Influence of Lone-Pair Cations on the Germanate Anomaly in Glass

ARTICLE pubs.acs.org/JPCC

Influence of Lone-Pair Cations on the Germanate Anomaly in Glass Emma R. Barney,*,† Alex C. Hannon,† Nattapol Laorodphan,‡ and Diane Holland‡ † ‡

ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, U.K. Physics Department, University of Warwick, Coventry, CV4 7AL, U.K. ABSTRACT: Neutron diffraction has been used to study the structure of a series of thallium germanate glasses, Tl2O
GeO2, containing up to 40 mol % Tl2O, as a means of investigating the influence of lone-pair cations on the germanate anomaly. As observed previously in alkali germanate glasses, the average Ge
O coordination number, nGeO, is found to rise above four as Tl2O is added to the glass. However, whereas for alkali germanates nGeO has its maximum value (∼4.36 ( 0.03) at ∼19 mol % R2O (e.g., R = Cs), for thallium germanates it continues to rise until 30 mol % Tl2O, with a higher maximum value of 4.44 ( 0.02. For low Tl2O content, most thallium cations are on modifier sites with a high coordination number (6 or greater). As the Tl2O content increases, glass former [TlO3] sites become increasingly common, and it is predicted from an extrapolation of the results that a glass with a composition of 50 mol % Tl2O would be composed entirely of [TlO3] and [GeO4] units. It is shown that the presence of [TlO3] units allows higher coordinated Ge units to share an oxygen, and this is why nGeO continues to rise beyond the composition for which it is a maximum in alkali germanates. There is thus an interplay between the germanate anomaly and the environment of the lone-pair cation—an effect which does not occur in alkali germanates.

’ INTRODUCTION The thermophysical properties of alkali germanate glasses (such as density and glass transition temperature) exhibit a composition dependence which has been called the germanate anomaly.1
3 As alkali oxide is added to the glass, at a specific composition there is a maximum or minimum in the properties. This behavior has been described as anomalous because it is unlike that observed for other well-known glasses, such as silicates, for which the properties show no such maxima or minima.4 Pure vitreous germania, v-GeO2, has a random network structure formed by corner-sharing between tetrahedral GeO4 units.5,6 It was proposed that the germanate anomaly is due to a growth and subsequent decline in a fraction of octahedral, GeO6, germanium atoms in the glass.1
3 Although for a time there were claims that the germanium coordination number, nGeO, does not change with composition,7
10 there is now clear experimental evidence, notably from neutron diffraction,4,11
17 which shows that nGeO does exhibit a growth and decline as alkali oxide is added to the glass. Nevertheless, it remains controversial whether the higher coordinated germanium is octahedral, GeO6, or 5-coordinated, GeO5.17
19 Lone-pair (LP) cations (e.g., Tl+, Pb2+, Bi3+) in glasses are of practical interest because of the attractive optical properties which they give to glass, but they have a more complex structural role than other cations. It has long been proposed,20,21 on the basis of structural information from crystallographic studies and the wide range of glass formation for glasses containing LP cations, that for low LP content they behave as network modifiers with high coordination numbers (∼6
12) but that for high LP content they behave as network formers with low coordination number r 2011 American Chemical Society

(∼3
5). An example of these two types of environments for Pb2+ has recently been shown in a study of crystalline Pb9Al8O21.22 However, a more mixed view of the structural role of lone pair ions in glasses is provided by relatively recent studies using modern structural probes: two different X-ray diffraction studies23,24 and a combined X-ray diffraction, neutron diffraction, and 29Si NMR study25 have all found evidence that Pb may be on asymmetric PbO3 sites for all lead silicate compositions. In addition, a neutron diffraction study of lead gallate glasses showed no evidence of Pb in high coordination number modifier sites,26,27 and a neutron diffraction study of tin silicates reports that, for compositions between 20 and 75 mol % SnO, the tin environment consists of [SnO3] and [SnO4] polyhedra,28 with no evidence of higher coordination. On the other hand, recent diffraction studies of borate glasses do indicate a change in the environment of the LP cations: An X-ray diffraction and 11B MAS NMR study of lead borate glasses has shown that the Pb
O coordination number is about 6 for low PbO content, falling to about 3 for high PbO content,29 while a recent neutron diffraction study of tin borates has also shown evidence for a change in the tin environment from symmetric high coordination sites to asymmetric SnO3 sites, as the SnO content is increased.30 Furthermore, a M€ossbauer, 119Sn NMR, and neutron diffraction study of tin germanate glasses has shown evidence for a similar change in the tin environment with composition.31 Thus, recent studies, using modern structural probes, give a mixed picture of the structural role of lone pair ions Received: March 10, 2011 Revised: June 14, 2011 Published: June 14, 2011 14997

