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lithium phyllosilicate
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ELSEVIER
Journal of Non-Crystalline Solids 176 (1994) 164-171
Extended structures in crystalline phyllosilicates: silica ring systems in lithium, rubidium, cesium, and cesium/lithium phyllosilicate B.H.W.S. de Jong a,,, P.G.G. Slaats a, H.T.J. Super a, N. Veldman b, A.L. Spek b a InstituteforEarth Sciences and VeningMeineszlnstitutefor GeodynamicResearch, University of Utrecht, Budapestlaan 4, 3508 TA, Utrecht, The Netherlands b Department of Chemistry and Bijvoet Institute for Bioraolecular Research, University of Utrecht, 3508 TA Utrecht, The Netherlands Received 25 May 1993; revised manuscript received 25 May 1994
Abstract The crystal structure of lithium phyllosilicate, (Li2Si205) has been refined and those of rubidium (Rb2Si2Os), cesium (Cs2Si205) and cesium/lithium (CSl.33Li0.67Si205) phyllosilicate have been determined. Lithium phyllosilicate is orthorhombic, Ccc2, a = 5.807(2), b = 14.582(7), c = 4.773(3) A with R = 0.042. Rubidium phyllosilicate is monoclinic, C2/c, a = 9.851(4), b = 8.3789(7), c = 14.753(4) .~,/3 = 90.09(3)° and R = 0.121. Cesium phyllosilicate is monoclinic, P21/c, a = 10.061(5), b---8.609(5), c = 18.414(6) A, /3 = 122.76(4) ° and R---0.088. Cesium/lithium phyllosilicate is monoclinic, P21/m, a = 10.9547(9), b = 8.4281(7), c = 18.962(2), /3 = 90.314(7) °, R = 0.0501. A review of all crystalline alkali-alkaline earth phyllosilicates known to date indicates that the topology of the silica sheets for lithium, a-, 13-, 8-sodium and barium phyllosilicate consists of six-membered rings in chair, boat or mixed chair-boat conformations. The newly discovered rubidium and cesium phyllosilicate sheets consist of four- and eight-membered silica rings whereas the mixed cesium/lithium phyllosilicate sheet consists of four-, eight- and 12-membered rings.
1. Introduction Not much is known about the structure of glasses beyond second nearest neighbour atoms despite much work in the past 60 years (see, for example, Refs. [1-3]). Recently substantial efforts have been made to get a handle on this problem using molecular dynamics calculations as a minimalist means to formulate such extended structures in silica (see, for
* Corresponding author. Tel: + 31-30 535 065. Telefax: + 31-30 535 030. E-mail: [email protected].
example, Refs. [4-6]) and in particular glassy alkali disilicates (R20.2SiO2(R = Li, Na, K, Rb, Cs)) [ 7 13]. The reason for choosing the alkali disilicate system is that, besides the fact that all alkalis form crystalline phyllosilicates and that disilicate composition glasses are readily made, the alkali-oxygen interaction can be described by a simple potential as predicated by its unequivocal Lewis acid-base, i.e., electron acceptor-electron donor, character. A corollary to this acid-base, y i n / y a n character is the concept of alkalis as network modifier and silica as network former convincingly demonstrated by the orders of magnitude decrease in viscosity of silica by
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 4 3 1 - 5
B.H.W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
addition of any alkali (see, for example, Refs. [14151). Conventional molecular dynamics (MD) modelling of alkali disilicate glasses employs as initial configuration the atomic positions determined by X-ray diffraction for crystalline sodium or lithium phyllosilicate (cf. Ref. [16] for a general discussion on MD modelling and Adams [17] specifically for MD modelling of glassy lithium disilicate). Randomization is accomplished by 'heating' these crystalline atomic arrangements to temperatures far above their melting point, followed by 'cooling' to a local minimum in potential energy at a specified temperature using properly chosen, two- and three-body potentials. Accurate crystallographic data are a prerequisite for successful modelling of this type. Five crystalline alkali and alkaline earth phyllosilicate structures are known to date: Li2Si205 [18], or-, 13-, ~-Na2Si205 [19-21], and BaSi205 [22]. Of these structures, the correctness of the lithium phyllosilicate structure has been a matter of some debate [23]. To settle this issue, we have re-determined the crystal structure of this compound. Additionally we have determined the crystal structure of rubidium, cesium and cesium/lithium phyllosilicate in order to expand the number of usefull initial configurations for molecular dynamics simulation of alkali and mixed alkali disilicate glasses. We have verified the correctness of the crystal structure for lithium phyllosilicate [18] and the topological equivalence of its silica sheet with those of ct-, 13- and ~-sodium, and barium phyllosilicate, all of which are characterized by sixmembered silica rings in chair, boat or mixed chair-boat conformations. We have discovered that the silica sheets of rubidium and cesium phyllosili-
cate are topologically equivalent to one another, containing four- and eight-membered silica rings, but not to the sheet of the cesium/lithium phyllosilicate which has four-, eight-, and 12-membered rings.
2. Experimental techniques and results
Crystals were synthesized by programmed cooling from melts of the proper composition. Cesium and lithium phyllosilicate were made in this manner without any problem. Rubidium phyllosilicate precipitated from the melt only after repeated trials using different cooling and heating cycles. For potassium phyllosilicate, we did not succeed as yet in growing a crystal suitable for X-ray diffraction nor were we successful in growing a suitable R b / L i phyllosilicate. The starting composition of cesium and lithium used in making the mixed cesium/lithium phyllosilicate was one to one, the crystalline product with as composition CSl.33Li0.675i205 e x h i b i t i n g incongruent precipitation. All crystals showed the large blades characteristic of sheet silicates. Lithium phyllosilicate is not hygroscopic nor is cesium/lithium phyllosilicate. Rubidium and cesium phyllosilicate are on the other hand exceedingly hygroscopic, their hygroscopicity presumably being the primary impediment to their earlier crystal structure determination. Special measures are required to prevent dissolution of these phases in atmospheric moisture. Thus the batches containing glass with crystallites were immediately dropped in paraffin oil, and transported to the X-ray diffraction laboratory. All data were collected on an Enraf-Nonius CAD-
Table 1 Crystal data Phase:
Li2 Si 205
Space group: a (,~): b (.~,): c (,~): /3 (°): Z: R a.
