Phys. Chem. Glasses, 2003, 44 (2), 79–87
Infrared studies of borate glasses E. I. Kamitsos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens 116 35, Greece
This paper presents some recent contributions of infrared reflectance spectroscopy to the structural investigation of borate glasses. Case studies of single alkali and alkaline earth borate glasses are presented and highlight the compositional dependence of the borate network and the nature of sites occupied by metal ions. Results of infrared spectroscopy are compared with those of molecular dynamics simulations to help elucidate the nature of the local borate units constituting the sites of metal ions in modified borate glasses. Infrared spectroscopy (IR) is one of the most useful experimental techniques available for structural studies of glasses.(1) For the particular case of glasses modified by metal oxides, infrared is a powerful tool because it leads to structural aspects related to both the local units constituting the glass network and the anionic sites hosting the modifying metal cations. Borate glasses provide an ideal case in comparison to other glass forming systems, to demonstrate the effectiveness of infrared spectroscopy in glass science. First, boron has the smallest mass compared to other network forming elements and thus the main vibrational modes associated with the glass network appear well above 500 cm-1 in the mid-infrared.(2) These network modes are well separated from the metal ion site vibrational modes active in the far infrared region, i.e. below ~600 cm-1.(3–5) Second, borates have a rich chemistry because of the ability of boron to change its coordination with oxygen between three and four and this provides a range of anionic environments that can coordinate the modifying metal ions. For these reasons, the borate glass systems can be considered as prototypes for infrared investigations of the shortrange order structure of the network in the mid infrared, as well as of the metal ion site interactions in the far infrared region of the spectrum. Infrared reflectance spectroscopy is the most suitable technique for glass studies among the various sampling techniques employed in infrared measurements. A key advantage of this technique is the use of the same sample for data acquisition over a broad and continuous frequency range covering both mid and far infrared (~30–5000 cm-1), without the need of changing sample form or thickness, a problem usually ene-mail:
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Physics and Chemistry of Glasses Vol. 44 No. 2 April 2003
countered in transmission measurements. This advantage is combined with the capabilities of modern Fourier transform spectrometers, and the availability of software for proper analysis of the reflectivity data, to lead to the frequency dependent optical and dielectric properties of glass. The spectral profiles obtained from reflectance studies are free of band shape distortions, which are present in transmission spectra because of salt matrix and/or hydrolysis effects, and this allows the use of infrared reflectance for the quantitative investigation of glass structure.(6,7) In this work we review results of mid infrared spectroscopy on alkali borate glasses and present results from our recent studies on alkaline earth borates, with the purpose of examining the effect of metal oxide type and content on the short range order structure of the borate network. The structural findings obtained form mid infrared are compared with those of other experimental investigations including NMR, ultrasonic and neutron diffraction studies. The interactions between metal ions and their sites, and the range of the available metal ion sites, are explored by studying the far infrared spectra of selected alkali and alkaline earth borate compositions. To shed light on the microstructure of modified borate glasses we also present and discuss results from recent molecular dynamics studies on lithium borate glasses. The emphasis is placed here on the type of short range order structures formed as a function of lithium oxide content, on the nature of sites occupied by lithium ions and the corresponding lithium ion site vibrational properties. The results of molecular dynamics simulations are discussed in relation to infrared and NMR studies on the same glass compositions. Experimental Alkali and alkaline earth borate glasses were prepared from stoichiometric mixtures of anhydrous B2O3 and metal carbonates, or metal oxides in the case of magnesium and calcium borates. The thoroughly mixed starting materials were melted in platinum crucibles for about half an hour in the temperature range 1000– 1300°C depending on composition. Glass samples with good surfaces were obtained by splat quenching the melt between two polished copper blocks. Infrared spectra were recorded in the specular reflectance mode on a Fourier transform spectrometer (Bruker IFS 113v). 79
PROC. FOURTH INT. CONF. ON BORATE GLASSES, CRYSTALS AND MELTS 18
M2O-3B2O3
14
-1
Absorption Coefficient (10 cm )
15
3
3
12
9
Li 6
Na
12 10 8
Li 6 4
Na
K
3
K 2
Rb Cs
0 0
200 400 600 800 1000 1200 1400 1600 -1
Wavenumber (cm ) Figure 1. Infrared absorption spectra of M2O–3B2O 3 glasses obtained by the Kramers–Kronig analysis of the specular reflectivity spectra. All spectra, except that of the Cs glass, have been offset to facilitate comparison
Each spectrum is the average of 200 scans at room temperatures and covers the range 25–5000 cm-1 with 2 cm-1 resolution. The reflectance data were analysed by the Kramers–Kronig transformation to yield the absorption coefficient spectra presented in this work. Details on glass forming regions and spectral analysis can be found in earlier publications.(3–5,7–9) Results and discussion Short-range order structure of borate networks Alkali borate glasses Much of the early knowledge on the structure of alkali borate glasses, xM2 O– (1-x)B2O3 where M=alkali, originates from the pioneering works of Krogh-Moe by infrared,(2) Bray and co-workers by NMR(10–12) and Konijnendijk & Stevels by Raman spectroscopy.(13) Early studies on the structure of borate glasses have been reviewed by Griscom.(14) The purpose of those studies was to elucidate the nature and relative population of the borate units building the glass network and thus to provide an explanation for the nonmonotonic variation of physical properties with alkali oxide content, a phenomenon known widely as the ‘boron anomaly’. Addition of alkali metal oxide to B2O3 was found to cause the transformation of neutral borate triangles, BØ3, into charged borate tetrahedral, BØ-4 , where Ø indicates an oxygen atom bridging two boron atoms. For x