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in glasses. On the basis of these reports, we suggest that LP cations undergo a change in behavior for glasses in which there are changes in the glass former cation coordination number (e.g., borate and germanate glasses), but otherwise (e.g., silicate glasses) the LP cations behave solely as a glass former. Although the basic concepts concerning the germanate anomaly and the structural role of LP cations are not new, it is only relatively recently that experimental techniques for probing the structure of glass have developed to a point where they can be investigated in detail. Here we report density and neutron diffraction (ND) measurements on thallium germanate glasses with a range of compositions. The aims of this study are 2-fold: first to investigate how the structural role of the LP cations in glass changes with concentration, and second to investigate the influence of the LP cation on the germanate anomaly. In fact, as will be shown, these two aspects of the structure are closely related. In a limited preliminary report, we have shown that for a germanate glass with 30 mol % Tl2O the value of nGeO is 4.44 ( 0.02,32 very much higher than for an alkali germanate glass with the corresponding composition (30 mol % Cs2O), for which the coordination number is 4.08 ( 0.03.17

’ THEORY A neutron diffraction (ND) experiment measures the differential cross section, (dσ)/(dΩ) = I(Q) (where Q is the magnitude of the scattering vector for elastic scattering), given by Wright.33 dσ ¼ IðQ Þ ¼ I s ðQ Þ + iðQ Þ dΩ

ð1Þ

where Is(Q) is the self-scattering, which can be calculated within an approximation. The distinct scattering, i(Q), can be Fourier transformed to give the total correlation function in real-space Z 2 ∞ QiðQ ÞMðQ ÞsinðrQ ÞdQ ð2Þ TðrÞ ¼ 4πFo ð cl b̅ l Þ2 + π 0 l

∑

where Fo is the atomic number density; cl and bl are, respectively, the atomic fraction and the coherent neutron scattering length for element l; and M(Q) is a modification function which is used to reduce termination ripples in the Fourier transform. The result from a diffraction experiment is not element specific, and T(r) is a weighted sum of all possible partial correlation functions, tll0 (r) TðrÞ ¼

∑ll cl b̅ l b̅ l tll ðrÞ 0

0

0

ð3Þ

The summation is over all the pairwise combinations of elements in the sample. For a peak in T(r) due to a particular correlation between atoms of element l and l0 , the coordination number, nll0 , can be calculated from the area, All0 , and position, rll0 , of the peak and the coefficient for tll0 (r) according to nll0 ¼

rll0 All0 ð2
δll0 Þcl b̅ l b̅ l0

ð4Þ

where δll0 is the Kronecker delta.

’ EXPERIMENTAL DETAILS Eight thallium germanate glass samples were prepared over a compositional range of 5
40 mol % Tl2O, at 5 mol % increments, using Tl2CO3 (Alfa Aesar, 99.99%) and GeO2 (Alfa

Table 1. Density of the Thallium Germanate Samples, Together with Both the Nominal Composition of the Glass, and the Revised Composition Calculated from the Mass Lost during the Melt nominal composition (mol % Tl2O)

mass loss composition ((0.1 mol % Tl2O)

density ((0.01 g cm
3)

5

4.3

4.34

10

9.8

4.94

15

14.0

5.43

20

19.4

5.81

25

24.5

6.15

30 35

29.4 34.5

6.49 6.77

40

39.7

6.98

Aesar, 99.98%) as starting ingredients. Batches (25 g) were heated in Pt/Rh crucibles at 10
/minute, held at a maximum temperature for 15 min, and then plate-quenched between copper sheets. The maximum temperature was chosen to be 100
above the melting temperature given in the thallium germanate phase diagram.34 Attempts were made to produce glasses with 45 and 50 mol % Tl2O, but they were not used in the study due to the presence of small black flecks in the glass. A very small number of these inclusions were also observed in 40 mol % glass, and these were removed before any measurements were made. Density measurements for each sample were made using a Micromeritics Accupyc 1330 pycnometer, with helium as the working fluid. The neutron diffraction data were measured using the GEM diffractometer at the ISIS pulsed neutron source, Rutherford Appleton Laboratory, UK.35 The glass samples were crushed to small pieces (5 mm across or smaller) and held in 8.3 mm diameter cylindrical containers. The containers were made of thin 25 μm vanadium foil to minimize corrections arising from the container. The data were reduced and corrected for attenuation and multiple scattering using standard Gudrun and ATLAS software.36 A quadratic of the form A + BQ2 was fitted to i(Q) at low Q to extrapolate the data to Q = 0 before being Fourier transformed using the Lorch modification function with a maximum momentum transfer, Qmax, of 40 Å
1 to reduce termination ripples.37 The corrected data, in both reciprocal- and real-space, are available from the ISIS Disordered Materials Database.38

’ RESULTS The density of each sample was measured, and the results, given in Table 1, are compared with previously published values in Figure 1a.39,40 There is reasonable agreement between the three data sets, but the values measured by Riebling39 and Nassau40 diverge from those measured in this study for high Tl2O content. One possible cause for this difference is volatilization of thallium oxide from the melt which would result in glasses with a reduced Tl2O content. However, careful weight loss measurements were made during the glass-making process, and the excess mass loss (i.e., that not due to removal of CO2) was found to correspond to a change in composition of 30 mol % Tl2O), the average germanium coordination number decreases, returning to tetrahedral environments (see Figure 6), while the fraction of Tl atoms which are in three coordinated environments continues to increase (see Figure 7). In this regime, Figure 6 shows that there are increasing numbers of NBOs (where the term NBO refers to the Ge
O network and denotes that the oxygen is bonded to only one Ge atom) that need to form additional bonds to thallium atoms to achieve charge balance. In crystalline Ge5Tl8O14, 80% of the [GeO4] units have two NBOs, while 20% have one NBO. In this structure, each NBO is bonded to three thallium atoms, which form the corner of a distorted