" R : {EIFo-IF~ II}/E&
Rb 2Si 205
Cs 2Si z05
Cs 1.33t i 0.67Si 2O5
Ccc2
C2/c
P2Jc
P2l/m
5.807 (2)
9.851 (4)
10.061 (5)
10.9547 (9)
14.582 (7) 4.773 (3)
8.3789 (7) 14.753 (4) 90.09 (3) 8 0.121
8.609 (5) 18.414 (6) 122.76 (4) 8 0.088
8.4281 (7) 18.962 (2) 90.314 (7) 12 0.0501
4 0.042
165
B.H. W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
166
Table 2 Positional Parameters for Li2Si208, Rb2Si2Os, Cs2Si205 and CSl.33Li0.67Si205 x
y
z
EV, j ~
Li2Si205 Si 1 O1 O2 03 Li I
0.1552(3) 0.3295(9) 0.0932(8) 1/4 0.346(2)
0.14836(12) 0.0718(3) 0.1377(3) 1/4 0.0584(8)
0.13520 b 0.0491(13) 0.4785(14) 0.0781(18) 0.625(5)
- 4.29(IV) 2.03(1V) 2.17(III) 2.23(I1) - 1.02(IV)
0.3737(8) 0.1313(8) 0 0.251(9) 1/2 0.357(2) 0.136(3) 0.379(3) 0.3396(2) 0.0881(3)
0.1883(10) 0.0670(1) 0.105(6) 0.119(11) 0.159(5) 0.380(3) 0.144(5) 0.113(4) 0.3888(4) 0.3724(5)
0.3090(7) 0.1954(7) 1/4 0.256(6) 1/4 0.300(3) 0.1002(17) 0.4068(17) 0.08193(19) 0.4093(2)
- 4.87(IV) - 5.10(IV) 2.52(II) 2.61(II) 2.66(II) 2.29(II) 1.94(IV) 1.86(1V) - 0.91(VII) - 0.86(VII)
0.0741(12) 0.4384(12) 0.4239(11) 0.0610(12) 0.001(3) 0.510(4) 0.433(4) 0.544(4) - 0.043(3) 0.063(3) 0.256(3) 0.064(4) 0.237(3) 0.548(3) 0.2663(3) 0.1814(3) 0.6830(3) 0.2412(3)
0.9236(10) 0.9392(10) 1.3165(9) 1.2995(10) 1.362(3) 0.875(3) 1.131(3) 1.374(3) 0.854(3) 1.111 (3) 0.876(3) 0.856(3) 1.352(2) 0.891(3) 0.0988(2) 0.6325(2) 0.1207(2) 0.1257(2)
0.1965(8) 0.3017(7) 0.3065(7) 0.1954(7) 0.103(2) 0.399(2) 0.294(3) 0.406(2) 0.106(2) 0.200(2) 0.2285(19) 0.279(2) 0.2720(2) 0.2642(16) 0.41886(19) 0.08939(19) 0.0912(2) 0.0840(2)
-4.05(IV) - 3.76(IV) - 3.94(IV) - 4.17(IV) 1.87(V) 1.80(V) 2.23(IV) 1.73(V) 1.96(V) 2.38(V) 2.20(IV) 2.24(IV) 2.23(IV) 2.28(IV) - 1.04(VII) -0.99(VII) - 1.07(VII) - 1.00(VIII)
0.24923(14) 0.00476(14) 0.01750(14) 0.25519(14) 0.49519(14) 0.4855 0(14) 0.3589(4) 0.2693(4) 0.2339(5) 0.1265(3) 0.0008(6) - 0.1145(4) - 0.0107(3) - 0.0190(6) 0.0895(4)
0.0592(2) 0.0579(2) 0.0582(2) 0.0598(2) 0.0640(2) 0.0623(2) 0.0336(5) 0.0256(6) 1/4 0.0034(6) 1/4 0.0051(6) 0.0036(6) 1/4 0.0081(6)
0.09537(9) 0.16525(9) 0.32896(9) 0.41235(9) 0.33517(9) O.17017(9) O.1536(2) 0.0228(2) 0.0876(4) 0.1364(3) 0.1681(4) 0.1208(3) 0.2469(2) 0.3252(4) 0.3745(3)
-
Rb2Si205 Si 1 Si 2 O1 O2 O3 04 05 06 Rb 1 Rb 2
Cs2Si205 Si I Si 2 Si 3 8i 4 01 02 03 04 05 06 07 O8 09 O1o Cs i Cs 2 Cs 3 Cs 4
Cs1.33Lio.67Si205 Si~ Si 2 Si 3 Si 4 Si 5 Si 6 01 02 03 04 05 06 07 O8 09
-
-
4.07(IV) 4.16(IV) 4.13(IV) 4.10(IV) 4.13(IV) 4.14(IV) 2.08(111) 1.85(V) 2.03(VI) 2.14(IV) 2.17(VIII) 1.90(VI) 2.19(IV) 2.22(VIII) 1.93(VI)
B.H. W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
167
Table 2 (continued) x
y
z
EV/j a
CsI 33Lio.67Si205 Ow O 11 O12 O13 O14 O15 O16 O17 Ois Li 1 Li 2 Cs a Cs 2 Cs 3 Cs 4 Cs 5 Cs 6 Cs 7 Cs 8
-
0.1531(4) 0.2375(5) 0.2468(4) 0.3870(4) 0.4535(6) 0.6232(4) 0.5156(4) 0.4488(6) 0.5951(4) 0.7397(8) 0.2384(9) 0.77477(5) 0.70829(5) 0.27506(5) 0.20649(5) 0.45871(5) 0.02947(5) 0.51763(5) 0.03920(5)
0.3549(3) 0.4217(4) 0.4844(2) 0.3719(2) 0.3412(4) 0.3669(3) 0.2522(2) 0.1689(4) 0.1218(3) 0.0726(6) 0.4197(6) 0.22908(3) 0.22500(3) 0.27803(3) 0.26565(3) 0.48610(3) 0.51656(3) 0.00831(3) 0.01696(3)
0.0014(5)
1/4 - 0.0287(6) 0.0393(6) - 1/4 - 0.0256(6) -0.0162(6) -1/4 - 0.0168(6) 0.5111(13) 0.5164(15) 3 /4 1 /4 1 /4 3 /4 1 /4 1 /4 1 /4 3 /4
a YW,j is the valence calculated according to the parameterization in Ref. [27].
4T/rotating anode s~(stem using Mo Ko~ X-ray radiation (A = 0.71073 A). Lithium and cesium/lithium phyllosilicate were measured at room temperature, rubidium and cesium phyllosilicate at 150 K to minimalize moisture adsorption. The intensity data were corrected for Lp and absorption (DIFABS, [24]). The structure was refined on F by full matrix least squares (SHELX 76, [25]). All atom positions were refined with anisotropic displacement parameters. Geometrical calculations were carried out with PLATON [26] on a DEC5000 cluster. The results of our crystal structure determinations are given in Tables 1 and 2. Included in Table 2 is also a valence calculation based on the parameterization of Brese and O'Keeffe [27].
