UDC 548.736.6

CRYSTALLOGRAPHY

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

CRYSTAL STRUCTURE OF Y-OXYSILICATE $\mathrm{Y}_2[\mathrm{SiO}_4]\mathrm{O}$

The initial data on synthetic oxysilicate $\mathrm{Y}_2\mathrm{SiO}_5$ are given in $(^{1-4})$. In $(^4)$, from the mappings of three layers $hk0$, $hk1$, and $hk2$ (CuK$_\alpha$ radiation—

Table 1

$\mathrm{Y}_2\mathrm{SiO}_5$. Coordinates of the basis atoms and isotropic thermal corrections

* The results of the authors of $(^4)$ are given in parentheses.

Table 2

Interatomic distances in the structure of $\mathrm{Y}_2[\mathrm{SiO}_4]\mathrm{O}$

$\mathrm{Y_I}$-octahedron

$\mathrm{Y_{II}}$-octahedron

Si-tetrahedron

…200 nonzero structural amplitudes), a preliminary determination of the crystal structure of \(\mathrm{Y_2SiO_5}\) was given. The coordinates of 8 independent atoms found in \((^4)\) and the interatomic distances calculated from them are presented in Tables 1 and 2. Among several unsatisfactory ones, one \(\mathrm{Y—O}\) distance, equal to \(2.064\ \text{Å}\), stands out, and in the Si tetrahedron \(\mathrm{O—O} = 2.36\ \text{Å}\).

In our refinement of the atomic coordinates of the crystal structure of \(\mathrm{Y_2SiO_5}\), 800 nonzero structural amplitudes from 7 layers, \(hk0 — hk5\) and \(h0l\), were used. The intensities (Mo \(K_{\alpha}\) radiation,

\[ \max \sin \vartheta / \lambda = 0.9\ \text{Å}^{-1} \]

) were estimated visually using a \(\sqrt[4]{2}\) blackening scale.

Fig. 1. Structure of \(\mathrm{Y_2SiO_5}\), projection \(xz\). Along the long axis \((c)\) extend serrated ribbons of deformed Y-octahedra. The light and dark ribbons are located in two successive layers at different levels. \(\mathrm{SiO_4}\) tetrahedra connect both translationally identical ribbons and ribbons from different layers. The morphological cell is shown by a dashed line; the cell from work \((^4)\), by a dotted line.

The entire set of intensities was obtained from a crystal elongated along the \(c\) axis, with dimensions \(0.08 \times 0.1 \times 0.3\ \mathrm{mm^3}\). Single-crystal samples of the compound \(\mathrm{Y_2SiO_5}\) were obtained by hydrothermal crystallization in the system \(\mathrm{K_2O—Y_2O_3—SiO_2—H_2O}\) at \(450^\circ\) and \(\sim 1500\) atm. According to X-ray spectral analysis (V. V. Lider, Institute of Crystallography), the crystals studied contain no impurities. The quantitative composition, confirmed by the results of chemical analysis \((\mathrm{Y_2O_3}\ 79.21\%;\ \mathrm{SiO_2}\ 20.43\%;\ \mathrm{K_2O}\ 0.006\%;\ \mathrm{H_2O}\ 0.2\%;\ \Sigma = 99.9\%,\) analyst V. S. Bykova), also raises no doubts.

The parameters of the monoclinic cell, refined on a DRON-I diffractometer (Cu \(K_{\alpha}\) radiation), are:

\[ a = 10.410 \pm 0.003\ \text{Å}, \qquad b = 6.721 \pm 0.002\ \text{Å}, \qquad c = 12.490 \pm 0.005\ \text{Å}; \]

\[ \beta = 102^\circ 39' \pm 2' \]

and coincide with the results of \((^3)\). \(Z = 8\) \((d = 4.6)\). The space group is \(C_{2h}^{6} = I2/c(a)\).

From the entire three-dimensional array, a three-dimensional function was calculated…

Patterson, the analysis of which agrees well with the results of (⁴). The refinement of the structure of Y₂SiO₅ was carried out by the least-squares method.

