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The Germanate Anomaly in Alkaline Earth Germanate Glasses Oliver L G Alderman, Alex C Hannon, Steve Feller, Richard Beanland, and Diane Holland J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12372 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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The Journal of Physical Chemistry

The Germanate Anomaly in Alkaline Earth Germanate Glasses Oliver L. G. Alderman,a*1 Alex C. Hannon,b† Steve Feller,c Richard Beanland,a Diane Hollanda a. Department of Physics, University of Warwick, CV4 7AL, UK b. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK c. Physics Department, Coe College, Cedar Rapids, IA 52402, USA * [email protected] † [email protected] Abstract A report is given of the first detailed investigation of the relation of structure to the germanate anomaly in alkaline earth germanate glasses. Pulsed neutron diffraction and high-energy x-ray diffraction measurements on calcium germanate glasses (22 to 41 mol% CaO, roller-quenched) show that the average Ge-O coordination number, nGeO, and average bond length, rGeO, exhibit broad maxima at ∼28 mol% CaO. Maxima in mass density, atom number density and glass transition temperature, also occur at a similar composition, and are characteristic of the germanate anomaly. The anomalies occur at a higher modifier oxide content for alkaline earth (Ae) germanate glasses than alkali (A) germanate glasses, attributed to the higher bond strengths within Ae-O polyhedra compared to A-O polyhedra. Consequently, the stabilisation of [GeO6] octahedra and/or bridges between higher coordinated [GeOm] units (m=5, 6) are a feature of AeO-GeO2 glasses, in contrast to the existence of only [GeO4] tetrahedra and [GeO5] units proposed in alkali (Cs) germanate glasses. At high CaO content, nGeO behaves as in thallium germanate glasses due to the very similar metaloxygen bond strengths within [CaO7] polyhedra and [TlO3] units. The composition dependence of atom number density at low modifier content follows a common trend for Ca, Sr, and Ba germanate glasses, as modifier cations fill germanate network cages. Our results show that the germanate anomaly for calcium germanate glasses is related to both a packing effect and a structural change in the germanate network; there is not a simple dichotomy whereby only one of these factors is of significance for the germanate anomaly.

1

Now at: Materials Development, Inc., Arlington Heights, IL 60004, USA. and: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.

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1. Introduction Germanate glasses, in general, have advantageous optical properties, including their wider infrared transmission windows, compared to silicates; calcium germanates, specifically, have attracted attention owing to their photoluminescence and ability to be prepared as both crystalline1 and amorphous2 nanowires. Doped alkaline earth (Ae) germanate glasses have shown promise for application as fibres for broad-band amplification and laser tuning,3 doped Ca gallogermanate glasses4-5 as tuneable solid-state laser sources, Ba gallogermanate glasses6 as windows in high energy laser systems,7 and Sr gallogermanate glasses as non-toxic fibres for medical laser applications.8 Continuing development of these materials would benefit from an improved understanding of the structure of alkaline earth germanate glasses and, in particular, their propensity to exhibit the “germanate anomaly”. Binary germanate (and also borate) glasses exhibit extrema in physical properties with change in composition, and this marks them as anomalous compared to their silicate analogues – hence the term “germanate anomaly”. Such extrema have been attributed to structural changes such as: change in coordination number of the network forming cation (B,9 Ge10-11); changes in network ring statistics, typically 4 to 3 membered [GeO4] ring conversions,12 or conversions between borate superstructural ring types;13 a combination of the aforementioned,14-19 as well as modifier cation effects. Identifying structural changes requires the successful determination of the type and number of structural units present. This is typically addressed either experimentally, using probes such as NMR, neutron and x-ray diffraction, EXAFS and vibrational spectroscopies, or theoretically, using relatively simple models for the distributions of structural units present, or with classical or ab initio molecular dynamics simulation. Determination of the actual structure-property relation is often very challenging, as discussed by Weinberg et al.20 in the context of alkali borate thermal expansion coefficient minima. Nonetheless, some progress has been made in describing glass density maxima in terms of partial volumes of structural units in borate21-24 and germanate18-19 systems. More recently, there have been attempts to apply temperature dependent constraint theory25 to relate alkali borate glass transition temperatures to the glass structure.26 Wright27 has discussed the various models developed for binary borate systems comprising structural species [BO3], [BO4]–, [BO3]– etc., whilst Hannon et al.28-29 have derived analogous charge avoidance models for alkali (A) germanate glasses based on the conversion of [GeO4] into [GeO5]– or [GeO6]2– in the absence of nonbridging oxygen (NBO) atoms at low modifier content.28-29 This process saturates at the limit of an alternating network of the two Ge–O coordination species present, and is followed by depolymerisation of the germanate network when NBOs are introduced as Q3 units, where Qn denotes a tetrahedral [GeO4] unit with (4 – n) NBOs. Importantly, the saturation point, and hence the maximum average Ge–O coordination number, nGeO, occurs at different glass compositions depending on whether [GeO5] (18.2 mol% A2O) or [GeO6] (28.6 mol% A2O) are present. This fact was used by Hannon et al.28 to infer the presence of [GeO5] in caesium germanate glasses, as opposed to [GeO6]. To date, whilst there have been numerous structural studies of binary A2O-GeO2 glasses, including neutron and x-ray diffraction measurements of nGeO,28, 30-33 there have been very few studies of divalent alkaline earth germanate, AeO-GeO2, glasses, possibly due to their tendency to phase separate.34-37 Schlenz et al.38 used high energy and anomalous x-ray scattering and electron diffraction to study a single phase 28.2BaO.71.8GeO2 glass. Two (unresolved) Gaussian peaks were