3. Discussion Six-membered silica rings are the most common silica conformation found in phyllosilicates. Four main types of such ring systems can occur as illustrated in Fig. 1. Here the silicon atoms are positioned on the corners of the honeycomb network omitting the oxygen atoms connecting these atoms. The flat sheet shown in Fig. I(A) is the one commonly
-
2.11(V) 2.14(VIII) 1.92(IV) 2.11(IV) 2.12(IV) 1.91(V) 2.22(V) 2.15(VI) 1.91(VI) 1.09(III) 1.09(III) 1.07(XI) 0.96(XII) 0.94(IX) 1.00(IV) 0.88(VIII) 0.92(IX) 0.95(X) 0.89(X)
b Fixed.
encountered in micas. Folding this flat sheet along the dotted lines results in three types of corrugated sheets shown in Figs. I(B)-(D). In Fig. I(B) all silica rings are in boat conformation, in Fig. I(C) all rings form chairs and in Fig. I(D) boats and chairs coexist. The sheet with all silica rings in chair conformation is the one encountered in lithium and ~and /3-sodium phyllosilicate. The lithium phyllosilicute sheet is most puckered followed by that of ~and ~3-sodium phyllosilicate. This decrease in sheet puckering with higher mass alkalis is well known and discussed by Liebau [28]. ~-sodium phyllosilicute [21] has a silica sheet with all silica rings in boat conformation as illustrated in Fig. I(B), whereas mixed boat and chair conformations such as illustrated in Fig. I(D) occur in high sanbornite, BaSi205 [22]. In three-dimensional silica networks, mixed forms of chairs and boats can also occur. Thus, whereas cristobalite contains only chairs, tridymite contains sheets made from chairs which are connected to one another by boats. In the latter structure, the ratio of chairs to boats is one. The second silica sheet topology is illustrated in Fig. 2 and consists of interconnected four- and eight-membered rings of silica. This coexistence of four and eight-membered ring systems is much rarer,
168
B.H. W.S. de Jong et al. /Journal of Non-CrystaUine Solids 176 (1994) 164-171
occuring only in the mineral apophyllite albeit with a different topology [29]. Although the combination 4 - 8 is rare, this should not be construed to mean that varying ring sizes in sheet silicates do not occur. Thus 4 - 6 - 8 - , 4 - 5 - 6 - 8 - and 4-6-12-ring systems have been discovered [28,30]. The rings in rubidium are more regular than those in cesium phyllosilicate. Both are puckered differently from those observed in lithium and sodium phyllosilicate as illustrated in Fig. 3. The observed 4-8-ring size of the heavier
Fig. 2. Topology of the silica sheet in Rb and Cs phyllosilicate. The actual configuration shown is the one for Rb phyllosilicate. The grey large spheres represent Si, the small black spheres represent O. Note the four- and eight-membered silica rings.
Fig. 1. The topology of six-membered rings in sheet silicates. (A) Honeycomb of six-membered rings with on each corner a Si atom. The O atoms above or below the plane are omitted as well as those between the Si atoms in the sheet. (B) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which all six-membered rings occur in boat conformation occurs in &Na2Si205. (C) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which all six-membered rings occur in chair conformation is characteristic for crystalline Li and a- and 13-Na phyllosilicate. (D) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which six-membered rings occur in chair as well as boat conformation is characteristic for crystalline Ba phyliosilicate.
alkali phyllosilicates was of course unanticipated in view of the identical six-ring size for lithium as well as barium phyllosilicate, i.e., the lightest and heaviest of the alkali and alkaline earth phyllosilicates. Since chair and boat designations only pertain to six-membered rings, some other designation is needed to describe eight-membered rings in sheets. The upward pucker of four of the eight silica tetrahedra in an eight-membered ring can be described in analogy with boranes as possessing a nest-like conformation. We designate the downward pucker as possessing a sidewinder (trail of a rattlesnake-like) conformation. This last conformation is the one observed for crystalline rubidium and cesium phyllosilicate. The third silica sheet topology is illustrated in Fig. 4 and consists of interconnected four-, eightand 12-membered rings of silica. This combination of ring systems is novel to the best of our knowledge. A side view of the structure in Fig. 3 shows the strong deviation from what one usually encoun-
B.H. W.S. de Jong et aL /Journal of Non-Crystalline Solids 176 (1994) 164-171
ters as a flat silica sheet. Here the silica sheet folds itself around the cesium atoms, the end of the cage being locked up by three-fold coordinated lithium atoms as illustrated in Fig. 5. The silicate sheet structure resembles that of a crash-zone built into car hoods and consists of tunnels, the cesium atoms being encapsulated in a silicate tunnel with lithium atoms closing it. Two tunnel types exist, one being closed off by one Li atom, the other by two. The extreme tortuosity of what is in essence a silica surfaces suggests possible very different surface structures from the ones considered hitherto. Note the 4-8-12-membered rings as opposed to the 4 - 6 8-membered rings which one might expect based on a simple mixing of the lithium and cesium phyllosilicate endmember topologies. Besides this strong puckering of the sheet, attention should be paid to
Fig. 3a: Rb2Si205
.Yk./k.rkik./-k Fig. 3b Li2Si205
wzrLr Fig 3c Cel 33Li067Si205
Fig. 3. Side view of silica sheets for Li, Rb and mixed C s / L i phyllosilicate. Only the Si atoms are drawn. (A) The sidewinder configuration in Rb and Cs phyllosilicate is indicated by the grey connections between Si atoms. (B) The grey shading of siliconsilicon connections in Li and et-13-Na phyllosilicate shows a side view of the chair configuration. (C) The crash-zone-like arrangement of the silica sheet in Cs-Li phyllosilicate.
