A. N. Khlapova

NEW DATA ON THE POLYMORPHISM OF SODIUM CARBONATE*

(Presented by Academician I. I. Chernyaev, 10 VI 1957)

Jaffé and Marten (¹), using methods of thermography, dilatometry, and electrical conductivity, established that anhydrous sodium carbonate has two polymorphic transformations: at 360 and 480°. According to X-ray diffraction analysis, the crystal structure of Na₂CO₃ changes only at 480°; at 360° the authors observed no structural changes.

Fig. 2. Calculated diagrams of X-ray powder patterns of anhydrous sodium carbonate:
a — α-Na₂CO₃ — low-temperature modification; the X-ray pattern was obtained at room temperature;
b — β-Na₂CO₃ — at 350°;
c — γ-Na₂CO₃ — at 500°;
d — δ-Na₂CO₃ at 566°.

A. I. Lazareva (²) also noted, on differential heating curves of Na₂CO₃, endothermic effects corresponding to phase transitions at 340–350 and 470–475°.

We carried out an X-ray diffraction and thermographic study of the process of polymorphic transformations of anhydrous sodium carbonate as a function of temperature and of the preceding history of the specimen.

* E. S. Kovaleva took part in the experimental work.

For the study, sodium carbonate of the “Beker’s Analysed” grade, composition 99.78% \(\mathrm{Na_2CO_3}\), was used.

X-ray diffraction analysis was carried out by the powder method in a cylindrical camera specially adapted for obtaining X-ray photographs at different temperatures. The construction of such a camera and the method of investigation were described by V. G. Kuznetsov \((^3)\). Before X-ray photographing, the initial \(\mathrm{Na_2CO_3}\) was melted in a platinum crucible at a furnace temperature of 860–880°. The remelted sodium carbonate was ground in an agate mortar to a powder, with a grain size of approximately \(10^{-4}\)—\(10^{-5}\) cm. Then the substance was placed in an open thin-walled quartz capillary 0.5–0.7 mm in diameter. Before irradiation the salt was kept for 45–60 min at a strictly definite temperature, after which, at this same temperature, X-ray photographing was carried out with unfiltered iron radiation, with an exposure of 3–4 hours and rotation of the specimen. X-ray photographs of the powders were obtained from one and the same specimen successively at the temperatures: room temperature, 80, 102, 122, 164, 204, 244, 350, 402, 450, 500, 566°, and again room temperature. Each time the specimen was heated from room temperature to the specified temperature. The results obtained, summarized in Tables 1 and 2 and in Figs. 1 and 2, show that \(\mathrm{Na_2CO_3}\) in the temperature interval from room temperature to melting exists

Table 1

Phase state of \(\mathrm{Na_2CO_3}\) as a function of temperature according to X-ray diffraction data

Table 2

Values of interplanar spacings \((d)\) and relative intensities \((I)\) of X-ray lines of the \(\alpha\)-, \(\beta\)-, \(\gamma\)-, and \(\delta\)-modifications of \(\mathrm{Na_2CO_3}\)

* Annealing at 200°, 7 days.

Fig. 1. Reproductions of X-ray diffraction patterns of powders of four modifications of anhydrous sodium carbonate: a — \(\alpha\)-Na\(_2\)CO\(_3\); the X-ray diffraction pattern was obtained at room temperature in an ordinary cylindrical X-ray camera; the specimen was annealed first at \(120^\circ\) for 22 h, then at \(200^\circ\) for 173 h; b — \(\alpha\)-Na\(_2\)CO\(_3\); the X-ray diffraction pattern was obtained at room temperature in a high-temperature camera from the specimen after X-ray photography at \(566^\circ\); c — \(\beta\)-Na\(_2\)CO\(_3\); the X-ray diffraction pattern was obtained at \(350^\circ\); \(\gamma\)-Na\(_2\)CO\(_3\), at \(500^\circ\); \(\delta\)-Na\(_2\)CO\(_3\), at \(566^\circ\).

not in two crystalline forms, as has hitherto been believed, but in four, each phase having its own characteristic structure, namely: $\alpha = \mathrm{Na_2CO_3}$—the low-temperature modification (Figs. 1a, 2a); $\beta = \mathrm{Na_2CO_3}$—the form existing above 350° (Figs. 1b, 2b); $\gamma = \mathrm{Na_2CO_3}$—above 485° (Figs. 1c, 2c); and $\delta = \mathrm{Na_2CO_3}$—above 566° (Figs. 1d, 2d).

The results presented are in agreement with the data of a thermographic study, which are given below.

Fig. 3. Heating curves of $\mathrm{Na_2CO_3}$ after: a—melting of the salt; b—storage for about a year in a jar with a ground-in stopper; c—storage for more than a year in air under laboratory conditions; as a result of absorption of moisture from the air, $\mathrm{Na_2CO_3}$ was converted into $\mathrm{Na_2CO_3 \cdot H_2O}$

Differential thermographic analysis was carried out on an N. S. Kurnakov pyrometer. Platinum–platinum-rhodium thermocouples were used. The resistance in the circuit of the simple thermocouple was 17,000 ohms, and that of the differential thermocouple was 1,000 ohms. Calcined $\mathrm{Al_2O_3}$ served as the standard for the differential recording. Heating curves of $\mathrm{Na_2CO_3}$ were recorded for samples with different prior histories.

Table 3

Temperatures of phase transitions of $\mathrm{Na_2CO_3}$ as a function of the preceding history of the sample

As is evident from Table 3 and Fig. 3, the thermograms of all the investigated $Na_2CO_3$ samples reveal not only the previously known thermal effects characterizing phase transitions at 340–360 and 470–485°, but also an endothermic effect at 560–588 or 620° (depending on the treatment of the sample), corresponding to a third polymorphic transformation of sodium carbonate, previously undescribed in the literature, from the $\gamma$- to the $\delta$-form. This phase transition of $Na_2CO_3$ was first established by us by the X-ray powder method during X-ray photography in a high-temperature chamber and was described above. The observed discrepancy between the temperatures of the phase transformations according to X-ray and thermographic data is apparently caused by the different heating rates of the substance, different grain size, and also by the previous history of the sample.

Thus, as a result of the present investigation, new data have been obtained on the polymorphism of anhydrous sodium carbonate. By X-ray and thermographic methods it has been established:

1. The existence of $Na_2CO_3$ in the temperature interval from room temperature to melting in four crystalline forms:

a) $\alpha$-$Na_2CO_3$—a low-temperature modification that exists up to 340–350°;

b) $\beta$-$Na_2CO_3$—exists in the temperature interval from 340–350 to 470–485°;

c) $\gamma$-$Na_2CO_3$—exists within the limits from 470–485 to 565–620°;

d) $\delta$-$Na_2CO_3$—a high-temperature modification existing above 560–620°.

The temperature range of existence of the phase depends on the treatment of the sodium carbonate samples.

1. A change in the crystal structure during the phase transition from the $\alpha$- to the $\beta$-form at 340–350°.

Institute of General and Inorganic Chemistry
named after N. S. Kurnakov
Academy of Sciences of the USSR

Received
23 IV 1957

REFERENCES CITED

1. J. Jaffray, P. Martin, C. R., 236, No. 18, 1755 (1953); RZhKhim, No. 1, 10262 (1954).
2. A. I. Lazareva, Candidate’s dissertation, Moscow, 1954.
3. V. G. Kuznetsov, Zhurn. neorg. khim., 1, issue 7, 1548 (1956).