J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 5 ( 1 9 8 6 ) 965 968

Infrared and X-ray diffraction studies of TiO2-GeO2 glasses

M. N. KHAN Department of Physics, University College of Bahrain, Bahrain

A. E. M O H A M E D - O S M A N Department of Physics, Brunel University, Uxbridge, Middlesex, UK

Transition metal glasses consist of inorganic oxides containing significant amounts of transition metal ions which may enter the glass structure in at least two different valency states. We have already reported [1 5] various properties of germanate glasses of V2Os-GeO2, GeO2-TeO2, Na20 GeO2-TiO2 and NazO-GeO2 systems.

The annealing of glass over extended periods of time is known to affect such properties as colour and structure. With the increased use of glass as a practical material in electronics it is of the greatest importance to specify exactly the mode of preparat ion and anneal- ing of the glasses used before attempting practical application. These remarks apply particularly to those glasses which contain transition metal ions.

In this work, we have carried out measurements of infrared absorption spectra and crystallization behav- iour in order to study (i) coordination of Ti 4+ ions in TiO2-GeO2 glasses, (ii) whether the coordination state of Ge 4+ ions would change on addition of TiO2, and (iii) the effect of heat treatment on the parameters describing the course of crystallization.

Binary TiO2-GeO2 glasses were prepared from analytical-grade materials, melted in alumina cru- cibles. The melting temperature varied from 1500 to 1560°C for different glasses. The melts were stirred occasionally using an alumina rod, and were finally poured on to a clean stainless steel plate. The clear TiO2-GeO2 glasses containing up to about 5 mol % TiO 2 resulted. Two disks of each composition were cast, one on a stainless-steel plate which was at room temperature (unannealed sample) and the other disk was cast on a stainless steel plate and introduced to a furnace which was already at a desired temperature and, after various lengths of time, was then switched off to cool down to room temperature. The infrared absorption spectra were measured using the KBr pellet technique on a double-beam grating spectro- photometer, a purging device being attached to the instrument to maintain the atmosphere in the instru- ment free from moisture and CO2. The instrument was calibrated using a standard polystyrene sheet and the absorption bands were reproducible to + 2 cm -~ . Comparison of the OH stretching bands suggests that the water contents of the glasses were less than 0.01%.

Heat treatment was applied to several glasses in order to introduce a tendency towards ordering and even crystallization. The examination of micro- structural changes after the heat treatment was conduc- ted by means of a Cambridge stereoscan 250Mk 2 scanning microscope. Furthermore X-ray diffraction

patterns were also obtained in order to provide definite evidence of crystallinity. The devitrified glass samples were finally powdered and the X-ray diffrac- tion patterns were recorded with a Philips-type PW 1051 diffractometer using nickel filtered CuK~ radiation at 2 x- or 1 ° (20)min -~ from 20 to 120 °.

Infrared spectra of the TiO2 GeO2 glasses are shown in Fig. 1. Assignment of each absorption band can be found in Nakamoto [6]. Basically, the absorp- tion peak at 872 c m - 1 is assigned to the antisymmetric stretching mode involving bridged oxygens (i.e. G e - O - G e bond). A small hump around 817cm ~ is observed in the glasses containing TiO2. The pre- cipitation of hexagonal GeO2 crystal in which some portion of Ge 4+ ions are replaced by four-fold coor- dinated Ti 4+ ions and the similarity of the infrared spectra of the crystalline state as well as in the glassy state provide sufficient justification for the conclusion that Ti 4+ ions in the TiO2-GeO2 glasses are four-fold coordinated by oxygens. This is in agreement with the observation of Sakka and Kamiya [7] who found the linear relation between the change of lattice constants and TiO2 contents, indicating that Ti 4+ ions substitute for Ge 4+ ions in the a-quartz-type GeO2 and that Ti 4+ ions are also in four-fold coordination in that crystal.

Murthy and Kirby [8] have reported that the Ge 4+ ions in GeO2 glass change coordination state from four- to six-fold when alkali oxides are added. The infrared peak at 872cm I due to the Ge O - G e bond has been shown to shift towards lower wavenumbers on addition of an alkali oxide at a rate of about 100cm -l for 1 0 m o l % alkali oxide. As shown in Fig. 1 no new absorption bands are found with increasing concentration of TiO2 which suggests that the coordination state of the Ge 4+ ion from four to six would not change on addition of TiO2.

In order to study the crystallization kinetics the glasses studied in this system were heat treated for various lengths of time at three different temperatures: 350, 700 and 1000 ° C. The heat treatment was also given for various lengths of time at a fixed temperature of 1000 ° C. The heat treatment at 1000° C for 2 h did not show any appreciable change in infrared spectra. However, new peaks which were not observed in the spectrum of the untreated glasses and glasses treated for 2h, appeared at about 514 and 584cm -~ for 4h and 515, 547 and 584cm 1 wavenumber for 8h in the infrared spectra of the glasses containing 5mo1% TiO 2 as depicted in Fig. 2. The appearance and the growth of these infrared absorption peaks suggest the

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Figure 1 The infrared absorption spectra of the TiO~ GeO 2 glasses.

