CHEMISTRY

I. A. BONDAR’, T. F. TENISHEVA, Yu. F. SHEPELEV,
Corresponding Member of the Academy of Sciences of the USSR N. A. TOROPOV

A NEW RARE-EARTH DIORTHOSILICATE, $\mathrm{K}_3\mathrm{Eu}(\mathrm{Si}_2\mathrm{O}_7)$

As reported earlier ($^1$, $^2$), we have synthesized and investigated a series of single crystals of silicates of rare-earth elements: oxyorthosilicates of lanthanum, samarium, neodymium, europium, and ytterbium ($\mathrm{Ln}2\mathrm{O}[\mathrm{SiO}_4]$), diorthosilicates of europium and ytterbium ($\mathrm{Ln}_2\mathrm{Si}_2\mathrm{O}_7$), yttrium silicate with the garnet structure $\mathrm{Y}_4\mathrm{Si}_3\mathrm{O}$, and others.

In the present communication, single crystals of europium and potassium diortho-(pyro)-silicate—$\mathrm{K}_3\mathrm{Eu}(\mathrm{Si}_2\mathrm{O}_7)$—are described. The synthesis of this silicate was carried out from a solution in a KF melt in a platinum-rhodium furnace with programmed slow cooling (the average cooling rate was $\sim 1^\circ$/hour). The duration of the experiment was 16 days. As a result, single crystals were obtained that made it possible to carry out microscopic, goniometric, spectroscopic, and X-ray investigations.

Fig. 1. Crystals of europium and potassium silicate
$\mathrm{K}_3\mathrm{Eu}(\mathrm{Si}_2\mathrm{O}_7)$

Fig. 2. General appearance of the crystals

The crystals are shown in Fig. 1. They have the form of equiaxial plates with refractive indices $n_o = 1.713$; $n_e = 1.709$, weak birefringence (0.004); the crystals are optically positive, uniaxial. The density, determined pycnometrically in kerosene, is $d^{25} = 3.41$ g/cm$^3$. Goniometric measurements showed that the crystals belong to the hexagonal system, of the dihexagonal-dipyramidal symmetry type ($6/mmm$). In their external faceting the silicate crystals have well-developed faces of the dihexagonal prism $(11\bar{2}0)$, the hexagonal pyramid $(10\bar{1}1)$, and the pinacoid $(0001)$. The general appearance of such a crystal is shown in Fig. 2.

The infrared spectrum, shown in Fig. 3, reveals, along with groups of bands in the ranges 1000–850 and 550–400 cm$^{-1}$, a single

a band of medium intensity at 685 cm(^{-1}). This band may be assigned to the symmetric stretching vibration of the Si—O—Si bond ((\nu_s) SiOSi) and, consequently, indicates the presence of condensed silico-oxygen tetrahedra in the structure. At the same time, the existence of only one band in the frequency region of (\nu_s) SiOSi makes it possible to assume the presence of diortho-groups (\mathrm{Si_2O_7}) ((^3,{}^4)), which agrees well with the Si:O content ratio, very close to (1:3.5), according to chemical analysis.

Fig. 3. IR absorption spectrum of (\mathrm{K_3Eu(Si_2O_7)}). (IKS-14 spectrometer, sample—a pellet in KBr, weight 2 g, diameter 25 mm, substance content 5 mg)

It should also be noted that the activity of the (\nu_s) SiOSi vibration in the IR spectrum indicates non-centrosymmetry of the (\mathrm{Si_2O_7}) ions ((\angle \mathrm{SiOSi} < 180^\circ)).

Table 1 gives the assignment of frequencies for the symmetry of the (\mathrm{Si_2O_7}) ion (C_{2v}=mm).

From rotation radiographs and Weissenberg photographs it was established that the crystal belongs to the hexagonal system—Laue class (6/mmm). The unit-cell parameters are (a=9.98 \pm 0.01) Å, (c=14.44 \pm 0.02) Å. Comparison of these data with the spectroscopically established presence of (\mathrm{Si_2O_7}) ions makes it possible to propose the formula of the compound obtained as (3\mathrm{K_2O}\cdot \mathrm{Eu_2O_3}\cdot 4\mathrm{SiO_2}=2[\mathrm{K_3Eu(Si_2O_7)}]). This formula agrees satisfactorily with the chemical-analysis data:

Found (products of two different syntheses), %: (\mathrm{K_2O}) 28.91, 28.68; (\mathrm{Eu_2O_3}) 43.24, 43.61; (\mathrm{SiO_2}) 26.61, 26.52

(\mathrm{K_3Eu(Si_2O_7)}). Calculated, %: (\mathrm{K_2O}) 32.30; (\mathrm{Eu_2O_3}) 40.24; (\mathrm{SiO_2}) 27.46

Some difference between the calculated and found oxide contents can be explained by the possibility of replacement of potassium positions by europium in the diorthosilicate.

