CHEMISTRY

L. M. KOVBA, E. A. IPPOLITOVA, Yu. P. SIMANOV
and Corresponding Member of the Academy of Sciences of the USSR Vikt. I. SPITSYN

X-RAY STUDY OF URANATES OF THE ALKALI ELEMENTS

The literature contains information on the structures of the monouranates of magnesium (^1), calcium (^2), strontium (^2), barium (^3), and β-UO₂(OH)₂ (^4). In the structures of the monouranates of magnesium, barium, and β-UO₂(OH)₂, somewhat distorted UO₆ octahedra form either pseudotetragonal layers (barium uranate, β-UO₂(OH)₂) or chains (magnesium uranate) of composition (UO₂)O₂. Hexagonal layers of composition (UO₂)O₂ are present in the structures of the monouranates of calcium and strontium. In this case the uranium atoms are surrounded by oxygen atoms at the vertices of a somewhat distorted cube. In the structures of the monouranates of lithium, sodium, and potassium, hexagonal or pseudohexagonal layers, analogous to those present in calcium monouranate (^5), were also found; however, more detailed information on the structures of the monouranates of the alkali elements has not been published. No experimental data whatever are available on the structures of diuranates.

We obtained single crystals of the normal uranates of lithium (α-modification), sodium (β-modification), and of the diuranates of sodium, potassium, and rubidium. The indicated compounds were studied by the Laue, rotation, and powder methods. α-Na₂UO₄, K₂UO₄, Rb₂UO₄, and Cs₂UO₄, single crystals of which could not be obtained, were studied by the powder method. In all cases the intensities of reflections were determined visually on a ten-point scale from Debyegrams. All data are given in angstroms.

Table 1

Some X-ray data for mono- and diuranates of the alkali elements

* In the literature (^7) for Li₂UO₄, Na₂UO₄, and K₂UO₄, the data given are (respectively) 6.61; 5.06; 4.98. The last two figures are underestimated; the first, apparently, is overestimated.
** For the rhombohedral cell.

Table 1 gives the values of the lattice parameters of the uranates studied, their space groups, and the density values calculated from the X-ray data and determined pycnometrically.

To determine the structures of the indicated uranates we used the trial method, with the use of geometrical and crystal-chemical con-

reflections. Calculation of the intensities confirms the structures described below.

In the structures of $\alpha$-Li$_2$UO$_4$, $\beta$-Na$_2$UO$_4$, K$_2$UO$_4$, Rb$_2$UO$_4$, and Cs$_2$UO$_4$, tetragonal or pseudotetragonal layers $(\mathrm{UO}_2)\mathrm{O}_2$ were found, analogous to those present in the structures of BaUO$_4$ and $\beta$-UO$_2$(OH)$_2$ ($^{3,4}$). The alkali-element atoms are located between the layers.

Normal uranates of potassium, rubidium, and cesium are isostructural with K$_2$NiF$_4$ ($^6$):

$2\mathrm{U}$ in $(a)$; $4\mathrm{Me}^{\mathrm{I}}$ in $(e)$; $4\mathrm{O}_{\mathrm{I}}$ in $(e)$; $4\mathrm{O}_{\mathrm{II}}$ in $(c)$ (here and below $\mathrm{O}_{\mathrm{I}}$ is oxygen belonging to the uranyl grouping). The values of the parameters $Z_{\mathrm{Me}^{\mathrm{I}}}$ ($\mathrm{Me}$—alkali element) and $Z_{\mathrm{O}_{\mathrm{I}}}$, as well as the interatomic distances $\mathrm{U—O}$, $\mathrm{Me}^{\mathrm{I}}—\mathrm{O}$ and the shortest $\mathrm{O—O}$ distances, are given in Table 2.

Table 2

Some data on the structures of $\alpha$-Li$_2$UO$_4$, $\beta$-Na$_2$UO$_4$, K$_2$UO$_4$, Rb$_2$UO$_4$, and Cs$_2$UO$_4$

$^{1}$ Shortest distance. $^{2}$ Li in $8(i)$. $^{3}$ Li in $8(f)$. $^{4}$ Li—2O$_{\mathrm{II}}$.

The structure of $\beta$-Na$_2$UO$_4$ is an orthorhombically distorted structure of the K$_2$NiF$_4$ type: $4\mathrm{U}$ in $(a)$; $8\mathrm{Na}$ in $(i)$; $8\mathrm{O}_{\mathrm{I}}$ in $(i)$; $8\mathrm{O}_{\mathrm{II}}$ in $(e)$. The values of the parameters $Z_{\mathrm{Na}}$, $Z_{\mathrm{O}_{\mathrm{I}}}$ and of the interatomic distances are given in the same Table 2. In lithium uranate, a somewhat different arrangement of the alkali-element atoms than in $\beta$-Na$_2$UO$_4$ is also possible, namely, $8\mathrm{Li}$ in $(f)$. The arrangement of the other atoms in $\beta$-Na$_2$UO$_4$ and $\alpha$-Li$_2$UO$_4$ coincides. Table 2 gives data for both variants of the arrangement of the lithium atoms.

