Li2Si3O7: Crystal structure and Raman Spectroscopy∗

Hannes Kruger†, Volker KahlenbergInstitut fur Mineralogie und Petrographie

Leopold-Franzens-Universitat6020 Innsbruck, Austria

Reinhard KaindlChristian-Doppler-Laboratory for Advanced Hard Coatings

at the Institute of Mineralogy and PetrographyUniversity of Innsbruck, Austria

2006

Abstract

The crystal structure of metastable Li2Si3O7 wasdetermined from single crystal X-ray diffractiondata. The orthorhombic crystals were found toadopt space group Pmca with unit cell parame-ters of a = 19.648(3)A, b = 5.9969(8)A and c =4.8691(6)A. The content of the cell is Z = 4. Theobtained structural model was refined to a R-valueof 0.035. The structure exhibits silicate sheets,which can be classified as {uB ,2,12∞}[Si6O14] usingthe silicate nomenclature of Liebau. The layers arebuild up from zweier single chains running parallelto c. Raman spectra are presented and comparedwith other silicates. Furthermore the structure isdiscussed versus Na2Si3O7.

1 Introduction

Crystalline lithium silicates are of special impor-tance for certain areas of materials science andsolid state chemistry. Recent fields of applicationsinclude, for example, glass-ceramics (Holand &Beall 2002), CO2 absorbing materials (Essaki, Nak-agawa, Kato & Uemoto 2004) and solid electrolytes

∗published in Journal of SolidState Chemistry 180(3), 922–928,http://dx.doi.org/10.1016/j.jssc.2006.12.015

†Email: Hannes.Krueger@uibk.ac.at

in lithium batteries (Jackowska & West 1983, Kho-rassani & West 1984). With this regard it is in-teresting to note that the historically first devel-oped glass-ceramic was based on Li2Si2O5 (Stookey1953). In addition to studies of the crystalline com-pounds numerous investigations were aimed to elu-cidate the glass formation and the crystallisationbehaviour in the binary glasses of the system Li2O–SiO2 (Vogel 1963, McMillan 1979, James 1985).Furthermore, the binary glasses have been also usedas comparatively simple model systems to under-stand the internal structure, short range orderingand ion transport mechanisms in amorphous sys-tems (Beier & Frischat 1984, Habasaki & Hiwatari2004, Lammert, Kunow & Heuer 2003, Soltay &Henderson 2005).

According to the phase diagram (Levin, Rob-bins & McMurdie 1969) three thermodynamicallystable crystalline phases have to be distinguished:Li4SiO4, Li2SiO3 and Li2Si2O5, respectively. Thecrystal structures of these compounds are knownfor a long time. However, there are indicationsthat metastable phases such as Li2Si3O7 can beobtained as well. The crystallisation of Li2Si3O7

from a stoichiometric glass was firstly reportedby Matveev & Velya (1959). A more detailedstudy was carried out by West & Glasser (1970),who gave optical data, IR spectra as well as in-dexed X-ray powder data along with a sugges-tion for the space group Pbcm or Pca21 de-

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termined from single crystal X-ray photographs.From the geometric similarity between the unitcells of α-Na2Si2O5 (Liebau 1961b) and Na2Si3O7

(Jamieson 1967) on the one hand and that be-tween Li2Si2O5 (Liebau 1961a) and Li2Si3O7 onthe other hand, they deduced a probable struc-ture for the lithium trisilicate, showing the sametype of layers as the sodium trisilicate. Both trisil-icates are thermodynamically metastable. Whereaslithium trisilicate is reported to persist indefinitely(West & Glasser 1970), the sodium compounddecomposes within months at ambient conditions(Kruger, Kahlenberg & Friese 2006).

This work reports the crystal structure ofLi2Si3O7 solved from single crystal X-ray diffrac-tion data, which basically confirms the assumptionof West & Glasser (1970), although some structuraldifferences will be discussed in detail later. More-over, the structural investigation is accompanied bya study of the Raman spectra, which includes a dis-cussion of the similarities in the spectra of Li2Si3O7

and the mineral orthoenstatite.

