Vibrational investigation of lithium metaborate-metaaluminate glasses and crystals

]OURNALOF

ELSEVIER

Journal of Non-Crystalline Solids 217 (1997) 278-290

Vibrational investigation of lithium metaborate-metaaluminate glasses and crystals G.D. Chryssikos *, M.S. Bitsis, J.A. Kapoutsis, E.I. Kamitsos Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Ave., Athens 11635, Greece

Received 31 December 1996; revised 29 April 1997

Abstract

Lithium metaborate-metaaluminate glasses xLiA102 • (1 - x)LiBO 2, (0 _
I. Introduction

Additions of alumina, a conditional glass-former, to alkali or alkaline earth borate networks produce glasses over broad composition ranges [1-4]. Of particular interest is the fact that A120 3 can induce or enhance the glass forming ability of metaborate melts [5,6]. There have been numerous structural investigations of modified boroaluminate glasses. Early ~lB and 27A1 N M R measurements by Bishop et al. [2] and Gresch et al. [3] on calcium and sodium boroaluminate glasses (CaBAl and NaBA1, respectively) agreed that increasing the substitution of boron by aluminum results in a decrease of N 4, the fraction

* Corresponding author. Tel.: +30-1 724 9483; fax: +30-1 724 9104; e-maih [email protected].

of four-coordinated boron atoms in the glass. Yet, these studies disagree on the coordination environment of the aluminum atoms. The presence of aluminate octahedra is indirectly inferred in CaBAl glasses [2], but is considered 'extremely improbable' in their NaBAI analogues [3]. A pioneering Raman investigation of potassium boroaluminate glasses by Konijnendijk and Stevels [7] suggested that aluminum participates to the network as A104 (0, oxygen atom bridging two network forming cations). The recent studies on alkaline earth boroaluminates by Bunker et al. [8,9] claim the existence of four-, fiveand six-coordinated aluminum, with relative abundance depending on composition. At the heart of these discrepancies is the fact that the various network polyhedra are chemically bonded to each other, and thus their manifestation in spectro-

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G.D. Chryssikos et al./ Journal of Non-Crystalline Solids 217 (1997) 278-290

scopic data cannot be always resolved [10]. The possibility then rises that spectroscopic trends may result from alterations in the way(s) these polyhedra are combined to form the network, rather than from direct coordination changes. In an attempt to study further the role of alumina additions in borate systems we present here a vibrational investigation of lithium boroaluminate glasses and crystals which belong to the metaborate-metaaluminate join, xLiAIO 2 • (1 - x)LiBO 2. The metaborate chemistry can be described at the local structural level by the isomerization between metaborate triangles and tetrahedra (Eq. (1)) and its cation- and temperature-dependent shifts [ 11-14]: BO20- ~ BO[,

(1)

where O denotes non-bridging oxygen atoms. Furthermore, the knowledge of the corresponding crystalline compounds and their vibrational spectra has been used for the structural description of intermediate range order in glass [15,16]. In the particular case of lithium metaborate (LiBO 2) this structural understanding is most extensive and relies on thorough NMR [ 17,18], vibrational [ 19-22] and devitrification [21] experiments. In a preliminary investigation of the xLiA10 2 - (1 - x ) L i B O 2 glass system [23], it was shown that 1

420

i

i

i

x LiAIO 2 .

(l-x)

01=

°.3

i

][

410

LiBO 2

400

oO 39O I"

380

370

360

0;

o11

I

I

0,

x, m o l e f r a c t i o n

Fig. 1. Glass transition temperatures of xLiAIO2.(1- x)LiBO2 glasses taken from Ref. [23]. The solid nine is guiding the eye.

279

increasing x induces the non-monotonic variation of the glass transition temperature (Tg) with a minimum at x = 0.25, Tg = 360°C (Fig. 1). This trend could originate from structural alterations of the metaborate network induced by the increasing aluminate substitution, as well as from non-monotonic changes of the aluminum bonding in the glass. The understanding of such phenomena can benefit from the systematic vibrational investigation of these glasses.

2. Experimental Lithium metaboroaluminate glasses, xLiA102 • (1 - x ) L i B O 2, were prepared from appropriate mixtures of reagent grade Li2CO3, B203 and AI203 (corundum). Melting of 5 g batches for ~ 30 min at 1100-1250°C in Pt crucibles yielded clear, homogeneous glass disks upon quenching between two copper plates. The samples were stored in desiccator and used for subsequent experiments without further treatment. This synthetic routine allows continuous glass formation for batch compositions in the range 0 < x < 0.42, and, according to Kim and Hummel [1], results in negligible evaporation losses. Increasing x was found to improve the glass forming ability of the x = 0 glass by decreasing the tendency for a-LiBO 2 precipitation, but LiAIO 2 contents larger than x = 0.42 gave inhomogeneous liquids at the highest melting temperatures employed. The LiA102-LiBO 2 phase diagram was investigated in this work by melt crystallization, glass devitrification and solid state synthesis, employing Raman and infrared spectroscopy as structural probes. All thermal treatments have been performed in an electrical furnace (Nabertherm) controlled within + I°C. The reproducibility of each thermal run was determined by measurements on several samples and the temperatures reported are accurate within + 5°C. Two mixed boroaluminate compounds Li3B2A106 and Li2BAIO 4, with stoichiometries corresponding to x = 0.33 and 0.50, respectively, were prepared in polycrystalline form. The first, Li3BzA106, can be formed either by cooling slowly the x = 0.33 melt, or by solid state reaction of a finely ground and pelletized stoichiometric mixture of Li2CO 3, B203 and A1203 which is kept at 750°C for 24 h. The crystal structure of

