CRYSTALLOGRAPHY

B. M. KUPTSEV, Yu. A. KHARITONOV, E. A. POBEDIMSKAYA,
Academician N. V. BELOV

CRYSTAL STRUCTURE OF BASIC Li,Cd MOLYBDATE LiCd\[MoO₄\]OH

Transparent, slightly yellowish crystals of the double Li,Cd molybdate were obtained by hydrothermal synthesis at the Institute for the Synthesis of Mineral Raw Materials. With a specific gravity of 4.26 g/cm³ and parameters of the monoclinic cell (RKOP camera)

\[ a = 7.91 \pm 0.1;\quad b = 5.95 \pm 0.2;\quad c = 9.77 \pm 0.1\ \text{Å};\quad \beta = 107^\circ 30' \pm 30' \]

chemical analysis (Institute of Crystallography, Academy of Sciences of the USSR) gives the material content of the cell as

\[ \mathrm{Li}_{3.22}\mathrm{Cd}_{4}[\mathrm{MoO}_{4}]_{4}\cdot(\mathrm{OH})_{3.92}(\mathrm{H}_{2}\mathrm{O})_{0.65}. \]

Table 1 gives the interplanar spacings calculated from a powder photograph on a URS-50I diffractometer with a scintillation counter (Cu radiation, Ni filter).

Table 1

LiCd\[MoO₄\]OH. Interplanar spacings

Preliminary structural indications were also obtained from the data of thermal analysis and infrared spectroscopy. The thermogram (recorded and interpreted by G. O. Pilyan, IGEM, Academy of Sciences of the USSR) showed the presence of hydroxyl groups in the structure and the absence of H₂O particles. The fixed amount, 3.7% OH, agrees with the chemical analysis. The diffuse form of the thermal effect in differential thermal analysis indicates a statistical distribution of the deficiency. Infrared absorption spectra taken (by I. I. Plyusnina) at the Department of Crystallography of Moscow University first of all confirmed, in agreement with the formula given above, tetrahedral coordination of Mo (absorption maxima 840 and 786 cm⁻¹ in the NaCl-prism region). Bands at 1160, 1097 cm⁻¹ in the same region make it possible to speak of fourfold (tetrahedral) coordination of Li atoms, and, finally, the absorption maximum at 3430 cm⁻¹ is usually attributed to stretching vibrations of OH⁻ (hydrogen bond). Bands of molecular H₂O are absent.

If the constant deficiency in the chemical analyses of Be and Li is taken into account, these results make it possible to accept as the number of chemical units in the cell

\[ Z = 4\ \text{units of } \mathrm{LiCd}[\mathrm{MoO}_{4}]\mathrm{OH}. \]

For the crystal-structure analysis, two series of X-ray goniometric scans were recorded (Mo radiation, owing to the low quality of the reflections, \(\max \sin\vartheta/\lambda = 0.73\ \text{Å}^{-1}\)): five layer lines along the \(b\) axis and two along \(c\). From the systematic extinctions, the centrosymmetric Fedorov group was determined fairly unambiguously as

\[ C_{2h}^{5} = P2_{1}/a. \]

The intensities were estimated visually on a \(2^{0,25}\) scale of blackening marks.

In solving the structure by the heavy-atom method, Patterson projections and, subsequently, projections and the three-dimensional distribution of electron density were used. To localize the O atoms, three-dimensional difference syntheses of electron density were required. The final averaged coordinates of all atoms are given in Table 2. The origin of the coordinates is at the center of symmetry on the common O—O edge of two Cd octahedra.

The interatomic distances corresponding to the refined coordinates are given in Table 3*. The discrepancy factor is \(R = 16\%\).