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fitted to the Ge–O correlation, yielding a total nGeO = 3.9(1) though the ratio of the two fitted peak areas was used to infer a ratio of 6-fold to 4-fold coordinated germanium of 1:3, which itself implies nGeO = 4.5. The discrepancy is due in part to the fitting of Gaussian functions to the radial distribution function extracted by high energy (100 keV) x-ray total scattering, which neglects the broadening arising from the x-ray form factors and the truncation in reciprocal space. This resulted in the exclusion of the areas under the clearly visible termination ripples from the calculation of nGeO, which would otherwise yield a value greater than 4.38 A decrease in the Ge-O-Ge stretching vibration frequency with addition of AeO, measured by infra-red absorption spectroscopy of AeO-GeO2 (Ae = Ca,Sr,Ba) glasses,39-41 was interpreted as due to the presence of a fraction of Ge in octahedral (6-fold) coordination to oxygen in all cases. Furthermore, for barium germanate glasses, Pernice et al.39 observed a maximum for both the glass transition temperature and the glass mass density at 20 mol% BaO, although these density data are not wholly in agreement with the values reported by Kasymova,42 which show a maximum at 27.5 mol% BaO (see Fig. 1a). Binary borate systems have received much greater attention than germanates due to the straightforward quantification of the relative abundance of [BO3] and [BO4] using solid-state 11B NMR, which is not yet possible with 73Ge NMR 43-44 for [GeO4], [GeO5] and [GeO6] germanate species. Hence we have used diffraction as one of the very few experimental techniques capable of quantitatively determining nGeO. In this paper we present results on glass formation, using rapid twin-roller quenching, and structural analyses by means of high-resolution neutron and x-ray total scattering, as well as density and glass transition temperature measurements for a wide range of CaO-GeO2 glasses. These results represent the first high-precision measurements of the Ge–O coordination numbers for any series of alkaline earth germanate glasses, and provide instructive comparison to existing results on alkali and other binary germanates, as well as to the more well known Ca silicate and borate systems. This paper is based on work presented in the PhD thesis of one of us (OLGA).45 2. Experimental 2.1 Glass Preparation Glasses of composition xCaO.(100 – x)GeO2 were prepared by rapid twin-roller quenching of the melts at Coe College, USA.46 CaCO3 (Sigma-Aldrich, ≥ 99.0%) and GeO2 (Alfa Aesar, 99.98%) powders were mixed in 20 cm3 platinum crucibles in sufficient quantities to yield 5 g of glass with nominal compositions from 5 to 55 mol% CaO, as given in Table S1. The crucibles were placed into an electric furnace and held, in most cases, at a temperature of 1450oC. This temperature was judged to be sufficiently above the liquidus line of the equilibrium phase diagram35 for all compositions, except x = 50 and 55, which were melted at 1500oC, and x = 5, which was remelted at the same temperature, being too highly viscous to pour from the crucible at 1450oC. No compositions beyond x = 55 were attempted on account of the rapidly rising liquidus. After 20 minutes, crucibles were removed from the furnace and weighed in order to determine if mass loss exceeded that expected from evolution of CO2 gas. CaO is highly refractory and so any additional mass loss was assumed to be due to volatilisation of GeO2. The resulting glass compositions calculated under this assumption are given in Tables 1 and S1. After weighing, crucibles were returned to the furnace for a further 10 minutes. The melts were poured into a 30 μm gap between two steel cylinders counter-rotating at 590 rpm, resulting in a cooling rate of order 105 oC s-1.46