280

G.D. Chryssikos et al. / Journal of Non-Crystalline Solids 217 (1997) 278-290

Li3B2A106 has been reported by Abdullaev and Mamedov [24]. The second lithium metaboroaluminate compound, Li2BA104, was prepared by solid state reaction at 750°C for 24 h or at 600°C for ~ 2 months. A compound with the same stoichiometry is included in the phase diagram of Kim and Hummel [ 1], but its structure remains unknown, to our knowledge. In order to allow spectroscopic comparisons, the high temperature form of lithium metaaluminate, LiA102, was prepared also by solid state reaction of ground and pelletized Li2CO 3 and A1203 at 750°C for 24 h [25]. The Raman spectra of glasses and crystals were obtained on a double monochromator spectrometer (Jobin Yvon HG2S) at 90 ° scattering geometry, employing for excitation ,-, 500 mW of the 488.0 nm line of an argon-ion laser (Spectra Physics 165). The spectral resolution was approximately 5 c m - l and the accuracy for the determination of peak maxima was + 1 cm -1 and + 4 cm -I for crystalline and vitreous samples, respectively. The infrared spectra were collected on a Fourier transform spectrometer (Bruker IFS 113v) in the specular reflectance mode (11 °). Appropriate selection of sources, beamsplitters and detectors ensured the continuous coverage of the 30 to 3000 c m - ] range. The reflectance spectra were averaged over 200 scans at 2 c m - l resolution, subjected to Kramers-Kronig inversion and presented in the absorption coefficient formalism [22]. The accuracy in determining infrared peak maxima was ___2 cm - I , unless otherwise stated.

2

4

22 L

~ 1218 A

xLiAIO2 ' ( / - x ) L i a O 2

} o

200

400

600

800 1000 1200 1400 1600

f r e q u e n c y (cm "1)

Fig. 2. Infrared absorption coefficient spectra of representative xLiA102.(1- x)LiBO 2 glasses. The spectra are off setted for clarity.

due to vibrations of metaborate tetrahedra [19,22]. A smaller band at ~ 730 cm -1 is in the region of deformation modes of the above units [19]. Finally, I

I

I

,

I

X

I

I

8

3. Results

c

I

x LiAIO 2 . (l-x) LiBO 2 983

470 j~

to.,,

1410 ,230

~V Wl~

3.1. Infrared and Raman spectra o f x L i A l O 2 " (I x)LiBO 2 glasses

The infrared absorption coefficient and Raman scattering spectra of representative lithium metaboroaluminate samples, with LiA10 2 content ranging from x = 0 to x = 0.42, are shown in Figs. 2 and 3, respectively. The infrared spectrum of glassy lithium metaborate ( x = 0, Fig. 2) has been presented and analyzed in previous publications [19,21,22]. Strong bands at about 1240 and 1430 c m - 1 denote the existence of triangular borate units, while absorption in the 800 to 1100 cm i range is

I

200

i

I

400

,

I

,

I

i

I

,

;

,

600 800 1000 1 00 1400 1600 Raman shift (cm "1)

Fig. 3. Raman spectra of representative xLiAlO2'(l- x)LiBO2 glasses.

G.D. Chryssikos et aL / Journal of Non-Crystalline Solids 217 (1997) 278-290

the broad absorption envelope below 550 cm-~ was assigned to the localized vibrations of the Li + ions in their equilibrium sites [19,21,22]. The corresponding Raman spectrum (x = 0, Fig. 3) shows relatively large scattering maxima at about 555 and 960 c m due to BO 4 units, bands at 765 and ~ 1100 c m which are typical of arrangements containing both metaborate triangles and tetrahedra, as well as two broad features peaking at about 1250 and 1490 c m attributed to the stretching modes of triangular units [19,21]. More specifically, the highest frequency Raman envelope (1350 to 1600 cm -1) covers the localized vibrational modes of B - O - terminal bonds of B 0 2 0 - triangles [15,16,19,21]. Increasing LiAIO 2 content results in systematic changes of the infrared spectra (Fig. 2). The high frequency bands due to borate triangles (1242 and 1428 c m - ~, x = 0) gain in amplitude and shift progressively to smaller frequencies (1219 and 1400 cm -~ for x = 0 . 2 0 ; 1218 and 1376 cm -1 for x = 0.42). The two bands due to borate tetrahedra at ~ 960 and 1050 cm -1 which are prominent in the spectrum of the x = 0 glass, diminish in amplitude with increasing x. The spectra of glasses with x > 0.33 have only a low absorption feature around 1000 cm -~. New bands grow in amplitude at smaller frequencies (784 and 855 c m - l for x = 0.42) and may be associated with the presence of aluminate

281

polyhedra. Finally, the far infrared envelope centered at ~ 465 cm -1 for x = 0, increases in amplitude with x and shifts to larger frequencies (540 c m for x = 0.42). The corresponding changes in the Raman spectra can be seen in Fig. 3. The envelope of B - O stretching modes shifts progressively to smaller frequencies (from 1490 cm -~ for x = 0, to 1410 c m for x = 0.42). A second band typical of borate triangles ( ~ 1250 cm -1 for x = 0) also shifts to ~ 1230 cm-1 for x = 0.42. A small feature of the x = 0 glass at ~ 1100 cm-1 decreases in amplitude with x and almost vanishes at x = 0.33. The remaining bands observed in the x = 0 sample decrease in relative amplitude with increasing x. New features emerge at 470 and 983 cm -1. The spectrum of the x = 0.42 glass also has smaller bands at about 865, 730 and 645 cm-~ which can be traced back to samples with smaller x. In order to deduce the structural role of the alumihate centers from the above spectral changes, more detailed assignments of the various bands are required. Such assignments can be provided by the structural and spectroscopic investigation of the relevant crystalline compounds. For this reason, prior to any further discussion on the interpretation of the glass spectra, data on the crystalline compounds of the LiA102-LiBO 2 join will be presented.

