UDC 541.123.25

CHEMISTRY

Corresponding Member of the Academy of Sciences of the USSR P. P. BUDNIKOV, V. I. KUPAKOVSKII,
V. S. BELEVANTSEV

STUDY OF THE SYSTEMS $\mathrm{Gd_2O_3}$—$\mathrm{Al_2O_3}$ AND $\mathrm{Sm_2O_3}$—$\mathrm{Al_2O_3}$

The interaction of aluminum oxide with gadolinium and samarium oxides has been studied repeatedly ($^{1-4}$). According to these investigations, in each system one chemical compound is formed ($\mathrm{GdAlO_3}$ or $\mathrm{SmAlO_3}$), possessing the perovskite structure. However, aluminum oxide with other rare-earth oxides and yttrium oxide forms a whole series of new chemical compounds: $2\mathrm{Y_2O_3}\cdot\mathrm{Al_2O_3}$, $3\mathrm{Y_2O_3}\cdot5\mathrm{Al_2O_3}$ ($^5$), $\mathrm{La_2O_3}\cdot12\mathrm{Al_2O_3}$ ($^6$). In this connection, investigations were carried out to study the interaction of aluminum oxide with samarium and gadolinium oxides below the solidus temperature. The oxide melts obtained earlier ($^4$) and coprecipitated mixtures were subjected to investigation. After thermal treatment, the mixtures were subjected to microscopic and X-ray analysis.

Table 1

Phase composition of oxide mixtures calcined at various temperatures

The mixtures were prepared by coprecipitation with ammonia from a nitric-acid solution of aluminum and gadolinium (samarium) hydroxides, followed by calcination of the precipitates obtained at various temperatures. The results of the phase X-ray analysis of the mixtures are given in Table 1. As follows from the data presented in Table 1, the reaction of formation of $\mathrm{SmAlO_3}$ is completed at 880°. In the $\mathrm{Gd_2O_3}$—$\mathrm{Al_2O_3}$ system, formation of the compound $\mathrm{GdAlO_3}$ proceeds through a new phase of unknown structure, traces of which are retained even after calcination at 1380°.

Melted specimens were obtained by thermal analysis, which was used to determine the melting temperatures ($^4$). In studying the interaction between aluminum oxide and the corresponding compound ($\mathrm{GdAlO_3}$ or $\mathrm{SmAlO_3}$), the melts were annealed at 1700°. No interaction between the components was detected. Microscopically, only coarsening of the eutectic structure was observed. The interplanar spacings of the aluminum oxide lattice and the lattice periods of the compounds $\mathrm{GdAlO_3}$ and $\mathrm{SmAlO_3}$ in the melts remained practically unchanged. The lattice period of $\mathrm{GdAlO_3}$ in annealed specimens containing 40 and 50 mol.% $\mathrm{Gd_2O_3}$ was, respectively, $3.732 \pm 0.005$ kX and $3.736 \pm 0.005$ kX, while the lattice period of $\mathrm{SmAlO_3}$ in analogous specimens was $3.727 \pm 0.005$ kX and $3.728 \pm 0.005$ kX, which indicates the absence of noticeable solubility of aluminum oxide in $\mathrm{GdAlO_3}$ and $\mathrm{SmAlO_3}$. In the study of annealed melts containing more than 50 mol.% rare-earth oxides, it was found—

the formation of new chemical compounds was established. According to microstructural studies, a single-phase structure arose in compositions containing about 66 mol.% rare-earth oxide. Figure 1 shows the microstructure of specimens before and after annealing.

Fig. 1. Microstructure of an alloy containing 66 mol.% gadolinium oxide, 200×.
Left—before annealing; right—after annealing at ~1900°.

The composition of the new compounds corresponds to the formulas $2\mathrm{Gd}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$ and $2\mathrm{Sm}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$. The compounds melt with decomposition at 1950° and 1920°, respectively.

Table 2

X-ray patterns of synthesized chemical compounds $2\mathrm{Gd}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$ and $2\mathrm{Sm}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$

* Apparently, the most intense lines were obtained on the X-ray pattern.

Table 2 gives the X-ray patterns of the new synthesized compounds. The agreement of the X-ray pattern of the chemical compound $2\mathrm{Gd}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$, synthesized at high temperatures, with the X-ray pattern of the compound found upon calcination of coprecipitated mixtures indicates that the formation of perovskite $\mathrm{GdAlO}_3$ at low temperatures proceeds through the 2 : 1 phase.

According to microstructural studies, the new compounds do not have a noticeable region of homogeneity. In the interval 50–66 mol.% rare-earth oxide, two phases, 1 : 1 and 2 : 1, coexist in equilibrium. Identification of the phases was carried out by X-ray and microscopic methods and by determination of microhardness. The microhardness of the compound $\mathrm{GdAlO}_3$ was found to be $1700 \pm 100\ \mathrm{kg/cm}^2$, and that of the compound $2\mathrm{Gd}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$ was $1100 \pm 100\ \mathrm{kg/cm}^2$. For the compounds $\mathrm{SmAlO}_3$ and $2\mathrm{Sm}_2\mathrm{O}_3 \cdot \mathrm{Al}_2\mathrm{O}_3$, the microhardness is respectively $1500 \pm 100$ and $1000 \pm$

± 100 kg/mm². In alloys containing more than 66 mol.% rare-earth oxides, at high temperatures the 2 : 1 compound and the rare-earth oxide, whose structure corresponds to the B-form, are in equilibrium. On the basis of the investigations carried out, phase diagrams were constructed for the systems

Fig. 2. Phase diagram of the systems $\mathrm{Gd_2O_3}—\mathrm{Al_2O_3}$ (A) and $\mathrm{Sm_2O_3}—\mathrm{Al_2O_3}$ (B)

$\mathrm{Gd_2O_3}—\mathrm{Al_2O_3}$ and $\mathrm{Sm_2O_3}—\mathrm{Al_2O_3}$, shown in Fig. 2. Low-temperature equilibrium in the indicated systems was not investigated.

Received
26 VI 1965

REFERENCES CITED

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