The structures and vibrational spectra of crystals and glasses in the silica-alumina system

Journal of Non-Crystalline Solids 53 (1982) 279-298 North-Holland Publishing Company

279

THE STRUCTURES AND VIBRATIONAL SPECTRA OF CRYSTALS A N D G L A S S E S IN T H E S I L I C A - A L U M I N A S Y S T E M Paul M c M I L L A N Department of Chemistry, Arizona State University, Tempe, Arizona 85287, USA

Bernard P I R I O U ER 60210 "Elements de transition dans les solides", CNRS Bellevue, 1 Place Aristide Briand, 92190 Meudon, France

Received 24 March 1982 Revised manuscript received 12 July 1982

Solar furnace melting and fast-quench techniques have been used to prepare SiO2-A1203 glasses to high alumina content (near 60 mol% A1203), which have been studied by Raman spectroscopy. These spectra may not be simply interpreted. The structures of crystalline compounds in the SiO2-AlzO 3 system are discussed in relation to their vibrational spectra. On the basis of this discussion and other considerations, a structural model for the silica-alumina glass system is proposed, which is consistent with the stable or metastable immiscibilitysuggested along this join. The essential features of this model include a modified silica structure at low alumina content, and "structure-broken" regions at high alumina compositions, with silicon in tetrahedral coordination, but ahiminium assuming a variety of bonding geometries. These are proposed to include aluminate tetrahedra with higher polymerization than simple corner-sharing, and less well-defined polyhedra of higher average coordination number.

1. Introduction The v i b r a t i o n a l a n d molecular structure of aluminosilicate glasses poses a m a j o r p r o b l e m of c u r r e n t interest to solid state science. Here we present results of a R a m a n spectroscopic study of glasses a n d crystals in the b i n a r y system S i O 2 - A 1 2 0 3 ( S - A ) , a n d discuss models for the behavior of a l u m i n i u m in the glass structure. The S - A b i n a r y is highly refractory, which has limited previous structural studies o n these glasses. K a t o [1] e x a m i n e d the R a m a n spectra of t r a n s p a r e n t S - A glasses with up to 6 mol% of a l u m i n a , a n d c o n c l u d e d that the added a l u m i n a had n o effect o n the spectrum of silica glass. However, K a t o ' s spectra for vitreous silica a p p e a r u n u s u a l when c o m p a r e d with those of other workers: the " V V " spectra quoted by K a t o are identical to the VH spectra of other studies, while his " V H " spectra correspond to very weak VV polarized spectra. T h e present study suggests that the most i m p o r t a n t modifications to the silica 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d

280

P. McMillan, B. Piriou / Crystals and glasses in the silica- alumina system

spectrum on initial addition of A1203 are only apparent in the VV spectra, so were not observed by Kato. Mysen and co-workers [2,3] obtained the Raman spectrum of a glass with 6 mol% A1203. They show only the high-frequency portion (above 900 cm i) of its unpolarized spectrum, and have fitted four components, at 950, 1060, 1120 and 1200 cm -1 to the band contour. Their deconvolution scheme appears rather arbitrary, and gives the weak 950 cm i band an exaggerated importance. These authors used the presence of this band, and the more obvious band at 1120 cm-~, to suggest that aluminium occupies a non-tetrahedral site as a "modifier" cation in this glass. The present study used fast-quench solar furnace techniques [4] to prepare S-A glasses to much higher alumina content, allowing a more complete view of systematic changes in spectra with composition.

2. Experimental The sample preparation and characterization techniques have been described in detail elsewhere [4-6]. A high-purity sample of vitreous silica ( S - N Q ) was kindly supplied by the Electro-Quartz Company, France. The crystal of natural kyanite was provided by P. Bariand of the Mineralogical Collection (Universit~ de Paris VI). All other glass and crystalline samples were prepared from oxide mixes at the French solar furnace (CNRS Odeillo). Glasses with up to nearly 30 mol% alumina were prepared by both normal quench (NQ: cooled in air; quench rate - 1 0 2 ° C S-1) and "super-quench" (SQ: splat cooling; quench rate - 1 0 6 ° C s - i ) , while glasses with more than 30 mol% alumina could only be prepared by super-quench. Temperatures at the sample could not be controlled or measured, but are estimated at 2000-2500°C (see [4-6]). Crystalline mullites were prepared by normal quench, and also by superquench for some high-alumina samples. Corundum was crystallized from an alumina melt. All samples were examined optically before Raman study, and crystalline phases were identified by X-ray diffraction. Chemical analyses were obtained from point count traverses using the Cameca MS-46 electron microprobe in the Chemistry Department at Arizona State University (ASU). Mean results of these analyses are given in table 1. The index H indicates the sample homogeneity [5,6]. A value of H much larger than 1 suggests that the sample is inhomogeneous in that oxide component. Samples found to be inhomogeneous were analyzed for their bulk composition by X-ray fluorescence, using the Phillips PW 1410 spectrometer in the Chemistry Department at ASU. Suitable samples were polished for 90 ° Raman scattering. Super-quenched samples generally consisted of thin plates, beads and fibers of glass which were extremely friable, and difficult to mount for oriented Raman spectroscopy. The spectra of these samples are partially or completely unpolarized. Samples were excited with the 4880 .~ line of an Ar ÷ laser. Raman spectra were