(b)

Fig. 4. Network structures of (a) Li2 B z A106 [23], and (b) CaBAIO4 or SrBA104 [25,26]. A13 ÷ and B 3 + network forming cations are shown by large and small black circles, respectively. Bridging and non-bridging oxygen atoms are shown grey and white, respectively. The charge balancing cations are omitted for simplicity.

282

G.D. Chryssikos et al. / Journal of Non-Crystalline Solids 217 (1997) 278-290 i

3.2. Synthesis and measurements of relevant crystals 3.2.1. Lithium metaborate compounds The crystalline lithium metaborate compounds have been the subject of many previous investigations (Ref. [21] and references therein). Three polymorphs have been detected at ambient conditions by the systematic devitrification of the corresponding glass. The highest temperature compound, a-LiBO~, consists solely of metaborate triangles, B ~ 2 0 - , and has the chain metaborate structure [26]. In an intermediate temperature compound, [3'-LiBO2, boron forms both B ~ 2 0 - and BO4 units in a hitherto unknown arrangement [21]. Finally the lowest temperature polymorph, 7-LiBO 2, has the zinc blende structure and consists solely of B ~ 4 tetrahedra [27]. The infrared and Raman spectra of these compounds have been published elsewhere (Ref. [21] and references therein). 3.2.2. Lithium metaboroaluminate compounds According to Abdullaev and Mamedov [24], Li3B2A106 (x = 0.33) consists of metaborate triangles and metaaluminate tetrahedra. A characteristic segment of its network is shown in Fig. 4a. Fourmembered rings of alternating corner-sharing B ~ 2 0 and A104 polyhedra are linked via the A1 centers by B~)20 spacers to form an infinite length 'ribbon'. All bridging oxygen atoms participate in B - O - A 1 linkages. The ribbons run parallel to each other and are held together by bands of lithium ions in tetrahedral coordination [24]. Four regions of infrared spectral activity are observed in crystalline Li3B2AIO 6 (Fig. 5). Above 1100 cm -1 bands due to triangular boron oxygen arrangements can be observed with maxima at ~ 1210 and 1420 cm '. The 850 to 1050 cm -~ range exhibits a well defined doublet at 951 and 994 c m - ~. Three bands at 738, 788 and 826 cm-~ are active between 650 and 850 cm -~ . Finally, a multicomponent envelope at ~ 540 cm- ] with local maxima at 481 and 626 cm -1 can be observed below 650 cm -j . The corresponding Raman spectrum can be described on the basis of the same frequency ranges (Fig. 5). Above 1100 c m - I the dominant feature is the localized B - O - stretching mode peaking at 1390 cm ~. Between 850 and 1050 cm -~, scattering maxima are observed at 956 and 1002 cm 1. Medium

i

I

~--~\~

i

i

i

' i /f-'~/

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/ I

' 49 I

I /'

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I/, " 481,I /

T

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956

I' /

/

/

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I

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778

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0 I

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v : ,All ,

0

,

,

200

400

; I

,

,L 600

800

t

;iulI 11a9 1000

frequency,

L

I),,

I

4,

1200

1400

1600

cm 1

Fig. 5. Infrared and Raman spectra of polycrystallineLi2B_~AIO6. Asterisks mark bands due to traces of a 7-LiA102 impurity.

amplitude bands at 688,736, 757, 778 and 803 c m - t are in the 650 to 850 cm I range. A complex envelope peaking at 517 and 536 c m - ' is active between 350 and 600 cm -~. Below 300 cm -1, the Raman spectrum contains a number of phonon bands at 115, 149, 167 and 218 cm -~. The slow cooling of liquids with x > 0.33 results in the appearance of the second lithium metaboroaluminate compound with increasing relative contribution to the spectra upon increasing x. Yet, even at the highest aluminate content which yields a homogeneous liquid (x = 0.42), the new compound could not be isolated free from Li3B2AIO 6 admixtures. The new compound was found identical to the product of a solid state reaction corresponding to x = 0.50 (see Section 2) and was identified by XRD as the Li 2BA104 phase reported in the diagram of Kim and Hummel [1]. The structure of Li2BAIO 4 is unknown to our knowledge. Two alkaline earth compounds of the same stoichiometry, CaBA104 and SrBA104, have the network structure shown in Fig. 4b [28,29]. Their network is made of three-membered rings of two A104 tetrahedra and one B O 2 0 - triangle. The rings are condensed and form a chain built along an aluminate backbone. Similarly to Li3B2AIO6, there are no BO 4- tetrahedra, and each BO20 triangle has two A104 neighbors. There are two types of oxygen bridges, A1-O-A1 and B - O - A 1 in 1:2 relative abundance [28,29].

