WAXS and NMR studies of intermediate and short range order in K2O-SiO2 glasses

F Meneau, R Winter, GN Greaves, Y Vaills; J Non-Cryst. Solids 293 (2001) 693
Abstract. Wide-angle x-ray scattering and 29Si NMR have been employed to investigate the medium-range structure of x K2O - (1-x) SiO2 glasses, with x varying in the limits 5% < x < 35%. The diffractograms show a First Sharp Diffraction Peak (FSDP) in the 1.4A-1 < q < 2.2A-1 range. The peaks broaden below x = 20%, and at the lowest K2O fraction, a bimodal line shape is found. This broadening is interpreted in terms of phase separation at low K2O fraction. The NMR spectra consist of several (usually three) Gaussian components assigned to the different Q species (SiO4 tetrahedra with different connectivity) present. All three components are uniformly deshielded as K2O is incorporated into the structure. The fraction of non-bridging oxygens (NBOs) derived from the distribution of Qn species matches the value obtained from the overall composition, except for the x = 17.41% sample, again indicating phase-separation at x < 20%. The inhomogeneities found by WAXS and NMR in the x K2O - (1-x) SiO2 glasses are interpreted in terms of broken bond-bending constraints at the NBOs. Constraint theory assigns the critical concentration for glass forming at xc = 20%, which may explain the tendency of the glasses to phase-separate at concentrations below xc.
Fig.1: Diffractograms. In this study, glasses along the join x K2O - (1-x) SiO2 were studied with modifier fractions up to x=0.35. Up to x=25% the glasses were found to be reasonably resistant to moisture if kept under dry conditions.

Fig. 1: Wide-angle x-ray scattering patterns of several glasses along the join x K2O - (1-x) SiO2. The respective molar fractions of K2O are indicated.

The First Sharp Diffraction Peak (FSDP) moves towards larger momentum transfer values as the K2O content increases. At the same time the width of the peak appears to decrease. The position of the FSDP corresponds to the inverse of a characteristic length of the structure. In silicate glasses, the characteristic length is commonly interpreted as the correlation length between voids formed in association with the free electron pairs of oxygen. The correlation length varies from 400pm (5% sample) to 320pm (25% sample).

Fig.2: Deconvolution of the 5% K<sub>2</sub>O diffractogram. Fig. 2: Diffractogram of the x=4.99% sample. The two overlapping lines required to fit the measured data are indicated by broken lines.

In the case of the 5% K2O sample, the line broadening trend is so predominant that no symmetric line shape (above theangle-dependent decaying background) such as a single Gaussian can fit the data. This suggests that the broadening is caused by the superposition of two FSDPs from two simultaneously present amorphous phases with different void correlation length.

Fig.3: FSDP positions and half widths. Fig. 3: WAXS peak positions (top) and line widths (bottom) as a function of the K2O fraction. The open symbols refer two the second component found in the x=4.99% sample. Errors attached to the fits (Gaussian plus linear background) are smaller than the symbols.

The decrease of the average distance between voids with increasing modifier content contradicts the idea that the voids are stuffed by added modifier, as has been suggested on the basis of simulations. The reason for this apparent discrepancy is that the modifier ions (K+) come with oxygen counter-ions, which in turn increases the amount of voids and hence reduces the correlation length. However, the creation of non-bridging oxygens (NBOs) due to modifier addition causes the network to become less strained. Larger voids can relax to form smaller ones and the local order is increased - hence the FSDP narrows.

Fig.4: NMR spectra. Fitting NMR spectra with several overlapping Gaussians reveals the fractions of Si atoms in SiO4 tetrahedra with varying number of NBOs (Qn species with n brigding oxygens).

Fig. 4: 29Si NMR spectra of the x K2O - (1-x) SiO2 glasses. The broken lines indicate the change in the chemical shift for the three Qn species when the K2O fraction increases.

In the 5% sample, only Q4 and Q3 species are present, while in the 35% sample, only Q2 and Q3 are found. In all intermediate glasses, all three species have a noticeable population. The chemical shift of each individual peak moves towards less negative values (i.e. deshielding) as the K2O fraction increases. All three species are displaced to an approximately equal extent, indicating that the modifier causes the overall network to relax rather than resulting only in local relaxation.

Fig.5: Q<sup>n</sup> fractions from NMR vs. composition. Fig. 5: Fractions of the different Qn species as function of the K2O fraction, as obtained from multiple-Gaussian fits to the spectra. Full circles denote Q4, open circles Q3, crosses Q2.

As the modifier content increases, the fraction of the Q3 component grows at the expense of Q4. There is, however, a discontinuity at 17% K2. For this glass, the Q3 component is considerably more predominant than in its neighbours. The Q3 fraction obtained from the analysis of NMR spectra is based on the assumption that all Q3 species are in similar environments, i.e. that a Gaussian distribution of electron densities around Si atoms in Q3 species exists.

Fig.6: NBO fractions from NMR and from stoichiometry vs. composition. Fig. 6: Dependence of the fraction of non-bridging oxygens on the K2O fraction. The line represents values calculated on the basis of the overall chemical composition, the circles correspond to the evaluation of the distribution of Qn species as found by NMR.

The NBO fraction can be computed independently from the Qn distribution and from the chemical composition of the sample (assuming one NBO per monovalent modifier). The high Q3 fraction in the 17% sample causes a discrepancy between the two values of the NBO fraction. This indicates that a single Q3 fit component is not accurate, and that the Q3 distribution is wider than a Gaussian line suggests. This points to the coexistence of two amorphous phases with different Qn distribution.

WAXS and NMR results indicate that K2O - SiO2 glasses with a modifier fraction below 20% are inhomogeneous. This behaviour can be explained by counting constraints, i.e. rigid bonds and angles which fixate the structure. If the number of bond angle constraints is reduced by one for each NBO, then a critical modifier concentration is found at xc=0.2, below which the structure is too rigid to form a glass readily. Following this concept, the rigidity of the structure would then drive the amorphous phase separation.
However, extrapolation to pure SiO2 would imply that silica is -contrary to observation- not a good glass former. Constraint theory was developed for more covalent systems such as tellurides, and the role of monovalent modifier ions is not accurately described by an effective coordination number of 1. However, it is hoped that a modified version of the constraint theory which takes ionic interactions into account properly, may help to understand phase separation in glasses.

Acknowledgements. We would like to thank Dr Nick Terrill, CLRC Daresbury Laboratory, Warrington, for help at beamline 8.2 and the EPSRC for synchrotron radiation beam time.
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