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system Table 1 Sample compositions Sample

mol% oxide

(a) glasses

A1203

SiO 2 NQ f) SiO 2 SQ SiO 2 SQ A-24 NQ A-24 NQ A-24 SQ A-29 n) NQ A-29 i) NQ A-29 j) NQ A-48 n) SQ A-49 i) SQ A-48 J) SQ A-59 SQ

0.2 0.7 g) 0.3

method c) SiO 2 100.0 e) 99.8 0.7 99.7

:~ d)

,,~ b)

98.7 1.7 96.7

homogeneity a) Al zO3

EMP

22

16

1

24

2

1

21

1

1

EMP

31

5

2

EMP

18

20

15

XRF

23.7 0.8 23.1

76.2 0.8 76.9

98.1 2.1 101.7

EMP

24.1 0.6 29.3

75.9 0.6 70.7

98.3 1.9 99.8

25.9 2.2 59.4 14.3 48.3

74.1 2.2 40.6 14.3 51.7

99.3 2.3 94110 99.2

48.3 0.5 58.6 5.2 59.0 0.4

51.7 0.5 41.4 5.1 41.0 0.4

98.9 0.9 100.5 4.1 101.2 1.3

EMP

23

1

1

EMP

11

7

5

EMP

24

1

1

45.4 55.0 66.5 66.0 77.2

54.6 45.0 33.5 34.0 22.8

98.9 99.8 101.2 98.3 99.5

XRF XRF XRF XRF XRF

XRF EMP XRF

XRF

(b) crystals A-45 A-55 A-66 A-66 A-77 ~) b) c) d) ¢) f) g) h) i) J)

NQ NQ NQ SQ SQ

S~O2

Homogeneity: index H calculated as in ref. [5]. See text. Sum of weight per cent oxides. Analysis method: EMP - electron microprobe, XRF - X-ray fluorescence. Number of points in EMP traverse. High-purity sample from the Electro-Quartz Company, France. Quench conditions: NQ - normal quench, SQ - super quench. Two standard deviations (20) of EMP analyses. Bulk sample. Matrix. Inclusions.

28

282

P. McMillan, B. Piriou / Crystals and glasses in the silica-alumina system

obtained with a Coderg pHo double monochromator at CNRS Bellevue, and with a Coderg T800 triple spectrometer in the laboratory of J. Etchepare at ENSTA, Palaiseau. Glass spectra were run using a 4 cm-~ slit width, while crystal spectra were obtained with 2 cm I slits. Further details of the experimental procedure may be found in refs. [5] and [6].

3. Crystalline compounds in the S - A system Before considering the Raman spectra of S - A glasses, it is useful to discuss the structures and vibrational spectra of crystalline phases along the silica-alumina binary.

3.1. s~o~ The phase diagram of pure silica is reviewed in [7]. Below around 90 kbar, the stable crystalline polymorphs are based on fully polymerized networks of corner-shared SiO4 tetrahedra [7,8]. A large number of both experimental and theoretical vibrational studies have been carried out for these structures (see refs. [5] and [6]). Although a detailed discussion is not necessary here, it is useful to distinguish three major regions in the spectra of these tetrahedral polymorphs which correspond to three general types of vibration. The highfrequency region, between 1000 and 1250 cm- l, corresponds to silicon-oxygen stretching motions, while bands around 800 cm i are due to vibrations of silicon about bridged oxygen, and the 400-500 c m - i region is mainly associated with vibrations of the bridged oxygens. At pressures greater than around 90 kbar, the stable form of silica is stishovite, with silicon in octahedral coordination. The infrared spectrum of stishovite has been reported [9,10]. The highest-frequency bands occur between 850 and 1000 c m - i , and are probably associated with silicon-oxygen stretching motions. These are at lower frequency than found in the tetrahedral silica polymorphs, consistent with longer silicon-oxygen distances in the octahedral silicate groups. The Raman spectrum of stishovite was investigated by Nicol and co-workers [11], who observed two low-frequency bands, but could not obtain a spectrum above 250 cm-l. Finally, another modification of silica prepared by vapour-phase deposition has been reported [12]. This fibrous polymorph is proposed to be based on chains of SiO4 tetrahedra, linked by edge-sharing of the tetrahedral groups. 3.2. Al20 ~

The only thermodynamically stable crystalline form of alumina is corundum (a-A1203), with a hexagonal close-packed oxygen structure and aluminium in octahedral sites [7,8]. Its polarized Raman and infrared spectra have been studied by a number of workers [13-15], and Iishi [16] has carried out a