G.D. Chryssikos et al. / Journal of Non-Crystalline Solids 217 (1997) 278-290

The infrared spectrum of polycrystalline Li2BAIO 4 (Fig. 6), exhibits above 1100 cm -~ two complex absorption envelopes with apparent maxima at 1199, 1248, 1404 and 1441 cm -~. In the 850-1050 cm-~ range, two features are observed peaking at 907 and 956 cm -]. Below 850 cm-1 the infrared spectrum has several medium amplitude bands with the most pronounced amplitude maxima observed at 751 and 493 cm -~. The corresponding Raman spectrum (Fig. 6) is better resolved and appears analogous (although simpler) to the spectrum of Li3B2A106 (Fig. 5). The characteristic B - O - localized stretching mode is active at 1362 cm -~. The 850 cm -~ to 1050 cm i range contains a band at 957 cm -1 and two smaller amplitude bands at 881 and 911 cm -~. Between 650 and 850 cm -~, two medium intensity well resolved bands are seen at 759 and 781 cm -~ Several bands are observed between 350 and 650 cm-1, the largest of which is at 485 cm -1. Finally, the low frequency phonon range is dominated by the bands at 119 and 148 c m - ~. In the context of their study of CaBAl glasses, Fukumi et al. have reported the Raman spectrum of crystalline CaBA104 [34]. This spectrum has a low signal to noise ratio, allowing only the observation of its largest amplitude bands at about 980, 690, 675 and 505 cm -~. 3.2.3. Lithium metaaluminate The only other compound obtained by liquid crystallization or solid state synthesis in the 0 < x < 0.50

II

LI

I Ij

gs~

!1

~ l

119

485

J

il

°1 ¢Y'

,

0

I 200

759

I 604

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, ~

01 e"F.

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1

I I

9o?11

~ II

1153

1362 ]

,I 400

600

800

frequency,

1000

1200

1400

1600

cm "1

Fig. 6. I n f r a r e d and R a m a n s p e c t r a o f polycrystalline L i 2 B A I O 4.

333

1~3

3?8

]

283

I

I

!r

580

I

L

5o71

'l,i~ I

.(-LiAIO 2 (high temp., tetragonal)

O) ti

269

!

112r I P (g

I

613

I

li

',

8,06 91z rgo ~3

'i

rA, i

200

400

600

800

1000

frequency, c m

I = I = i t 200 1400 1600

1

Fig. 7. Infrared and Raman spectra of polycrystalline LiAIO 2 (high temperature polymorph).

range was the high temperature polymorph of lithium metaaluminate, LiA102. To confirm its identification, this x = 1 compound was synthesized free of admixtures by solid state synthesis (see Section 2). The product of this synthesis was identified by X-ray diffraction as the 1:1 lithium aluminate phase reported by Hummel et al. [25], the ",/-LiAIO 2 compound reported by Marezio [30], as well as the LiA102-(I) polymorph in the Landolt-Boernstein catalog [31]. In this compound both AI and Li ions are four-coordinated to oxygen [30]. The infrared and Raman spectra of ",/-LiA102 are shown in Fig. 7. The main features of the Raman spectrum are a band at 507 cm -1, a triplet at 790, 808, and 843 cm ~, as welt as smaller bands at 613, 269 and 127 cm r. The Raman spectrum of calcium metaaluminate, CaAI204, whose structure also consists of A104 tetrahedra, shows its strongest band at 520 cm -1, and exhibits a feature at 790 cm-~ [32]. The infrared transmission spectrum of "y-LiAIO2 dispersed in a KBr matrix (not shown in Fig. 7) is identical to that reported earlier by Tarte [33]. In Fig. 7 we show instead the infrared absorption spectrum of the same compound obtained by specular reflectance measurements via Kramers-Kronig analysis [22]. The infrared spectrum of "y-LiAIOz is dominated by an absorption envelope between 830 and 920 cm 1 and contains also a number of medium intensity bands between 280 and 700 cm-~ (Fig. 7). Since this spectrum is obtained from a polycrys-

G.D. Chryssikos et aL / Journal of Non-Crystalline Solids 217 (1997) 278-290

284

talline, KBr-free, sample it is assumed to be free of hydrolysis as well as optical dispersion and saturation effects [21,22].

3.2.4. Thermal stability of crystalline compounds Although Li3B2A106 can be obtained free of admixtures only for x = 0.33, its crystallization field, determined by both solid state reaction and melt crystallization, covers the range 0 < x < 0.5. Above 790 + 5°C Li3B2AIO 6 dissociates into ot-LiBO 2 and Li2BA104. Below 7 9 0 _ 5 ° C , the formation of Li2BA104 was found to occur for 0.33 < x < 0.50, and between 790 + 5 and 890 + 5°C in the composition range 0 < x < 0.50. Above ~ 890°C, Li2BAIO 4 was found to dissociate into oL-LiBO2 and ~-LiAIO 2 in agreement with the report of Kim and Hummel [1]. The high temperature dissociation reactions of the two metaboroaluminate compounds are described by Eqs. (2) and (3). They are both reversible but sluggish, which presumably indicates that they occur via major bonding rearrangements: >__790 + 5°C

Li3B2AIO 6

--~

Li2BAIO 4 + ot-LiBO 2,

(2)

also that Li3B2AIO6 and Li2BAIO 4 are among the devitrification products of samples with 0 < x < 0.42 and 0.33 < x < 0.42, respectively. No evidence for low temperature polymorphism of these compounds has been found. Summarizing the above data on crystalline lithium metaboroaluminates we note that cooling the liquids with x < 0.50 below 900°C, favors the formation of mixed boroaluminate networks which consist of interconnected metaborate triangles ( B 0 2 0 - ) and metaaluminate tetrahedra (A104). Within the restrictions imposed by the B to AI ratio in the liquid, there is a tendency in maximizing the number of B - O - A 1 bridges. The formation of six coordinated aluminum arrangements is not favored, while devitrification products containing B 0 4 units are only present in compositions with small x, in arrangements of the 6'-LiBO 2 type, i.e., linked to B02 O - triangles. These structural trends are compatible with the predictions of a model by Bunker et al. [8] which is based on the balance of local charges in borate, aluminate and boroaluminate systems,

> 890 + 5°C

Li2BA104

~

~/-LiA102 + a-LiBO 2 .

