V. B. GLUSHKOVA, E. K. KELER

ON THE POLYMORPHISM OF LANTHANUM OXIDE

(Presented by Academician A. N. Frumkin, June 3, 1963)

According to Goldschmidt \((^1)\), lanthanum oxide \(\mathrm{La_2O_3}\) crystallizes in the hexagonal structure (\(A\)-form). In 1935 Loberg \((^1)\) reported the preparation of cubic lanthanum oxide (\(C\)-form \(\mathrm{La_2O_3}\)) by decomposition of the double ammonium–lanthanum nitrate salt when it was heated for 20 hours at \(450^\circ\). The X-ray diffraction pattern of this preparation was, however, very indistinct, so that only 4 lines could be indexed. After repeated heatings of the oxide

Fig. 1. X-ray diffraction patterns of the decomposition products of lanthanum nitrate (a) and lanthanum oxalate (b): 1—\(480^\circ\), 4 hours; 2—\(450^\circ\), 6 hours; 3—\(450^\circ\), 3 hours; 4—\(400^\circ\), 3 hours; 5—\(700^\circ\), 2 hours; 6—\(570^\circ\), 20 hours; 7—\(450^\circ\), 16 hours.

with additions of mineralizers, in order to obtain a better-crystallized product, no lanthanum oxide lines at all were found on the X-ray diffraction patterns. Only at a temperature of \(650^\circ\) did lines belonging to the \(A\)-form of \(\mathrm{La_2O_3}\) appear. Yandelli \((^3)\), who investigated the polymorphism of rare-earth oxides, mentions the appearance, at a temperature of \(350–500^\circ\), in the X-ray pattern of the \(A\)-form of \(\mathrm{La_2O_3}\) of some additional lines, which he attributed to the \(C\)-form of the oxide. Shafer and Roy \((^4)\) concluded that it is impossible to obtain the \(C\)-form at temperatures above \(500^\circ\). In a recently published work, Roth and Schneider (1960) \((^5)\) repeated Loberg’s experiments, but the \(C\)-form of \(\mathrm{La_2O_3}\) was not detected. They made an attempt to obtain the \(C\)-form of \(\mathrm{La_2O_3}\) by decomposing the hydroxide at low temperatures. However, upon decomposition of the hydroxide, as also of lanthanum nitrate, basic salts were formed as intermediate products; these are stable at least up to \(500^\circ\), a temperature too high for obtaining the \(C\)-form of \(\mathrm{La_2O_3}\). Roth and Schneider did not succeed in obtaining pure lanthanum oxide at temperatures below \(500^\circ\).

In our previous work we studied the decomposition of various lanthanum salts and its hydroxide during slow heating in air.

The results we obtained coincided with the data of Roth, Schneider, Wendland, and others \((^{5,7})\). Thus it was shown that it is impossible to obtain the low-temperature form \(C\)-\(\mathrm{La_2O_3}\) by decomposition of salts and the hydroxide upon heating them in air. It was of interest to study the decomposition of lanthanum salts in vacuum, since lowering the pressure could reduce the temperature for obtaining the pure oxide to the stability temperature of the \(C\)-form of \(\mathrm{La_2O_3}\). However, the first experiments at a pressure of 1 mm Hg did not lead to the desired results. In all cases, upon decomposition of the basic salts, the \(A\)-form of \(\mathrm{La_2O_3}\) was immediately formed.

Fig. 2. X-ray diffraction patterns of \(C\)- and \(A\)-\(\mathrm{La_2O_3}\)

Experiments at a lower pressure of the gas phase in the reaction volume were carried out in a vacuum furnace with a molybdenum heater. The substance under study (lanthanum salt or hydroxide) was placed in a platinum ampoule in the furnace and slowly heated to the required temperature. The temperature in the furnace was measured with a standard platinum thermocouple; the accuracy of maintaining the temperature was \(\pm 5^\circ\). The rate of heating of the substance was such that the pressure during the entire experiment did not rise above \(1 \cdot 10^{-3}\) mm Hg. After the required temperature had been reached, the samples were held at it for several hours. The products obtained after calcination were analyzed by X-ray and chemical analyses.

The decomposition of lanthanum carbonate, oxalate, hydrate, and nitrate was studied in vacuum \(1 \cdot 10^{-3}\)—\(1 \cdot 10^{-4}\) mm Hg. The results obtained are summarized in Table 1. As an example, Fig. 1 gives some X-ray diffraction patterns of products obtained in the decomposition of lanthanum nitrate and oxalate.

Fig. 3. X-ray diffraction patterns of lanthanum oxide calcined at temperatures:
\(a\)—\(500^\circ\), 4 hours; \(b\)—\(550^\circ\), 2 hours; \(v\)—\(600^\circ\), 30 min.; \(g\)—\(700^\circ\), 30 min.

Thus, upon decomposition of lanthanum oxalate and carbonate in vacuum, an amorphous product was formed containing a significant amount of carbon. The presence of carbon both in the basic salt and in the oxide impeded their crystallization up to a temperature of \(570^\circ\), when weak lines of \(A\)-\(\mathrm{La_2O_3}\) appeared. Upon decomposition of the hydrate, an amorph-

Table 1

... product which at a temperature of about 470° crystallizes in the cubic structure (C-form La₂O₃). However, already at 550° C-La₂O₃ begins slowly to transform into the A-form. The best-crystallized C-form of La₂O₃ is formed upon decomposition of lanthanum nitrate in vacuum.

Table 2

Figure 2 presents the X-ray diffraction patterns (Cu \(K_\alpha\) radiation), and Table 2 gives the interplanar spacings and intensities of the \(C\)- and \(A\)-forms of \(\mathrm{La_2O_3}\). As a result of indexing the X-ray diffraction patterns, the following values were obtained for the unit-cell parameters: the low-temperature \(C\)-form—body-centered fluorite-type lattice, \(a_0 = (11.36 \pm 0.02)\ \text{Å}\); the high-temperature \(A\)-form—hexagonal, \(a_0 = 3.937\ \text{Å}\), \(c_0 = 6.130\ \text{Å}\), \(c/a = 1.56\).

When heated in air, \(C\)-\(\mathrm{La_2O_3}\) at a temperature of about \(300^\circ\) absorbs carbon dioxide and water vapor from the air and is converted into a basic salt; upon further heating to \(650\)—\(700^\circ\), the product obtained decomposes, again forming the oxide, but now the \(A\)-form of \(\mathrm{La_2O_3}\). When heated in vacuum or in dry oxygen, \(C\)-\(\mathrm{La_2O_3}\) slowly transforms into \(A\)-\(\mathrm{La_2O_3}\) at temperatures above \(550^\circ\) (Fig. 3).

The \(C\)-form of lanthanum oxide, first obtained by us in pure form, is a metastable form of lanthanum oxide which, upon heating, like \(C\)-\(\mathrm{Nd_2O_3}\), transforms into the hexagonal form, the only stable form for these oxides. For the \(C\)-form of \(\mathrm{La_2O_3}\), the lattice constant and the temperature interval of its transition to the \(A\)-form of \(\mathrm{La_2O_3}\) have been measured.

Institute of Silicate Chemistry
named after I. V. Grebenshchikov
Academy of Sciences of the USSR

Received
28 V 1963

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