Since the absorption edge for yttrium coincides with the $K_\alpha$ series of Mo radiation, for the Y atom a correction for anomalous scattering was used, $f_{\mathrm{corr}} = f_0 + \Delta f'$. The initial $R$ factor, equal to 17.3% ($B = 0.32$, absorption was not taken into account; $\mu R = 0.9$), upon refinement of the positional parameters of the Y atoms decreased to 14.7%. Refinement of the lighter Si and O atoms led to a further decrease of the discrepancy coefficient to 10.1%. The final coordinates of the 8 independent atoms and the interatomic distances calculated from them are presented in Tables 1 and 2, together with the data of (⁴).

Fig. 2. Vertical (parallel to the elongation) chains of Y octahedra (toothed)

The crystal structure of yttrium oxysilicate Y₂SiO₅ may be regarded as built of orthotetrahedra SiO₄ and octahedra YO₆. In the Si tetrahedron the mean Si—O distance is 1.63 Å and O—O = 2.70 Å, with an insignificant spread: the limiting Si—O values are 1.60–1.66 and O—O = 2.52–2.76 Å. The yttrium atoms occupy two general crystallographic positions in distorted YO₆ octahedra. The mean Y—O distance is 2.3 Å, with limiting values of 2.21–2.33 Å in the Yᵢ octahedron and 2.15–2.39 Å in the Yᵢᵢ octahedron. The next distances in magnitude are 3.06 and 2.66 Å in the Yᵢ and Yᵢᵢ octahedra, respectively. The lengths of the oxygen edges of the Y octahedra vary over wide limits, 2.76–4.24 Å.

In Fig. 1 the principal architectural features of Y oxyorthosilicate YᵢYᵢᵢ[SiO₄]O are evident. Along the long axis $c = 12.49$ Å extend toothed ribbons of Y polyhedra connected by common edges. The core of the ribbon is a chain in which the Yᵢ octahedra alternate in pairs at two levels with a small difference in height. The teeth are Yᵢᵢ octahedra, connected to the core Yᵢ octahedra of their level by a common edge. Translationally identical ribbons along $a$ are connected into a layer, parallel to the $xz$ plane, by orthotetrahedra SiO₄. The next underlying layer is identical to the one just described, but is displaced by the oblique translation of the body-centered cell $(a + b + c)/2$. In Fig. 1 its dark ribbons appear between the light ribbons of the layer above. The flat layers in which the ribbons of octahedra lie are not isolated, but are connected with the ribbons of the layers above and below by toothed octahedra. These octahedra, connected by common edges, form columns along the $b = 6.721$ Å axis (Fig. 2), and it is they that determine the elongation of the crystals. The previously described, less effective chains parallel to the long $c$ axis make the elongated crystals platy, with indistinct cleavage on (100).

The inclusion in the coordination sphere of the Yᵢᵢ octahedron also of a 7th O with an elongated Y—O bond of 2.64 Å leads to the appearance of a common edge between the toothed octahedra of two neighboring translationally identical chains and to a less desirable common edge at the Si tetrahedron and Y octahedron. Both circumstances substantially worsen the valence balance. Transformation of the Y semipolyhedron into a distorted octahedron (we amputate one normal bond Y₂—Oᵢᵢ = 2.33 Å, but retain the elongated Y₂—Oᵢ = 2.66 Å!) and the subsequent idealization of the octahedra leads to the “manifestation” in the structure of an ideal closest cubic packing of O atoms, in which for every four Y octahedra there are six vacant ones (Fig. 3). The same ratio $^{2}/_{3}$ is obtained from the formula Y₂SiO₅, since in the case of close-

Fig. 3. Idealized motif of \( \mathrm{YO}_6 \)-octahedra. Occupied and empty octahedra alternate in the ratio \(4:6\)

With the densest packing of 5 O anions in the compound \( \mathrm{Y}_2\mathrm{SiO}_5 \), there must be 5 octahedral voids, 2 of which are filled by the two types of Y represented in the formula.

Institute of Crystallography
Academy of Sciences of the USSR

Received
22 VIII 1968

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