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2.2 Density Measurement Sample volumes were measured for a known mass of sample, using helium pycnometry in a Micromeritics Accupyc 1330 pycnometer, in order to calculate the mass densities, ρm. The values for ρm were used to derive molar volumes, VM (i.e. the volume of one mole of x′CaO.(1 – x′)GeO2, where x=100x′), and atom number densities, gtot = (3 – x′)NA/VM, where NA is Avogadro’s number and (3 – x′) is the number of atoms in a formula unit.

2.3 Energy Dispersive X-ray Spectroscopy Glass composition was measured using energy dispersive x-ray spectroscopy (EDX) in a Zeiss SUPRA 55-VP field emission gun, scanning electron microscope (FEGSEM) operating at an accelerating voltage of 20 kV. Quantification of the glass composition was based on the integrated intensities of the Ca K and Ge K lines of the spectra after ZAF correction, using the EDAX Genesis software, which employs internal standards. 2.4 Differential Thermal Analysis 100 mg of powdered glass sample and 100 mg of Al2O3 reference powder were heated in separate Pt/Rh crucibles from room temperature to above the melting point at a rate of 10 oC min–1. Although all exothermic crystallisation events and endothermic melting events were recorded, only the glass transition temperatures, Tg, taken as the intersection of a linear extrapolation of the baseline from below Tg with a tangent to the point of maximum slope of the event, are reported here. 2.5 Energy Filtered Transmission Electron Microscopy EFTEM (energy filtered electron transmission microscopy) images were obtained at the Ca L edge from a sample of nominal composition 10 mol% CaO, using a JEOL 2100 TEM combined with a Gatan Quantum SE post-column imaging energy filter. The three-window method47 was used to extrapolate the background contribution to the electron energy loss spectrum into the post-edge energy window, and this was subtracted to produce the EFTEM map. 2.6 Neutron Diffraction Time-of-flight neutron diffraction (ND) measurements were made using the GEM48 diffractometer at the ISIS Facility, Rutherford Appleton Laboratory, UK. The glasses, in the form of small flakes (few mm2 in area), were loaded into cylindrical, thin walled (25 μm) vanadium containers of internal diameter 5.0 mm. Data were collected with sufficient statistical accuracy to justify use of a maximum momentum transfer Qmax = 40.0 Å–1 for Fourier transformation. Measurements were also performed on an empty vanadium container and the empty instrument to allow for subtraction of background signals, and on an 8.34 mm diameter vanadium rod for normalisation purposes. A vitreous germania sample was also measured and the preparation and characterisation details for this sample have been reported elsewhere.49 2.7 X-ray Diffraction


