Structure of calcium aluminosilicate glasses: wide-angle X-ray scattering and computer simulation

]OURNA L OF

Journal of Non-Crystalline Solids 136 (1991)27-36 North-Holland

~'~1]1~

~0IlI~

Structure of calcium aluminosilicate glasses: wide-angle X-ray scattering and computer simulation B. H i m m e l a, j. W e i g e l t a, Th. G e r b e r a a n d M. N o f z b Fachbereich Physik, Universitiit Rostock, Universitiitsplatz 3, Rostock, 0-2500, Germany b lnstitut fiirAnorganische Chemie, Rudower Chaussee 5, Berlin, 0-1199, Germany

Received 9 April 1991 Revised manuscript received 18 July 1991 The structure of selected calcium aluminosilicate glasses (CaO. A1203.2SIO2 and CaO. A1203.4SIO2) was analyzed with respect to the short-range order as well as to the middle-range order using wide-angle X-ray scattering (WAXS) in combination with computer simulation calculations. A structure model of glassy anorthite is proposed which is characterized by a network topology resembling triclinic anorthite. The network disorder is created by the incorporation of topological defects resulting in a limited correlation range of ~ 1.2 nm within a homogeneous network. The glass structure of the composition CaO. A1203.4SIO2 could not be constructed by coexisting anorthite-like and SiO2-1ikeclusters. Investigation of glass samples with identical chemical compositions but prepared with different cooling rates indicates that the structures of glasses having a high fictive temperature are determined not only by a configurational part of disorder (topological defects) but also by a chemical one (concentration fluctuations). 1. Introduction

The dependence of the macroscopic properties on the atomic-scale structures in SiO2-A120 3based systems including alkaline or e a r t h - a l kaline constitutents is of great importance in glass and ceramic science but also in geology. In addition to the restricted possibilities of glass preparation from the melt, the sol-gel method has been the subject of increasing interest [1-4] which requires also a better structural understanding of melt-prepared glasses. A representative review of vitreous aluminosilicates was given by Mysen in 1988 [5]. Most information on the structure of aluminosilicate glasses have been acquired from spectroscopic methods such as 29Si and 27A1 M A S - N M R [6-8], R a m a n [9,10], XPS [11], E S R [12], and I R [13] as well as from wide-angle X-ray scattering (WAXS) using the radial distribution function ( R D F ) [1418]. The diffraction method, especially in combination with computer modelling, is capable of giving information about the middle-range order (network topology) as shown earlier [19,20].

Engelhardt et et. [6] reported high-resolution solid-state Z'~Si and 27A1 M A S - N M R investigations on about 50 calcium aluminosilicate glasses along seven selected joins of the ternary composition diagram and represented an analysis of the distributions of Qm building units for this system (m is the total number of bridging oxygen atoms and n the number of AI atoms bound via oxygen bridges with the SiO4/2 tetrahedron). Glasses of compositions SiO2, CaO . A 1 2 0 3 - 4 S I O 2 , and C a O ' A I 2 0 3 . 2 S i O 2 (under consideration in our study) were chosen as reference samples characterizing the chemical shifts of Q4, Q4 and Q4 groups, respectively. Nevertheless, the tetrahedral coordination of A1 with oxygen was established and a coupling of Ca 2 + ions to the AIO4/2 tetrahedra was presumed [6,8]. In addition to spectroscopic data [8], the results of thermochemistry [21] allow the conclusion that there exists a certain degree of (Si, A1) disorder in the anorthite glass. Seifert et al. [10] suggested the existence of four-membered interconnected A12Si20 ~- rings for compositions with AI/(A1 + Si) = 0.5. The ob-