(3)

In addition to solid state reactions and liquid crystallization, the development of crystalline phases was investigated by devitrification heat treatment procedures as a function of composition, temperature and time. Such treatments often allow the synthesis of low-temperature polymorphs not feasible by other techniques [21,35]. These devitrification studies demonstrate that the pronounced ability of glassy LiBO 2 (x = 0) to devitrify at sub-T~ temperatures [21] is reduced upon increasing x, and samples with x > 0.10 devitrify slowly at Tg. Lithium metaborate is among the devitrification products of samples with 0 < x < 0.33, treated below 790°C, as expected from stoichiometry. It is normally found as the a-LiBO 2 chain polymorph, even when the devitrification is induced at temperatures below 420°C. Only samples with x approaching zero devitrify into [3'-LiBO2 under sub-Tg conditions, and the lowest temperature polymorph, -~-LiBO 2, has not been observed among the devitrification products of lithium metaboroaluminate samples (cf. Refs. [21,35]). As an example, the devitrification of a x = 0.05 glass (7"8= 402°C, Fig. 1) at 370°C for 48 h yields mostly [3'-LiBO2 and traces of et-LiBO 2 and Li3B2A106. It was found

4. Discussion

4.1. Vibrational assignments and the structure of Li 2 BAIO4 The B - O stretching modes of triangular borate arrangements have infrared and Raman bands at frequencies greater than 1100 cm -1, while bending and deformation modes are active at < 800 cm -1 . As a result, oL-LiBO2 is transparent between 800 and 1100 cm - l [20,21]. Tetrahedral metaborate units exhibit their stretching modes at considerably smaller frequencies. For example, the B(OH) 4 species in solution or as part of crystalline structures is identified by vibrational modes in the frequency ranges: 900 and 1000 cm -1 (v 3, infrared and Raman active), 700-760 cm -1 (v l, Raman active), and 500-550 cm i (u4, infrared and Raman active) [36]. Frequency shifts of these modes as well as relaxation of the selection rules are often observed in crystalline compounds and glasses containing BO4 units, but in most cases do not alter the diagnostic value of the vibrational spectra [19,21,22,37]. The identification of aluminate tetrahedra and octahedra from their infrared and Raman spectra is

G.D. Chryssikos et al. / Journal of Non-Crystalline Solids 217 (1997) 278-290

more controversial. The most detailed investigation is that of Tarte [33] who studied the infrared spectra of several reference compounds and concluded that the largest frequency stretching modes of 'isolated' A104 units are observed between 650 and 800 cm -~, while the corresponding vibrations of AIO6 species are active between 400 and 530 cm 1. When these units are 'condensed', i.e., are part of an extended aluminate network, these vibrations shift to larger frequencies (700-900 cm -~ and 500-680 cm -~, respectively) [33]. Typical aluminates with 'condensed' tetrahedra, such as ~/-LiA102 and CaAlzO 4, have their largest frequency infrared and Raman bands (presumably due to stretching modes of A104 tetrahedra) in the 790 to 920 cm ~ range (Fig. 7 and Refs. [32,33]). Both compounds have their largest amplitude Raman bands in the 500-520 cm-~ range. In comparison, we note that 7-LiBO 2, which consists solely of interlinked B 0 4 tetrahedra and thus is the borate analog of ~/-LiAIO2 and CaAI204, has its largest amplitude Raman band at 575 cm-~ [21]. Guided by the above general assignments, it is evident that the high frequency infrared and Raman features of the mixed boroaluminate compounds (~, > 1100 cm -~ , Figs. 5 and 6) are due to the stretching modes of the metaborate triangles which participate in their networks. Of diagnostic importance is the highest frequency Raman feature (1350-1500 cm -~) which originates from the stretching of the B - O - dangling bond. Due to ax-delocalization effects, the higher the basicity of the units bonded to boron via the two bridging oxygen atoms, i.e., the stronger the B - O bonds, the longer the B - O - bond and the lower the frequency of its stretching mode [16,21,23,25]. Thus, while chains or rings of metaborate triangles have characteristic B - O - stretching frequencies in the 1450-1550 cm ~ range [15], the presence of BO£ tetrahedra neighboring the B O 2 0 'probe' decreases the frequency to the 1400-1420 cm -1 range [16,21,35]. The same mode in the spectrum of Li3B2AIO 6 appears between 1340 and 1390 cm-~ (Fig. 5), as expected from the presence of the more basic AIO4 units adjacent to B ~ 2 0 - (Fig. 4a). By the same token we conclude that in the unknown structure of Li2BAIO 4 (~,= 1362 cm - l , Fig. 6) every metaborate triangle is also bonded to two A104- units. Equally instructive is the association of the Ra-

