CHEMISTRY

HARI DEV BHARGAVA, L. M. KOVBA, L. I. MARTYNENKO,
Academician V. I. SPITSYN

ON NEW COMPOUNDS OF OXIDES OF RARE-EARTH ELEMENTS, STRONTIUM, AND BARIUM

We have already reported \((^{1})\) on the synthesis of compounds of the type \(\mathrm{SrLn_2O_4}\) (where \(\mathrm{Ln}=\mathrm{Sm}, \mathrm{Dy}\), and \(\mathrm{Yb}\)), having the \(\mathrm{CaFe_2O_4}\) structure \((^{2})\). In the present work the results are given of a study of the interaction of strontium oxide with oxides of the remaining rare-earth elements, and also of barium oxide with oxides of certain rare-earth elements.

The synthesis of the compounds was carried out by calcining mixtures of oxides or salts—carbonates, nitrates, oxalates—at a temperature of \(1100\text{–}1300^\circ\). Usually the mixtures, containing the interacting components in various

Table 1

Results of phase analysis

* The samples were calcined in a stream of hydrogen.
** For pure \(\mathrm{SrO}\), \(a=5.1588\pm0.0005\) Å was found.

ratios, were calcined in the form of pressed pellets. For the \(\mathrm{SrO — Sm_2O_3}\) system, annealing of samples at \(1700^\circ\) with subsequent quenching was also carried out. Mixtures containing praseodymium oxide and terbium oxide were calcined in a stream of hydrogen. X-ray diffraction patterns of the resulting preparations were recorded in RKD-57 and RKU-86 cameras; radiation \(\mathrm{Fe}K\) (Sm), \(\mathrm{Co}K_\alpha\) (Eu, Gd, Dy, Tb), and \(\mathrm{Cu}K_\alpha\) (La, Ce, Pr, Nd, Ho, Er, Tu, Yb, Lu) was used. In several cases the recording was carried out in a focusing camera–monochromator \((^{3})\).

The results of phase analysis, presented in Table 1, show that at \(1100^\circ\) the rare-earth oxides do not dissolve noticeable amounts of strontium and barium oxides and do not dissolve in them (the lattice parameter of \(\mathrm{SrO}\) is practically

does not change after calcination in the presence of Sm₂O₃). Strontium oxide reacts with rare-earth oxides in the series from neodymium to lutetium, forming compounds SrLn₂O₄. In this case the rate of interaction drops sharply with increasing ionic radius of the rare-earth element. Thus, in the case of the Nd₂O₃—SrO mixture, the reaction is completed at 1300° in 16 h, and at 1100° only in 36 h, whereas for complete

Table 2

Lattice parameters of the compounds (Sr,Ba)Ln₂O₄

interaction of the reagents in other cases, calcination at 1100° for 16 h is sufficient.

Barium oxide reacts with rare-earth oxides in the series from praseodymium to lutetium, i.e., the formation of BaLn₂O₄ compounds is possible for rare-earth elements with larger ionic radii than in the case of analogous strontium compounds. An attempt to synthesize the compounds SrPr₂O₄ and BaCe₂O₄ by reducing the corresponding tetravalent rare-earth compounds SrPrO₃ and BaCeO₃ with hydrogen at 1200° was unsuccessful. Under these conditions cerium is not reduced from BaCeO₃, while the compound SrPrO₃ decomposes, apparently with the formation of a mixture of Pr₂O₃ and SrO. By contrast, reduction of BaPrO₃ under these conditions leads to the formation of BaPr₂O₄.

Table 3

Results of indexing the Debye diagram of BaYb₂O₄

Almost all the synthesized compounds crystallize in the rhombic system (structure of the CaFe₂O₄ type, like SrSm₂O₄ (¹) and SrEu₂O₄ (⁴)). The lattice parameters are given in Table 2. Comparison of the lattice parameters and the intensities of the lines on the X-ray diagrams of the compounds obtained clearly shows that these substances are isostructural with calcium ferrite.

