UDC 548.736.453.622

CRYSTALLOGRAPHY

S. T. AMIROV, A. V. NIKITIN, V. V. ILYUKHIN, Academician N. V. BELOV

CRYSTAL STRUCTURE OF SYNTHETIC Na,Zn-DIORTHOSILICATE Na₂Zn₂[Si₂O₇]

In studying the system Na₂O—ZnO—SiO₂ under conditions of relatively moderate alkalinity (5–15 wt.% NaOH), by hydrothermal syntheses from a charge with ZnO : SiO₂ = 1 : 1 and Na₂O : (ZnO + SiO₂) = 1 : 1 (¹), a crystalline phase B was obtained of composition: Na₂O 18.65%, ZnO 45.65%, SiO₂ 34.17%, H₂O 0.50% (Σ = 98.97%, analysis by V. S. Bykova), which corresponds well to the formula Na₂Zn₂Si₂O₇. Biaxial crystals ($N_g = 1.633$, $N_m = 1.615$, $N_p = 1.614$), activated by Mn, luminesce in the green

Fig. 1. Na,Zn-diorthosilicate. Metachains $[\mathrm{Zn}_2\mathrm{O}_6]_\infty$ winding around the screw axis $2_1$, with $\mathrm{SiO}_4$ tetrahedra encrusting them; the latter are paired by a mirror plane into diorthogroups $[\mathrm{Si}_2\mathrm{O}_7]$.
a — frontal projection $ac$; b — plan projection $bc$

region of the spectrum; in addition, they exhibit green triboluminescence and X-ray luminescence.

The parameters of the rhombic cell found by us, $a = 5.17$, $b = 9.41$, $c = 13.73$ Å, are close to those reported earlier (²); $Z = 4$ formula units of Na₂Zn₂Si₂O₇ (specific gravity 3.82).

The X-ray group $mmmC--c$, established from systematic extinctions, corresponds, in addition to the holohedral group $Cmcm$, also to two acentric Fedorov groups: $Cmc2_1$ and $C2cm$. The choice in favor of the latter ($C_{2v}^4$) was made in the initial analysis of the three-dimensional Patterson function, constructed from 360 nonzero intensities recorded from Weissenberg photographs $0kl—3kl$ and $hk0$ (Mo radiation, $\max \sin \theta / \lambda = 0.86$ Å; $\sqrt[4]{2}$ scale of blackening marks). The absence of linkage peaks (³) in the Harker sections $u00$ and $uv\frac{1}{2}$ made impossible a plane $m$ in the first position of the $C$ symbol and an axis $2_1$ in the third.

The coordinates of the heavy Zn atom and of the average Si atom were determined directly from the three-dimensional Patterson function. In the subsequent refinement by successive approximations, a methodological difficulty arose in localizing three O atoms. The concentration of Zn and Si near the levels $0yz$ and $\frac{1}{2}yz$ made the latter, at the first stage, pseudosymmetry planes, whose negative—interfering—role was further inten-

was eliminated by placing the Na atoms (two 4-fold positions) and one O atom (one 4-fold position) on the same levels. As a consequence, parasitic peaks associated with the true glide plane of symmetry were present in the intermediate electron-density syntheses. These peaks, however, disappeared in zero difference syntheses \(\rho_0(xyz)\) by \(F'_e = F_e - F_{\mathrm{T}(\mathrm{Zn}+\mathrm{Si}+\mathrm{Na})}\). The discrepancy factor, which after cycle IV was 17%, after introduction of the thermal correction and least-squares refinement was 9.5%; individualization of the temperature factors reduced \(R\) over all nonzero reflections to 8%. The final coordinates of the 8 independent—basic—atoms (20 − 1 parameters) are given in Table 1, and the interatomic distances calculated from them are given in Table 2.

Table 1

Coordinates of the basis atoms in the structure of \(\mathrm{Na_2Zn_2Si_2O_7}\)
(phase B)

The Zn atoms, as in a number of other (zinc) silicates—clinohedrite \((^4)\), hodgkinsonite \((^5)\), willemite \((^6)\), phase D \((^7)\)—are located in large oxygen tetrahedra. The distances are Zn—O \(1.94\)–\(2.06\) Å (mean 1.99 Å). In slightly distorted Si tetrahedra: Si—O \(1.58\)–\(1.69\) Å (mean 1.64 Å), O—O edges \(2.62\)–\(2.76\) Å (mean 2.67 Å). The Si—O—Si angle for the two tetrahedra related by the mirror plane is equal to \(132^\circ\). Zn tetrahedra form the principal architectural details of phase B, namely infinite polar chainlets extending along the short axis \(a = 5.17\) Å with a link consisting of two tetrahedra \([\mathrm{Zn_2O_6}]_\infty\). The chainlets are similar to those in clinohedrite and hardystonite \((^8)\), but in their “morphology” they are closest to the \([\mathrm{Be_2O_6}]_\infty\) chainlets in barylite \(\mathrm{Ba_2Be_2Si_2O_7}\) (allowing for the scale factor—the parameter 5.17 in barylite \((^9)\) corresponds to 4.63 Å). As in barylite, so in phase B the chainlets are not only encrusted \((^9)\) by orthotetrahedra \([\mathrm{SiO_4}]\), but also (Fig. 1) these \(\mathrm{SiO_4}\) (from neighboring Zn chains) pair through the plane of symmetry into diortho groups \([\mathrm{Si_2O_7}]\). In barylite, however, chainlets with opposite polarity alternate (being linked by a pseudocenter of symmetry), whereas in phase B all chainlets have one polarity (a significant piezoelectric effect). Although in both barylite and phase B the Zn chainlets are discrete, nevertheless by orthogroups of tetrahedra they are linked into a three-dimensional framework; the essential difference of this from ordinary (alumo)silicate frameworks, or even from the zincosilicate framework in phase D \(\mathrm{Na_2ZnSiO_4}\) \((^7)\), is that here at most oxygen atoms not two but three tetrahedra meet: \(2\mathrm{Zn}(\mathrm{Be}) + 1\mathrm{Si}\) (as in willemite \(\mathrm{Zn_2SiO_4}\)).