Figure 13. (a) Values for nGeO (black squares) and nOGe (black circles) for the three compositions containing g30 mol % Tl2O. The linear fit to nGeO was used to calculate the composition at which the germanium network would be fully 4 coordinated (open square and circle). (b) The values for nOTl (black triangles) for compositions g20 mol % Tl2O. A polynomial fit to all of the data was then used to estimate nOTl at that composition (open triangle).

cube comprised of four edge sharing [TlO3] units, as shown in Figure 8b and Figure 10a. Figure 12d shows the environment of one of these NBOs, which has a total EBS of exactly two. This sort of oxygen is well charge-balanced and is favored in thallium germanates with high thallium content. Now the necessity for the formation of [GeO4] units with NBOs at high Tl2O contents can be understood; an oxygen bridging between two [GeO5] units can accommodate one Tl
O bond from a [TlO3] unit, whereas a NBO in a [GeO4] unit can accommodate three Tl
O bonds. Therefore, as the number of germanium atoms in the glass reduces and the number of thallium atoms in [TlO3] units increases, [GeO4] units with NBOs become more common. As shown in Figure 7, the extrapolation of the coordination number derived from the Tl
O peak at ∼2.5 Å in T(r) indicates that for a composition 51 ( 1 mol % Tl2O all thallium atoms are predicted to be in [TlO3] units. Furthermore, Figure 13a shows an extrapolation of the average Ge
O coordination number values between 30 and 40 mol % Tl2O which predicts that the germanium atoms in the glass would be fully 4 coordinated for 48 ( 4 mol % Tl2O. Therefore, a hypothetical glass with a composition of 50 mol % Tl2O (Tl2GeO3) is predicted to be comprised entirely of [TlO3] and [GeO4] units. The bonding in a glass containing 50 mol % Tl2O will be similar to that found in crystalline Ge5Tl8O14, where each NBO is bonded to three thallium atoms and one germanium atom. These assumptions have been used to develop a simple model for the connectivity of Tl2GeO3, given in Figure 14. For this model, the average oxygen-centered 15005

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nGeO to increase to larger values at higher Tl2O content, compared to alkali germanate glasses.

’ ACKNOWLEDGMENT The sample preparation and characterization was carried out at the University of Warwick Physics Department. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beam time allocation from the Science and Technology Facilities Council with RB number 820434. ’ REFERENCES

Figure 14. Model of the connectivity in a hypothetical glass with 50 mol % Tl2O, GeTl2O3. For this composition, the glass is predicted to contain only [GeO4] and [TlO3] structural units. The dashed lines indicate the formula unit for the glass, from which the average coordination numbers can be calculated.

coordination numbers are nOGe = 1.33 and nOTl = 2, and on average each [GeO4] tetrahedron contains two NBOs and two bridging oxygens. The extrapolated values for nOGe and nOTl in Figure 13a and 13b are in good agreement with this model. Thus far, it has not been possible to produce a glass sample containing 50 mol % Tl2O due to disproportionation of thallium in the melt.

’ CONCLUSIONS Neutron diffraction has been used to investigate the environment of germanium and thallium in a series of thallium germanate glasses, Tl2O
GeO2. For low Tl2O content, the average Ge
O coordination number, nGeO, is observed to rise above four as Tl2O is added to the glass, and the additional oxygen is accommodated mostly by a fraction of the Ge being in higher coordinated units [GeOn] with n greater than four. In alkali germanate glasses, the maximum value of nGeO occurs at ∼19 mol % R2O (e.g., R = Cs), but for thallium germanates nGeO continues to rise until 30 mol % Tl2O, at which composition nGeO has a value of 4.44 ( 0.02, which is significantly higher than is observed for alkali germanate glasses. For Tl2O content greater than 30 mol % Tl2O, nGeO declines toward a value of four. There is also a change in the thallium coordination; for low Tl2O content most thallium cations are on modifier sites with a high coordination number (6 or greater), but as the Tl2O content increases, there is a growing number of glass former [TlO3] sites. The fraction of Tl in [TlO3] units varies linearly with Tl2O content, and by extrapolating both the germanium and thallium coordination numbers, it is predicted that the glass would be entirely comprised of [TlO3] and [GeO4] units at a composition of 50 mol % Tl2O. For alkali germanates, the fraction of higher coordination germanium units, [GeOn], is limited by the inability of two such units to share an oxygen atom. However, we have used an electrostatic bond strength model to show that the presence of [TlO3] units in thallium germanates makes it possible for two higher coordination germanium units to share an oxygen, with the result that a greater concentration of [GeOn] units form. Thus, the presence of lone-pair cations on glass former sites has an influence on the germanate anomaly in thallium germanate glasses by allowing

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