Figure 2 The infrared spectra of TiO 2 GeO 2 glasses containing 5mo1% TiO 2 heat treated at 1000 ° C for different times.

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Figure3 A scanning electron micrograph TiO 2 GeO 2 glass annealed at 1000~'C for 8 h

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Figure 4 Powder X-ray diffraction patterns for TiO 2 GeO 2 glasses for (a) unannealed glass, (b) annealed at 350 ° C, (c) annealed at 700 ° C, (d) annealed at 1100 ° C, (e) starting crystalline powder mix- ture of TiO 2 GeO 2.

possibility of some type of microstructural change occurring after heat treatment. In order to detect these microstructural changes, scanning electron micro- graphs were taken on the glass sample, which was annealed at 1000° C for 8 h. One of the typical micro- structures is shown in Fig. 3 at a magnification of × 36 000, in which some distinct droplets can obviously be seen. The structure has morphology resembling spherical "modules" at selected sites throughout the microstructure, strongly implying their crystalline nature. It appears from the above that observable vestiges of crystallization occur after 8 h at 1000 ° C.

Despite the evidence from infrared spectra and scanning electron micrographs suggesting that crystal-

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Figure 5 Powder X-ray diffraction patterns of TiO 2 GeO 2 glasses heat treated at 1000 ° C for different times. (a) Untreated glass; (b) treated for 2 h; (c) 4 h; (d) 8 h; (e) 24 h.

lization occurred after heat treatment, it is still necess- ary to provide some direct evidence in order to prove that the petal-like particles which were observed are indeed of a crystalline nature. This crystallization is

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detected by X-ray diffraction (XRD) patterns. Fig. 4e shows the X-ray diffractogram of starting crystalline powder mixture of TiO2 + GeO2 containing 5 mol % TiO2, which is almost a superposition of the diffracto- grams obtained individually from TiO2 and GeO2. Comparison of the powder XRD patterns of the glass sample in Fig. 4 indicates the presence of additional peaks in the XRD of Fig. 4d. This probably shows that the additional peaks are due to the crystallization of a new phase from the glass. It is clear from the XRD of Figs. 4a to c that no appreciable new peaks are observed, indicating no change of phase at these temperatures. In the following we attempt to justify the change of phase from amorphous to crystalline in Fig. 5, which in turn justifies the results obtained in Figs 3 and 4.

The powder X-ray diffraction patterns of heat- treated glasses for various lengths of time at a fixed temperature of 1000°C are illustrated in Fig. 5 and analysis of the patterns indicates that the new peaks began to emerge in Fig. 5b. These peaks subsequently increased in magnitude with increasing annealing time. Annealing at an even higher temperature for 24 h for the same glass resulted in a remarkable growth of these peaks as depicted in Fig. 5e. The appearance and growth of these peaks suggest the possibility of some type of microstructural changes. This supports infra- red and SEM measurements.

On the basis of the infrared absorption spectra, it was found that Ti 4+ ions in the TiO2-GeO2 glasses are four-fold coordinated. No new absorption bands are found with increasing concentration of TiO2, which suggests that the coordination state of Ge 4+ ions would not change on addition of TiO2. This observa- tion is consistent with the results of Sakka and Kamiya, for the same glass system.

Crystallization of TiO2-GeO2 glass containing 5 mol % TiO2 occurs after annealing the specimens at a temperature of 1000 ° C. The distinct droplets, petal- like crystalline particles, appear in the heat-treated glass. These particles also contribute to the infrared absorption spectra with the appearance and growth of new peaks.

M.N.K. thanks Drs D. Holland and M. M. Ahmed for their hospitality and help with infrared obser- vations during his stay at Warwick University. The authors also thank Evelyn Thompson, A. Reynolds and S. Pearce for assistance with X-ray diffraction and microscopy, respectively, Professor C. A. Hogarth of Brunel University for discussions, Mukhtar A1-Hashimi for preparing the figures and Miss Maryam A1-Kadi for typing the manuscript.

References 1. M. N. KHAN,

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E. E. KHAWAJA, A. A. KUTUB and C. A. HOGARTH, Int. J. Electron. 56 (1984) 395. M. M. AHMED, C. A. HOGARTH and M. N. KHAN, J. Mater. Sci. 19 (1984) 4040. M. N. KHAN, D. SAVE and C. A. HOGARTH, Int. J. Electron. 55 (1983) 795. M. N. KHAN and E. E. KHAWAJA, Phys. Status Solidi (a) 74 (1982) 273. M. N. KHAN and E. E. KHAWAJA, J. Physique (1982) C9-319. K. NAKAMOTO, "Infra-red Spectra of Inorganic and Co- ordination Compounds" (Wiley, New York, 1963). S. SAKKA and K. KAMIYA, J. Physique (1982) C9-235. M. K. MURTHY and E. E. KIRBY, Phys. Chem. Glasses 5 (1964) 144.

Received 18 February and accepted 14 March 1986

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