2 Experimental details

2.1 Synthesis

The synthesis of lithium trisilicate was carried outfollowing the procedure given in (West & Glasser1970). Fine grained Li2CO3 (Merck, p.a.) andSiO2 (Alfa Johnson Matthey, 99.995%) were mixedaccording to the stoichiometric ratio of lithiumtrisilicate and filled in a platinum crucible. Thesample was inserted in a resistance heated cham-ber furnace at 673K. The furnace was then heatedup to 1673K using a rate of 5K/min and held atthis temperature for about 30 minutes. Subse-quently, the sample was quickly transferred to an-other chamber furnace, pre-heated at 1163K. Whileinserting the sample, the temperature of the sec-ond furnace dropped temporarily down to approx-imately 1020K, but stabilised after one minute at1163K. Because of the thermal inertia of the cru-cible/sample, the sample temperature probably hasnot dropped that low. The sample was kept for sixminutes at 1163K, before it was finally removedfrom the furnace and cooled to ambient conditions.After coarsely crushing the sample, transparentcrystals, exhibiting shapes of very thin platelets,

could be isolated from the bulk material for furthersingle crystal investigations. A part of the bulk ma-terial was powderised and scrutinised by means ofX-ray powder diffraction, which revealed Li2Si3O7,as well as the stable form of Li2Si2O5 (de Jong,Super, Spek, Veldman, Nachtegaal & Fischer 1998)and minor amounts of quartz as by-products. Thephase amounts (wt%) as roughly determined byRietveld analysis were 56, 42 and 2, respectively.West & Glasser (1970) reported, that the trisilicatecrystallises in a temperature range of 1123–1193K.Samples cooled below 1123K were reported to showgreatly reduced yields of Li2Si3O7. Higher amountsof the trisilicate may be obtained by optimising thethermal treatment.

2.2 Single crystal X-ray diffraction

One of the selected crystals was mounted at thetop of a glass fibre for single crystal X-ray diffrac-tion experiments on a Stoe IPDS II diffractome-ter. Table 1 presents further experimental details.The diffraction pattern could be indexed with anorthorhombic lattice. The data reduction processincluded corrections for Lorentz, polarisation andair absorption effects (STOE & Cie GmbH 2005a).Due to the pronounced anisotropic shape of thethin crystal a correction for absorption was per-formed by the integration method using 8 indexedcrystal faces (STOE & Cie GmbH 2005b). An in-ternal R value of 0.049 for the Laue group mmmconfirmed the orthorhombic symmetry. From theanalysis of systematically absent reflections the ex-tinction symbol P -ca could be derived. Possiblespace groups are P21ca (No. 29) or centrosym-metric Pmca (No. 57). The structure was solvedin Pmca using direct methods (Altomare, Burla,Camalli, Cascarano, Giacovazzo, Guagliardi, Mo-literni, Polidori & Spagna 1997) and refined us-ing the software Jana2000 (Petrıcek, Dusek &Palatinus 2000). The non-standard setting of thecell was chosen to stay conform to earlier struc-tural investigations on trisilicates (Jamieson 1967,Kruger et al. 2006). The refinement of atomic co-ordinates (Table 2) as well as anisotropic displace-ment parameters (Table 3) of all atoms results inthe final structural model.

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Table 1: Experimental detailsCrystal dataChemical formula Li2Si3O7

Mr 210.13Cell setting, space group Orthorhombic, PmcaTemperature (K) 298a, b, c (A) 19.648(3), 5.9969(8), 4.8691(6)V (A3) 573.70 (14)Z 4Dx (Mg m−3) 2.435Radiation type Mo Kα

No. of reflections for cell parameters 3544θ range (◦) 2.1–29.5µ (mm−1) 0.81Crystal form, colour plate, colourlessCrystal size (mm) 0.26×0.09×0.02Data collectionDiffractometer Stoe IPDS 2Data collection method rotation methodAbsorption correction IntegrationTmin 0.875Tmax 0.983No. of measured, independent and observed reflections 4610, 803, 649Criterion for observed reflections I > 3σ(I)Rint(mmm) 0.049θmax (◦) 29.2Range of h, k, l −26 ≤ h ≤ 26

