V. E. PLYUSHCHЕV, M. B. VARFOLOMEEV

TETRAHYDRATES OF THE PERRHENATES OF RARE-EARTH ELEMENTS AND YTTRIUM

(Presented by Academician I. V. Tananaev on May 6, 1964)

The perrhenates of the rare-earth elements (r.e.e.) have until recently belonged among compounds that are almost entirely unstudied. In the literature there were only fragmentary data on the synthesis of crystal hydrates of the perrhenates of lanthanum and cerium (¹) and neodymium (²), the composition of which was not confirmed by our recent investigation (³). We synthesized (³) crystal hydrates of the perrhenates of all the r.e.e. (with the exception of promethium) and of yttrium and established that the composition of the crystal hydrates of the perrhenates from La to Gd corresponds to the general formula Me(ReO₄)₃ · H₂O, while the crystal hydrates of the perrhenates from Tb to Lu and of yttrium correspond to the formula Me(ReO₄)₃ · 2H₂O. The indicated crystal hydrates were synthesized in the course of evaporating perrhenate solutions at 80° to dryness (to constant weight of the crystals) and, as a study of their properties showed, may serve as intermediate compounds for obtaining anhydrous perrhenates of the r.e.e. and yttrium of composition Me(ReO₄)₃, stable over a considerable temperature interval.

The route described (³) for the synthesis of crystal hydrates of r.e.e. perrhenates makes it possible to use the starting materials (r.e.e. oxides and HReO₄) most effectively, with a yield of not less than 95%. At the same time, according to the data from the study of the thermal properties of the r.e.e. perrhenates synthesized earlier, these compounds are obtained with a minimally constant content of water of crystallization, and were named by us lower crystal hydrates. Thus we did not exclude the possibility of the existence of crystal hydrates of the r.e.e. and yttrium perrhenates with a higher water content.

Indeed, as will be seen from what follows, compounds with the general formula Me(ReO₄)₃ · 4H₂O were obtained, the conditions for isolation of which and certain properties are described in the present communication.

The starting substances, the procedure for obtaining perrhenate solutions, and the control of the completeness of the reaction were described earlier (³), but crystallization from the filtered perrhenate solutions was carried out under other conditions, according to two parallel schemes. In one case crystals were removed from a solution evaporated at 75–80°; in the other, the solution was evaporated almost to saturation, and then the crystals separated on cooling to 20–25°. In this case crystallization proceeded very rapidly, and from a solution of volume 3–4 ml well-formed large crystals (sometimes up to 1 cm) were obtained. However, irrespective of the temperature regime of crystallization (from 20–25° and higher), the perrhenates separated from solution contain one and the same number of moles of water, but different from that established for the lower crystal hydrates of these compounds (³).

The newly synthesized crystal hydrates were analyzed for Me₂O₃ and Re₂O₇ by the procedure we described (³). The water content was determined by difference, by a chromatographic method (⁴), and was also estimated from the results of isothermal dehydration (see below).

The results of the analysis are given in Table 1 (the water content is the average of data from three methods). As can be seen, the crystal hydrates of the perrhenates of all r.e.e. and yttrium crystallize with 4 molecules of water and correspond to the formula Me(ReO₄)₃ · 4H₂O. The tetrahydrates of the perrhenates of the r.e.e. and yttrium are nonhygroscopic, coarse-crystalline substances, readily soluble in water and alcohol; their color is determined by the ion Me³⁺. The forms of crystall-

of the synthesized perrhenate tetrahydrates are shown in Fig. 1, from which it is evident that these compounds form 3 types of crystals. The difference in crystal forms in this case coincides with the structural differences of the compounds (to which we intend to devote special attention), which is clearly seen from the types of X-ray powder diffraction patterns (Fig. 2) obtained as a result of studying $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ by the powder method. The photographs were taken in an RKD-57 camera with a Cu anode at a voltage of 35 kV and a current of 12 mA*.

Table 1 gives data on the determination of the densities of $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ (pycnometric method using $\mathrm{CCl_4}$) and their refractive indices, found by the immersion method**. In this series of properties, discontinuous transitions are also observed, corresponding to the change in crystal forms.

The thermal stability of the tetrahydrates of the perrhenates of the rare-earth elements and yttrium was studied using the methods of isothermal dehydration, thermography, and thermogravimetry.

For carrying out isothermal dehydration, a salt sample of $\sim 0.5$ g was placed in a drying oven (in the range 30–200°; accuracy $\pm 1^\circ$) or in a muffle furnace (in the range 200–700°; accuracy $\pm 20^\circ$). After the sample reached constant weight, the temperature was raised by 10° (in the oven) or 50° (in the furnace), and the sample was again held to constant weight at the specified temperature. Isothermal dehydration showed that $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ begin to lose water above 50°, with compounds of the elements from lanthanum through gadolinium inclusive gradually converting to $\mathrm{Me(ReO_4)_3\cdot H_2O}$, while the perrhenate tetrahydrates of the elements in the terbium–lutetium series and of yttrium convert to $\mathrm{Me(ReO_4)_3\cdot 2H_2O}$. At the same time, the stability of $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ increases in the series from lanthanum to lutetium. Thus, if $\mathrm{La(ReO_4)_3\cdot 4H_2O}$ begins to lose water at 50° and rather rapidly (in 2–3 hours) converts at this temperature to $\mathrm{La(ReO_4)_3\cdot H_2O}$, then $\mathrm{Lu(ReO_4)_3\cdot 4H_2O}$ begins to lose water only at 65°, converting thereafter at this temperature to $\mathrm{Lu(ReO_4)_3\cdot 2H_2O}$. After the conversion of $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ to mono- or dihydrates, further dehydration with formation of anhydrous perrhenates proceeds under the conditions described earlier ($^3$). The conversion of the tetrahydrates of the perrhenates of the rare-earth elements and yttrium to their mono- or dihydrates and of these lower crystallohydrates to anhydrous compounds was confirmed by X-ray phase analysis.