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

28

B. Himmel et al. / Structure of calcium aluminosilicateglasses

served trends in the Raman spectra were attributed to mixtures of SiO2, AI2Si2 O2-, and A12042- interconnected, three-dimensional structures in melts on the join SiO2-CaA1204. Mysen [5] established that compositions with A1/(AI + Si) 4:0.5 might consist of clusters of structures resembling that of anorthite mixed with either SiO 2 or aluminate clusters. However, this hypothesis has not been proved by means of X-ray scattering. The first WAXS investigation on anorthite glass using the RDF method was made by Taylor and Brown [15,16]. They found good correspondance between the RDF (to about 0.4 nm in real space) of crystalline and glassy anorthite. Thus they drew the conclusion, that 'crankshaft'like and feldspar-like arrangements of four-membered rings of tetrahedra exist in both the crystalline and the glassy sample. Because of the limited visual field of the WAXS experiment rmax = r r / S m i n determined by the first significant measuring point at s = 10 nm-I, their discussion is strongly related to the short-range order. The present study discusses the structure of calcium aluminosilicate glasses with respect to the network topology using the WAXS method in combination with simulation calculations. It is shown that the examination of crystalline model compounds can give an orientation to find topological rules determining glassy networks and these rules can be successfully applied for the network generation within the simulation procedure used. Thus the procedure of simulation conceived is a combination between interactive network building and relaxation processes in contradiction to molecular dynamic simulations [22-24] where the successive cooling of a theoretical 'atomic gas' within a simulation box can result only in random structures. 2. Simulation procedure 2.1. M o d e l g e n e r a t i o n

A comprehensive understanding of a glass structure which is isochemical to a crystal could be gained by taking into account the crystal topology. That means it is often necessary to use certain topological units (e.g., polyhedra) for a

consistent construction of a glassy network with respect to the middle-range order [19,20]. This procedure must n o t be confused by a package of crystalline clusters (small crystallites), b u t should help to understand the essential rules determining a glassy network. That is why the isochemical crystalline structure will be analyzed in correspondence to their main topological units as a starting point in order to synthesize a model structure of the glassy network. In the course of the further simulation process, the crystalline long-range order will be destroyed by generating topological defects which distort the whole network which results in a limited correlation range in a continuous network. Up to 500 atoms had to be included in this simulation procedure. Our computer simulations were made in an interactive regime including network enlargement, maintenance of the short-range order via relaxation processes, and comparison with the experimental results by means of scattering and structure functions. An adequate pair potential used in this study was given by Keating [25]: N NN 3~ij

N NN 313i + ~i • ( r i j r i g - dijdik cos /~i)2 ' j,k>j 8dijdik

(1) where N is the number of atoms, N N is the number of next neighbours of the ith atom, olij is the stretching constant between the atoms i and j (av_o = 152 N m - l ; T ~ S i , AI), dij is the average bond length, /3i is the bending constant (/3o_v_ o = 13.8 N m -1, /3T_o_x = 21.0 N m-i), O i is the average bond angle, and rjj = r i - r i is the difference of the radii vectors. Deviations of the chemical bonds are considered by the first term of eq. (1). The second term corresponds to variations of the bond angles. Screening of the nearest neighbours caused by Coulomb interactions were considered by the Lennard-Jones potential according to Murray et al. [26]: U =

i

Pii

12 rij

r6

,

(2)

with the parameters p, T and v given in table 1.

B. Himmel et al. / Structure of calcium aluminosilicate glasses

29

Anorthite belongs to the feldspar family. Its structure can be described by a three-dimensional network of perfectly interconnected SiO4/2- and A1Oa/z-tetrahedra which are arranged to form characteristic four-membered rings. The CaZ+-ca tions balance the negative charges of the A13 + O2/2_tetrahedra. The triclinic symmetry is the only stable form of the existing crystalline phases (see table 2). The 3,-6 transformation to the low-temperature triclinic form (disorder-order transition) takes place slowly and continuously via processes of site interchange and diffusion processes [27,28]. How-

ever, it is likely that the high-temperature modification is preserved metastably at room temperature [28]. The cell dimension of anorthite (in the following the triclinic form) is given by Cole [29]: a = 0.81768 nm, b = 1.28768 nm, c = 1.41690 nm, a=93 ° 10',/3=115°51 ',3,=91°13 ',p=2.76g cm 3, and the coordinates of the 104 atoms in one unit cell are given by Kempster and coworkers [30,31]. Figures l(a)-(c) show some typical topological arrangements of anorthite which can be instrumental to a better understanding of the isochemical glass structure. The crystalline short-range order shows important deviations from the ideal tetrahedral angle of (109.37_+ 5.37) ° as well as from the average Si-O- and AI-O-bond lengths of (0.1615 + 0.0026) nm and (0.1749 _+ 0.0033) nm, respectively. Two different values of intertetrahedral angles at the oxygen atoms can be found: (132.5 _+ 6.1) ° and (165.7 + 4.1) ° with the lower one occurring most frequently. The medium- and the long-range order is characterized by a socalled 'crankshaft'-pattern of four-membered rings arranged as chains in the x-direction (figs. l(a) and l(b)). Neighbouring chains are interconnected by two oxygen-bridges between opposite four-membered rings in each case accompanied with the formation of six-membered cages. Figure l(c) shows elongated eight-membered planes terminated with two vertically arranged four-membered rings which appear as a feature of the y-direction only. The topological elements found in the x - y plane (four-membered rings, six-membered cages, eight-membered planes) recur in the z-direction.