−8 ≤ k ≤ 8−6 ≤ l ≤ 6

RefinementRefinement on FR[F 2 > 2σ(F 2)], wR(F 2), S 0.035, 0.040, 2.34No. of reflections 803 reflectionsNo. of parameters 59Weighting scheme Based on measured s.u.’s w = 1/(σ2(F ) + 0.0001F 2)(∆/s)max <0.0001∆ρmax,∆ρmin (e A3) 0.53, -0.50Extinction method B-C type 1 Gaussian isotropic (Becker & Coppens 1974)Extinction coefficient (×10−2) 4(2)Software used: Stoe X-Area, X-Red (STOE & Cie GmbH 2005a, STOE & Cie GmbH2005b); Sir97 (Altomare et al. 1997); Jana2000 (Petrıcek et al. 2000); DRAWxtl(Finger, Kroeker & Toby 2005)

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Table 2: Positional ParametersAtom x y z Uiso

Si1 0.25 0.34929(15) 0.57981(18) 0.0123(2)Si2 0.10440(3) 0.14615(10) 0.55364(12) 0.01098(16)O1 0.05547(8) 0.3389(3) 0.4566(3) 0.0156(4)O2 0.18419(9) 0.2016(3) 0.5144(4) 0.0194(5)O3 0.25 0.4260(4) 0.8970(5) 0.0206(7)O4 0.09068(8) 0.0865(3) 0.8801(3) 0.0141(4)Li1 0.0440(2) 0.3535(7) 0.0567(9) 0.0190(11)

Table 3: Anisotropic Displacement ParametersAtom U11 U22 U33 U12 U13 U23

Si1 0.0135(4) 0.0119(4) 0.0115(4) 0 0 -0.0001(3)Si2 0.0124(3) 0.0101(3) 0.0104(3) -0.0006(2) -0.0002(2) 0.0000(2)O1 0.0190(7) 0.0128(7) 0.0151(7) 0.0020(6) -0.0014(6) 0.0013(7)O2 0.0155(7) 0.0226(9) 0.0201(9) -0.0053(7) 0.0006(6) -0.0024(7)O3 0.0334(13) 0.0161(12) 0.0123(11) 0 0 -0.0001(10)O4 0.0195(7) 0.0119(7) 0.0108(7) 0.0024(6) 0.0011(5) -0.0001(6)Li1 0.0246(19) 0.0156(18) 0.0169(18) 0.0018(15) -0.0007(15) 0.0011(17)The anisotropic displacement factor exponent takes the form:−2π2

[h2a∗2U11 + · · ·+ 2hka∗b∗U12

]

2.3 Raman spectroscopy

Confocal Raman spectra were obtained with aHoriba Jobin Yvon LabRam-HR 800 Ramanmicro-spectrometer. Samples were excited by the487.9 nm emission line of a 30 mW Ar+-laser andan Olympus 100× objective (N.A. = 0.9). Thespectra were recorded without polarisation filter forthe incident laser and the scattered Raman light.Size and power of the laser spot on the surfacewas approximately 1 µm and 5 mW. The spectralresolution, determined by measuring the Rayleighline, was about 1.8 cm−1. The dispersed lightwas collected by a 1024× 256 open electrode CCDdetector. Confocal pinhole was set to 1000 µm.Spectra were recorded unpolarised in scanning-mode (Knoll, Singer & Kiefer 1990, Nasdala &Massonne 2000). All spectra were background cor-rected by subtraction of line segments and fitted toGauss-Lorentz functions. Accuracy of Raman lineshifts, calibrated by regular measuring the Rayleighline, was in the order of 0.5 cm−1. Simple sym-metry analysis was done by the aim of the web-based program package SAM (Aroyo, Perez-Mato,

Capillas, Kroumova, Ivantchev, Madariaga, Kirov& Wondratschek 2006).