169
the unequivocal three-fold coordination around the lithium atom to three O(non-bridging) atoms. We only once encountered such lithium coordination before in lithium nesosilicate for one of the twenty unique lithium atoms [31] where we used a tighter oxygen coordination criterion around Li as tends to be employed by other authors for topologically similar structures [32-34] The quality of the structure determinations can be gauged among other criteria from the R factor measuring the goodness of fit between observed and calculated structure factors F. A low R factor being desirable, it is clear from Table 1 that the Li, Cs and C s / L i phyllosilicate structure determinations vary between very good and acceptable. This is, however, not the case for Rb phyllosilicate which has such high R factor that the structure on itself would not be publishable. That this structure determination is not so hot can also be learned from inspection of the calculated valences. According to Pauling's second rule, the oxygen valence should be around 2, i.e., the charge donated by the cations to oxygen should be two electrons in order to complete its Lewis octet. Inspection of the oxygen valences in Table 2, calculated using Brese and O'Keeffe's [27] parameterization, show that their values fall within expected ranges except for the oxygen atoms in Rb phyllosilicate. The valence donated to the oxygen atom has to come from the surrounding cations and these calculated values are indicated with a negative sign in Table 2. Thus, silicon with four valence electrons can at most have a valence of - 4 . Again, the tabulated values in Table 2 indicate that the Si valence for the crystalline phyllosilicate structures varies around - 4 except for the rubidium phyllosilicate where Si valences around - 5 are found, indicating that not all is well with the silicon positions in this last structure. It is clear that, although the sheet image in Rb phyllosilicate is correct, better crystals with a better mozaic spread have to be grown in order to get precise structural information. The importance of these newly discovered variations in ring sizes in crystalline alkali-phyllosilicates for the structural elucidation of glassy alkali-disilicates is that in all likelihood small but in particular large rings systems are underestimated in their molecular dynamics models. It is not clear if the presence of such extended ring systems can be ascer-
170
B.H. W.S. de Jong et al. /Journal of Non-CrystaUine Solids 176 (1994) 164-171
~-a' ~-~ a-~ ~ - a - a - ~ a-a-a-~ II
I I I I
Silicon Oxygen
,-~ II ~-~
II ~-~
I I I I
~-~-,-~ / I l l 9-~-~-~
~-~
II ~-~
I
~-~-o-~ I / l l ~-~-O-~
~-~
Fig. 4. Top view on the sheet of Cs/Li phyllosilicate. The tortuosity of the sheet makes the actual topology difficult to discern in a projection. The sheet topology is drawn in the bottom right hand comer.
tained by conventional measures of goodness of fit between experiment and theory as provided by radial distribution analysis. It may prove to be a solid test of molecular dynamics modelling to ascertain if lithium disilicate glasses modelled with the crystalline lithium phyllosilicate atomic positions as starting parameters give a topological result similar to those using the rubidium, cesium or cesiumlithium phyllosilicate atomic positions in which the heavy alkalis are replaced by lithium. It should also be ascertained as a property of the potentials employed in MD modelling if these newly found crystal structures form indeed the most stable atomic configuration relative to all possible ones. What is clear is that in discussing the properties of mixed alkali glasses such variations in sheet topology may be of consequence as it may result in an inhomogeneous distribution of ring systems [35]. What is also clear is that, in describing the mixed alkali effect, strong alkali-alkali interactions have to be included since it is observed that, whereas, with the exception of lithium phyllosilicates, all single alkali phyllosilicates are hygroscopic, it is a typical
characteristic of mixed alkali disilicate glasses such as Li-Rb and Li-Cs disilicates that they are not hygroscopic at all [15]. It is this last observation which resulted in making non-hygroscopic soluble silicates and the manufacture of extra moisture resistant welding electrodes [36].
4. Conclusion The crystal structure of lithium phyllosilicate has been refined and those of rubidium, cesium and cesium-lithium phyllosilicate have been determined. The silica sheet topology of the low mass alkali phyllosilicates and of barium phyllosilicate, the only known alkaline earth phyllosilicate, are characterized by six-membered rings in chair or boat conformation. The sheet topology of the high mass alkali phyllosilicates is characterized by interconnected four- and eight-membered rings of silica. The sheet topology of a crystalline mixed cesium-lithium phyllosilicate consists of interconnected four-, eight- and 12-membered rings. It is therefore not a mixture
B.H. W.S. de .long et aL /Journal of Non-Crystalline Solids 176 (1994) 164-171
000Ooo~
°oo000
Cesium
0
Oxygen
Silicon
C)
Lithium
Fig. 5. Side view of the Cs/Li phyllosilicate conformation with the Cs and Li atoms drawn in. Note the folding of the silica sheet around the Cs atoms and how the Cs containing tunnels are closed off by three-fold oxygen coordinated Li atoms. Note that there are two types of tunnel: one type which is closed off by one Li atom, the other by two.
between the six-ring structure of crystalline Li2Si205 and the 4-8-ring structure of crystalline Cs2Si205.
References [1] Elliot, S.R., Structure of Amorphous Materials. Encyclopedia of Applied Physics, G.L. Trigged. VCH Weinheim Germany D-6940, vol 1 (1991) 559-592. [2] Wright, A.C., ed (1991). The structural chemistry of silicates. Trans. Am. Cryst. Ass. Evans volume, no 27, pp 329. [3] Catlow, C.R.A., ed. (1994) Defects and disorder in crystalline and amorphous solids, NATO ASI Series, vol 418, pp 511. Kluwer Ac. Pub. Dordrecht, the Netherlands. [4] Rustad, J.R., D.A. Yuen, F.J. Spera, (1991) Phys. Earth Plan. Int. 65, 210-230. [5] Rustad, J.R., D.A. Yuen, F.J. Spera, (1991) J. Geophys. Res. 96, 19665-19673. [6] Rustad, J.R., D.A. Yuen, F.J. Spera, (1991) Phys. Rev. B. 44, 2108-2121. [7] Vessai, B., G.N. Greaves, P.T. Marten, A.V. Chadwick, R. Mole, & S. Houde-Walters, Nature (1992) 356-358.