Table 2 Crystalline phases of anorthite according to ref. [27]

3. Experimental

Table 1 Parameters of the Lennard-Jones potential used for the relaxation ( p in units of a according to the Keating potential) Pairs of atoms

p

T (10 -6 nm 6)

c

Ca-Ca Ca-Si Ca-AI Ca-O

0.001c~ 0.001o~ 0.001 o~ 0.2a

2048 2048 2048 84

0 0 0 1

The force on each atom was obtained by calculating the gradient of the potential field. It was the aim of the relaxation to approach the shortrange order corresponding to the experimental values as a minimum of the communal energy, whereby a local energetic minimum cannot be excluded. The force constants used were choosen with reference to the literature for simple networks (e.g., SiO 2, Na20-SiO2). 2.2. Anorthite structure CaO .A1203 • 2 S i O 2

Phase

System

Remarks

ot-CaAl2Si20 s (metastable

hexagonal

/3-CaAI2Si20 s

rhombic

Crystallization from the melt; can be transformed into the triclinic form by heattreatment at 1200 ° C

y-CaAlzSi208 (higb-anorthite)

triclinic

Statistical distributed Siand Al-tetrahedra

6-CaAI2Si20 s (Iow-anorthite)

triclinic

Alternating ordered Siand Al-atoms

Two compositions were choosen from the ternary glass system C a O - A 1 2 0 3 - S i O 2 (fig. 2) for our investigations. Glasses were prepared by melting of CaCO 3, AI20 3, and SiO 2 at a temperature of about 1600 ° C in a platinum crucible [6]. Both glasses were cooled through the region of glass transition with a cooling rate of 0.5 K / m i n . In order to reach optimal conditions for our X-ray scattering investigations, plates of ~ 1 mm

30

B. Himmel et aL / Structure of calcium aluminosilicate glasses a

q

Z

o13

o 0 • •

oxygen aLuminium silicon calcium

mot % At 203 Fig. 2. Ternary diagram of the system CaO-AI203.SiO 2 with compositions of glasses investigated.

ature were obtained by fibre spinning from the remelted glass. The compositions determined by wet chemical analysis as well as some macroscopic properties are given in table 3. The WAXS experiments were performed in symmetrical transmission using AgK~-radiation monochromatized with a graphite crystal. Further details of measurement and data treatment have been described previously [19,32,33].

4. Results

C

Figures 3 and 4 show intensity distributions and the corresponding structure functions of the two glasses A and B (for compositions see table 3). The first measuring point of Smin 1.3 nm-1 guarantees a maximal visual field of approximately 2.5 nm in every case. The absence of an intensity increase in direction to lower s-values (fig. 3(a)) gives evidence for a homogeneous net=

Fig. 1. View of typical topological arrangements in triclinic anorthite illustrating (a) the 'crankshaft' pattern of four-membered rings, (b) the six-membered cages and (c) the eightmembered planes terminated with two four-membered rings.

thickness were cut from the bulk materials in each case. Samples with identical chemical compositions but having very different fictive temper-

Table 3 Compositions (values of analysis) and properties of the glasses CaO'A1203-4 SiO 2 (glass A) and CaO.AI203.2 SiO 2 (glass

B) Composition (mol%)

glass A glass B

CaO

AI20 3

SiO 2

16.8 24.4

16.6 24.6

66.6 51.0

Tf ( o C)

a (10 -7 K l)

P20 (g cm -3)

867 860

41.5 50.1

2.4668 2.6053

B. Himmel et al. / Structure of calcium aluminosilicate glasses

50

,~ 4 0 0

A--si°2

,,.)