285

man bands in the 480 to 550 c m - J range (Figs. 5-7) with the presence of A1Oj- tetrahedra (see above). The frequency of this mode appears to be sensitive to the next neighbors of A104. We observe this mode at 507 c m - I in the spectrum of ~/-LiAIO2 (Fig. 7, A1-O-A1 bridges) and at ~ 525 cm -1 in the spectrum of Li3B2A106 (Fig. 5, A 1 - O - B bridges). The fact that the spectrum of Li2BAIO 4 in the same frequency range has bands at 485 and ~ 530 cm-~ (Fig. 6), presumably indicates the presence of both A 1 - O - B and A1-O-A1 types of bridge in its boroaluminate network. Li3BzA106 and Li2BA104 have two more ranges of spectral activity which can be related to internal vibrational modes of the network polyhedra. They are easily resolved in the Raman spectra at 900-1000 cm-1 and 700-800 cm -~, but they can be identified also in their infrared counterparts (Figs. 5 and 6). We argue that bands in the 900-1000 cm-~ range have a contribution from A10Z stretching modes. The fact that they are observed at frequencies higher than the corresponding envelope of "y-LiA102 (790-840 cm - / , Fig. 7) is presumably due to mass and charge effects arising from the coupling of A104- and B 0 2 0 - species in the boroaluminate networks. Finally, Raman scattering and infrared absorption between 700 and 800 cm ~ can be attributed to the out-of-plane bending mode of metaborate triangles. Its splitting into components can result from a decrease of its ideally degenerate symmetry (E), as Table 1 Ranges of vibrational activity of typical structural arrangements encountered in metaborate-metaaluminatecrystals. All frequencies are in cm- ). For details see text Infrared Raman Comment 1150-1500

B020- stretching 02 B- O- stretching: 1450-1500 1400-1420 1340-1390

900-1100 900-1000 830-920 700-800

950-1000 780-850 700-800 510-540 480-500

- B O 2O - - B O 2 0 - -BO 2 O- -

-B04 -

B02 O - -B04

-

-A104 - BO 20 - -AI04 BOa stretching AIO4 asymmetricstretch: -B02 O -A/O4-B020 -AI04 - a104 -AI04- B020- out of plane bending AIO4 stretching-bending: -B020 - AIO4 - B O 2 0 - -AlOe-- AI¢)~--AlOe--

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well as from crystal field effects. For convenience, the main ranges of vibrational activity of the structural species found in lithium metaborate and metaaluminate crystals are listed in Table 1. Also, the structural information on Li~BA104, obtained from the vibrational spectra, may be used to guide a future crystallographic study of this compound and is summarized as follows: (a) LizBA104 consists solely of AI~ 4 and B O 2 0 - species; (b) every B O 2 0 - triangle is bonded covalently to two AI~ 4 tetrahedra; (c) there is a strong possibility that both A 1 - O - B and A1-O-A1 bridges are present. All these criteria are fulfilled by the known structure of CaBAIO 4 (Fig. 4b and Ref. [28]). However, CaBA104 and Li2BA104 have differences in their Raman spectra (Fig. 6 and Ref. [34]) and, therefore, do not have the same structure. Instead, the similarities of the Raman spectra of Li2BAIO 4 and Li3B 2 AIO6 (Figs. 5 and 6) suggest that the networks of two compounds are closely related despite their different boron to aluminum ratios.

4.2. The structure (?f xLiAlO 2 • (1 - x)LiBO: glasses

The most straightforward application of the above in discussing the spectra of lithium metaboroaluminate glasses (Figs. 2 and 3) concerns the nature of the borate polyhedra present in the network as a function of x. The decrease of infrared absorption in the 900-1100 cm-~ range upon increasing x (Section 3.1 and Fig. 2) correlates with the decreasing presence of BO4- tetrahedra. These units are still present at x = 0.20, but beyond x = 0.33 their characteristic stretching bands at about 960 and 1055 cm ~ disappear and give place to a small feature at ~ 990 c m - i (Fig. 2). Although this feature could be attributed in principle to four-coordinated metaborate units [22], we note that stretching modes of AIO2 units are also active in the same frequency range (c.f. Fig. 5). The Raman profile of the B - O - stretching mode in the spectra of glasses (1350-1600 cm l, Fig. 3) is also indicative of the destruction of B O 4 units with x. Glassy LiBO 2 (x = 0) has in its Raman spectrum an asymmetric envelope of such stretching modes

peaking at ~ 1490 cm-~ with a shoulder at ~ 1400 cm -I (Fig. 3) [21]. The high frequency component of this envelope has been associated with - [ B 0 2 0 - BO20--B(~20-]segments of the ct-LiBO 2 type in the glass, while its lower frequency counterpart has been taken to indicate - [ B O ~ - B 0 2 0 - - B 0 4 ]arrangements of the 13'-LiBO 2 type [21]. Increasing x in the xLiAlO 2 • (1 - x)LiBO 3 system results in the continuous shift of the B - O - stretching envelope to lower frequencies, till it appears centered at ~ 1410 cm-~ for x = 0.42 (Fig. 3). The trend indicates the systematic lengthening of the B - O - bond, which is typical of the enhanced involvement of the empty B p: orbital in w-bonding with increasingly basic bridging oxygens [16,21,35]. At large x, such oxygens cannot be provided by B 0 4 since the concentration of these units in the glass is decreasing (see above). In Section 4.1 it was demonstrated that the presence o f - [ A I O 4 - B O 2 0 - - A 1 0 4 ] - network sequences leads to B - O - stretching frequencies in the 1360-1390 c m - ~ range. Therefore, the decreasing frequency of the B - O - mode ought to be related to the formation of - [ A I O 4 - B 0 2 O - ]- arrangements. In the Raman spectrum of the x = 0.33 glass the B - O - stretch is observed at ~ 1420 cm -1, i.e., 30 c m ~ greater than in the corresponding crystal, (Li3B2AIO6). This difference may indicate that the glass contains - [ A I O 4 - B O 2 0 - - B O 2 O - ] - segments which are not present in the structure of crystalline Li3B2AIO 6. Oxygen bridges between metaborate triangles are only favored at high temperatures as a result of the decomposition of Li3B 2 A I O 6 (Section 3.2.4, Eq. (2)). Thus, the existence of - [ A I O 4 - B O 2 0 - - B O 2 0 - ] - segments in the glass with x >__0.33 is a metastable structure arrested by the vitrification of the metaboroaluminate liquid. The amplitude decrease of the x = 0 Raman bands at 555, 765 and 960 cm ~ upon increasing x (Fig. 3), is compatible with the elimination of B 0 2 units, since these features are associated with the presence of BOa-Containing, [3'- and "y-LiBO 2 type arrangements [21]. The fact that y-LiBO 2 is not among the low temperature devitrification products of the x 4~ 0 glasses, and that [3'-LiBO2 appears only up to x = 0.10 and at temperatures less than in the x = 0 glass enhances this argument. However, the growth of new bands at 983 and ~ 470 c m - ~ upon increasing LiAIO 2 content prohibits the determination of a