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[8] Soules, T.F., Glass Science and Technology, vol 4a, D.R. Uhlmann & N.J. Kreidl eds. Ac Press, Boston (1990) 267338. [9] Kubicki, J.D. & A.C. Lasaga, Am. Min. (1988) 73, 941-955. [10] Kubicki, J.D. & A.C. Lasaga, Adv. Phys. Geochem. (1990) 8, Chpt 1, J. Ganguly ed., Springer. [11] Kubicki, J.D. & A.C. Lasaga, Phys. Chem. Min. (1991) 17(8), 661-673. [12] Huang. C. & A.N. Cormack, J. Mat. Chem. (1992) 2(3), 281-287. [13] Huang, C. & A.N. Cormack, J. Chem. Phys. (1991) 3634. [14] Bansal, N.P. & R.H. Doremus, (1986) Handbook of glass properties Ac. Press, New York, pp 680. [15] de Jong, B.H.W.S., Glass. Ullmann's Encyclopedia of Industrial Chemistry, (VCH, Weinheim, 1989) A12, pp. 365-432. [16] Dove, M.T., (1993) Introduction to Lattice Dynamics, Cambridge Univ. Press, pp 258. [17] Adams, J.W., (1994) Morphogenesis in lithium disilicate glass: internal nucleation in amorphous structures. Thesis, Cambridge University, pp 170. [18] Liebau, F., Acta Cryst. (1961) 14, 389-395. [19] Pant, A.K., Acta Cryst. (1968) B 24, 1077-1083. [20] Pant, A.K. & D.W.J. Cruickshank, Acta Cryst. (1968) B 24, 13-19. [21] Jacobsen, H., (1991) Neue Untersuchungen an Natriumdisilikat, Diplomarbeit, Hannover, pp 140. [22] Hesse, K.-F., F. Liebau, (1980) Z. Kristallgr. 153, 33-41. [23] Dupree, R., D. Holland, & M.G. Mortuza, J. Non-Cryst. Solids (1990) 116, 148-160. [24] Walker, N. & D. Stuart, (1983) Acta Cryst. A 39, 158-166. [25] Sheldrick, G.M., (1976) SHELX 76. A program for crystal structure determination, Cambridge University, Great Britain. [26] Spek, A.L., (1990) Acta Cryst. A46, C34. [27] Brese, N.E. & M. O'Keeffe, (1991) Acta Cryst. B47, 192197. [28] Liebau, F., Structural chemistry of silicates, Springer (1985) pp 347. [29] Taylor, W.H. & St. Naray-Szabo, Z. Krist. (1931) 77, 146158. [30] Haile, S.M., B.J. Wuensch, R.A. Laudise, R. Opila, T. Siegrist, & B.M. Foxman, (1991) Trans. Am. Cryst. Ass. 27, 77-94. [31] de Jong, B.H.W.S., D. Ellerbroek, & A.L. Spek, (1994) Acta Cryst B (in press) [32] Hong, H. Y-P., (1978) Mat. Res. Bull. 13, 117-124. [33] Baur, W.H. & Ohta, T., (1982) J. Solid State Chem. 44, 50-59. [34] Tranqui, D, R.D. Shannon, H.-Y. Chen, S. lijima, & W.H. Baur, (1979). Acta Cryst. B 35, 2479-2487. [35] Uchino, T., T. Sakka, Y. Ogata, & H. lwasaki, J. Non-Cryst. Solids (1992) 146, 26-42. [36] de Jong, B.H.W.S. & H. Kasteel, (1986) Extra Moisture Resistance Technology. Smitweld Techn Bull, pp 31.
ELSEVIER
Journal of Non-Crystalline Solids 176 (1994) 164-171
Extended structures in crystalline phyllosilicates: silica ring systems in lithium, rubidium, cesium, and cesium/lithium phyllosilicate B.H.W.S. de Jong a,,, P.G.G. Slaats a, H.T.J. Super a, N. Veldman b, A.L. Spek b a InstituteforEarth Sciences and VeningMeineszlnstitutefor GeodynamicResearch, University of Utrecht, Budapestlaan 4, 3508 TA, Utrecht, The Netherlands b Department of Chemistry and Bijvoet Institute for Bioraolecular Research, University of Utrecht, 3508 TA Utrecht, The Netherlands Received 25 May 1993; revised manuscript received 25 May 1994
Abstract The crystal structure of lithium phyllosilicate, (Li2Si205) has been refined and those of rubidium (Rb2Si2Os), cesium (Cs2Si205) and cesium/lithium (CSl.33Li0.67Si205) phyllosilicate have been determined. Lithium phyllosilicate is orthorhombic, Ccc2, a = 5.807(2), b = 14.582(7), c = 4.773(3) A with R = 0.042. Rubidium phyllosilicate is monoclinic, C2/c, a = 9.851(4), b = 8.3789(7), c = 14.753(4) .~,/3 = 90.09(3)° and R = 0.121. Cesium phyllosilicate is monoclinic, P21/c, a = 10.061(5), b---8.609(5), c = 18.414(6) A, /3 = 122.76(4) ° and R---0.088. Cesium/lithium phyllosilicate is monoclinic, P21/m, a = 10.9547(9), b = 8.4281(7), c = 18.962(2), /3 = 90.314(7) °, R = 0.0501. A review of all crystalline alkali-alkaline earth phyllosilicates known to date indicates that the topology of the silica sheets for lithium, a-, 13-, 8-sodium and barium phyllosilicate consists of six-membered rings in chair, boat or mixed chair-boat conformations. The newly discovered rubidium and cesium phyllosilicate sheets consist of four- and eight-membered silica rings whereas the mixed cesium/lithium phyllosilicate sheet consists of four-, eight- and 12-membered rings.
1. Introduction Not much is known about the structure of glasses beyond second nearest neighbour atoms despite much work in the past 60 years (see, for example, Refs. [1-3]). Recently substantial efforts have been made to get a handle on this problem using molecular dynamics calculations as a minimalist means to formulate such extended structures in silica (see, for
* Corresponding author. Tel: + 31-30 535 065. Telefax: + 31-30 535 030. E-mail: [email protected].
example, Refs. [4-6]) and in particular glassy alkali disilicates (R20.2SiO2(R = Li, Na, K, Rb, Cs)) [ 7 13]. The reason for choosing the alkali disilicate system is that, besides the fact that all alkalis form crystalline phyllosilicates and that disilicate composition glasses are readily made, the alkali-oxygen interaction can be described by a simple potential as predicated by its unequivocal Lewis acid-base, i.e., electron acceptor-electron donor, character. A corollary to this acid-base, y i n / y a n character is the concept of alkalis as network modifier and silica as network former convincingly demonstrated by the orders of magnitude decrease in viscosity of silica by
0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 4 ) 0 0 4 3 1 - 5
B.H.W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
addition of any alkali (see, for example, Refs. [14151). Conventional molecular dynamics (MD) modelling of alkali disilicate glasses employs as initial configuration the atomic positions determined by X-ray diffraction for crystalline sodium or lithium phyllosilicate (cf. Ref. [16] for a general discussion on MD modelling and Adams [17] specifically for MD modelling of glassy lithium disilicate). Randomization is accomplished by 'heating' these crystalline atomic arrangements to temperatures far above their melting point, followed by 'cooling' to a local minimum in potential energy at a specified temperature using properly chosen, two- and three-body potentials. Accurate crystallographic data are a prerequisite for successful modelling of this type. Five crystalline alkali and alkaline earth phyllosilicate structures are known to date: Li2Si205 [18], or-, 13-, ~-Na2Si205 [19-21], and BaSi205 [22]. Of these structures, the correctness of the lithium phyllosilicate structure has been a matter of some debate [23]. To settle this issue, we have re-determined the crystal structure of this compound. Additionally we have determined the crystal structure of rubidium, cesium and cesium/lithium phyllosilicate in order to expand the number of usefull initial configurations for molecular dynamics simulation of alkali and mixed alkali disilicate glasses. We have verified the correctness of the crystal structure for lithium phyllosilicate [18] and the topological equivalence of its silica sheet with those of ct-, 13- and ~-sodium, and barium phyllosilicate, all of which are characterized by sixmembered silica rings in chair, boat or mixed chair-boat conformations. We have discovered that the silica sheets of rubidium and cesium phyllosili-
cate are topologically equivalent to one another, containing four- and eight-membered silica rings, but not to the sheet of the cesium/lithium phyllosilicate which has four-, eight-, and 12-membered rings.