P. McMillan, B. Piriou / Crystals and glasses in the silica- alumina system

283

vibrational calculation. None of its modes may be simply related to internal vibrations of the A106 octahedra groups, but the bands between 600 and 750 cm-~ involve mainly aluminium-oxygen stretching motions with major displacement of oxygen, while those between 300 and 500 cm i are mainly associated with aluminium displacements. A considerable number of metastable modifications of alumina are known [7], all of which have structures with aluminium in both four- and six fold coordinations [17,18,29,30]. It has been suggested that polymorphs prepared at higher temperature have a larger proportion of fourfold coordinated aluminium [18]. The powder infrared spectrum of y-A1203 has been reported [19], and shows strong bands near 820, 750 and 592 c m - t. That at 820 c m - ~ occurs as a shoulder of varying relative intensity in MgO-A1203 spinels [19], and may be related to vibrations of tetrahedral aluminate units (B. Piriou, unpublished results). The powder IR spectrum of 0-A1203 also shows strong bands near 830 and 580 cm i, and weaker bands in the 700-800 c m - i region [20]. 3.3. A I , S i O 5

At this composition are found the three polymorphs sillimanite, andalusite and kyanite [21,22]. Only sillimanite appears on the one-atmosphere liquidus, melting incongruently to mullite and liquid. Below around 850°C, andalusite is the stable low-pressure phase, while kyanite appears at higher pressures. The crystal structures of these polymorphs have been studied in some detail [22,23]. Sillimanite may be described as edge-shared ribbons of AIO6 octahedra aligned in the c-direction, cross-linked by corner-shared A104 and SiO4 tetrahedra. Four A1 atoms per unit cell are in octahedral sites, while four are tetrahedral. The low-temperature polymorph andalusite is also based on edge-shared A106 ribbons, but cross-linked by irregular A105 polyhedra and SiO4 tetrahedra. There is one A106 octahedron per fivefold coordinated aluminium in the structure. Finally, the most dense high-pressure phase, kyanite, has a network of A106 octahedra and SiO4 tetrahedra. These A12SiO5 polymorphs are commonly classed mineralogically as orthosilicates, with isolated SiO4 units embedded in an aluminate matrix. This definition is useful for a simple consideration of their high-frequency vibrational spectra, which may be compared with those of the olivines and related orthosilicates. The highest-frequency modes of these structures are derived from the p~ and s,3 internal vibrations of the isolated SiO 4 unit. In order to apply this to the aluminosilicates, it must be assumed that the SiO4 stretching vibrations may be distinguished from those of the aluminosilicate framework. This implies that aluminium acts only as a perturbation on these silicate stretching vibrations, as do the metal cations in other orthosilicates. There is some justification for this, as discussed elsewhere [5,6,24]. The unpolarized Raman spectrum of kyanite is shown in fig. l, along with the Raman-active modes found by other workers for andalusite [24] and sillimanite [25]. The modes for all three polymorphs are collected in table 2. The triclinic kyanite

284

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system

Table 2 R a m a n b a n d s for A I 2 S i O 5 p o l y m o r p h s a n d mullite K y a n i t e a)

A n d a l u s i t e b) BIg

As 180 w 210 w 224 w 281 w 295 w 307 m 330m 364m 375 w 381 w 389 m 397 m 409m 423 w 442m 452 w 493 s 514 w 592 w 609 w 640w 658 w 673 w 753 w? 895 w , b 906m, b 955m, b 966 sh 1004 w, b

Sillimanite f) Bzg

B3g 187 e) 203

192 242

243 ¢) 277

291

Mullite a,g)

237 s 295 ~

322 360

337 362 ¢)

325 d)

310 325 ~

310 s

310 s, asym

390 393 w 410 d~

452

474 515 550 c)

553 573

490 d)

405 420

425 w

487

462m 489 w

605

410 s

599m 610 w

629 649

665

721 c)

718 798 834 921 951

833 c~ 891 953 c)

714 792 877 912

m w, b s m

710 w 880 sh

931 980

967m, b

960 s

1044 1065 1113

1040 sh 1132 w 1160 w

A b b r e v i a t i o n s : s - strong; m - m e d i u m ; w - weak; b - b r o a d ; sh - shoulder; a s y m - asymmetric. All frequencies are in c m - t. a) This work. b) F r o m ref. [24]. ~ M a y be parasites from A s spectrum (our comment). d) R e g a r d e d as uncertain by lishi et al. [24]. e) M i s p r i n t e d as 87 c m - 1 in table 3 of lishi et al. [24]. f) F r o m ref. [25]. g) The mullite values are averaged for the various spectra. sample vector, from

was and an

oriented its spectrum

oriented

with

its c-axis

obtained

single-crystal

nearly

by reflection, study

[24],

parallel

to

the

incident

while the andalusite and

the

unpolarized

electric

modes

are

sillimanite

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system

285

spectrum was obtained with a R a m a n microprobe [25]. Both andalusite and sillimanite are orthorhombic, with four SiO4 units of C~ symmetry per unit cell, as for the olivines [26,27]. Eight Raman-active, high1000

500

o

j

u o o o • g r o g a n •4nOlO • o

•

I Sill

•

•

O0

•

•

O•*•

, 1000 I , , , , L J , I 500 , Raman

shift

•

•

o

c m "1

Fig. 1. Unpolarized Raman spectrum of kyanite, and observed Raman bands of sillimanite and andalusite from refs. [24] and [25]. Arrows indicate the major Ag bands of andalusite. (See table 2.)