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Wiggler beamline BW550-51 on the synchrotron radiation source DORIS III, HASYLAB at DESY, was used for x-ray diffraction (XRD) measurements of the powdered glasses, which were held inside 1.5 mm diameter silica glass capillaries (20 μm wall thickness). Measurements of an empty capillary and the empty instrument were made to allow removal of background scattering. The x-ray energy of 84.486 keV (wavelength λ = 0.14675 Å), makes accessible a maximum momentum transfer, Qmax = 4πsin(θmax)/λ = 23.60 Å–1, at the maximum scattering angle of 2θmax = 32.0o. Data were collected in three angular ranges using different pre-detector slit widths to ensure that the count rate in the Ge detector did not saturate, despite the strong angular variation of the signal which arises from the form factor. All sets of data on a given sample were combined after omission of bad points, deadtime correction, normalisation to the incident beam monitor counts, correction for the geometrical arrangement of the detector and sample, and scaling as required for datasets with different predetector slit widths. A vitreous germania sample was also measured.49 2.8 Total Scattering Formalism The definitions of the real- and reciprocal-space scattering functions are described in detail in our previous work.52 The real-space total correlation function is defined by

T R (r ) = T R,0 (r ) +

2

π

∞

∫ Qi (Q )M (Q )sin (rQ )dQ R

(1)

0

where R (= N or X) denotes the radiation type, iN(Q) is the measured distinct53 neutron scattering, whereas

i X (Q ) =

i(Q )

(∑

)

c f (Q ) i =1 i i n

(2)

2

is the measured distinct x-ray scattering (after division by a sharpening54 function, used to approximately eliminate the Q-dependence of the scattering, which arises from the x-ray form factors, fi(Q)55). Subscripts i denote the n elements in the sample, and ci are atomic fractions. In equation (1), M(Q) is a modification function, used to reduce the termination ripples that arise from the finite upper integration limit, Qmax, for the experimental data. In this study the Lorch56 modification function was used. TR,0(r) represents the average scattering density terms, given by

T N,0 (r ) = 4πg tot r

(∑

n

cb i =1 i i

)

2

and T

X,0

(r ) = 4πgtot r

(3)

where bi are bound coherent neutron scattering lengths,57 and gtot is the atom number density. The distinct scattering functions, iR(Q), may be written as sums of Faber-Ziman58 partial structure factors, Sij(Q), and the correlation functions, TR(r), as sums of partial pair correlation functions, tij(r),53 and it is convenient to define the pair weighting factors

wijN (Q ) = ci c jbib j and wijX (Q ) =

ci c j fi (Q ) f j (Q )

(∑

)

c f (Q ) i =1 i i n

2

,

(4)

such that in the neutron case


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tij ( r ) =

c jT N ( r ) (2 − δ ij )wijN

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(5)

over any regions of r to which only a single pair term (i-j) contributes to the total TN(r). δij is the Kronecker delta. 3. Results 3.1 Glass Formation and Property Measurements Mass densities of the roller quenched CaO-GeO2 glasses are given in Table S1 and compared with literature data in Fig. 1. The mass density passes through a maximum, a characteristic of the germanate anomaly,10-11 between 27 and 30 mol% CaO. Kamiya et al.59 measured the density for three glass compositions close to the density maximum, and these values are in good agreement with those in the present study. The density data of Margaryan and Wai60 also display a maximum at approximately the same glass composition, but the densities are systematically lower than those measured for the roller quenched series. The earlier measurements made by Margaryan and Narekatsyan61 are mostly in accord with those of the present study, other than for their two lowest CaO content compositions (36 and 40 mol% CaO). A possible reason for the discrepancies may be the compositional changes arising due to volatilisation of GeO2 from the melt. These depend strongly on the melting conditions, such as melt surface area and partial vapour pressures within the furnace, and may have been exacerbated by the use of an argon atmosphere.60-61 Of further note are the measurements made by Kasymova42 on strontium and barium germanate glasses, both of which show broad maxima at about the same composition (∼30 mol% AeO) as found for the calcium germanate series. In Fig. 1, and throughout the ensuing analyses, the glass compositions used are those calculated from the mass loss during melting (Tables 1 and S1), assuming volatilisation of GeO2, and these are mostly in quantitative agreement with the CaO rich (compared to nominal) compositions measured by EDX spectroscopy (Table S1). EDX measurements at different points on the glass surfaces revealed apparent inhomogeneity of the samples containing nominally less than 21 mol% CaO. Table S2 records observations made, during cooling, of the material remaining in the crucible (inertially cooled) and also crystalline phases (later identified by diffraction) formed in the roller quenched products. From these initial observations, the 27 and 30 mol% CaO glasses appear to be the most stable, based on their vitrification under inertial cooling, with compositions either side displaying recalescence – visible light emission associated with the exothermic crystallisation of the (supercooled) melt. For compositions below 21 mol% CaO, opalescence was observed, indicative of glass-in-glass phase separation on a length scale of order 0.1 to 1 μm or greater, capable of scattering visible light. The present results are in accord with previous observations of phase separation in CaO-GeO2 glasses35-36, 62 which report a region of stable immiscibility, above the liquidus, between about 5 and 15 mol% CaO. EFTEM mapping allowed direct observation of the phase separation in a clear portion of the rollerquenched glass sample with nominal average composition 10 mol% CaO, as shown in Fig. 2, which reveals heterogeneity on a length scale of ∼50 nm, with bright regions corresponding to the Ca rich phase. The bi-percolating morphology is consistent with spinodal type phase decomposition, which