-2 (lJ

31

@

l

E C"

300

25

o

0

200 '7

-25

100

-50

40

2'0

O0

40

80

S [ n m -1 ]

120

160 5 [ n m -1 ]

200

Fig. 3. s-dependent intensity distributions of the investigated glasses with s = 47r/3. sin(6)), where 3. is the wavelength and 6) is the scattering angle. (a) Intensity normalized to absolute units using the theoretical independent scattering functions (e.u./u.c. means electron units per unit of composition). (b) Reduced intensity distribution i(s)= (lcor(S)- lcoh(s)-- Iincoh(S))/gZ(s), where /cor is the corrected measured intensity distribution, Icoh and lincoh are the coherent and incoherent independent scattering, respectively, and g2 is the sharpening function.

work structure which was confirmed by SAXS measurements down to s = 0.07 nm-1. The well-known short-range order determined by (Si, Al)-tetrahedra is proved by the average atomic distances at 0.165 nm, 0.265 nm and 0.309 nm for sample A and at 0.167 nm, 0.267 nm and 0.312 nm ( T - O , O - O , T - T distances) for sample B (fig. 4(b)). Due to their broad distribution the

'E

TE 2O

3

:&

2

,j

Si-O- and the Al-O-distances in first coordination cannot be distinguished. Note that this first maximum is shifted to higher r-values with increasing AI content not only due to the greater average A1-O distance but also as a result of the Coulomb interaction between the AIOa/2-tetrahedra and the CaZ+-cations. Also, the correlation range, which equals 2.2 nm for vitreous silica [19],

o

I

1

2

10

a~ 0 c) e,l L

-2

-~o

~-3

"4"

0.1 0.2 0.3 0.4 0:5 1.2 1./. 1.6 r [nm] r [nm] Fig. 4. Structure functions calculated with the resolving limits of 8r = 0.02 nm for sample A (curve 1) and sample B (curve 2). (a) Distance distribution functions 4arrEC(r) with

02

0. z,

0.6

0.8

10

C(r)= ~ 21 r f~;l(s)ssin(sr)ds. (b) Radial distribution functions in the representation of the difference between radial and average electron density DIF(r): 2T¢

DIF(r) = 7 f si(s) s i n ( s r ) ds.

32

B. Himmel et al. / Structure of calcium aluminosificate glasses

_o F

~300

®

.°.2oo "c HIO 0 i

I

20

40

~-3.o 6

0.2

0.4

0.6

0.8

s [ n m -T]

1.0

1.2 1.4 r [nm]

Fig. 5. Intensity distributions (a) and distance distribution functions(b) of the glass A prepared with a cooling rate of 0.5 K min(curve 1) and by fibre spinning(curve 2).

becomes smaller with increasing A1 content (1.5 nm and 1.2 nm in the case of sample A and sample B, respectively) indicating a higher disorder of the whole network (fig. 3(b)). Further, the remarkable shift of the first scattering maximum to higher s-values (fig. 3(a)) and the decreasing period of the distance distribution function (fig. 4(a)) indicate that the continuous network is constructed by smaller topological units if the (CaO + Al203) content increases. Figure 5(a) shows the scattering curves of two glasses having the composition A which were cooled from the melt with different rates. One sample was cooled under normal conditions and its fictive temperature runs to Tf = 867 o C. The second sample having a higher fictive temperature was prepared by fibre spinning. The most striking differences could be found in the low-angle region of scattering: the level of fluctuation

'E

6

o

4

scattering increases with increasing cooling rate. However, the average short-range order does not change significantly with fictive temperature (fig.

5(b)). 5. Discussion 5.1. C a O - A l 2 0 3 - 2 S i 0 2

Taking glassy anorthite as an example, the question: which topological units participate in the network formation?, should be answered. In fig. 6(a), a comparison of the scattering function of a triclinic anorthite crystal having the size of 2x x 2y x l z cell units (416 atoms) with that of the corresponding glass sample is shown. By reason of clarity in relation to the significant differences, our attention should be focused on the

®

__ 2O E

t3 5 lO 0

.-K.

-2

o

L

~,-lo

-4

20

40

60

80

I00

s [ n m -~ ]

01.1

0.2

013

0.4

0.5 0.6 r[nm]

Fig. 6. Comparison between triclinic anorthite ( ) and the glass having the same composition ( . . . . . . ) in terms of the intensity distribution (a) and the DIF(r) function (b).