3 Description of the structure

The structure contains two symmetrically indepen-dent silicon atoms, which are each four-coordinatedby oxygen atoms. The coordination tetrahedra ofthe two silicon atoms form a zweier single chain(Liebau 1985) each. The smallest identity periodof the structure along [001] represents the directionof the chains (Figure 1). In Figure 2a it is shown,how these chains are connected to build [Si3O7]

2−-layers. The darker chain is formed by Si1 and asthe ‘central’ chain in the layer the connectivity ofits tetrahedra is Q4. The oxygen O2 is shared bythe different chains. Tetrahedra belonging to the‘outer’ chain (formed by Si2, light gray in the fig-ures) are Q3-connected and their O1 is not sharedbetween silicon tetrahedra. Therefore, the bondingdistance Si2-O1 is significantly shorter (see Table4) compared to an average Si-O bonding distance(Liebau 1985). This shorter bond to the termi-

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Figure 1: Silicate zweier single chains of a sin-gle [Si6O14] layer. Dark tetrahedra represent Si1,the brighter tetrahedra represent Si2 coordinationpolyhedra. The displayed region extends fromx = 0 to x = 0.5. Li displacement ellipsoids aredrawn at 95% probability.

nal oxygen O1 induces a higher distortion of theQ3- compared to the Q4-tetrahedra. A quantitativemeasure of the distortion is proposed by a work ofRobinson, Gibbs & Ribbe (1971). The mean tetra-hedral quadratic elongation ⟨λtet⟩ is 1.001(1) and1.002(1), the tetrahedral angle variance σ2

θ(tet) is

2.8(1) and 9.8(2) for the Q4- and Q3-tetrahedra,respectively. A difference in the distortion param-eters of Q3- and Q4-species in a layered silicatestructure was also observed in Na6Si8O19 (Kruger,Kahlenberg & Kaindl 2005). However, the observeddistortion parameters are in good agreement withthe linear correlation between σ2

θ(tet) and ⟨λtet⟩ ob-served by Robinson et al. for a variety of silicatesand aluminosilicates (Robinson et al. 1971). Dis-tances, angles and bond valence sums (Brown &Altermatt 1985) are given in Table 4.

The lithium atoms are located between the sil-icate layers and are also four-coordinated by oxy-gens. The coordination tetrahedra involve threeoxygen atoms belonging to the closest silicate layer,the fourth one is attributed to the other neigh-bouring layer. Compared to the silicate tetrahe-dra, they are highly distorted [σ2

θ(tet)=111(1)]. The

[LiO4]-tetrahedron also forms chains (see Figure 3),but the [LiO4] tetrahedra of adjacent chains shareedges, so that their linkedness is two. Since each

tetrahedron shares two corners and two edges withneighbouring ones, its connectedness is four withregard to [LiO4] tetrahedra. The one unshared oxy-gen exhibits the longest Li-O distance (see Table 4).

4 Results of the Raman spec-troscopy

The Raman spectrum of a thin, sheet-like Li2Si3O7

crystal is given in Figure 4. Raman shifts and rel-ative intensities in percent, related to the ampli-tude of the most intense Raman band at 387 cm−1,are listed in Table 5. 30 Raman bands with rela-tive intensities ≥ 1 could be detected. The high-wavenumber region of the spectrum from 800–1300cm−1 contains two intense bands at 928 and 1097cm−1 and five bands of lower intensity. The mid-and low-wavenumber regions below 800 cm−1 showtwo bands of almost 100% relative intensity at 387and 516 cm−1 and several other bands (139, 150,307, 359 cm−1) with intensities around 20%. Alarge number of low-intensities bands occur below300 cm−1, some also at higher wavenumbers. Thetheoretical number of possible vibrational modesof Li2Si3O7, which crystallises in the space groupPmca (point group mmm or D2h) can be deducedby symmetry analysis using the web-based programpackage SAM (Aroyo et al. 2006) . 127 vibrationalmodes are possible according to the following irre-ducible representations:

