Physics and Chemistry of Glasses

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Oct 5, 2010 - is they that determine the differences between the network structures .... stabilisation energy compensates the endothermic formation energy .... Potassium Rubidium. Cæsium ..... silicon sp3 hybrid orbitals directed tetrahedrally in space and ..... formed from a combination of its 4s, 4p and two of its unfilled 4d ...
ISSN 1753-3562

October 2010 Volume 51 Number 5

Physics andChemistry ofGlasses

European Journal of Glass Science and Technology Part B

B2O3-I

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B2O3-0

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B2O3 liquid

Volume 51 Number 5

October 2010

Physics and Chemistry of Glasses European Journal of Glass Science and Technology B Contents

The European Journal of Glass Science and Technology is a publishing partnership between the Deutsche Glastechnische Gesellschaft and the Society of Glass Technology. Manuscript submissions can be made through Editorial Manager, see the inside back cover for more details. Regional Editors Professor J. M. Parker Professor L. Wondraczek Professor A. Duran Professor R. Vacher Dr A. C. Hannon Professor M. Liška Professor S. Buddhudu Professor Y. Yue Abstracts Editor Professor J. M. Parker Managing Editor D. Moore Assistant Editor S. Lindley

233 Review: Borate versus silicate glasses: why are they so different? A. C. Wright & G. Dalba, F. Rocca & N. M. Vedishcheva 266 Aluminium-26 and oxygen-18 tracer diffusion measurements in an aluminosilicate glass: temperature dependence M. Lepke, P. Fielitz, G. Borchardt, G. H. Frischat, A. Goß & E. Pösl 271 Impact of cationic diffusion on properties of iron-bearing glass fibres M. M. Smedskjaer, Yuanzheng Yue, J. Deubener & S. Mørup 281 Spectroscopic studies of vanadyl doped MB4O7 (M=Zn, Cd) glasses K. S. N. Murthy, P. Narayana Murty, Ch. Rama Krishna, P. S. Rao & R. V. S. S. N. Ravikumar 284 Phthalocyanine templates joined through basic ligands to a ruthenium sulfonate–aqua complex onto glassy surfaces N. A. Wiederkehr 291 Conference diary A47 Abstracts

Society of Glass Technology Unit 9, Twelve O’clock Court 21 Attercliffe Road Sheffield S4 7WW, UK Tel +44(0)114 263 4455 Fax +44(0)114 263 4411 E-mail [email protected] Web http://www.sgt.org The Society of Glass Technology is a registered charity no. 237438. Advertising Requests for display rates, space orders or editorial can be obtained from the above address. Physics and Chemistry of Glasses: European Journal of Glass Science and Technology, Part B ISSN 1753-3562 (Print) ISSN 1750-6689 (Online) The journal is published six times a year at the beginning of alternate months from February. Electronic journals: peer reviewed papers can be viewed by subscribers through Ingenta Select http://www.ingentaconnect.com The editorial contents are the copyright © of the Society. Claims for free replacement of missing journals will not be considered unless they are received within six months of the publication date.

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Cover: Thermally and mechanically induced transformation pathways in B2O3, from Understanding the anomalous behaviours of B2O3 glass, by John Kieffer, Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, October 2009, 50 (5), 294–300.

21/10/2010 12:24:53

Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, October 2010, 51 (5), 233–265

Review Borate versus silicate glasses: why are they so different? Adrian C. Wright* & Giuseppe Dalba Dipartimento di Fisica, Università di Trento, I-38050 Povo (Trento), Italy

Francesco Rocca CNR Istituto di Fotonica e Nanotecnologie, Via alla Cascata 56/c, 38100 Povo (Trento), Italy

Natalia M. Vedishcheva Institute of Silicate Chemistry of the Russian Academy of Sciences, Ul. Odoevskogo, 24, Korp. 2, Sankt Petersburg, 199055, Russia Manuscript received 25 November 2009 Revised version received 13 September 2010 Accepted 27 September 2010