The barium oxide compound with ytterbium oxide has a structure of another type—it apparently crystallizes in the hexagonal system. All the intense lines of the X-ray diagram are indexed (Table 3) assuming a hexagonal cell with a volume equal to 111,9, which is very close to the value of the sum of the volumes of BaO and Yb₂O₃ per one formula unit (113 ų). The X-ray diagram contains only lines with indices satisfying the condition \(h - k \pm l = 3n\), i.e., the coordinates of the heavy atoms are \(000\), \(1/3, 2/3, 1/3\), and \(2/3, 1/3, 2/3\). Ordered

the arrangement of the heavy atoms leads to the appearance of a superstructure. It was not possible to determine the parameters of the true cell; the parameters of the subcell are: \(a = 6.038\) Å, \(c = 3.545\) Å, \(V = 111.9\) Å\(^3\).

As indicated above, strontium and samarium oxides at 1100° practically do not dissolve in one another. In the case of samples quenched from 1700°, however, considerable solubility of strontium oxide in samarium oxide was found (up to 30 mol.% SrO). Conversely, \(\mathrm{Sm_2O_3}\) is practically insoluble in SrO. The change in the lattice parameters of the solid solution of SrO in \(\mathrm{Sm_2O_3}\) as a function of the composition of the mixtures is shown in Table 4. These data present

Table 4

Lattice parameters of the solid solution of SrO in \(\mathrm{Sm_2O_3}\)

great interest in connection with the presumed existence of a region of homogeneity in \(\mathrm{Sm_2O_3}\) (the \(\beta\)-form) \((^5)\). Our data confirm the possibility of the existence of a considerable region of homogeneity in \(\mathrm{Sm_2O_3}\), since we have shown that the oxide of a divalent metal (SrO in the present case) dissolves in \(\mathrm{Sm_2O_3}\) at high temperatures.

The interaction of rare-earth-element oxides with strontium and barium oxides and the formation of compounds of the type \((\mathrm{Sr}, \mathrm{Ba})\mathrm{Ln_2O_4}\) may be regarded as a manifestation of amphoteric properties by the rare-earth oxides, since the atoms of the divalent and trivalent metals in the \(\mathrm{CaFe_2O_4}\) structure differ substantially in the type of coordination. The absence of interaction in systems with \(\mathrm{La_2O_3}\) and \(\mathrm{CeO_3}\) (and also \(\mathrm{Pr_2O_3}\) in the case of strontium oxide) can be explained by crystallographic features of the \(\mathrm{CaFe_2O_4}\) structure. Indeed, the rare-earth atom in a compound of the \(\mathrm{CaFe_2O_4}\) type has sixfold coordination—an octahedron. Such a type of coordination polyhedron is observed in metastable modifications of the oxides of neodymium, samarium, and other rare-earth elements having a smaller ionic radius, but is not characteristic of lanthanum and cerium.

We studied the stability of the synthesized compounds toward the action of water vapor, moist and dry \(\mathrm{CO_2}\), at room temperature. In all three cases no formation of a new phase was detected. For the \(\mathrm{SrYb_2O_4}\) preparation, kept in a stream of moist \(\mathrm{CO_2}\) for 20 h, a gain in weight amounting to 0.02% of the weight of the initial preparation was observed. Thus, partial decomposition nevertheless did occur.

Moscow State University
named after M. V. Lomonosov

Received
19 XI 1964

CITED LITERATURE

1. Kh. D. Bkhargava, L. M. Kovba et al., DAN, 153, 1318 (1963).
2. B. E. Decker, J. S. Kasper, Acta crystallogr., 10, 332 (1957).
3. Yu. P. Simanov, V. K. Trunov et al., in: New Machines and Instruments for Testing Metals, Moscow, 1961.
4. H. Bärnighausen, G. Brauer, Acta crystallogr., 15, 1059 (1962).
5. V. S. Kutsev et al., ZhNKh, 8, 1049 (1963).