frequency modes are expected for each, derived from v~- and v3-type silicate stretching vibrations. The symmetric stretch v~ gives rise to one Ag, while v3 asymmetric stretching gives two Ag crystal modes. Each Ag mode is related to a Big mode by the Davydov splitting. Finally, B2g and B3g modes are derived from v3-type stretching [24,26]. These modes may be identified in the spectrum of andalusite [24]. The three A g modes are readily attributed to the bands at 1065, 951 and 921 cm -1, and the B2g and B3g modes to 931 and 980 cm 1 respectively [24]. The highest-frequency Big is observed at 1113 cm l, giving a Davydov splitting of 48 cm-~, large compared with that for the olivines with 10-15 cm-1 [26]. Iishi and co-workers [24] find the other Big modes at 953 and 1044 cm-~. The 953 c m - J band is probably a parasite of the strong 951 c m Ag mode, and it might be better to assign the weak shoulder near 970 cm (ref. [24], fig. 2(c)) as the Big associated with the 921 cm - l Ag mode. The second Big is quoted at 1044 c m - l in table 3 of ref. [24], but seems to occur at significantly lower frequency in their fig. 2(c). However, it is readily seen that the Raman-active phonons of andalusite derived from internal stretching

286

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system

modes of the SiO 4 tetrahedra occur between 921 and 1113 c m - i . Fewer Raman-active bands were observed for sillimanite [25]. The strongest high-frequency mode at 877 c m - I, and the highest-frequency band observed at 1132 cm -I, are probably Ag modes derived from u I and u3 of the S i O 4 tetrahedra. A weaker Big mode is expected at even higher frequency, related to the 1132 cm i Ag by the Davydov splitting. From this, the sillimanite modes derived from silicate stretching motions lie between 877 and at least 1132 c m - i . Finally, kyanite is triclinic with space group P1 and four S i O 4 groups per unit cell, and eight high-frequency Raman modes are expected for silicate stretching vibrations. These form a group between 895 and 1004 c m - i (fig. 1 and table 2). In the olivines and related orthosilicates, the frequency range covered by the silicate stretch vibrations, or the site-group splitting, and the degree of u ~ - u 3 coupling were found to increase with the degree of distortion of the SiO4 units from the tetrahedral symmetry [26,27]. The largest site-group splitting found in these olivines was of the order of 150 cm 1 for Raman-active modes. In sillimanite, the Si-O bond lengths range from 1.574 to 1.645 A, while in andalusite, these lie between 1.618 and 1.645 ,~ at room temperature [23]. The range in andalusite is similar to that for the most distorted tetrahedra in olivines, but for sillimanite is rather larger. This is consistent with the large site-group splitting of at least 255 cm-1 found in sillimanite, compared with 190 cm J for andalusite. This large splitting may also be observed in the high-frequency infrared bands of sillimanite, compared with andalusite [10]. It is not possible to simply compare these with the high-frequency bands of kyanite, of different crystal symmetry. It was noted above for andalusite that the Davydov splitting of u~- and p3-derived SiO4 vibrations was much larger than for the olivines. This may be due to a stronger coupling between SiO4 groups within the unit cell through the A13+ cations, which are more electropositive than metal cations in the olivine structures. Finally, the silicate stretch vibrations in these aluminosilicates occur at generally higher frequency than found in other orthosilicates. This could be due to addition of aluminium-oxygen stretching force constants to the silicon-oxygen stretching force constants, which might result in a higher silicate stretching frequency [5,28]. Consistent with this, Iishi and co-workers found a small component of aluminium-oxygen stretching coupled to silicate stretching vibrations for andalusite [24]. The combination of site-group and Davydov splitting and the perturbation of silicate stretching causes the highest-frequency bands for these aluminosilicates to appear at abnormally high frequencies for silicate structures, which has already been noted for sillimanite [ 10]. The vibrational calculation for andalusite [24] found no vibrations which could be described as internal to the aluminium-oxygen polyhedra, although a band group near 700 c m - ~, which is absent in other orthosilicates, is associated with a large component of aluminium motion. The polarized infrared spectra of andalusite show strong B2u bands at 690 and 673 c m - i , with experimental

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system

287

T O - L O splittings of 65 and 22 c m - i respectively. Iishi et al. [24] assign the higher-frequency mode to p4-type motions of the SiO4 tetrahedra, and the other to movements of aluminium. The vibrational calculation does not reproduce well the observed T O - L O splitting for either band, and we suggest that it might be better to attribute large aluminium displacements to both modes. Similarly, the R a m a n modes at 798 and 700 c m - i, both assigned to ~'4 motions of the SiO 4 groups, are poorly reproduced by the calculation [24], and may have a large component of aluminium-oxygen stretching. Iishi and co-workers concluded that the vibrational structure of andalusite was best described by covalent SiO 4 groups embedded in a more ionic aluminate framework, which is consistent with the present discussion, and supports a vibrational model proposed earlier for glasses in the system SiO2-CaA1204 [5,6].