With 6 formula units of (\mathrm{K_3Eu(Si_2O_7)}) entering the unit cell, the calculated density is (3.50\ \mathrm{g/cm^3}), which is in good agreement with the experimental value. The presence in X-ray goniometric surveys of systematic extinctions (hh0l) for all (l=2n+1) leads to three possible space groups: (C_{6v}^{3}=P6_3cm), (D_{3h}^{2}=P\overline{6}C2), (D_{6h}^{3}=P6_3/mcm); however, the symmetry of the external form of the crystal argues in favor of the last. Since the possible local symmetry of the non-centrosymmetric (\mathrm{Si_2O_7}) ions (i.e., the symmetry of the special positions of the “bridging” O atoms) is limited to the groups (mm), (m), and (2), the placement of six of these ions in a crystal with symmetry (P6_3/mcm) is unambiguously determined by a sixfold set of special positions with symmetry (mm). It follows from the above that the crystal under investigation is potassium europium diorthosilicate, (\mathrm{K_3Eu(Si_2O_7)}); therefore, the analogy between this compound and the recently described pyrosilicates and pyrogermanates of the type (\mathrm{X_2Pb(Z_2O_7)}), where (\mathrm{X}=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs}); (\mathrm{Z}=\mathrm{Si}, \mathrm{Ge}) (5), is of some interest. The structure of (\mathrm{K_2Pb_2(Si_2O_7)}), studied

Table 1

Frequencies of absorption maxima in the IR spectrum of (\mathrm{K_3Eu(Si_2O_7)})

Note. v. s. — very strong, s. — strong, med. — medium, w. — weak, sh. — shoulder, v. w. — very weak shoulder, sh. — “shoulder” on the slope of a stronger band.

Narai-Szabó (⁶), is characterized by layers of composition Pb₂Si₂O₇ with centrosymmetric Si₂O₇ groups, which are linked to one another by Pb atoms in trigonal coordination with respect to oxygen. The layers are separated by large potassium cations (coordination number = 9). There are grounds for assuming the existence of layers of composition EuSi₂O₇ in the crystal K₃Eu(Si₂O₇), but the Si₂O₇ groups are noncentrosymmetric. From the standpoint of the considerations expressed earlier concerning the dependence of the magnitude of the SiOSi angle in Si₂O₇ ions on the degree of covalency of the bonds of the “terminal” oxygen atoms with cations (⁴, ⁷), the decrease in ∠SiOSi on going from K₂Pb₂(Si₂O₇) to K₃Eu(Si₂O₇) can be explained by a decrease in the covalency of the Eu—O bonds in comparison with Pb—O and by an increase in the relative content of K⁺ ions.

A complete determination of the structure of K₃Eu(Si₂O₇) will be carried out later.

The authors express their gratitude to M. M. Piryutko and E. T. Khomutova for carrying out the chemical analyses, and to A. N. Lazarev and Yu. I. Smolin for taking part in the discussion of the present work.

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

Received
10 IX 1964

REFERENCES CITED

¹ N. A. Toropov, I. A. Bondar, M. M. Piryutko, DAN, 156, No. 3, 619 (1964). ² N. A. Toropov, I. A. Bondar, DAN, 157, No. 6 (1964). ³ A. N. Lazarev, Optics and Spectroscopy, 9, 195 (1960). ⁴ A. N. Lazarev, T. F. Tenisheva, Izv. AN SSSR, OKhN, 1961, 964. ⁵ A. Durif-Varrambon, J. Lajzerowicz, Bull. Soc. franc. minéral. et cristallogr., 86, 88 (1963). ⁶ T. Náray-Szabó, Phys. and Chem. Glasses, 4, 38A (1963). ⁷ A. N. Lazarev, Izv. AN SSSR, ser. khim., 1964, 234.