Full Text
Chemistry
Corresponding Member of the Academy of Sciences of the USSR A. V. NOVOSELOVA, Yu. M. KORENEV
and Yu. P. SIMANOV
STUDY OF THE SYSTEM KF—ZrF₄
Zirconium fluoride forms double salts of complex composition with alkali-metal fluorides. Thus, in the system NaF—ZrF₄, studied by Barton et al. (^1), two congruently melting compounds, Na₃ZrF₇ and Na₇Zr₆F₃₁, were found; three compounds melting incongruently—Na₅Zr₂F₁₃, Na₂ZrF₆, and Na₃Zr₄F₁₉; and one compound—Na₃Zr₂F₁₁—existing only in the solid state.
According to the literature data, KF and ZrF₄ form the compounds: K₃ZrF₇, K₂ZrF₆, and KZrF₅. The fusibility diagram of the system KF—ZrF₄ (up to 65% ZrF₄) was investigated in works (^2,^3), apparently by the visual-polythermal method, since in work (^2) the phase fields below the solidus line are not indicated on the diagram of this system. In work (^3) there are likewise no indications of transformations in this system in the solid state.
In the present work we carried out differential thermal analysis and an X-ray phase study of the system KF—ZrF₄. According to the data of work (^4), zirconium fluoride undergoes a polymorphic transformation at 405 ± 5°. Schulze (^5) determined the parameters of the low-temperature monoclinic modification of ZrF₄: $a = 9.46$ Å; $b = 9.87$ Å, $c = 7.64$ Å; $\beta = 94^\circ 30'$. According to Barton’s determination (^1), ZrF₄ melts at 912°. Sense et al. (^6) determined the dependence of the vapor pressure of ZrF₄ on temperature; according to their data, the sublimation temperature of ZrF₄ is 903°.
Starting substances. For the study of the system we obtained potassium fluoride by dehydrating KF·2H₂O; zirconium fluoride was prepared from $(\mathrm{NH}_4)_3\mathrm{ZrF}_7$ by distilling off ammonium fluoride in a stream of CO₂. The synthesis of $(\mathrm{NH}_4)_3\mathrm{ZrF}_7$ was carried out by the method described in work (^7).
For the preparation of melts containing less than 33.3% ZrF₄, we used potassium fluoride and the available preparation K₂ZrF₆. Melts containing more than 33.3% ZrF₄ were prepared by fusing calculated amounts of K₂ZrF₆ and $(\mathrm{NH}_4)_3\mathrm{ZrF}_7$ in a stream of CO₂. Because of the volatility of ZrF₄, melts with a content of it greater than 75% were not studied. The results of analyses of the starting preparations and the melts obtained for potassium, zirconium, and fluorine agreed well with the calculated values. The errors of determination did not exceed 0.2–0.25%.
Thermal and X-ray phase analysis of the system KF—ZrF₄. Differential thermal analysis was carried out on an N. S. Kurnakov pyrometer using a platinum–platinum-rhodium thermocouple. Aluminum oxide served as the standard. Melts for thermal analysis were taken in an amount of 0.5 g. For melts containing up to 33.3% ZrF₄, cooling and heating curves were recorded. For melts with a ZrF₄ content greater than 33.3%, only cooling curves were recorded, since during slow heating of the melts their composition changed noticeably. In order to reduce the loss of ZrF₄, the melts were placed in a furnace preheated to a temperature slightly exceeding the melting temperature of the melt, and then after several minutes the furnace was switched off and the cooling curve was recorded. The temperatures were noted from the beginning of the effect on the differential curve.
For the X-ray investigation, the alloys were ground and, with the aid of zapon lacquer, applied to a Pyrex whisker. Potassium fluoride and alloys with a high content of it were photographed in sealed capillaries made of Pyrex glass. X-ray patterns were recorded in RKD-57 cameras using Fe radiation.
In view of the fact that a number of effects connected with its transformations were detected on the heating curves of $K_2ZrF_6$, we recorded X-ray patterns of $K_2ZrF_6$ at 260 and 340° in a Unicam camera (8) on a BSVLT tube with a copper anode. At a higher temperature it was impossible to carry out the recording because of the interaction of $K_2ZrF_6$ with quartz.*
The phase diagram of the system $KF$—$ZrF_4$, constructed by us on the basis of thermal and X-ray phase analysis, is presented in Figs. 1 and 2.