In the structure of $\alpha$-Na$_2$UO$_4$ there are infinite chains of UO$_6$ octahedra joined by common edges. $2\mathrm{U}$ in $(a)$; $4\mathrm{Na}$ in $(f)$; $4\mathrm{O}_{\mathrm{I}}$ in $(i)$ with $y=0.195$; $4\mathrm{O}_{\mathrm{II}}$ in $(h)$ with $x=0.245$; $\mathrm{U—2O}_{\mathrm{I}}=1.90$; $\mathrm{U—4O}_{\mathrm{II}}=2.24$; $\mathrm{Na—2O}_{\mathrm{II}}=2.44$; $\mathrm{Na—4O}_{\mathrm{I}}=2.32$; the shortest $\mathrm{O—O}$ distance is 2.73.

The structures of lithium, sodium, and potassium monouranates described above differ from the structures of these compounds studied by Zachariasen ($^5$).

The structures of the diuranates of sodium, potassium, and rubidium are defective structures of the CaUO$_4$ type: $\mathrm{U}$ in $(a)$; $\mathrm{Me}^{\mathrm{I}}$ in $(b)$; $2\mathrm{O}_{\mathrm{I}}$ and $1\frac{1}{2}\mathrm{O}_{\mathrm{II}}$ in $(c)$. The parameters $X_{\mathrm{O}_{\mathrm{I}}}$ and $X_{\mathrm{O}_{\mathrm{II}}}$, together with the values of the interatomic distances, are given in Table 3. In these structures, hexagonal layers of composition UO$_{3.5}$ were found.

Table 3

Some data on the structures of the diuranates of sodium, potassium, and rubidium

Oxygen atoms can be partially replaced by fluorine with the formation of a fluorouranate, for example, $\mathrm{NaUO_3F}$, likewise isostructural with $\mathrm{CaUO_4}$. On calcination in air, $\mathrm{NaUO_3F}$ gradually transforms into sodium diuranate. The samples remain single-phase throughout, so that, probably, a continuous region of solid solutions exists between the fluorouranate and sodium diuranate.

By reduction of sodium and potassium diuranates at 450–500° we obtained sodium and potassium uranates (V) of composition $\mathrm{Me^IUO_3}$, which were studied by the powder method. Both compounds belong to the perovskite structural type. For $\mathrm{NaUO_3}$, $a\sin\beta = b = c\sin\beta = 4.129 \pm 0.002$; $\beta = 88^\circ36' \pm 6'$; $\rho_{\mathrm{rent}} = 7.38$; for $\mathrm{KUO_3}$, $a = 4.290 \pm 0.001$; $\rho_{\mathrm{rent}} = 6.85$. Using $\mathrm{KUO_3}$ as an example, it has been shown that uranates (V) have a narrow homogeneity range (the lattice constant does not change either in the presence of an excess of potassium (upon reduction of $\mathrm{K_2UO_4}$) or in the case of its deficiency—the latter samples, along with $\mathrm{KUO_3}$, always contained uranium dioxide). Uranates (V) dissolve readily in nitric acid and slowly in acetic acid. Thus, uranates (V) are not analogues of “tungsten bronzes.” Upon reduction of $\mathrm{Li_2UO_4}$ and $\mathrm{Na_2UO_4}$, uranates (V) are not formed, in agreement with the data of Rüdorff and Leutner (⁷). As a result of reduction, $\mathrm{Rb_2UO_4}$ forms $\mathrm{Rb_2UO_3}$ with cubic pseudocell parameter $d = 4.316$. $\mathrm{Cs_2UO_4}$ on reduction does not give uranates (V) or (IV).

Moscow State University
named after M. V. Lomonosov

Received
11 II 1958

REFERENCES CITED

¹ W. H. Zachariasen, Acta Cryst., 7, 783 (1954).
² W. H. Zachariasen, Acta Cryst., 1, 281 (1948).
³ S. Samson, L. G. Sillén, Ark. Kemi. Min., Geol., 25A, 21, 16 (1947).
⁴ G. Bergström, G. Lundgren, Acta Chem. Scand., 10, 673 (1956).
⁵ W. H. Zachariasen, Manh. Prakt. Rep., CP—2611, 14 (cited according to Acta Cryst., 7, 795 (1954)).
⁶ D. Balz, K. Plieth, Zs. Elektrochem., 59, 545 (1955).
⁷ W. Rüdorff, H. Leutner, Zs. anorg. Chem., 292, 193 (1957).