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is to be expected from the proximity of the average glass composition to the centre of the immiscibility dome (at ∼10 mol% CaO35-36, 62). 50 nm corresponds to vacuum UV wavelengths, well below the wavelength limit for visible light scattering, and is consistent with the predominantly transparent appearance of the material (Table S2). Glass transition temperatures, Tg, measured by DTA (Table S1 and Fig. 3) show a maximum at around 30 mol% CaO, another characteristic of the germanate anomaly.63 Shelby,64 however, did not observe this anomaly for his inertially-cooled calcium germanate glasses but reported two, composition-independent (10 to 30 mol% CaO) Tg events, a consequence of phase separation: the lower temperature event (∼560oC) being due to a GeO2-rich phase and the higher temperature event (∼675oC) being due to a CaO-rich phase. In contrast, only a single Tg event was observed for the current roller-quenched glass compositions (>10 mol% CaO) indicating single-phase glasses and therefore composition-dependent Tg values. Thus roller-quenching exceeds the critical cooling rate for circumventing phase separation by rapidly traversing the metastable immiscibility region. The 10 mol% CaO sample deviates from the observed trend in Tg because of phase separation, as confirmed by EFTEM (Fig. 2), leading to two Tg events. The higher Tg value (673(5)oC) is in close agreement with that observed by Shelby, and comparison with the Tg trend for the glasses with more than 10 mol% CaO (Fig. 3) shows that this Tg may correspond to a phase with ∼22 mol% CaO. The strong, low temperature Tg event (between 540 and 560oC) observed by Shelby64 for the 10 mol% CaO glass is just detectable in the roller-quenched glass at 549(5)oC, and the 673oC event is also weak and broad. The difference is almost certainly due to the different cooling rates applied during glass formation: the large cooling rate achieved by roller-quenching results in a higher glass fictive temperature, and so a less distinct calorimetric glass transition. On consideration of all of the measurements and observations made, only the samples with nominal compositions of 21 to 40 mol% CaO can be considered homogeneous, single-phase glassy materials. 3.2 Distinct Scattering and Correlation Functions The program GudrunX65-66 was used for the correction of the x-ray diffraction data for the effects of polarisation, absorption and multiple scattering, the removal of experimental backgrounds and the Compton scattering, normalisation using the Krogh-Moe67 and Norman68 method, and extraction of iX(Q) by subtraction of the self-scattering and sharpening. The GudrunN66 program and the Atlas53 suite of programs were used to correct the ND data for absorption, multiple scattering, inelasticity effects, and backgrounds, yielding the normalised iN(Q). The distinct scattering functions, iX(Q) and iN(Q), for the calcium germanate glasses are shown in Fig. 4. For the 10 mol% CaO sample, iX(Q) shows five small Bragg peaks from quartz GeO2.69 Since pure GeO2 material does not crystallise during rapid quenching, it is possible that these crystallites were nucleated at the interfacial regions between the two immiscible supercooled liquids. Glass samples with nominal compositions of 45, 50 and 55 mol% CaO were all found to contain some CaGeO3 wollastonite70 (Table S2) and therefore the data collected have not been analysed beyond the identification of the crystalline phase. The real-space correlation functions, TR(r) (R = N or X), are shown in Fig. 5. The most obvious systematic change with glass composition is the growth of intensity at ∼2.4 Å, which is attributed to Ca–O bonds for Ca–O coordination numbers of 6 to 7, based on bond-valence71-72 considerations. Despite the lower real-space resolution of the X-ray results, the Ca–O peak is better resolved in TX(r)