Table 2

Interatomic distances in the structure
of \(\mathrm{Na_2Zn_2Si_2O_7}\) (phase B)

* The asterisk indicates atoms related to the basis atoms by symmetry operations. The number of bonds is given in parentheses.

In the structure of barylite, \(\mathrm{BaBe_2Si_2O_7}\), the cavities of the three-dimensional framework are occupied by divalent Ba cations; correspondingly, in the larger-scale structure of phase B = \(\mathrm{Na_2Zn_2Si_2O_7}\), the cavities here contain twice the number of monovalent Na cations of two crystallographic types with different coordination. The four atoms \(\mathrm{Na_1}\), lying on twofold axes, have four neighboring O atoms at distances close to the sum of the ionic radii (2.37 and 2.43 Å). The large tetrahedron formed by them may be regarded as the first “coordination sphere.” Four more O atoms are farther from \(\mathrm{Na_1}\), but not so far as to exclude them from the coordination environment (2.81 and 2.88 Å), which, “in total,” is described as a twisted Thomson cube.

The coordination of the \(\mathrm{Na_2}\) cation lying in the mirror plane is somewhat unexpected: five close neighbors at distances 2.33 (\(\times 2\)),

Fig. 2. A trellis net of \(\mathrm{Na_1}\)-octavertices. Frontal projection

Fig. 3. Combination of the trellis net of \(\mathrm{Na_1}\)-polyhedra with discrete \(\mathrm{Na_2}\)-pentavertices

2.62 (\(\times 2\)), and 2.58 Å form a pseudorhombic pyramid—half of an irregular octahedron, whose sixth vertex is removed to 3.22 Å and therefore can be assigned to \(\mathrm{Na_2}\) only with considerable strain. A similar “hemimorphy”—the collapse of Na into one half of its “ideal” polyhedron—was encountered in the structure of Zn-chkalovite, \(\mathrm{Na_2Zn(Cd)Si_2O_6}\) \((^{10})\). As is usual for voluminous polyhedra around cations, they occupy a large part of the area in the drawing.

Thomson cubes, through common vertices (but not edges), are linked into a very open “trellis” wall (Fig. 2), parallel to the centered face \(C = ab\). It appears very interesting to see here features similar to garnet; namely, along the \(c\) axis each twisted cube has two edges (related by a twofold axis) in common with two Si tetrahedra; however, between two successive Thomson cubes (along \(c\)) there is not a single Si tetrahedron, but a diortho group, the two halves of which are related by a plane of symmetry. The two shorter edges of the trigonal prism in which the \([\mathrm{Si_2O_7}]\) group is inscribed are the basal edges of two \(\mathrm{Na_2}\) pyramids; the third long edge is divided in half by the “fifth” vertex of the neighboring \(\mathrm{Na_2}\) pyramid (Fig. 3). The reality of chains of Thomson cubes and Si tetrahedra (of quasi-garnet type) is emphasized by the parallelism of their axes to the elongation of the crystals.

Table 3

In Table 3 the valence balance according to Pauling is given. It is quite satisfac—

...satisfactory from the standpoint of the usual tolerances, but becomes still better if one takes into account the remark made above concerning the two “coordination spheres” around $\mathrm{Na}_1$, with four distances $\mathrm{Na}_1—\mathrm{O}_3$ considerably greater than the distances $\mathrm{Na}_1—\mathrm{O}_2$ and $\mathrm{Na}_1—\mathrm{O}_4$, and if the corresponding contributions of $\mathrm{Na}_1$ to the valence balances of the oxygen atoms are estimated as $1/8-$ and $1/8+$. The same also applies to the atom $\mathrm{Na}_2$, with four nearer neighbors ($1/5+$) and one more distant one ($1/5-$).

Institute of Crystallography
Academy of Sciences of the USSR

Institute of Inorganic and Physical Chemistry
Academy of Sciences of the Azerbaijan SSR

Received
13 VII 1967

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