3.4. Si02-Al203 mullites These form a series of nonstoichiometric compounds with compositions between A12SiO 5 and Al203 to give the major liquidus phase in the S - A binary [21,22,29,30]. Their structure is derived from sillimanite by creation of an oxygen vacancy at the previously shared apex of cross-linking A104 and S i O 4 tetrahedra. Two new tetrahedral sites are formed about this vacancy, in each of which one oxygen now serves as common apex for three cross-linking tetrahedra [22]. A number of polycrystalline mullite samples were obtained by normal and super-quench from aluminosilicate melts in the present study. Slight differences in their X-ray diffraction patterns suggested small compositional variations in these mullites. This could not be verified by chemical analysis, since a number of these mullites were intergrown with a glass matrix, and the crystals were small for accurate electron microprobe analysis. Samples A-45, A-55, and A-66 gave mullites on normal quenching (NQ), while A-66 also crystallized mullite with super-quench (SQ). Sample A-77 gave an inhomogeneous product on both normal and super-quenching. The N Q sample showed a mixture of corundum and high-silica glass, as did regions of the SQ sample. Other parts of the SQ sample gave the mullite spectrum shown here. The bulk compositions of these samples are given in table 1, while their Raman spectra are shown in fig. 2 and table 2. These mullite spectra have extremely broad bands, which is common for crystals with a vacancy structure [31,32]. They have a weak band near 1160 cm i and a strong peak at 960 cm 1, with weak shoulders at 1040 and 880 cm i A broad asymmetric band appears near 710 cm i with another better-defined at 610 cm -1. Two strong bands are observed at 410 and 310 cm i. The 310 band is asymmetric to higher frequency. The band near 700 cm ~ appears to increase in relative intensity for samples with higher bulk alumina content, and is better-defined for sample A-66 N Q than for the super-quenched sample of the same bulk composition. As noted above, additional aluminium occupies a new tetrahedral site in the mullite structure, and it is possible that the 700 c m - I band is related to vibrations of these A 1 0 4

P. McMillan, B. Piriou / Crystals and glasses in the sifica-alumina system

288

groups. Sample A-66 SQ was observed to contain between 30 and 50% glass of high alumina content which is probably responsible for the rising background in its R a m a n spectrum, as discussed later. The 700 cm - ] band for the 1000

500

0

,, 1

mullites

I I

A-77 SQ

A-6~.Q

f/

II.,tI'1'

A-66 SQ ,1~ ~, ,~P,!~¢,tt'i~tl ~t'~ 'I

A-55 NQ

" I /,,

',

.;

/A '

\.

,, f

~v,,~Nji ,,'

s,,1, ,,I,, L.,,IJi,iIj,,, 1000

1'll~J]

\..,, I '~1

, ,I,, II 500

0

Raman shift cm "1 Fig. 2. Unpolarized Raman spectra of polycrystalline mullites, compared with observed Raman bands of sillimanite from ref. [25].

P. McMillan, B. Piriou / Crystals and glasses in the silica - alumina system

289

coexisting mullite is weak, suggesting a composition near sillimanite. On normal quenching, the composition has crystallized entirely to give a higher alumina mullite (A-66 NQ). Also shown in fig. 2 and table 2 are the major Raman bands of sillimanite, from ref. [25]. Some similarities may be observed, and the mullite bands at 1160, 880, 710, 610 and 310 c m - i may correspond to the sillimanite peaks at 1132, 877, 714, 599 and 310 cm 1 consistent with the close relation between their structures. The strong mullite band at 410 cm i has no counterpart in the sillimanite spectrum, nor have the bands at 1040 and 960 c m - I . The highfrequency bands are probably associated with silicon-oxygen stretching, and their appearance for mullite reflects the creation of a new tetrahedral site for silicon in that structure. As noted above, it is tempting to assign the band group near 700 c m - I in sillimanite and mullite to stretching vibrations of tetrahedral A104 groups, but bands are also observed in this region for kyanite, andalusite and corundum, none of which have tetrahedral aluminate units. A number of general conclusions may be drawn from the above discussion of crystal structures on the S-A binary. Silicon remains in tetrahedral coordination to oxygen, except at pressures in excess of 90 kbar. In silica, the SiO4 tetrahedra polymerize by corner-sharing, with one exception, where edge-sharing is reported. Aluminium shows variable coordination behavior, with examples of regular and distorted four-, five- and sixfold coordinated polyhedra, even for pure A1203. These considerations are useful for a discussion of glass structures on the S-A binary, in relation to their Raman spectra.

4. The SiO2-AI203 glass system Samples with bulk composition of up to nearly 30 mol% alumina formed transparent glasses on normal quenching, and their polarized Raman spectra are shown in fig. 3(a). The super-quench technique allowed the preparation of glasses with up to 60 mol% alumina, whose unpolarized or partially polarized Raman spectra appear in fig. 3(b). Comparison of these figures shows that quench rate has no effect on the spectrum of glass A-29 (NQ and SQ), and only a small effect on that of vitreous silica, which will not be considered here. As seen from fig. 3(a), the addition of up to 30 mol% alumina has only a minor effect on the silica glass spectrum. A weak, polarized band is observed to appear near 1100 c m - l (see also fig. 4), with possibly a weak feature around 950 cm-J. This is in agreement with the results of earlier studies [1-3]. The contour of the asymmetric 800 c m - 1 band is slightly changed, while the sharp bands at 500 and 600 cm ~ are suppressed in intensity. The pseudoband near 100 cm-~ [33,34] becomes more prominent with increasing alumina content. This modified silica spectrum may still be recognized in the spectrum of super-quenched sample A-48 SQ (fig. 3(b)). The dominant 430 band forms the broad, asymmetric structure, while the 500 and 600 c m - l peaks appear as weak maxima. The asymmetric 800 band is still present, while the high-

P. McMillan, B. Piriou / Crystals and glasses in the silica- alumina system

290

~

1000

r~-]-- ~

[

[

500 I l,

l l l

I ~

I 1 [ 1 1 1 1

mol %oxide

tool % o x i d e SiO, ~ - - - . .