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threshold value of x beyond which no B04- units would be present in the glass. Searching for the signatures of aluminate species in the infrared and Raman spectra of glasses with increasing x, we note first that the 983 cm- 1 Raman band of the glasses (Fig. 3) has a width that encompasses the characteristic vibrations in the Raman spectra of both Li3B2AIO6 (956, 1002 cm 1, Fig. 6) and Li2BAIO4 (957 cm -1, Fig. 7) crystals, which are associated to A104 species linked to B ~ 2 0 triangles. Also, absorption in the infrared spectra of samples at ~ 785 cm-~ (Fig. 2), parallels infrared bands at similar frequencies in the spectra of Li3B2A106 and Li2BAIO 4 (Figs. 6 and 7), which are attributed to out-of-plane vibrations of B ~ 2 0 - triangles. The broad Raman band at ~ 470 cm- ~ in the spectra of samples with increasing x (Fig. 3), corresponds to strong bands in the Raman spectra of Li3B2A106 ( ~ 525 cm -j, Fig. 5), Li2BAIO4 (485 and 530 cm -I, Fig. 6) as well as "y-LiA102 (507 c m - l , Fig. 7), and points also to the formation of A104 species in glass. Following arguments presented in Section 4.1, the relatively low frequency of the 470 cm- ~ Raman band may be a sign of cornersharing between adjacent aluminate tetrahedra in the glass network. This interpretation is supported by two more similarities between the spectra of aluminum-containing glasses and those of -/-LiA102 (cf. Figs. 2 and 3 and 7). First, the presence in the Raman spectra of glasses of a band at 865 cmcorrelates with well defined Raman bands of ~LiA102 in the 790 to 843 cm -j range. Second, infrared absorption at ~ 865 cm-~ in the spectra of samples corresponds to the high frequency infrared envelope of -/-LiA102. We note that the presence of - [ A I O 4 - A 1 0 Z ] - sequences is not favored in the crystalline lithium metaboroaluminate compounds which are stable at room temperature and correspond to x_< 0.33. The fact that the presence of such segments is inferred from the spectra of samples with x < 0.33 is also considered as a high temperature structural characteristic (cf. Eqs. (2) and (3)) frozen upon vitrification of the liquid. The above analysis leads so far to the following description for the structure of lithium metaboroaluminate glass networks. (a) There is evidence for the presence of A104 species.

287

(b) Increasing xLiA102 leads to the isomerization of BO 4- tetrahedra into BO~- triangles, i.e., shifts Eq. (1) to the left. (c) Although upon cooling the network tends to maximize the concentration of B - O - A I bridges, there is evidence that an excess of B - O - B and A1-O-A1 'hot' links persists in the glassy state. It is interesting to compare these trends with data from analogous systems. Fukumi et al. have published the Raman spectra of CaBAl glasses including samples on the metaboroaluminate join [34]. Comparison of these spectra to those in Fig. 3 indicates that Ca- and Li-metaboroaluminate glasses have similar networks. This similarity was expected from the similarities in the local structure of the corresponding crystals (see Section 4.1), and can be attributed to similarity of Li + and Ca 2÷ field strengths [15]. The structural similarities do not extend to other alkali or alkaline earth boroaluminates. Fukunaga et al. comment on the structural differences between Ca and Mg boroaluminates [4], while comparisons between the Li-glass spectra of Fig. 3 and those of their sodium counterparts [38] indicate that the aluminate substitution proceeds in these cases via different structural routes. As a consequence of the above, when seeking structural data to complement those on lithium metaboroaluminates, it is meaningful to consider mainly their calcium analogs. Indeed, both the network participation of aluminum as AI~ 4 and the reduction of the fraction of four-coordinated boron atoms as the aluminate content increases are confirmed in CaBAl glasses [2,9,34,39]. However, some of these works on CaBAl glasses discuss the presence of sites where aluminum is bonded to more than four oxygen ligands. Bishop and Bray [2] reported the l rB and 27A1 broad-line NMR spectra of such glasses including three compositions along the xCaA1204 • ( 1 x)CaB204 pseudobinary. Although they determined indirectly six-coordinated aluminum (A106) in samples with small modifier content, they found no evidence for their presence along the metaboroaluminate join. More recently, Bunker et. al. reported JiB and 27A1MAS NMR spectra on various alkaline earth boroaluminate glasses [9]. Their study includes one calcium composition with x = 0.50, i.e., with stoichiometry analogous to that of the samples investigated here. The 27A1 MAS NMR spectrum of this