2. Experimental techniques and results
Crystals were synthesized by programmed cooling from melts of the proper composition. Cesium and lithium phyllosilicate were made in this manner without any problem. Rubidium phyllosilicate precipitated from the melt only after repeated trials using different cooling and heating cycles. For potassium phyllosilicate, we did not succeed as yet in growing a crystal suitable for X-ray diffraction nor were we successful in growing a suitable R b / L i phyllosilicate. The starting composition of cesium and lithium used in making the mixed cesium/lithium phyllosilicate was one to one, the crystalline product with as composition CSl.33Li0.675i205 e x h i b i t i n g incongruent precipitation. All crystals showed the large blades characteristic of sheet silicates. Lithium phyllosilicate is not hygroscopic nor is cesium/lithium phyllosilicate. Rubidium and cesium phyllosilicate are on the other hand exceedingly hygroscopic, their hygroscopicity presumably being the primary impediment to their earlier crystal structure determination. Special measures are required to prevent dissolution of these phases in atmospheric moisture. Thus the batches containing glass with crystallites were immediately dropped in paraffin oil, and transported to the X-ray diffraction laboratory. All data were collected on an Enraf-Nonius CAD-
Table 1 Crystal data Phase:
Li2 Si 205
Space group: a (,~): b (.~,): c (,~): /3 (°): Z: R a.
" R : {EIFo-IF~ II}/E&
Rb 2Si 205
Cs 2Si z05
Cs 1.33t i 0.67Si 2O5
Ccc2
C2/c
P2Jc
P2l/m
5.807 (2)
9.851 (4)
10.061 (5)
10.9547 (9)
14.582 (7) 4.773 (3)
8.3789 (7) 14.753 (4) 90.09 (3) 8 0.121
8.609 (5) 18.414 (6) 122.76 (4) 8 0.088
8.4281 (7) 18.962 (2) 90.314 (7) 12 0.0501
4 0.042
165
B.H. W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
166
Table 2 Positional Parameters for Li2Si208, Rb2Si2Os, Cs2Si205 and CSl.33Li0.67Si205 x
y
z
EV, j ~
Li2Si205 Si 1 O1 O2 03 Li I
0.1552(3) 0.3295(9) 0.0932(8) 1/4 0.346(2)
0.14836(12) 0.0718(3) 0.1377(3) 1/4 0.0584(8)
0.13520 b 0.0491(13) 0.4785(14) 0.0781(18) 0.625(5)
- 4.29(IV) 2.03(1V) 2.17(III) 2.23(I1) - 1.02(IV)
0.3737(8) 0.1313(8) 0 0.251(9) 1/2 0.357(2) 0.136(3) 0.379(3) 0.3396(2) 0.0881(3)
0.1883(10) 0.0670(1) 0.105(6) 0.119(11) 0.159(5) 0.380(3) 0.144(5) 0.113(4) 0.3888(4) 0.3724(5)
0.3090(7) 0.1954(7) 1/4 0.256(6) 1/4 0.300(3) 0.1002(17) 0.4068(17) 0.08193(19) 0.4093(2)
- 4.87(IV) - 5.10(IV) 2.52(II) 2.61(II) 2.66(II) 2.29(II) 1.94(IV) 1.86(1V) - 0.91(VII) - 0.86(VII)
0.0741(12) 0.4384(12) 0.4239(11) 0.0610(12) 0.001(3) 0.510(4) 0.433(4) 0.544(4) - 0.043(3) 0.063(3) 0.256(3) 0.064(4) 0.237(3) 0.548(3) 0.2663(3) 0.1814(3) 0.6830(3) 0.2412(3)
0.9236(10) 0.9392(10) 1.3165(9) 1.2995(10) 1.362(3) 0.875(3) 1.131(3) 1.374(3) 0.854(3) 1.111 (3) 0.876(3) 0.856(3) 1.352(2) 0.891(3) 0.0988(2) 0.6325(2) 0.1207(2) 0.1257(2)
0.1965(8) 0.3017(7) 0.3065(7) 0.1954(7) 0.103(2) 0.399(2) 0.294(3) 0.406(2) 0.106(2) 0.200(2) 0.2285(19) 0.279(2) 0.2720(2) 0.2642(16) 0.41886(19) 0.08939(19) 0.0912(2) 0.0840(2)
-4.05(IV) - 3.76(IV) - 3.94(IV) - 4.17(IV) 1.87(V) 1.80(V) 2.23(IV) 1.73(V) 1.96(V) 2.38(V) 2.20(IV) 2.24(IV) 2.23(IV) 2.28(IV) - 1.04(VII) -0.99(VII) - 1.07(VII) - 1.00(VIII)
0.24923(14) 0.00476(14) 0.01750(14) 0.25519(14) 0.49519(14) 0.4855 0(14) 0.3589(4) 0.2693(4) 0.2339(5) 0.1265(3) 0.0008(6) - 0.1145(4) - 0.0107(3) - 0.0190(6) 0.0895(4)
0.0592(2) 0.0579(2) 0.0582(2) 0.0598(2) 0.0640(2) 0.0623(2) 0.0336(5) 0.0256(6) 1/4 0.0034(6) 1/4 0.0051(6) 0.0036(6) 1/4 0.0081(6)
0.09537(9) 0.16525(9) 0.32896(9) 0.41235(9) 0.33517(9) O.17017(9) O.1536(2) 0.0228(2) 0.0876(4) 0.1364(3) 0.1681(4) 0.1208(3) 0.2469(2) 0.3252(4) 0.3745(3)
-
Rb2Si205 Si 1 Si 2 O1 O2 O3 04 05 06 Rb 1 Rb 2
Cs2Si205 Si I Si 2 Si 3 8i 4 01 02 03 04 05 06 07 O8 09 O1o Cs i Cs 2 Cs 3 Cs 4
Cs1.33Lio.67Si205 Si~ Si 2 Si 3 Si 4 Si 5 Si 6 01 02 03 04 05 06 07 O8 09
-
-
4.07(IV) 4.16(IV) 4.13(IV) 4.10(IV) 4.13(IV) 4.14(IV) 2.08(111) 1.85(V) 2.03(VI) 2.14(IV) 2.17(VIII) 1.90(VI) 2.19(IV) 2.22(VIII) 1.93(VI)
B.H. W.S. de Jong et al. /Journal of Non-Crystalline Solids 176 (1994) 164-171
167
Table 2 (continued) x
y
z
EV/j a
CsI 33Lio.67Si205 Ow O 11 O12 O13 O14 O15 O16 O17 Ois Li 1 Li 2 Cs a Cs 2 Cs 3 Cs 4 Cs 5 Cs 6 Cs 7 Cs 8
-
0.1531(4) 0.2375(5) 0.2468(4) 0.3870(4) 0.4535(6) 0.6232(4) 0.5156(4) 0.4488(6) 0.5951(4) 0.7397(8) 0.2384(9) 0.77477(5) 0.70829(5) 0.27506(5) 0.20649(5) 0.45871(5) 0.02947(5) 0.51763(5) 0.03920(5)
0.3549(3) 0.4217(4) 0.4844(2) 0.3719(2) 0.3412(4) 0.3669(3) 0.2522(2) 0.1689(4) 0.1218(3) 0.0726(6) 0.4197(6) 0.22908(3) 0.22500(3) 0.27803(3) 0.26565(3) 0.48610(3) 0.51656(3) 0.00831(3) 0.01696(3)
0.0014(5)
1/4 - 0.0287(6) 0.0393(6) - 1/4 - 0.0256(6) -0.0162(6) -1/4 - 0.0168(6) 0.5111(13) 0.5164(15) 3 /4 1 /4 1 /4 3 /4 1 /4 1 /4 1 /4 3 /4
a YW,j is the valence calculated according to the parameterization in Ref. [27].