B. Himmel et al. / Structure of calcium aluminosilicate glasses

first scattering region reflecting particularly the effects of middle-range order. Regarding the peak positions and the common oscillations, a sufficient similarity is established and becomes stronger for greater s-values. The first scattering maximum of the glass (s = 18.20 nm -1) is also closely attached to the main crystal reflection of triclinic anorthite. Nevertheless, there are some striking differences between the scattering curves of the glassy and crystalline samples (fig. 6(a)): (i) the higher intensity of the main reflection engendered by the crystalline long-range order; (ii) the additional peak at s = 35 nm-1; (iii) the pronounced structurization of the whole intensity distribution; and (iv) the remarkable small-angle scattering caused by the small crystal size. As pointed out by Taylor and Brown [15,16], the short-range order of anorthite remains preserved in the glass. The increasing deviations for r > 0.35 nm (fig. 6(b)) can only be eliminated by a comprehensive modelling including middle-range order. An analogous comparison to the hexagonal modification of anorthite brings out important differences over all length scales, wherefore certain building units of that symmetry must completely be excluded. As a first approach to the glass structure of CaO "A120 3 • 2 SiO 2, the alternating order of (Si, A1)-tetrahedra in low-anorthite was interrupted and then an energy relaxation was carried out to adjust the corresponding shortrange order. This kind of disorder influences the intensity of the maximum at s - - 3 5 n m - I (fig. 6(a)). It should be re-emphasized that topological defects must be included when generating a network with (i) a real middle-range order, (ii) a limited correlation length, and (iii) an unchanged average short-range order. For instance, the feldspar-like four-membered rings, where front and back rings are joint through opposite tetrahedra as was proposed previously [15], cannot participate in the network generation because of their closely-meshed and relatively rigid structure. In particular, the intensity distribution in the region of the main scattering maximum (fig. 7) collides with the experimental curve. A more realistic structure modelling including the middle-range order could be achieved only by the incorporation of topological defects such as

~4

33

400 300

2OO

lO0 ~°°,°°°,°" I

I

20

40

60 s [nm-~]

Fig. 7. Intensity function of anorthite glass ( . . . . . . ) in comparison to that of a structure model ( ) characterized by feldspar-like cages of four-membered rings which was proposed in ref. [15].

adjacent three- and five-membered rings in the crystalline network. By that procedure, the regular 'crankshaft' propagation as well as the order of the elongated eight-membered planes could be interrupted. These defects bring strain into the network resulting in a limited correlation length. Figure 8 shows the experimental scattering functions of the glass having the composition of anorthite once again compared with that of the final model having an average density of about 2.5 g cm -3. The theoretical intensity distribution reproduces the experimental one sufficiently, also for details (fig. 8(b)). The short-range order demonstrated in fig. 8(c) by means of the difference distribution function could be recovered only with a broad and asymmetric distribution of the T - O - T bond angles (T stands for Si or AI). This distribution arises from a smearing over the tail of the bimodal crystalline bond angle distribution mentioned above (see section 2.2.). Taking ordering processes, e.g., a structural relaxation of the glassy network to the metastable equilibrium of the undercooled melt at T < Tf, into consideration, the concept of topological 'defect' and 'antidefect' including the feasibility of healing is very helpful. Since this process, which is accompanied by bond breaking, is a volume effect, its activation energy must correspond to that of the viscous flow (54 kcal/mol [5]). The network construction of the anorthite glass according to certain topological rules may also ex-

B. Himmel et al. / Structure of calcium aluminosilicate glasses

34

plain the volume crystallization producing the triclinic phase with heat treatment under normal pressure as experimentally observed. From the thermochemical data, it was also concluded that there is some degree of (Si, A1)-disorder in the anorthite glass [5,21]. This conclusion followed

3OO 200

.o. ,~ 100 I

20

4 s [ n m -~ ]

40O %t,

~- 3 0 0

I

E 20

@

U

= 10

200

~ 100

0

I

I

L

~ -10

,°°°oo°O*°° l

I

20

40

I

I

I

0.I

0.2

0.3

60 s [ n m -1]

=

--2

1.5 ~

120

160 200 ,5 [nm -~ ]

20 "7 E ::Ik 0 10

© i

.4

1.0

1.4

1

-1.5

0.2

80

0.5

0

®

4O

I

0.~ r[nm]

0.4

06

0.B

1.2 r[nm]

Fig, 9. Results of a weighted superposition of the scattering functions for silica and anorthite glass 0.3184SIO2+0.6816 CaO.ml203. 2SiO 2 (curve 2) in order to obtain the scattering as well as the structure functions of the glass having the composition CaO-AI203-4SiO 2.

from the observation that the heat of solution in the system CalA1204-SiO 2 was found at a minimum at AI/(AI + Si) slightly greater than 0.5.

•

L

5.2. C a O - A l 2 0 3 - 4 S i O 2

O

-1 0 0.2

0.4

0,6

0.8

1.0

r [rim]

Fig. 8. Results of simulation calculations for a glassy network model of the composition CaO-AI203.2SiO ~ ( ) in comparison with the experimental functions ( . . . . . . ).