ΓV ib = 19Ag + 19B1g + 17B2g + 17B3g

+17B1u + 19B2u + 19B3u

72 of them (all g-modes) are Raman and therest are IR-active. Band assignment to vibra-tional modes can be done by analogy by comparingLi2Si3O7 with the spectra of known related struc-tures. Bands of alkali and alkaline earth metalsilicates in the high-wavenumber range from 800to 1300 cm−1 are generally caused by symmetricstretching vibrations of the SiO4 groups (νSi−O)(Akaogi, Ross, McMillan & Navrotsky 1984, You,Jiang, Hou, Chen, Wu & Xu 2005). The vibra-tional wavenumber of these modes is approximatelyrelated to the type of the silicon-oxygen tetrahe-dron and to the different structural building units(Akaogi et al. 1984, You et al. 2005, Sharma, Virgo

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a)

b)

Figure 2: [Si6O14] layers formed by two independent zweier single chains. (a) in Li2Si3O7, lithiumatoms represented by 95% probability displacement ellipsoids. Dark and bright tetrahedra represent Si1and Si2 coordination polyhedra, respectively. (b) Different arrangement of the layers and two distinctorientations of the central chain in the averaged structure of modulated Na2Si3O7 (Kruger et al. 2006).Sodium atoms depicted by spheres.

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Table 4: Distances and AnglesAtoms Distance [A] Angle [◦] BVSSi1-O2 1.5993(19)Si1-O2i 1.5993(19)Si1-O3 1.612(3)Si1-O3ii 1.615(3)O2-Si1-O2i 107.88(10)O2-Si1-O3 110.43(9)O2-Si1-O3ii 110.63(9)O2i-Si1-O3 110.43(9)O2i-Si1-O3ii 110.63(9)O3-Si1-O3ii 106.86(14)Si1 4.20(1)Si2-O1 1.5759(18)Si2-O2 1.6139(19)Si2-O4 1.6515(17)Si2-O4iii 1.6531(18)O1-Si2-O2 113.96(10)O1-Si2-O4 110.36(9)O1-Si2-O4iii 111.47(9)O2-Si2-O4 108.50(9)O2-Si2-O4iii 105.77(9)O4-Si2-O4iii 106.41(9)Si2 4.02(1)Li1-O1 1.962(5)Li1-O1ii 1.921(5)Li1-O1iv 1.957(4)Li1-O4v 2.036(5)O1-Li1-O1ii 106.3(2)O1-Li1-O1iv 98.4(2)O1-Li1-O4v 109.4(2)O1ii-Li1-O1iv 98.7(2)O1ii-Li1-O4v 126.5(2)O1iv-Li1-O4v 113.6(2)Li1 1.033(7)Symmetry codes: (i) 1/2 − x, y, z; (ii) x, 1 − y,−1/2 + z; (iii) x,−y,−1/2 + z; (iv)x, y,−1 + z; (v) −x, y, 1/2− z

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Figure 3: Chain of [LiO4] coordination tetrahedra including their bonds to silicon atoms. The dis-played region extends from x = 0.3 to x = 0.8. Coordination tetrahedra are displayed transparent.Displacement ellipsoids are shown at 95% probability.

Figure 4: Raman spectrum of a thin plate-like crystal of Li2Si3O7.

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Table 5: Raman shifts (cm−1) and intensities (amplitude) of bands in the spectrum of Li2Si3O7 (Lith.)in percent relative to the highest intensity band at 387 cm−1, compared with experimental data oforthoentstatite (En.) (Choudhury et al. 1998) and Na6Si8O19 (Sod.) (Kruger et al. 2005). Abbreviations:weak (w), medium (m), strong (s), shoulder (sh).