Fig. 1. Crystalline forms of the tetrahydrates of the perrhenates of the rare-earth elements (20×).
$a$ — La and Ce, $b$ — from Pr to Dy, $c$ — from Ho to Lu and Y

* Debye patterns of $\mathrm{Me(ReO_4)_3\cdot 4H_2O}$ were obtained in the X-ray laboratory of the V. I. Vernadsky GEOKHI.

** Earlier ($^3$) these refractive indices were mistakenly placed in the work devoted to lower crystallohydrates of rare-earth perrhenates; the corresponding data for the latter are given in the correction published in ($^5$).

Table 1

Composition and some properties of tetrahydrates of rare-earth element and yttrium perrhenates

* Accuracy of determination ±0.005.
** Accuracy of determination ±0.003; values of $N_m$ for lanthanoid tetrahydrates in the La–Dy series are not given because of the unreliability of their determination (owing to the closeness of $N_g$ or $N_p$, which lies within the limits of the accuracy of the determination).

It is interesting to note the intermediate position of Tb(ReO₄)₃·4H₂O and Dy(ReO₄)₃·4H₂O in the series of other crystalline hydrates of rare-earth-element perrhenates: in their crystal structure they are similar to the tetrahydrates of perrhenates of elements located to the left of them in the periodic system, while in thermal stability they are similar to the analogous compounds of elements located to the right (with higher atomic numbers).

Fig. 2. X-ray line diagrams of tetrahydrates of rare-earth-element perrhenates. a — La and Ce (using the cerium compound as an example), b — from Pr to Dy (using the gadolinium compound as an example), c — from Ho to Lu and Y (using the erbium compound as an example)

The results of thermographic and thermogravimetric investigations* of Me(ReO₄)₃·4H₂O are illustrated in Figs. 3 and 4. The thermogram and thermogravigram of Fig. 3 are typical for Me(ReO₄)₃·4H₂O in the La—Gd series, i.e., those elements that also form Me(ReO₄)₃·H₂O, while the thermogram and thermogravigram of Fig. 4 are characteristic for Me(ReO₄)₃·4H₂O in the Tb—Lu (and Y) series—elements that also form Me(ReO₄)₃·2H₂O. Dehydration of Me(ReO₄)₃·4H₂O on the thermograms

* Heating rate 5–6 deg/min.

(Figs. 3, a; 4, a) is expressed by two endothermic effects: the first effect corresponds to the loss of 3 moles of water for compounds of the La—Gd series and 2 moles for compounds of the Tb—Lu (and Y) series; the second characterizes the further removal of water. The thermograms also show effects at 865 and 905°, arising as a result of melting of the formed and slightly decomposed anhydrous perrhenates. In Fig. 4, a, another effect at 580°, pertaining to an already dehydrated compound, is clearly visible. The appearance of similar

Fig. 3. Thermogram (a) and thermogravitogram (b) of samarium perrhenate tetrahydrate

Fig. 4. Thermogram (a) and thermogravitogram (b) of holmium perrhenate tetrahydrate

effects in this temperature region (580—620°) is observed for all perrhenates, beginning with terbium perrhenate, and is apparently associated with a polymorphic transformation of the corresponding anhydrous perrhenates.

A comparison of Figs. 3 and 4 shows that the data of thermographic and thermogravimetric studies of \(\mathrm{Me(ReO_4)_3 \cdot 4H_2O}\) agree well with one another, although, for entirely understandable reasons, they differ from the data of isothermal dehydration, since only the latter correspond to the equilibrium process of water removal.

Thus, as a result of the present work and our previous investigation \((^3)\), the existence of intermediate anhydrous perrhenates and their tetrahydrates has been established for all rare-earth elements and yttrium. Dehydration of the latter occurs through the formation of intermediate crystalline hydrates, the water content of which is determined by the position of the rare-earth element. There are two dehydration schemes for \(\mathrm{Me(ReO_4)_3 \cdot 4H_2O}\):

\[ \mathrm{Me(ReO_4)_3 \cdot 4H_2O \rightarrow Me(ReO_4)_3 \cdot H_2O \rightarrow Me(ReO_4)_3} \qquad \text{(in the La—Gd series)} \tag{1} \]

\[ \mathrm{Me(ReO_4)_3 \cdot 4H_2O \rightarrow Me(ReO_4)_3 \cdot 2H_2O \rightarrow Me(ReO_4)_3} \qquad \text{(in the Tb—Lu and Y series)} \tag{2} \]

Moscow Institute of Fine Chemical Technology
named after M. V. Lomonosov

Received
4 V 1964

CITED LITERATURE

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5. V. E. Plyushchev, V. M. Amosov, M. B. Varfolomeev, DAN, 152, No. 2, 254 (1963).