500

1000

.

S °

AI~O~ !

50

I

VV

\~/~

/

VH

SiO 2 NQ

/

vv+v.

VV+VH A-24 SQ

/ ~

7

/ VV

,

VH "V

i i , I

~'/

A-48 SQ

w,÷v. j,#!¢ il A-59 SQ

, E

,,!,

I

!

1000 Raman

I

I 500

shtft

Ca)

I

I

I

I

~ooo Raman

shtft

5oo cm "1

o

cm "1

(b/

Polarized Raman spectra of high-silica glasses (NQ samples). (b) Raman spectra of S Q glasses to high alumina content. The spectrum of sample A-48 is partially polarized.

F i g . 3. (a) S-A

frequency region consists of a broad, asymmetric band between 880 and 1250 c m - i with its maximum neat 1050 c m - 1. The spectrum is weak compared with the low-frequency pseudo-band, whose tail provides the baseline for the modified silica spectrum as described above. This is clearly seen in the partially depolarized spectrum obtained for sample A-48 SQ, and shown in this figure. The spectrum of sample A-59 SQ is one of the most remarkable obtained in the present study, and shows almost no Raman band structure. The spectrum is extremely weak, and consists of broad bands superimposed on a descending

P. McMillan, B. Piriou / Crystals and glasses in the silica- alumina system

291

tail from the p s e u d o - b a n d at low-frequency. A weak, b r o a d b a n d is observed b e t w e e n 880 a n d 1250 c m - ] with its m a x i m u m near 1050 c m - l , a n d a weaker b a n d is a p p a r e n t near 780 c m - 1. The spectrum was reduced [35,36] to remove the effects of thermal p o p u l a t i o n (fig. 5). The n o r m a l i z e d spectrum shows one b r o a d b a n d covering the frequency range from zero to near 1000 c m - ], with its

Raman shift cm"1 1000

500

r ~ L I I ' I I

U

,1

VV

SiO2 VH

vv

~l

i ~

J I J l1000 tllll

A-29

500

Fig. 4. High-frequency detail of the polarized Raman spectra of S i O 2 and A-29 NQ glasses, compared with the high-frequency detail of the potassium aluminosilicate glass AN-211 (SiO295 : KA102-5) from ref. [6]. The polarized band near 1100 cm- ) is indicated by the arrow.

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maximum in the 500-600 cm-i region. The weak 780 and 1050 cm-l bands, although broad, are much better defined than this near-continuum of vibrational modes. We consider that two major types of structural effect are responsible for the

OBSERVED

(vv

INTENSITY + VH)

I--I

i

I

L

I

500

1000 Raman

SAMPLE

shift

cm "1

A-59

N O R M A L I Z E D INTENSITY (vv + VH)

0

500

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1000

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c m "1

Fig. 5. Raman spectrum of high-alumina A-59 SQ glass from fig. 3(b), normalized to remove the temperature dependence.

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above evolution of R a m a n spectra with composition along the S - A glass join. One gives rise to the "modified silica" spectrum observed at higher silica content, and the other is reponsible for the near-continuum of vibrational modes appearing as the alumina content is increased.

4.1. The modified silica spectrum Initial addition of A1203 to vitreous silica causes only minor modification of its Raman spectrum, as noted in earlier studies [1-3,5]. This modified silica spectrum changes little on further addition of alumina, but decreases in relative intensity, to disappear near 50 mol% A1203. Our interpretation of the Raman spectrum of vitreous silica and its relation to the glass structure has been considered elsewhere [6]. Although the detailed interpretation of its vibrational spectrum is still controversial, some general agreement has been reached, similar to the earlier discussion of vibrations of the crystalline tetrahedral silica polymorphs. The strong, polarized mode near 450 c m - l in the glass (fig. 3) may be described as a symmetric motion of the bridged oxygens in the planes bisecting the S i - O - S i bonds, with little or no silicon displacement. The band group near 800 cm i is due to symmetrical motion of silicons about their bridged oxygen probably with some associated oxygen displacement. The high-frequency bands are associated with bond stretching motions of silicon relative to oxygen, involving both silicon and oxygen movement. The initial addition of A1203 to silica glass causes the appearance of a polarized band near 1100 c m - i , superimposed on the depolarized bands of silica at 1060 and 1200 c m - l , and perhaps a weaker band near 950 cm (figs. 3 and 4, and refs. [1-3,5]). The 1100 band is similar to that observed for a silica glass with 5 mol% KA102 component (fig. 4), and other alkali and alkaline earth aluminosilicate glasses with high silica content [1,2,5,6,37]. This band has been assigned to the symmetric stretch vibration of a silicate tetrahedron with three apices bridged to other silicate tetrahedra, and one oxygen apex shared with an aluminate polyhedron [5,6]. It was proposed that this stretch vibration is best described as a motion of a silicate tetrahedron with one "non-bridged" oxygen, perturbed by interaction between the nonbridged oxygen and aluminium. This was written as the vibrating unit = S i ( O A I ) [5,6]. A similar interpretation is suggested for the polarized 1100 band in the present S - A glasses. The weak band observed near 950 c m - i could correspond to a stretching vibration of =Si(OA1) 2 molecular units, with two silicate tetrahedral apices bound to aluminium [5,6]. As the alumina content is increased, the region covered by these predominantly silicate stretching vibrations is observed to extend to progressively lower frequency, to give a broad band from 880 to near 1250 c m - l for sample A-59 SQ. We suggest that this is due to the appearance of less-polymerized silicate species (-Si(OA1) 3 and perhaps Si(OA1)4 ) as the alumina content is increased. Some changes may also be observed in the band group near 800 c m - l of vitreous silica as A1203 is added. The band broadens, becomes more asymmet-