Lith. Int. En. Sod. Int. Lith. Int. En. Sod. Int.106 1 - 107 7 - - - 481 59113 6 113(w) - - 486 21 475(w) 486 10123 1 - 123 6 - - - 494 92139 19 132(m) - - 516 97 516(w) 511 45150 27 - 149 4 - - 519(m) - -153 2 154(w) - - 521 8 525(w) - -- - 160(w) - - - - 527(w) - -

163 1 165(w) 167 18 541 10 545(w) 540 54189 2 193(w) 178 3 - - 538(w) - -- - 196(m) 196 4 - - 552(w) 569 8- - - 204 6 - - 581(m) - -- - - 226 12 - - 595(sh) - -

232 10 234(m) 232 5 643 33 - - -- - 237(w) - - - - 663(s) 706 3- - 242(w) - - - - 687(s) - -

248 10 243(m) 248 3 759 4 750(w) 766 14259 3 - - - 804 9 - - -283 2 - 288 2 - - 846(w) - -- - 301(w) - - - - 891(w) - -

307 27 306(w) - - 928 25 927(m) 927 <1- - 322(w) 324 5 - - 960(m) 964 5- - 339(w) - - - - - 975 3

352 7 343(s) - - - - - 995 12359 29 358(w) 356 86 - - 1014(sh) - -- - 379(w) 361 22 1026 5 1033(s) 1027 7

387 100 403(s) 381 3 - - 1043(m) 1049 24- - 409(w) 412 4 1051 4 1062(w) 1060 100- - 419(w) - - 1097 67 - 1095 11- - 422(s) - - 1106 2 - 1109 12

438 17 442(w) - - - - - 1130 10- - 448(m) 453 4 - - - 1177 7- - 460(w) - - 1226 4 - - -

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& Mysen 1978, Iguchi, Kashio, Goto, Nishina &Fuwa 1981).Experimental and theoretical studies of binary

sodium silicates suggest that the intense band at1097 cm−1 results from νSi−O of tetrahedra withthree bridging oxygens (Q3) (You et al. 2005).In pyroxenes and NaYSi2O6, which are based onzweier and vierer single chains, modes between1000–1050 cm−1 were observed and assigned tosymmetric and asymmetric Si-O stretching vibra-tions of tetrahedra with non-bridging oxygens inthe chains (Choudhury et al. 1998, Yang, Finger,Conrad, Prewitt & Hazen 1999, Tobbens, Kahlen-berg & Kaindl 2005). The mode frequencies ofνSi−O of Li2Si3O7 are slightly higher comparedto the chain silicate orthoenstatite (Table 5), con-sistent with slightly lower Si-O bond lengths be-tween 1.58-1.65A. Na6Si8O19, which consists of fourdifferent zweier single chains forming a sequenceof tetrahedral layers, shows a νSi−O mode withextremely high intensity at 1060 cm−1 (Krugeret al. 2005). Q4-species are Raman inactive in thehigh wavenumber range, but the high intensity ofthe νSi−O mode at 1097 cm−1 may be attributedto increased intensities of Q3-species with adjacentQ4-species (You et al. 2005).In the intermediate wavenumber range from 400

to 900 cm−1, high-intensity Raman bands emergeby the symmetric bending and rocking vibrationsof the Si-O-Si (δSi−O−Si) and νSi−O of the Si-Obridging oxygen of adjacent SiO4 tetrahedra (Youet al. 2005, Choudhury et al. 1998, Yang et al. 1999,Karlsson, Zanghellini, Swenson, Roling, Bowron& Borjesson 2005, Kirkpatrick, Yarger, McMillan,Yu & Cong 1997). Also bending modes of thenon-bridging oxygens (δOnb) contribute to bandsin this spectral region (Karlsson et al. 2005, Bell,Dean & Hibbins-Butler 1971). For binary sodiumsilicates, a relation between the δSi−O−Si vibra-tional wavenumber and the Si-O-Si angle was es-tablished (You et al. 2005). According to them,δSi−O−Si are expected around 600–700 cm−1, cor-responding to the angle variation in the two Q3-and Q4-connected chains of Li2Si3O7 of 130–140◦.In Li2Si3O7 several bands from 438–643 cm−1 withrelative intensities between 10–97 were observed.In orthoenstatite, two strong modes at 663 and687 cm−1 correspond to two kinds of tetrahedralchains with 19◦ kinking angles apart (Table 5)(Choudhury et al. 1998). Several intense bands be-

tween 480 and 540 cm−1 were described in the spec-trum of Na6Si8O19, which consists of a sequence oftetrahedral layers built up by zweier single chainsand shows a considerable scatter in Si-O-Si an-gles from 136 to 161◦ (Kruger et al. 2005). Wetherefore suggest the band at 643 cm−1 to be re-lated to νSi−O of both chains, which does not splitdue to the low angle variations. The other bandsin this wavenumber region correspond probably toδSi−O−Si or to δOnb vibrations of the Si-O layers.

