UDC 548.7

CRYSTALLOGRAPHY

B. A. MAKSIMOV, Yu. A. KHARITONOV, V. V. ILYUKHIN, Academician N. V. BELOV

CRYSTAL STRUCTURE OF Li METASILICATE Li(_2)SiO(_3)

Transparent, well-faceted single crystals of Li(_2)SiO(_3) were obtained (hydrothermal synthesis) at 450° and (\sim 1500) atm. The parameters of the orthorhombic cell (a = 9.38) Å, (b = 5.40) Å, (c = 4.68) Å coincide with those already known ((^1)), (Z = 4). The space group is (Cmc2_1).*

The structure was solved (independently) from x-ray goniometric scans of two zones: the acentric (h0l) and the centrosymmetric (hk0)

Fig. 1. Li(_2)SiO(_3). Difference synthesis of electron density (\sigma(xz)) with the Li atoms singled out

Fig. 2. Li(_2)SiO(_3). Difference Patterson synthesis with retained Si—Si and Si—Li vectors

(Mo radiation, (\max \sin \theta/\lambda) 1.36 and 1.0 Å(^{-1}), respectively). Intensities were estimated on a (\sqrt[4]{2})-scale of blackening marks.

The Si and O atoms were determined with sufficient reliability from the Patterson maps (p(xz)) and (p(xy)). Three cycles of electron-density syntheses led to coefficients (R_{h0l} = 20\%), (R_{hk0} = 16.6\%). The light Li did not appear distinctly in the ordinary projections (\sigma(xz)) and (\sigma(xy)): primarily because of overlap with the heavier O atoms (in both projections, as became clear later) and, in addition, because the contribution of the Li atom did not change the sign of the structural amplitudes—the Fourier-series coefficients, for even without allowing for ionization, lithium accounts for only 6% of the scattering matter in Li(_2)SiO(_3).

In the acentric projection (xz), for localization of Li, the difference synthesis (\sigma_\Delta(xz)) with the contribution of the heavier Si and O excluded proved sufficient (Fig. 1). The height of the ghosts is less than 1 electron/Å(^2). In the centrosymmetric projection (xy) (for the reasons indicated), only the difference Patterson synthesis with the initial

[
(F_\Delta)^2 = [F_{\mathrm{e}} - F_{\mathrm{B}}(\Sigma O_i)]^2,
]

on which only the nonoverlapping Si—Si and Si—Li vectors were retained, was effective,

* Zeemann ((^1)) proposed a variant of the structure of Li(2)SiO(_3) on the basis of the solution (Cu radiation, (\max \sin \theta/\lambda = 0.65) Å(^{-1}), (R = 0.27)) of one (xy) projection and the assumption of a crystallochemical analogy between Li(_2)SiO(_3) and Na(_2)SiO(_3) ((^2)).

the coordinates of the light Li were fixed unambiguously (and with sufficient reliability) (Fig. 2).

The coordinates of the independent atoms of the structure, averaged over both projections, are given in Table 1. The coefficients (R_{hk0}=10.8\%), (R_{h0l}=15.4\%) for isotropic thermal factors (B_{hk0}=B_{h0l}=0.1\text{ Å}^2). Refinement by the least-squares method scarcely changed the (R)-factors, but worsened the interatomic distances, especially in the Li polyhedron (Fig. 3 and Table 2).

Fig. 3. (\mathrm{Li}_2\mathrm{SiO}_3). Si- and Li-tetrahedra with the distances Si—O, Li—O, and O—O indicated

In fairly regular Si tetrahedra the distances (\mathrm{Si—O}=1.63_5—1.66_0) Å deviate little from the mean, 1.65 Å. The spread of the tetrahedron edges is likewise small: (\mathrm{O—O}=2.66—2.76) Å, mean 2.70 Å.

In the larger, but likewise only slightly distorted, Li tetrahedra the distances (\mathrm{Li—O}) do not go beyond the limits given in the literature*: 1.92—2.18 Å (mean 2.01 Å), but are shorter than the corresponding distances given in (1): 2.02—2.14 Å (mean 2.08 Å). One (\mathrm{Li—O}) distance nevertheless stands out against the rest (cf. (4) and (1)). In the loose Li tetrahedron the spread of the edges is relatively small: 3.10—3.44 Å.

Like the two Be silicates, barylite (5) and bertrandite (6), deciphered in our laboratory, the structure of (\mathrm{Li}_2\mathrm{SiO}_3) provides a good illustration of the principle of closest anion packing. Table 3 compares the coordinates of the basis atoms of (\mathrm{Li}_2\mathrm{SiO}_3) and of the related (\mathrm{Na}_2\mathrm{SiO}_3), as well as their deviations (\Delta) from the ideal model within a two-layer (hexagonal) closest packing. In (\mathrm{Li}_2\mathrm{SiO}_3) the Li and Si cations occupy almost exactly the centers of the tetrahedral voids of the packing (columns 1 and 2 of Table 3), but the larger Li cations push the O anions apart; the hexagonal lattice is distorted: the ideal axial ratio

[
a:b:c=1.732:1:0.866
]

is replaced by the rhombic

[
1.737:1:0.816.
]

From the small value of (c) it follows that it is specifically the horizontal axes that expand:

[
a:b=b\sqrt{3}:b.
]

Table 1

Coordinates of the basis atoms in the structure of (\mathrm{Li}_2\mathrm{SiO}_3)

In the similarly constructed Na metasilicate (\mathrm{Na}_2\mathrm{SiO}_3), the larger Na, taking the place of Li (Table 3, (2, 6)), distorts the hexagonal lattice still more sharply ((a:b:c=1.73:1:0.793)), adding to its immediate environment an additional O atom from the lower-lying layer, and thus the coordination polyhedron of Na becomes a trigonal bipyramid (c.n. (=5)).