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tic, and moves to slightly lower frequency with increased alumina. A weak band remains in this region at high alumina content, even when the modified silica spectrum may no longer be recognized. As discussed earlier, there is no general agreement on the precise atomic displacements responsible for this band in the silica spectrum, but they involve mainly a symmetric motion of two silicons about their bridged oxygen. It is expected that such modes would be modified as the symmetric of the S i - O - S i arrangement disappears as AI is substituted for Si. However, previous work on other glass systems [2,5,6] and the earlier discussion of crystalline S - A compounds suggest that modes due to aluminium-oxygen stretching motions might also be expected in this region, which further complicates its interpretation. Finally, the 450 cm-1 band of vitreous silica is observed to broaden as alumina is added, and moves to slightly higher frequency. It has been suggested that the transverse oxygen vibrations discussed earlier for S i - O - S i in silica occur at higher frequency for S i - O - A l linkages [5,6]. This would be consistent with the formation of S i - O - A l linkages in the present glasses, as A1203 is added to silica. Note that the coordination of aluminium involved in such linkages is unspecified, and is discussed below. Since the initial addition of A1203 to silica has little overall effect on its R a m a n spectrum, it is suggested that the structure of the modified silica discussed above is similar to that of pure silica. The changes in the Raman spectrum are consistent with formation of S i - O - A I linkages in ~Si(OA1) and possibly ~Si(OA1)2 units. The aluminium in these groups may either substitute tetrahedrally for silicon, or have a nontetrahedral site. For samples on the SiO2-Al203 join, there is insufficient oxygen for a fully-polymerized, cornershared network of SiO4 and AIO 4 units. If the added aluminium is in tetrahedral coordination, the modified silica structure must have oxygen vacancies, or have tetrahedra more polymerized than corner-sharing, by edge- or facesharing, or by sharing of one oxygen apex between more than two tetrahedra. The present interpretation of the Raman spectra of the high-silica S - A glasses can not distinguish between these possibilities, and cannot be used to determine the coordination of aluminium. It is noted that a number of studies have been carried out on crystalline quartz, with aluminium as a substitutional impurity [38-40]. These suggest that AI 3+ may substitute for Si 4+ o n tetrahedral sites, to give a defect tetrahedral aluminate site associated with a hole. It is possible that this behaviour may also occur in SiO 2 glass on initial addition of A1203 component, which would be consistent with the observed slight modifications to the silica spectrum for high-silica S - A glasses. 4.2. Loss of band structure at high alumina content

In general, silicate glasses have well-defined R a m a n band structure associated with internal modes of silicate molecular units [1-3,5,6,26,32-37]. Within selected compositional ranges, aluminate and aluminosilicate glasses also show

