 under HPHT has been studied in the past few years [1,4-11]. In previous works, the effect of densification on the crystallization process and thermal properties of LS2 glass was investigated [
Nuclear Magnetic Resonance (NMR) results obtained by Fuss et al. [
Adams & De Jong [
Nesbitt et al. [
According to Sawyer et al. [
In this context, the aim of this work was to investigate the effect of high pressure on the chemical environment of Si 2p, O 1s and Li 1s atoms of the LS2 glass structure using XPS to help elucidate the mechanisms responsible for the early stages of the formation of the metasilicate structure induced by densification. For comparison, the chemical environment of these atoms was also investigated after complete crystallization of the glass structure under high pressure and high temperature.
Lithium disilicate glass of stoichiometric composition Li2O∙2SiO2 (LS2) was prepared using standard reagent grade Li2CO3 (Aldrich Chem. Co., 99+%) and ground quartz (<99.9% SiO2). The 200 g batch was melted in a Pt crucible at 1450˚C during 2 hours in an electric furnace. The melt was poured on a steel plate, annealed below the glass transition temperature, at 430˚C, during 1 h and cooled down slowly to room temperature.
The HP experiments were performed in a toroidal high pressure apparatus. The specific configuration and type of high-pressure chamber used in the experiments were optimized for providing quasi-hydrostatic pressure [
During the HPHT experiments, a monolithic LS2 sample was placed inside a hexagonal boron nitride (hBN) capsule which acted as a soft pressure transmitting medium. The hBN capsule was placed inside a graphite cylinder which acted as a heater element. For experiments performed at HP and room temperature (HPRT), the capsule of hBN was replaced by a lead capsule, which is a softer pressure transmitting compared to hBN.
In the first group, the samples were submitted to 2.5 GPa, 4 GPa and 7.7 GPa at room temperature during 5 min inside the lead capsule. In the second group, the samples were submitted to HP (2.5, 4.0 and 7.7 GPa) inside the hBN capsule and, simultaneously, to the following thermal treatment: 500˚C during 2 h for nucleation followed by 610˚C during 0.5 h for crystal growing. For comparison, a pristine glass sample and a sample submitted to the same heat treatment at 1 atm were also investigated.
According to a previous work [
After HPRT and HPHT processing, the surface of the samples was grounded on SiC abrasive paper up to #1200 and polished with CeO2 slurry for XPS analyses.
XPS analyses were performed in an Omicron-SPHERA station using Al Kα radiation (1486.6 eV). The anode was operated at 225 W (15 kV, 15 mA). Survey spectra were recorded with a 50 eV pass energy. Si 2p, O 1s, and Li 1s regions were recorded with high resolution (pass energy of 20 eV). The detection angle of the photoelectrons (Θ) with respect to the sample surface (take-off angle) was fixed at 53˚ for all measurements. The C 1s signal from adventitious carbon at 284.6 eV was used as an internal energy reference. All spectra were fitted assuming a Shirley background. Lines were fitted by 70% Gaussian + 30% Lorentzian functions with set values of full width at half maximum for each line.
Figures 1-3 show high resolution XPS spectra for Si 2p, O 1s and Li 1s, respectively, after processing at high pressure compared to the pristine sample. It is clearly seen that the BE of the three elements changed irreversible after densification, revealing the strong influence of high pressure in the chemical environment due to the structural freedom of the glassy LS2.
According to
while after processing at 4 and 7.7 GPa there are two components. In these cases, the component with higher intensity corresponds to BO while the lower intensity component corresponds to NBO. For Li atoms (
Figures 4-6 show the XPS results for the samples crystallized under high pressure. In this case there was practically no shift in the BE as a function of pressure.
[
The BE measured for the pristine glass sample and for the crystalline sample obtained at atmospheric pressure are similar to the results found in the literature [12,13,18], as shown in
The large shift of the BE toward higher energies observed for Si 2p after processing at 2.5 and 4 GPa (
In the case of O 1s atoms (
Considering the XPS spectra for crystalline samples produced under high pressure,
For O 1s (
Two peaks related to BO atoms, of higher BE, were observed for all pressure range in the crystalline samples. The most intense component was probably related to Si-O-Si and the other one may be related to BO-Li. Ching et al. [
Fuss et al. [
Densification of lithium disilicate at room temperature under high pressure induced large and irreversible modifications in the chemical environment of Si 2p, O 1s and Li 1s. These modifications would be related to the rearrangement of the atoms induced by high pressure due to the structural freedom of the glass structure, even at room temperature. XPS showed to be a suitable technique to detect the structural changes induced by densification at room temperature. For samples crystallized under high pressure, it was observed that a similar behavior for pressures up to 4 GPa, related to the formation of monoclinic and orthorhombic lithium disilicate. The interpretation of the XPS results for Si 2p and O 1s for the sample crystallized at 7.7 GPa was consistent with the formation of lithium metasilicate and SiO2.
The authors would like to thank CNPq, CAPES and FAPERGS for the financial support and to LaMaV for the fusion of the samples.