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(compared to TN(r)) because of the low weighting of the overlapping O–O term for XRD. It is also notable that the intensity of TX(r) at around 3.6 Å increases markedly with CaO content; for the 40 mol% CaO germanate glass, there is a distinct peak at this distance, which is assigned to Ca-Ge correlations. The neutron diffraction data presented here are available from the ISIS Disordered Materials Database.73 There are systematic changes in the shape of the first Ge–O peak in the correlation functions, and these are more clearly shown by the function tGeO(r) (eq. 5), shown in Fig. 6. With the addition of CaO, the Ge-O peak becomes asymmetric, with an excess of longer Ge–O bonds, and this is due to an increase in the average Ge–O coordination number, nGeO. The experimental values of nGeO given in Table 1 and shown in Figure 7 were determined by integration of rTN(r) from 1.52 Å up to the first minimum (after the Ge-O peak maximum) which occurs at about 2.10 Å, approximately independent of glass composition; these values provide strong evidence for average Ge–O coordination numbers higher than four. Also shown in Fig. 7 are theoretical curves, from a charge avoidance model,29 for the formation of either [GeO5] (continuous line) or [GeO6] (dashed line). For modifier contents of 18 mol% CaO or less, the CaO-GeO2 glasses are phase separated and inhomogeneous, and nGeO is not expected to follow a model based on an assumption of homogeneity. However, the experimental points within the homogeneous glass forming region (Fig. 7) also fail to follow the charge avoidance model29 for the formation of either [GeO5] or [GeO6]. Instead, nGeO increases on going from 21 to 24 mol% CaO, at which point it is in quantitative agreement with the [GeO5] model. On further CaO addition, nGeO remains approximately constant until it approaches the [GeO6] model prediction at 35 mol% CaO, and then it drops sharply, in accord with that model, as the CaO content approaches 40 mol%. It is notable that the calcium germanate glasses have much lower nGeO values compared to the ambient pressure crystals,70, 74-75 all of which contain [GeO4] tetrahedra and [GeO6] octahedra, but no [GeO5] polyhedra. The glasses containing nominally 27, 30, 35 and 40 mol% CaO all have nGeO in excess of the [GeO5] model,29 which implies that they contain [GeO6] species, or else violate assumptions of the model and contain corner-sharing [GeOm], m = 5, 6, units, or [OGe3] triclusters. Integration methods were also applied to rTN(r) to obtain the average Ge–O bond length, rGeO, and its standard deviation, σGeO (Table 1 and Fig. 8). These values show the same trend as nGeO, and notably they both pass through a maximum, as a function of CaO content, something which has not been observed for caesium germanate glasses,28 or indeed for any other binary alkali germanate glass series,76 although a similar phenomenon has been measured for thallium germanate glasses.77 As was the case for nGeO (Fig. 7), the values of rGeO for ambient pressure calcium germanate crystals70, 7475, 78 are in excess of those measured for the glasses. 3.3 Correlation Function Peak Fits To gain quantitative information, particularly concerning the Ca–O coordination, fitting of Gaussian distributions (convoluted with the appropriate real-space resolution function) to peaks in the experimental total correlation functions was performed. For the neutron correlation function, only the 40CaO.60GeO2 glass has a Ca-O peak which is sufficiently well resolved from the neighbouring Ge-O and O-O peaks to be amenable to fitting. Fig. 9 shows the peak fits to the neutron and x-ray total correlation functions for this sample. The neutron correlation function was first fitted with a Ge-O peak between 1.0 and 1.8 Å (leading edge), and then residuals were fitted sequentially for GeO (up to 1.95 Å) and Ca-O (up to 2.462 Å). Finally, this fitting process resulted in a very small residual