4T/rotating anode s~(stem using Mo Ko~ X-ray radiation (A = 0.71073 A). Lithium and cesium/lithium phyllosilicate were measured at room temperature, rubidium and cesium phyllosilicate at 150 K to minimalize moisture adsorption. The intensity data were corrected for Lp and absorption (DIFABS, [24]). The structure was refined on F by full matrix least squares (SHELX 76, [25]). All atom positions were refined with anisotropic displacement parameters. Geometrical calculations were carried out with PLATON [26] on a DEC5000 cluster. The results of our crystal structure determinations are given in Tables 1 and 2. Included in Table 2 is also a valence calculation based on the parameterization of Brese and O'Keeffe [27].
3. Discussion Six-membered silica rings are the most common silica conformation found in phyllosilicates. Four main types of such ring systems can occur as illustrated in Fig. 1. Here the silicon atoms are positioned on the corners of the honeycomb network omitting the oxygen atoms connecting these atoms. The flat sheet shown in Fig. I(A) is the one commonly
-
2.11(V) 2.14(VIII) 1.92(IV) 2.11(IV) 2.12(IV) 1.91(V) 2.22(V) 2.15(VI) 1.91(VI) 1.09(III) 1.09(III) 1.07(XI) 0.96(XII) 0.94(IX) 1.00(IV) 0.88(VIII) 0.92(IX) 0.95(X) 0.89(X)
b Fixed.
encountered in micas. Folding this flat sheet along the dotted lines results in three types of corrugated sheets shown in Figs. I(B)-(D). In Fig. I(B) all silica rings are in boat conformation, in Fig. I(C) all rings form chairs and in Fig. I(D) boats and chairs coexist. The sheet with all silica rings in chair conformation is the one encountered in lithium and ~and /3-sodium phyllosilicate. The lithium phyllosilicute sheet is most puckered followed by that of ~and ~3-sodium phyllosilicate. This decrease in sheet puckering with higher mass alkalis is well known and discussed by Liebau [28]. ~-sodium phyllosilicute [21] has a silica sheet with all silica rings in boat conformation as illustrated in Fig. I(B), whereas mixed boat and chair conformations such as illustrated in Fig. I(D) occur in high sanbornite, BaSi205 [22]. In three-dimensional silica networks, mixed forms of chairs and boats can also occur. Thus, whereas cristobalite contains only chairs, tridymite contains sheets made from chairs which are connected to one another by boats. In the latter structure, the ratio of chairs to boats is one. The second silica sheet topology is illustrated in Fig. 2 and consists of interconnected four- and eight-membered rings of silica. This coexistence of four and eight-membered ring systems is much rarer,
168
B.H. W.S. de Jong et al. /Journal of Non-CrystaUine Solids 176 (1994) 164-171
occuring only in the mineral apophyllite albeit with a different topology [29]. Although the combination 4 - 8 is rare, this should not be construed to mean that varying ring sizes in sheet silicates do not occur. Thus 4 - 6 - 8 - , 4 - 5 - 6 - 8 - and 4-6-12-ring systems have been discovered [28,30]. The rings in rubidium are more regular than those in cesium phyllosilicate. Both are puckered differently from those observed in lithium and sodium phyllosilicate as illustrated in Fig. 3. The observed 4-8-ring size of the heavier
Fig. 2. Topology of the silica sheet in Rb and Cs phyllosilicate. The actual configuration shown is the one for Rb phyllosilicate. The grey large spheres represent Si, the small black spheres represent O. Note the four- and eight-membered silica rings.
Fig. 1. The topology of six-membered rings in sheet silicates. (A) Honeycomb of six-membered rings with on each corner a Si atom. The O atoms above or below the plane are omitted as well as those between the Si atoms in the sheet. (B) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which all six-membered rings occur in boat conformation occurs in &Na2Si205. (C) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which all six-membered rings occur in chair conformation is characteristic for crystalline Li and a- and 13-Na phyllosilicate. (D) Corrugated sheet formed by folding the flat sheet shown in Fig. I(A) along the dotted lines. This type of sheet in which six-membered rings occur in chair as well as boat conformation is characteristic for crystalline Ba phyliosilicate.