The Cd atoms in the structure of \(\mathrm{LiCd[MoO_4]OH}\) occupy the centers of octahedra somewhat flattened along the pseudo-fourfold axis, i.e., they have four longer equatorial distances of 2.31–2.45 Å and two shortened ones, 2.23–2.24 Å. It is of interest to note that the octahedral coordination of Cd in the double molybdate separates it from \(\mathrm{Cd[MoO_4]}\) with the scheelite structural type \(\mathrm{CaWO_4}\), i.e., with Cd in eightfold coordination (1), and, conversely, the double molybdate approaches \(\mathrm{CdWO_4}\), which crystallizes in the wolframite type (2), i.e., with Cd in octahedra; moreover these octahedra (both in the tungstate and in the double molybdate) are joined by common edges into rocking columns extending along the axis \(b = 5.95\) Å. Mo orthotetrahedra adjoin the columns on both sides through common vertices.

Table 2

\(\mathrm{LiCd[MoO_4]OH}\). Coordinates of the basis atoms

Table 3

\(\mathrm{LiCd[MoO_4]OH}\). Interatomic distances

The Li atoms, since they account for only 2.2% of the scattering matter, almost do not appear on the electron-density maps; nevertheless, their positions are clearly established from an unexpected analogy of the structure of the basic molybdate \(\mathrm{LiCd[MoO_4]OH}\) with the structure of the basic zinc silicate, hutchinsonite \(\mathrm{Zn_2Mn[SiO_4](OH)_2}\) (3), from the “molecule” of which, leaving aside for the moment the qualitative difference, one cation in a tetrahedron (Zn) and one anion OH are thrown out.

These two structures in projection along the short axis \(b\) are shown in Figs. 1A and 1B; in both, in addition to the “economic” cells (with an angle \(\beta\) close to \(90^\circ\)) appearing in the original articles on hutchinsonite and above in the present work for the double molybdate, elongated cells are also indicated (for the double molybdate \(a' = a = 7.91\), \(b' = b = 5.95\), \(c' = (2a + c) = 15.90\) Å; \(\beta = 144^\circ 06'\)), with the aid of which the basic structural principle of both structures appears more clearly: two layers (along the axis \(a = 7.71\) Å directed obliquely to them) of the densest hexagonal packing of anions, of length (along the new axis \(c'\)) of 6 O—O edges in hutchinsonite and only 5 O—O edges in the double molybdate. In the six and, respectively, five

* All calculations were carried out at the Computing Center of Moscow University.

the outer edges are the ridges of the end octahedra in the Mn-column of hutchinsonite and the Cd-column in the molybdate. Along the obliquely running axis \(a\), in each successive layer of the packing these ends are shifted by one edge, and the columns represented by them become the steps of an infinitely wide staircase (Fig. 2). The space between the staircases, corresponding to five and four

Fig. 1. Comparison, in the \(xz\) projection, of the structures of hutchinsonite \((A)\) and Li, Cd molybdate \((B)\). In both projections, along the axis \(c\), the cells are bounded by “staircases” of occupied octahedra. In the tetrahedral material between the staircases, the light tetrahedra are the ends of the metachains \([\mathrm{Zn}_2\mathrm{O}_6]_\infty\) in \(A\) and \([\mathrm{Li}_2\mathrm{O}_6]_\infty\) in \(B\). The solid black ones are the encrusting Si-tetrahedra in \(A\) and Mo-tetrahedra in \(B\).

edges, is filled by Zn- and Si-tetrahedra in hutchinsonite, and by Mo- and Li-tetrahedra in the double molybdate.

Immediately adjacent to the rocking columns of occupied octahedra (with only one common vertex) are Zn-tetrahedra in hutchinsonite and Mo-tetrahedra in the double molybdate. One step farther (from the column of octahedra) in hutchinsonite we encounter a Zn-tetrahedron, likewise an end tetrahedron in the metachain \([\mathrm{Zn}_2\mathrm{O}_6]_\infty\), extending along the axis \(b = 2_1\) parallel to the column of octahedra. The metachains are encrusted by Si-orthotetrahedra. The pecul—

…of these chains is also that at each of their steps along the axis \(2_1\) (\(\parallel b\)) the Zn tetrahedra are doubled, and the second Zn tetrahedron is precisely the one that directly adjoins the column of Mn octahedra. This is shown in Fig. 2, where it is also seen that the second halves, the second tetrahedra, of the chain \([\mathrm{Zn}_2\mathrm{O}_6]_\infty\) are located at another level of the closest packing, and the “additional” Zn tetrahedron is already linked with the left-hand swinging column of Mn octahedra. The chains are polar along the axis \(2_1=b\), but in alternating layers of the closest packing they alternately change their polarity, being reflected with a shift in the glide plane \(a\). The chains nearest to the axis \((2_1)\)