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peak at 2.04 Å which was also fitted and assigned to the Ge-O contribution. The total Ge-O coordination number, nGeO, obtained by summing the contributions from the three fit components, was 4.15(3), in excellent agreement with the value of 4.14(3) obtained by integration. Note, however, that the individual Ge-O fit components do not represent discrete, well-defined interatomic distances, but instead are a convenient way of parameterising the distribution of distances in a way that takes into account the real-space resolution function. The parameters obtained in this way for the first Ca–O peak are given in Table 2. The average Ca–O bond length and coordination number (rCaO = 2.398(4) Å, nCaO = 6.9(2) from TN(r)) are consistent with each other according to bond-valence71-72, 79 predictions, and are similar to the values found in crystalline calcium germanates. The width of the distribution of Ca-O distances (root mean square variation 2 uCaO

½

2

= 0.155(4) Å) is unusually large (for example, in calcium aluminate glasses,80 uCaO

½

is of

N

order 0.11 Å). For glasses with less than 40 mol% CaO, the Ca-O peak in T (r) at ~2.4 Å is not sufficiently well resolved to be fitted, and fits to the correlation functions for these glasses were limited to the Ge–O leading edge. Peak fitting to TX(r) was attempted, initially using just one Ge–O peak, and one Ca–O peak, as illustrated in Fig. 9 for the 40CaO.60GeO2 glass. The use of a single Ge–O peak was based on the lower spatial resolution in the x-ray case, and led to Ge-O coordination numbers in agreement with those measured by ND for 35 and 40 mol% CaO germanate glasses, see Fig. 7. For the 40 mol% CaO glass, the Ca–O peak parameters (see Table 2) are also in fair agreement. However, as the CaO content decreased, the fits to the Ca–O peak became increasingly broad, due to overlap with the O– 2

O and Ge–Ge peaks on the high r side, and reasonable nCaO and uCaO

½

parameters could not be

extracted. On the other hand, the values of rCaO for glasses with 30 and 35 mol% CaO (Table 2) were not significantly different from the value for the glass with 40 mol% CaO. Fitting to the leading edge of the Ge–O peak revealed peaks in the residual attributable to longer Ge-O bonds, but fitting sequential residuals, as performed for TN(r), was not reliable due to the complex and broad peak shape function for XRD.81 4. Discussion 4.1 Ge-O Coordination Numbers The behaviour of nGeO in CaO-GeO2 glasses is discussed in the context of what is known from studies of alkali germanate glasses and calcium germanate crystals. An extensive set of nGeO values has been measured for the Cs germanate glasses,28 and Fig. 7 shows that these are similar to the predictions of a charge avoidance model in which the higher coordinated species is [GeO5], rather than [GeO6].29 In this model the maximum value of nGeO occurs for an alternating network of [GeO4] and [GeOm], and the composition at which this occurs is the highest modifier content for which NBOs are absent from the network. Values for nGeO in Na2O-GeO233 and K2O-GeO232 glasses, measured by ND, are also in accord with the same model and the presence of [GeO5] species. However, the values of nGeO in CaO-GeO2 glasses differ in two respects. Firstly, whilst the measured Ge–O coordination numbers and distributions of Ge–O bonds (Figures 6 and 7) show that the calcium germanate glasses contain higher coordinated [GeOm] (m = 5, 6) species, these must co-exist with NBOs. The presence of NBOs is apparent from the way that the average Ge–O coordination numbers fall below the 4 + 2x/(100 – x) prediction29 (Fig. 7), which is based on the assumption that no NBOs are present. Table 1 lists the


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fraction of oxygens that are non-bridging, fNBO = 2 – nOGe, where nOGe is the O–Ge coordination number and is obtained from the relation nGeOcGe = nOGecO. This shows that there are NBOs present in all of the CaO-GeO2 glasses. Even at the composition of the anomaly maximum, ~20% of the oxygens are non-bridging (fNBO~0.2). There is also evidence from 17O NMR spectroscopy82-83 on sodium germanate glasses for the presence of NBOs at compositions close to the anomaly maximum, but the fraction is markedly less (fNBO

The Germanate Anomaly in Alkaline Earth ... - ACS Publications

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