alkali phyllosilicates was of course unanticipated in view of the identical six-ring size for lithium as well as barium phyllosilicate, i.e., the lightest and heaviest of the alkali and alkaline earth phyllosilicates. Since chair and boat designations only pertain to six-membered rings, some other designation is needed to describe eight-membered rings in sheets. The upward pucker of four of the eight silica tetrahedra in an eight-membered ring can be described in analogy with boranes as possessing a nest-like conformation. We designate the downward pucker as possessing a sidewinder (trail of a rattlesnake-like) conformation. This last conformation is the one observed for crystalline rubidium and cesium phyllosilicate. The third silica sheet topology is illustrated in Fig. 4 and consists of interconnected four-, eightand 12-membered rings of silica. This combination of ring systems is novel to the best of our knowledge. A side view of the structure in Fig. 3 shows the strong deviation from what one usually encoun-
B.H. W.S. de Jong et aL /Journal of Non-Crystalline Solids 176 (1994) 164-171
ters as a flat silica sheet. Here the silica sheet folds itself around the cesium atoms, the end of the cage being locked up by three-fold coordinated lithium atoms as illustrated in Fig. 5. The silicate sheet structure resembles that of a crash-zone built into car hoods and consists of tunnels, the cesium atoms being encapsulated in a silicate tunnel with lithium atoms closing it. Two tunnel types exist, one being closed off by one Li atom, the other by two. The extreme tortuosity of what is in essence a silica surfaces suggests possible very different surface structures from the ones considered hitherto. Note the 4-8-12-membered rings as opposed to the 4 - 6 8-membered rings which one might expect based on a simple mixing of the lithium and cesium phyllosilicate endmember topologies. Besides this strong puckering of the sheet, attention should be paid to
Fig. 3a: Rb2Si205
.Yk./k.rkik./-k Fig. 3b Li2Si205
wzrLr Fig 3c Cel 33Li067Si205
Fig. 3. Side view of silica sheets for Li, Rb and mixed C s / L i phyllosilicate. Only the Si atoms are drawn. (A) The sidewinder configuration in Rb and Cs phyllosilicate is indicated by the grey connections between Si atoms. (B) The grey shading of siliconsilicon connections in Li and et-13-Na phyllosilicate shows a side view of the chair configuration. (C) The crash-zone-like arrangement of the silica sheet in Cs-Li phyllosilicate.
169
the unequivocal three-fold coordination around the lithium atom to three O(non-bridging) atoms. We only once encountered such lithium coordination before in lithium nesosilicate for one of the twenty unique lithium atoms [31] where we used a tighter oxygen coordination criterion around Li as tends to be employed by other authors for topologically similar structures [32-34] The quality of the structure determinations can be gauged among other criteria from the R factor measuring the goodness of fit between observed and calculated structure factors F. A low R factor being desirable, it is clear from Table 1 that the Li, Cs and C s / L i phyllosilicate structure determinations vary between very good and acceptable. This is, however, not the case for Rb phyllosilicate which has such high R factor that the structure on itself would not be publishable. That this structure determination is not so hot can also be learned from inspection of the calculated valences. According to Pauling's second rule, the oxygen valence should be around 2, i.e., the charge donated by the cations to oxygen should be two electrons in order to complete its Lewis octet. Inspection of the oxygen valences in Table 2, calculated using Brese and O'Keeffe's [27] parameterization, show that their values fall within expected ranges except for the oxygen atoms in Rb phyllosilicate. The valence donated to the oxygen atom has to come from the surrounding cations and these calculated values are indicated with a negative sign in Table 2. Thus, silicon with four valence electrons can at most have a valence of - 4 . Again, the tabulated values in Table 2 indicate that the Si valence for the crystalline phyllosilicate structures varies around - 4 except for the rubidium phyllosilicate where Si valences around - 5 are found, indicating that not all is well with the silicon positions in this last structure. It is clear that, although the sheet image in Rb phyllosilicate is correct, better crystals with a better mozaic spread have to be grown in order to get precise structural information. The importance of these newly discovered variations in ring sizes in crystalline alkali-phyllosilicates for the structural elucidation of glassy alkali-disilicates is that in all likelihood small but in particular large rings systems are underestimated in their molecular dynamics models. It is not clear if the presence of such extended ring systems can be ascer-
170
B.H. W.S. de Jong et al. /Journal of Non-CrystaUine Solids 176 (1994) 164-171
~-a' ~-~ a-~ ~ - a - a - ~ a-a-a-~ II
I I I I
Silicon Oxygen
,-~ II ~-~
II ~-~
I I I I
~-~-,-~ / I l l 9-~-~-~
~-~
II ~-~
I
~-~-o-~ I / l l ~-~-O-~
~-~
Fig. 4. Top view on the sheet of Cs/Li phyllosilicate. The tortuosity of the sheet makes the actual topology difficult to discern in a projection. The sheet topology is drawn in the bottom right hand comer.
tained by conventional measures of goodness of fit between experiment and theory as provided by radial distribution analysis. It may prove to be a solid test of molecular dynamics modelling to ascertain if lithium disilicate glasses modelled with the crystalline lithium phyllosilicate atomic positions as starting parameters give a topological result similar to those using the rubidium, cesium or cesiumlithium phyllosilicate atomic positions in which the heavy alkalis are replaced by lithium. It should also be ascertained as a property of the potentials employed in MD modelling if these newly found crystal structures form indeed the most stable atomic configuration relative to all possible ones. What is clear is that in discussing the properties of mixed alkali glasses such variations in sheet topology may be of consequence as it may result in an inhomogeneous distribution of ring systems [35]. What is also clear is that, in describing the mixed alkali effect, strong alkali-alkali interactions have to be included since it is observed that, whereas, with the exception of lithium phyllosilicates, all single alkali phyllosilicates are hygroscopic, it is a typical
characteristic of mixed alkali disilicate glasses such as Li-Rb and Li-Cs disilicates that they are not hygroscopic at all [15]. It is this last observation which resulted in making non-hygroscopic soluble silicates and the manufacture of extra moisture resistant welding electrodes [36].
4. Conclusion The crystal structure of lithium phyllosilicate has been refined and those of rubidium, cesium and cesium-lithium phyllosilicate have been determined. The silica sheet topology of the low mass alkali phyllosilicates and of barium phyllosilicate, the only known alkaline earth phyllosilicate, are characterized by six-membered rings in chair or boat conformation. The sheet topology of the high mass alkali phyllosilicates is characterized by interconnected four- and eight-membered rings of silica. The sheet topology of a crystalline mixed cesium-lithium phyllosilicate consists of interconnected four-, eight- and 12-membered rings. It is therefore not a mixture
B.H. W.S. de .long et aL /Journal of Non-Crystalline Solids 176 (1994) 164-171
000Ooo~
°oo000
Cesium
0
Oxygen
Silicon
C)
Lithium
Fig. 5. Side view of the Cs/Li phyllosilicate conformation with the Cs and Li atoms drawn in. Note the folding of the silica sheet around the Cs atoms and how the Cs containing tunnels are closed off by three-fold oxygen coordinated Li atoms. Note that there are two types of tunnel: one type which is closed off by one Li atom, the other by two.
between the six-ring structure of crystalline Li2Si205 and the 4-8-ring structure of crystalline Cs2Si205.
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