A comparative review is presented of a range of physical and chemical properties of borate and silicate glasses. Most attempts to explain the difference between the properties of these glasses, in terms of the so-called borate (or boron oxide) anomaly, have focussed on the change in co-ordination number of some of the boron atoms from three to four. The origin of this co-ordination number change is discussed at a fundamental level, together with the important role played, both by superstructural units and by network-modifying cations, in determining borate glass properties. In addition, the Krogh-Moe–Griscom superstructural unit model for the structure of borate glasses is updated in line with more recent experimental data, and used to account for the way in which the fraction of 4-fold co-ordinated boron atoms in lithium, sodium and silver borate glasses varies with composition. It is concluded that the “anomalous” properties of vitreous B2O3 and binary M2O–B2O3 glasses are entirely consistent with respect to the electron configuration and bonding of the boron atoms and to the presence of superstructural units. Extrema in the properties of the binary glasses, when plotted as a function of composition, arise due to the interplay between two (or more) opposing factors. The major remaining question concerns the origin and relative magnitudes of the stabilisation energies for the various superstructural unit species.

1. Introduction Borate glasses are frequently considered to be anomalous in comparison to silicate glasses, both in terms of their structure and their properties. Following a brief review of some of their important physical and chemical properties, the present paper will attempt to explain, at a fundamental level, why borate and silicate glasses behave so differently. The structures of borate glasses and crystalline phases have recently been reviewed in detail elsewhere,(1) and so only the most relevant aspects will be briefly summarised here. In addition, the discussion of binary systems will be limited to those containing monovalent network-modifying cations; viz M2O–B2O3. However, whereas the previous paper concentrated on structure, the objective of this sequel is to demonstrate that the properties of borate glasses and melts are a direct consequence of their chemical bonding and structure and, as such, are not at all anomalous. The convention employed in this paper for the * Corresponding author. Permanent address: J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, UK. Email [email protected]

formulæ of (super)structural units follows that of Ref. 1, in that the symbol Ø is reserved solely for those bridging oxygen atoms that are shared between adjacent (super)structural units, whilst bridging oxygen atoms that are situated completely within a superstructural unit are denoted O, as are negatively charged nonbridging oxygen atoms that form part of a (super)structural unit. The advantage of this representation is that it allows equations describing the various reactions/equilibria involving (super)structural units to be balanced, since Ø is then equivalent to half of an (O) oxygen atom. Structural unit fractions are denoted xn, where n is the number of bridging oxygen atoms, and the symbols for the fractions of the boron atoms in the various superstructural units and the tetraborate group are as follows: boroxol group, xB; pentaborate group, xP; tetraborate group, xTe; triborate group, xTr; di-pentaborate group, xDP; diborate group, xD and di-triborate (NBO) group, xDT. The fractions of independent BØ3 and BØ4− structural units (i.e. those not incorporated into superstructural units) are indicated by x3I and x4I, respectively. Reference 1 has a comprehensive table of references for

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 51 Number 5 October 2010

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A. C. Wright et al: Review: Borate versus silicate glasses: why are they so different?

the structures of M2O–B2O3 crystalline phases, and so the references for individual structures will not be repeated here. A very much more detailed account of the high temperature crystal chemistry of borates and borosilicates has also recently been published by Bubnova & Filatov.(2) To fully understand the structure and properties of borate glasses, it is necessary to explain why the initial addition of a network modifier to vitreous B2O3 leads to an increase in the co-ordination number of some of the boron atoms from 3 to 4, rather than to the introduction of nonbridging oxygen atoms (NBO), and also the important contribution made Figure 1. Fraction of 4-fold co-ordinated boron atoms in by superstructural units in the formation of borate borate glasses. Blue closed circles, Jellison et al(6) 1. networks although, as demonstrated in Ref. 1, theseFigurelithium (NMR); red closed squares, Kroeker and co-workers(7,8) two unusual aspects of borate structures are in fact (NMR); green closed triangles, Bray & O’Keefe(5) (NMR); closely related. Similarly, the role of the networkmagenta inverted closed triangle, Ratai et al(9) (NMR) and modifying cations must not be forgotten, since it cyan open circle, Cormier et al(10) (neutron diffraction). The is they that determine the differences between the black solid line denotes the fit described in Section 7.1, and network structures of the various M2O–B2O3 glasses the black dashed line is the continuation of Equation (3) at any given composition.