As indicated above, it is commonly supposed that compositions with A1/(A1 + Si) < 0.5 probably consist of anorthite-like and SiO2-1ike clusters. Figure 9(a) shows the result of a mixed scattering curve obtained by a superposition of

B. Himmel et al. / Structure of calcium aluminosilicate glasses

the scattering parts of both, SiO 2 and C a O . A120 3 • 2SiO 2 glass (see fig. 3(a)), weighed due to the composition of the glass CaO • A120 3 • 4 SiO 2. There is a surprising good conformity between both intensity curves for s > 20 n m - I which is clearly mirrored in the representation si(s) of the reduced intensity function. That means in the picture of real space, that an identical short-range order could be obtained on the average as demonstrated in fig. 9(b). However, the significant differences for s < 20 nm-1 (fig. 9(a)) are clear indications that the SiO2-rich and the anorthite-rich regions connected together to build a unique homogeneous network cannot be larger than approximately 0.5 nm as shown by the distance distribution functions in fig. 9(c). With increasing fictive temperature, the level of fluctuation scattering increases as in vitreous silica [20] and as in vitreous B203 [34]. The connection between the l(s --* 0) value and the tictive temperature of a glass, given by [35]

I(0)

=p2kBT[~T(T),

with

T = Tf,

(3)

where Pe -2 is the average electron density, k B the Boltzmann constant and T the temperature, is exactly valid for a system without fluctuations in concentration. Using this equation in order to calculate the fictive temperature of the quickly cooled glass sample via the experimental I(0) values, we obtain an unrealistically high value. It was shown by Bhatia and Thornton [36] that the scattering function I(s) for a binary system can be expressed by a superposition of three structure factors. For the limit of s - ~ 0, these structure factors are defined according to ref. [36] by

SNN(O)

=

((AN))/N,

Scc(O) = N ( ( A c ) )

and

Sue(O) = (ANAc),

(4)

where ( ( A N ) 2) is the mean square fluctuation in the number of particles in the volume V, ((Ac) 2) the mean square fluctuation in the concentration, and ( A N Ac) the correlation between the two fluctuations Ac and AN. In relation to our results, that means that not only a configurational

35

part of disorder (topological defects) was frozen in but also a chemical part of disorder (concentration fluctuations). The first scattering maximum becomes slightly higher which can be caused by an intensity increase due to the chemical fluctuation scattering.

6. Concluding remarks Using WAXS in combination with computer simulation experiments, we have found that the structure of glassy anorthite can best be described as a short-range order and a network topology resembling those of triclinic anorthite. Our model of the anorthite glass is characterized by the following structural features within a continuous network. (1) The structure is built by completely interconnected TO4/z-tetrahedra (T = Si, AI) and Ca 2+ cations located in the vicinity of the A13+oZ/2-tetrahedra for charge-balancing. The distribution of the T - O - T bond angles is broad and asymmetric. (2) The alternating order of (Si, Al)-tetrahedra is interrupted. (3) T h e topology is d e t e r m i n e d by a 'crankshaft' arrangement of four-membered rings as well as by six-membered cages. (4) Disorder of the network is produced by topological defects like adjacent three- and fivemembered rings limiting the correlation length to 1.2 nm and producing strain in the network. The regular arrangement of the elongated eight-membered planes found in crystalline anorthite should be destroyed by additional tetrahedra. The assumption that SiO2-1ike and anorthitelike clusters determine the glass structures along the join Ca05AiO2-SiO2 for A1/(A1 + Si) < 0.5 [5] could not be confirmed by means of X-ray scattering. SAXS-measurements of the CaOAI20 3. 4 SiO 2 glass have indicated a homogeneous network. Similarities between the scattering curves of that glass and a model curve constructed by mixing the data for SiO 2 and anorthite glass are only observed for the short-range order (r < 0.5 nm).

36

B. Himmel et al. / Structure of calcium aluminosilicate glasses

The most striking changes in the scattering curve for CaO-A1203-4 SiO 2 samples with different fictive temperature were found in the region of the fluctuation scattering. It is suggested that the strong increase of the fluctuation level due to an increasing fictive temperature is caused not only by configurational fluctuation (increasing part of topological defects) but also by chemical fluctuation. A further structural understanding of the dependence on the fictive temperature by computer simulation can be achieved only with enlarged models having more than 1000 atoms.

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