In the low wavenumber range below 400 cm−1,Raman active vibrations are caused by lattice vi-brations of the framework and by the bonding ofcation and oxygen (Li-O) (You et al. 2005). Ra-man shifts and intensities of bands in that rangeare similar to orthoenstatite and Na6Si8O19 (Table5) (Choudhury et al. 1998), supporting that latticevibrations of both the layers and the zweier sin-gle chains they are composed of contribute to thespectrum in that range.

The most frequent full width at half maximumof virtually all Raman bands around 10 cm−1 anda maximum <20 cm−1 and the presence of bands<400 cm−1 indicate a well ordered structure, long-distance ordering and the absence of structural dis-order like short-range Si-O-Si bridging angle disor-der in sodium silicate glasses (You et al. 2005, Zo-tov, Marinov & Konstantinov 1996).

5 Related structures and dis-cussion

The type of [Si6O14]-layers found in Li2Si3O7 istopologically equivalent to the silicate sheets foundin Na2Si3O7 (Jamieson 1967). A more recent study(Kruger et al. 2006) showed, that the sodium trisil-icate structure is more complex involving aperiodicchanges in the orientation of the ‘central’ chain.However, the classification of the silicate layer af-ter Liebau’s nomenclature (Liebau 1985, Lima-de-Faria, Hellner, Liebau, Makovicky & Parthe 1990)should be the same for both structures. A clas-sification of the Li2Si3O7 structure utilising thesoftware Crystana (Klein 2001, Goetzke, Klein &Kandzia 1988) ranks the chains in Li2Si3O7 as un-branched, thus leading to Li4{uB ,2,12∞}[Si6O14] asthe structural formula after Liebau.

As already mentioned, the compound Na2Si3O7

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exhibits very similar layers. As can be seen inFigure 2a, the chains of one layer in Li2Si3O7 areall of the same type, e.g. their vertices connect-ing along the chain direction point in the same di-rection for one layer. The neighbouring layer isequivalent by a centre of symmetry. Therefore,the connecting vertices point to the opposite di-rection. In the sodium compound (see Figure 2b)the centre of symmetry is located within the ‘cen-tral’ chain. Consequently the connecting verticesof the ‘outer’ chains of one layer point in oppo-site directions. The ‘central’ chain can adopt twoequivalent orientations. The sequence of these ori-entations within one layer shows aperiodic orderand breaks the lattice periodicity along the b di-rection. A detailed description of the incommensu-rate structure of the metastable sodium trisilicatecan be found in (Kruger et al. 2006). The largersodium atoms between the layers are coordinatedby five oxygens causing a different configuration ofthe [Si6O14] sheets in the sodium compound. An in-teresting subject for further investigations is a pos-sible solid solution series between the sodium andthe lithium compound as found in the metasilicates(although very limited) (West 1976).

Two more compounds are known, whichmay adopt similar [Si6O14] silicate sheets.Na2Cu(Si3O7)2·5H2O (Hubert, Jordan, Guth &Kalt 1977), which shows a doubled b and c latticeparameter (compared to Li2Si3O7). This couldbe caused by the presence of vierer chains in themore complex layer. The incorporation of watermolecules and copper atoms between the silicateanions may be expressed by the increased latticeparameter a. The second compound was describedby Guth, Hubert, Jordan, Kalt, Perati & Wey(1977): a hydrated compound (H2Si3O7), whichis reported to show two different modifications.Although there are good indications, that thesecompounds adopt similar silicate layers, thefinal evidence of structural investigations is stilloutstanding.

Acknowledgements

The authors are grateful to Friedrich Liebaufor very helpful discussion regarding the classifi-cation of the silicate layer. Furthermore, the au-thors would like to thank three anonymous referees,

whose valuable comments improved the paper.

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