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well-defined bands due to internal aluminate and aluminosilicate vibrations [2,3,5,6,36]. Although these bands are broad compared with internal mode vibrations in crystalline aluminosilicates [24-27], none are nearly as broad as the band observed near 500 c m - l in the present S-A glass series (figs. 3(b), 5). The Raman band width is controlled by a number of factors, including the range in molecular geometry for a particular vibrating unit, and the efficiency of the vibrational coupling to other units. Band broadening in glassy systems relative to corresponding crystal structures is not well understood, but must be partly due to the increased range in molecular geometry in the glass, and orientational disorder reducing the coherence of vibrations between adjacent units. The extreme broadening observed in the high alumina S - A glasses A-48 and A-59 suggests either destruction of short-range order, or greatly reduced vibrational coherence between adjacent groups, or some combination of these. Although our knowledge of the dynamical behavior of disordered systems is not yet sufficient to fully explore this problem, we propose a number of structural models which might be consistent with the observations. Aluminium is known to substitute tetrahedrally for silicon in aluminosilicate crystals, and most of the structural studies on aluminosilicate glasses to date have been consistent with tetrahedral aluminate units [2,3,5,6,41,42]. However, for compositions along the SiO2-AI203 join, insufficient oxygen is available for both aluminium and silicon to be tetrahedrally coordinated, with only corner-shared tetrahedra. In the crystalline SiO2-A1203 compounds, this is expressed by an increase in the average coordination of aluminium as the aluminium content is increased. However, although the aluminium in pure corundum is sixfold coordinated, a number of metastable aluminas are known with substantial tetrahedral aluminium, as discussed earlier. If the ideal formula for a fully-polymerized network of corner-shared tetrahedra is TO 2, as in SiO 2, then the stoichiometry of alumina, A10~. s, is not far from this requirement. In a system with no crystal structural constraints, a large proportion of aluminium could be tetrahedral, even at high alumina content. It is of interest that Oka and coworkers [43] have obtained X-ray radial distribution functions for amorphous anodic A1203 films, which they concluded to have structures with a distribution of four-, five- and sixfold coordinated aluminate polyhedra. Secondly, the earlier discussion of the binary aluminosilicate crystals showed that while aluminium displayed variable coordination behavior, silicon remained tetrahedral throughout. In the present high alumina glasse~, (A-48 and A-59: fig. 3(b) and (5), the broad band at high frequency covers the region expected for silicon-oxygen stretching vibrations, where silicate tetrahedra are coordinated to aluminium [2,3,5,6]. This suggests that silicon also remains tetrahedral in the S - A glasses, and we propose that any change in coordination or coupling behavior required by the bxygen deficiency in these S - A glasses is restricted to the aluminate units. As discussed above for the modified silica structure, aluminium may remain tetrahedral, either by edge- or face-sharing or by coordination of oxygen to more than two tetrahedra, or may change coordination. Although tetrahedral

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aluminium with oxygen vacancies may be an important feature of the modified silica structure with its highly-constrained network, it is unlikely in the high-alumina glasses. Highly-condensed aluminate tetrahedra might lead to enough geometric distortion and loss of vibrational coherence to cause the observed loss of band structure, and is a possible model for aluminium behaviour in these high-alumina glasses. Finally, the average aluminium coordination may increase with alumina content, as observed in the crystals. This may occur continuously, over a range of non-integral coordinations, or discrete populations may exist. Either might explain the observed loss of band structure. A range of non-integral coordination would imply a distribution in molecular geometry and a reduction in average short-range order. Discrete populations of individual polyhedra would have high short-range order, but could have considerable orientation disorder between polyhedral units, with resulting loss of vibrational coherence. A number of molecular dynamics calculations have recently been carried out for "tetrahedral" silicate and fluoroberyllate liquids and glasses [44-48], which predict the occurrence of both transient and static fivefold coordinated silicon and beryllium. In recent pressure-simulation experiments [48], the average static coordination of silicon was found to increase smoothly towards higher coordination, and no discrete populations were observed. We suggest that the coordination behavior of aluminium in these high-alumina S - A glasses is similar, with a range of coordination polyhedra increasing in average coordination with alumina content. We expect these polyhedra to be interrelated by distortion parameters, rather than discrete populations of well-defined polyhedral groups.

5. Conclusion

We propose the following structural model for the SiO2-AI203 glass series. At low alumina content, the silica network is preserved, and aluminium probably resides in a tetrahedral site associated with an oxygen vacancy. This cannot continue to high alumina content, and structures with highly condensed aluminate tetrahedra and aluminate polyhedra of higher average coordination will appear. These polyhedra have non-integral coordination, derived initially from distortion of tetrahedral units, which will increase continuously with alumina content. The resulting glass network is much less well defined than the modified silica discussed above. It is noted that the Raman results may not be used to determine the coordination of aluminium, neither in the modified silica, nor in these structure-broken regions. We suggest that the modified silica units may show a tendency to cluster in glasses where poorly-defined structure-broken regions have started to form. The modified silica network may only stabilize up to a limiting concentration of A1203, after which excess alumina forms the structure-broken aluminosilicate network with high alumina content. In the present study, the high silica glasses A-24 and A-29 were found to be inhomogeneous, with lensoid inclu-

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sions whose composition was near 60 mol% alumina. These inclusions were all completely crystalline, but their shape could indicate that these were unmixed droplets in the liquid state. A number of previous studies have suggested the presence of one of more miscibility gap in the S-A glass system [49-51 ], with poles near 10-20 mol%, and 50-60 mol% alumina. This would be consistent with unmixing of "modified silica" and "structure-broken" regions, as suggested from the present study. We thank the National Science Foundation (Grants EAR-780995402, INT7926523 and INT-8006965 to J.R. Holloway and A. Navrotsky at ASU), the French CNRS and the program PIRPSEV for funding this project. Jean-Pierre Coutures supervised the preparation of samples at the solar furnace, while Jean Etchepare kindly allowed the use of his Raman laboratory. Helpful discussions were had with J. Holloway, A. Navrotsky and C. Capobianco at various stages of the interpretation, and we thank A. Navrotsky for her comments on the original manuscript which was improved by an anonymous review. Samples were analyzed at the electron microprobe and X-ray fluorescence facilities at ASU, with the expert help of J. Clark and A. Yates, and the manuscript was ably typed by N. Dagon and M. Palitz.

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