In the architectural motif of (\mathrm{Li}_2\mathrm{SiO}_3) there appears a polarity of structures made up of tetrahedra, well known from the crystal chemistry of sulfides: in the closest packing, that half of all tetrahedra is occupied which is determined by the uniformity of orientation of their tetrahedra with their tips in one direction along the (c) axis (in Fig. 4 this direction is downward). As regards the distribution of Li- and Si-tetrahedra in the ratio 2:1, here there is the repeatedly emphasized [[unclear: continuation cut off]]

* (\mathrm{Li—O}=1.92—1.99) Å in (3); 1.85—2.04 Å and 2.02—2.14 Å in (1).

the authors (^8^) not only the crystallochemical affinity of lithium with beryllium, similar to the bonding of aluminum with silicon, in accordance with which the Li tetrahedra in the metasilicate repeat the motif of Be(+Si)-tetrahedra in bertrandite, Be(_2)SiO(_7)(OH)(_2), namely (Fig. 4) they form a corundum motif of tetrahedra (lacework) with six-membered rings of Li tetrahedra, for which, according to Fig. 4, in each of the three rows that can be distinguished in the close packing, there alternate, in accordance with the ratio Li : Si = 2 : 1, two lithium tetrahedra and one non-lithium tetrahedron. The latter here become Si tetrahedra, which in this way center the basic corundum—ring motif of Li tetrahedra (Fig. 4).

Fig. 4. Li(2)SiO(_3). Wurtzite-like (polar) structure of tetrahedra. Rings of the larger Li tetrahedra are centered by Si tetrahedra. The latter form a metachain ([{\rm Si}_2{\rm O}_6]\infty), with a ((2_1)) axis perpendicular to the packing layers.

Table 2

Interatomic distances (in Å) in the structure of Li(_2)SiO(_6)

Note. Asterisks mark atoms obtained from the basis atoms by symmetry operations.

Si tetrahedra from superposed layers turn out to be linked into the principal structural detail for a silicate—the metasilicate chain ([{\rm Si}2{\rm O}_6]\infty = [{\rm SiO}3]\infty). This chain (Fig. 4) has the same repeat period along the axis—two tetrahedra—as in the better-known pyroxene chain; but whereas in the latter the chain axis lies in the plane of the close-packing layer, the axis of the metachain in Li(_2)SiO(_3) is perpendicular to the plane of the (ABAB) layers, as in barylite and bertrandite, and therefore the repeat period with со-

Table 3

shortened to ~4.7 Å (two tetrahedron heights) versus ~5.2 Å in the pyroxenes (two edges).

The structure of Li₂SiO₃ explains rather well the elongation of the crystals coinciding with the chain axis, the perfect cleavage in the direction parallel to the needle axis, and the piezoelectric effect. Despite the fourfold coordination of Li, the valence balance (according to Pauling) is satisfactory: of the four O atoms of the Si tetrahedron, at two vertices there meet, in addition to Si, three Li tetrahedra, and at the other two—two Si and two Li. The sums of the bond strengths are, respectively, 1¾ and 2½.

In describing the structure of Li₂SiO₃, a crystallochemical analogy in the behavior of Li and Be was indicated. More essential, however, is the chemical analogy, which is especially characteristic in the known—up to 15%—substitution in beryl of beryllium by lithium with Na compensation in the channel (Be²⁺ → Li¹⁺ + Na¹⁺, analogous to Si⁴⁺ → Al³⁺ + Na¹⁺).

Some doubt is caused, however, by the considerable difference in ionic radii: 0.35 and 0.70 Å. Therefore it seems natural to turn to the crystallochemical analogue of Be, namely Zn, where the difference in radii disappears. And indeed, recently (⁹), for hutchinsonite, in which there are metachains ([ \mathrm{Zn}2\mathrm{O}_6 ]\infty), a synthetic analogue with metachains ([ \mathrm{Li}2\mathrm{O}_6 ]\infty) was found, in which the ability of Li to replace divalent ions in tetrahedra is once again demonstrated, i.e., when divalent ions perform an “anion” function. Let us note the well-known fact (which also receives a crystallochemical explanation) of a substantial improvement in the growth of single crystals of zincite ZnO in the presence of LiOH.

Institute of Crystallography
Academy of Sciences of the USSR

Received
10 VIII 1967

REFERENCES

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