Fig. 2. Hodgkinsonite (A) and Li,Cd molybdate (B). Arrangement in a two-layer closest packing of the basic metachains (light tetrahedra) \([\mathrm{Zn}_2\mathrm{O}_6]_\infty\) in (A) and \([\mathrm{Li}_2\mathrm{O}_6]_\infty\) in (B). Solid black: the incrusting Si tetrahedra in (A) and Mo tetrahedra in (B).

Zn tetrahedra are linked only with one another, with the \(\mathrm{Zn}_2\) tetrahedron, and with their own Zn tetrahedra. To the latter, however, there immediately adjoin two Si from different metachains, and thus the tetrahedra \(\mathrm{Zn}_1\), \(\mathrm{Zn}_2\), and Si are tied into a thick two-dimensional net clamped between walls of Mn octahedra. In the double molybdate \(\mathrm{LiCd}[\mathrm{MoO}_4]\mathrm{OH}\), the metachains \([\mathrm{Zn}_2\mathrm{O}_6]_\infty\) are entirely excluded, but the “second” Zn tetrahedra remain in place and are replaced by Mo tetrahedra. At the same time, the previously incrusting Si tetrahedra come closer together and join into a metachain which, after replacement of Si by Li, acquires the form \([\mathrm{Li}_2\mathrm{O}_6]\), while the former \(\mathrm{Zn}_2\), and now Mo, tetrahedra incrust the “basic” chain \([\mathrm{Li}_2\mathrm{O}_6]_\infty\).

We have before us a new example of the manifestation of the “anion” (anion-forming) function of lithium, which was emphasized by one of the authors in analyzing the structure of the garnet-like F analogue of cryolithionite \(\mathrm{Na}_3\mathrm{Al}_2\mathrm{Li}_3\mathrm{F}_{12}\). The less certain Li coordinates are given (in Table 2) in parentheses. The Li—O distances \((1.92–2.07\ \text{Å})\) agree well, even with respect to their scatter, with those usually cited \((^5)\).

The valence balance makes the atoms \(\mathrm{O}_\mathrm{V}\) undoubted hydroxyls, since they form the common edges of Cd octahedra, and in addition only three Cd octahedra meet at each O \((3\cdot 2/6 = 1)\). However, the Cd octahedron has one more common edge with a neighboring Cd, and since Mo tetrahedra are also attached to its ends, the corresponding \(\mathrm{O}_\mathrm{IV}\) no longer include H. At the anion \(\mathrm{O}_\mathrm{III}\), besides one Cd octahedron, Mo and Li also meet. Nevertheless each Cd also has a third OH vertex, likewise \(\mathrm{O}_\mathrm{V}\); this is the one by which the Cd octahedron abuts against the common edge of two neighboring Cd. The strain in the balance at those O atoms \((\mathrm{O}_\mathrm{I})\) at which \(1\mathrm{Mo}\), \(1\mathrm{Cd}\), and \(1\mathrm{Li}\) meet \((\Sigma = 2\frac{1}{4})\) lengthens the Li—O distance to \(2.07\ \text{Å}\).

Moscow State University
named after M. V. Lomonosov

Received
10 VIII 1967

REFERENCES

1. A. V. Chichagov, V. V. Ilyukhin, N. V. Belov, DAN, 166, No. 2 (1965).
2. A. V. Chichagov, L. N. Dem’yanets, V. V. Ilyukhin, N. V. Belov, Kristallografiya, 11, 685 (1966).
3. L. P. Solov’eva, N. V. Belov, DAN, 152, No. 2; 153, No. 3 (1963).
4. N. V. Belov, Mineralogical Collection of Lviv University, 12, 39 (1958).
5. F. Liebau, Acta crystallogr., 14, 389 (1961).