2. Borate glass structure This section briefly summarises those aspects of the structure of borate glasses that are important to the sections that follow. For a much more complete discussion of borate structures, both crystalline and vitreous, the reader is directed to Ref. 1.

2.1. Boron co-ordination number The addition of a network modifier to B2O3 initially results in the conversion of BØ3 triangles into BØ4− tetrahedra ½M2O + BØ3 Æ M+ + BØ4−

(1)

i.e. it acts as a network compacter via an increase in nB(O) from 3 to 4. Further additions of a network modifier eventually lead to the formation of negativelycharged nonbridging oxygen atoms ½M2O + BØ3 Æ M+ + BOØ2−

(2)

etc.(1) Thus, at higher network-modifier mole fractions, xM, the network modifier acts as a network breaker. Thermodynamic data suggest that the conversion of a BØ3 triangle into a BØ4− tetrahedron is slightly endothermic {ΔH≈4·5 kcalmol−1,(3) ~19 kJmol−1, or ~195 meV per structural unit (s.u.)}, whereas the interaction between the network modifying cation and its surrounding borate network is strongly exothermic, such that the overall result is an exothermic reaction. In the absence of nonbridging oxygen atoms, the fraction of 4-fold co-ordinated boron atoms, x4, is given by(4) x4=R=xM/(1−xM)

(3)

in which R is the ratio of M2O to B2O3, as used in NMR studies. The greater electronegativity of oxygen, relative to boron, means that the negative charge on the BØ4− tetrahedron is delocalised over the entire structural unit. Hence it is energetically unfavourable for BØ4− tetrahedra to be immediate bonded neighbours. {Note that the heteropolar B–Ø bonds of the BØ3 structural units in vitreous B2O3 are already polarised, such that there is a (small) negative charge (δ-) on the oxygen atoms 46 and a balancing charge of 3 ⁄2δ+ on the boron atoms. The addition of the negative charge on conversion to a BØ4− tetrahedron results in the boron atom becoming negatively charged and an increased negative charge being delocalised on to the four oxygen atoms. Thus the electrostatic component of the network-modifying cation interaction with the borate network includes the negative charges on both the boron and the oxygen atoms.} The determination of the fraction of 4-fold coordinated boron atoms, x4, was one of the early successes of NMR spectroscopy(5), and data for the Li2O–B2O3 system(5–10) are summarised in Figure 1 to illustrate the general form of the variation of x4 with xM, which is similar for all five alkali borate systems.(5) At low xM, x4 follows the curve of Equation (3), but starts to fall significantly* below this value at xM~0·28, due to the formation of nonbridging oxygen atoms {Equation (2)}. There is a maximum in x4, at a value of xM that depends slightly on the network modifying cation, after which x4 decreases with increasing xM. In the case of the Li2O–B2O3 system, the maximum occurs at xM~0·37, and x4 extrapolates to * For the present purposes, a significant deviation from Equation (3) is taken as 0·01, which is the uncertainty on the best NMR data. For the fit to the Li2O–B2O3 data in Figure 1, this occurs at xM=0·277.

234 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 51 Number 5 October 2010

A. C. Wright et al: Review: Borate versus silicate glasses: why are they so different?

zero at xM~0·72, between the pyroborate (xM=2⁄3) and orthoborate (xM=¾) compositions. It is useful to divide the composition range into four different regions, according to the predominant conversion of the structural unit species present(1): Region I (Formation of BØ4− tetrahedra): BØ3 Æ BØ4− (0≤xM≤1⁄3) Region II (Break up of superstructural units): BØ3 Æ BOØ2− (1⁄3≤xM≤½) Region III (Network destruction): BOØ2− Æ BO2Ø2− (½≤xM≤2⁄3) Region IV (Invert glasses): BO2Ø2− Æ BO33− (2⁄3≤xM≤¾) In defining these regions, it is assumed that the repulsion between negatively-charged nonbridging oxygen atoms on the same boron atom means that significant numbers of BO2Ø2− and BO33− units are not formed until it is unavoidable, as supported both by spectroscopic and thermodynamic data. The above scenario only applies fully to the smaller, more highly polarising, network-modifying cations. In Region IV, the larger cations favour the formation of BO2Ø23− tetrahedra with two nonbridging oxygen atoms, as will be discussed in Section 4.4.