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sample suggests the existence of three different aluminate species with chemical shifts about 55, 30 and 0 ppm relative to AI(OH)63- and fractional populations 0.61, 0.28 and 0.12 (+0.15-0.20), respectively. The main peak of the 27A1 NMR spectrum at ~ 55 ppm is attributed to four-coordinated aluminum atoms, in agreement with the results of our vibrational investigation. Also, the peak of nearly zero chemical shift is attributed to six-coordinated aluminum [9] as in corundum [40]. Along these lines we note that Tarte [33] reports infrared activity due to AI06 arrangements at 600 to 700 cm-1 in the case of A1203, and 500-800 cm -1 in the case of low temperature-high pressure, octahedra containing, LiA10 z. The infrared spectra of the samples investigated here (Fig. 2) exhibit bands in these frequency ranges. However, the compositional dependence of these bands does not allow us to ascertain the presence of six-coordinated aluminate species from the vibrational spectra. The remaining 27A1 peak at ~ 30 ppm has been attributed by Bunker et al. [9] to the presence of five-coordinated aluminum oxygen sites. Relatively regular five-coordinated aluminum sites of trigonal bipyramidal symmetry (A1-O bonds in the range 1.76 to 1.92 A) as in barium aluminate glycolate [41] give a 27A1 NMR resonance at ~ 35 ppm [42]. Similar results are obtained for aluminosilicate and aluminogermanate crystals which also contain well defined five-coordinated aluminum sites [43,44]. On the basis of the findings of Bunker et al. on calcium boroaluminate glasses, we cannot exclude the presence of trigonal bipyramidal aluminum in their lithium metaboroaluminate analogs despite the lack of supporting vibrational evidence. In this respect we note that a very recent 27A1 MAS NMR study of calcium aluminate glasses by McMillan et al. reports that the concentration of the five- and six-coordinated aluminate species decrease upon increasing CaO content, and fall below the NMR detection limit at the metaaluminate composition [45]. Given the uncertainty in determining the presence of aluminate polyhedra with coordination higher than four in lithium metaboroaluminate samples, it is important to examine whether an account of the observed dependence of Tg on x could be found by considering solely AI~ 4, BCl4, and B~20- species. It has been shown that the extrema in the T~

versus composition of borate glasses coincides with extrema of the same sign in the average number of bridging oxygens per network forming polyhedron, N b [46,47]. Employing similar principles, Araujo had accounted for the occurrence of a minimum in the variation of the softening point of sodium aluminoborosilicate glasses with increasing aluminate content [48]. The simple structural model where every aluminate center introduced to glassy lithium metaborate participates in the structure as A104 and induces simultaneously the transformation of one B 0 4 tetrahedron into a B O : O - triangle, yields the following expression for Nb: Nb=4x+a(l-x)(Na,0-x × ( 1 - N4,0 + x),

)+2(1-x) (4)

where N4.0 is the fraction of four coordinated boron atoms in the x = 0 glass. The three terms of Eq. (4) account for the contribution of aluminate tetrahedra, borate tetrahedra and borate triangles, respectively, to N b. Since x denotes the molar fraction of LiA102 in the pseudobinary glass, N b reaches the limiting values 2(N4,0 + 1) at x = 0, and 4 at x = 1. Eq. (4) depends quadratically on x, and N b exhibits a minimum at Xmin = N4,o/2. Experimentally derived values of N4,0 -~ 0.39, obtained by l°B and ZlB NMR techniques [17,18], lead to the prediction that in xLiA102 • (1 - x)LiBO 2 glasses the minimum value o f N b o c c u r s at Xmin ~-- 0.2, in good agreement with the observed minimum of Tg (Fig. 1). Naturally, treatments of the above type do not rule out the presence of aluminate polyhedra with coordination number greater than four in the glass. The model simply shows that such polyhedra are not required in order to account for the compositional dependence of Tg in lithium metaboroaluminate glasses.

5. Conclusions The crystallization chemistry of our samples is dominated by two mixed-network crystals, Li3B2A106 and Li2BAIO4, which do not have low temperature polymorphism, as well as by the hightemperature, end-member polymorphs a-LiBO 2 and ~-LiA10:. The vibrational spectra of LizBAIO4 indicate that its network consists also of B O 2 0 - and

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A104 units linked to form intermediate range order structures similar to those of Li3B2A106. The same chemical trends are recorded in the structure of our glass samples. Their infrared and Raman spectra indicate that increasing LiA102 substitution results in the formation of A104 tetrahedra, and the parallel transformation of B 0 4 species into their isomeric B~J20- triangles. Given that glassy LiBO 2 (x = 0) has ~ 39% of the boron centers in fourfold coordination, the above structural scheme leads to the occurrence of a minimum of the average number of bridging oxygens per network cation at x--0.20. In turn, this behavior correlates well with the x-dependence of the glass transition temperature, which was found earlier to show a minimum at x -- 0.25. The vibrational spectra of xLiA102 • ( 1 x)LiBO 2 glass samples bear many similarities to those of the corresponding crystalline compounds. These similarities signal structural analogies at the local and intermediate ranges. Thus, vitrification results in the formation of B-O-A1 linkages although an excess of B - O - B and A1-O-A1 bridges is also observed and assumed to be a high temperature structural property of the samples. There is no direct vibrational spectroscopic evidence for the formation of five- or six-coordinated aluminate polyhedra in the lithium metaboratemetaaluminate glass family. The presence of such species cannot be ruled out, but the relatively high Li20 content of the glasses and the structure of the relevant crystalline compounds argue against their formation in appreciable fractions.

Acknowledgements The authors are indebted to Dr V. Psycharis of NCSR 'Demokritos' for the identification of the various crystalline phases by XRD techniques. The project was funded by NHRF.

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