2.2 Superstructural units Superstructural units comprise well-defined arrangements of the basic borate structural units, with no internal degrees of freedom in the form of variable bond or torsion angles, and those of particular relevance to the present paper are illustrated schematically in two dimensions in Figure 2. The tetraborate group (B8O10Ø62−; Figure 3), is not a superstructural unit, since its constituent pentaborate {B5O6Ø4−; Figure 2(B)} and triborate {B3O3Ø4−; Figure 2(C)} groups are joined by a single bridging oxygen atom with variable bond and torsion angles. However, tetraborate groups are only found in the crystalline state at the tetraborate composition, and sodium tetraborate melts with little decomposition.(11) Hence Krogh-Moe(11) concludes that the melt does not contain separate pentaborate and triborate groups, but rather tetraborate groups; i.e. that a triborate group must therefore (nearly) always appear in such melts associated with a pentaborate group, presumably due to some form of association energy. The fact that borate crystallography up to the diborate composition is totally dominated by superstructural units indicates that their formation is not just random, due to the proximity of the equilibrium ˆ –B bond angle to that required for planar 3-memB–Ø bered rings, but that they must have a favourable formation/stabilisation energy. Irrefutable evidence for the existence of superstructural units in borate glasses is provided by x-ray and neutron diffraction studies, and from optical, NMR and neutron spec-

(A) (D)

(B)

(C)

(E)

(F)

Figure 2. A, B3O3Ø3 boroxol group; B, B5O6Ø4− pentaborate group; C, B3O3Ø4− triborate group; D, B5O7Ø32− di-pentaborate (NBO) group; E, B4O5Ø42− diborate group and F, B3O4Ø32− di-triborate (NBO) group. (Key: blue, tetrahedral boron; cyan, trigonal boron; red, bridging oxygen atom and magenta, nonbridging oxygen atom.) 47 troscopy, as summarised in Ref. 1. In particular, both diffraction and NMR data indicate that the network of pure vitreous B2O3 is composed of approximately equal numbers of boroxol groups {B3O3Ø3; Figure 2(A)} and independent BØ3 triangles. A detailed survey of the structures of crystalline alkali and related borates(1) indicates that, for every crystalline phase with xM≤1⁄5 (tetraborate composition), all of the BØ4− tetrahedra are incorporated into superstructural units, and hence that their creation is facilitated by the superstructural unit formation/ stabilisation energy (i.e. the superstructural unit stabilisation energy compensates the endothermic formation energy for the BØ4− tetrahedron). Independent BØ4− tetrahedra first appear at xM=5⁄24 (0·208; 5M2O.19B2O3) and adjacent pairs of BØ4− tetrahedra within a superstructural unit begin at xM=¼ (triborate composition), with the diborate group {B4O5Ø42−; Fig-

Figure 3. B8O10Ø62− tetraborate group. (Key as Figure 2) Figure 3.

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 51 Number 5 October 2010

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A. C. Wright et al: Review: Borate versus silicate glasses: why are they so different?

Table 1. Network-modifying cation (M+) data

Lithium

Sodium

Potassium

Electron Configuration (He)2s1 (Ne)3s1 (Ar)4s1 Electronegativity(13) 1·0 0·9 0·8 Ionic radius (nM(O))* (Å) 0·59 (4) 1·02 (6) 1·38 (6) M+ −O– bond ionicity** 0·53 0·58 0·64 * Effective ionic radii based on a radius for the O2− anion of 1·40 Å(14) ** Value for M2O.3B2O3(12)

ure 2(E)} in α-Na2O.3B2O3. Note that the unfavourable B[4]–O–B[4] linkage within the diborate group, where B[4] represents a tetrahedral boron atom, is also offset by the superstructural unit stabilisation energy(1). External B[4]–Ø–B[4] bridges (i.e. those not within a superstructural unit) are not observed until xM=3⁄10. In the ambient-pressure crystalline state, nonbridging oxygen atoms (BOØ2− structural units) do not occur until xM=1⁄3 (diborate composition); e.g. as part of the di-triborate (NBO) group {B3O4Ø32−; Figure 2(F)} in the structure of α-Na2O.2B2O3. They are relatively rare for xM