The zirconium fluoride synthesized by us was a monoclinic modification, described in (5). On its heating curve, endothermic effects at 612 and 685°, corresponding to transformations in the solid state, were found; on repeated heating the effect at 612° was absent.
In view of the strong volatility of $ZrF_4$ on heating, beginning at 880° a strong deviation of the differential curve is noted on the thermogram. The endothermic effect corresponding to the melting of $ZrF_4$ is found at $903 \pm 5^\circ$. If a closed platinum crucible with $ZrF_4$ is placed in a furnace preheated to 930–940° and then, after switching off the furnace, the cooling curve is recorded, then the effect of solidification of $ZrF_4$ is observed at $903 \pm 5^\circ$; on further cooling a polymorphic transformation occurs at 685°. Potassium fluoride, according to our data, melts at 850°. In the system
Fig. 1. System $KF$—$ZrF_4$. Phase fields: 1 — $KF +$ liq.; 2 — $K_3ZrF_7 +$ liq.; 3 — $K_5Zr_2F_{13} +$ liq.; 4 — solid solution $+$ liq.; 5 — $\delta$-$K_2ZrF_6 +$ liq.; 6 — $\gamma$-$K_2ZrF_6 +$ liq.; 7 — $\gamma$-$KZrF_5 +$ liq.; 8 — $\delta$-$K_2ZrF_6 +$ liq.; 9 — $\alpha$-$ZrF_4 +$ liq.; 10 — $KF + K_3ZrF_7$; 11 — $K_3ZrF_7 + K_5Zr_2F_{13}$; 12 — solid solution; 13 — solid solution $+ \delta$-$K_2ZrF_6$; 14 — solid solution $+ \gamma$-$K_2ZrF_6$; 15 — solid solution $+ \alpha$-$K_2ZrF_6$; 16 — $\gamma$-$K_2ZrF_6 + \gamma$-$KZrF_5$; 17 — $\gamma$-$K_2ZrF_6 + \beta$-$KZrF_5$; 18 — $\gamma$-$K_2ZrF_6 + \alpha$-$KZrF_5$; 19 — $\gamma$-$K_2ZrF_6 + K_7Zr_6F_{31}$; 20 — $\alpha$-$K_2ZrF_6 + \beta$-$K_3Zr_2F_{11}$; 21 — $\gamma$-$K_2ZrF_6 + \alpha$-$K_3Zr_2F_{11}$; 22 — $\alpha$-$K_2ZrF_6 + \alpha$-$K_3Zr_2F_{11}$; 23 — $\beta$-$K_3Zr_2F_{11} + K_7Zr_6F_{31}$; 24 — $\alpha$-$K_3Zr_2F_{11} + K_7Zr_6F_{31}$; 25 — $\alpha$-$KZrF_5 + K_7Zr_6F_{31}$; 26 — $\alpha$-$KZrF_5 + \alpha$-$ZrF_4$; 27 — $\beta$-$KZrF_5 + \alpha$-$ZrF_4$; 28 — $\gamma$-$KZrF_5 + \alpha$-$ZrF_4$.
* When the present work had been completed, a paper (9) appeared in print on the thermal stability and polymorphism of $K_2ZrF_6$.
In the KF—ZrF₄ system the following compounds were found: K₃ZrF₇, K₅Zr₂F₁₃, K₂ZrF₆, K₃Zr₂F₁₁, K₇Zr₆F₃₁, KZrF₅. The fluorozirconates K₃ZrF₇ and KZrF₅ melt congruently at temperatures of 923 and 455°, respectively. The fluorozirconates K₅Zr₂F₁₃ and K₂ZrF₆ are formed by peritectic reactions:
\[ \mathrm{K_3ZrF_7} + \text{melt }(\mathrm{KF}, \mathrm{ZrF_4}) \ \underset{}{\stackrel{848^\circ}{\rightleftarrows}}\ \mathrm{K_5Zr_2F_{13}}, \]
\[ \mathrm{K_3ZrF_7} + \text{melt }(\mathrm{KF}, \mathrm{ZrF_4}) \ \underset{}{\stackrel{585^\circ}{\rightleftarrows}}\ \mathrm{K_2ZrF_6}. \]
The fluorozirconates K₃Zr₂F₁₁ and K₇Zr₆F₃₁ exist only in the solid state below 327 and 380°, respectively. The eutectic between KF and K₃ZrF₇ lies at 760° and 13% ZrF₄; between K₃ZrF₇ and KZrF₅ at 430° and 47% ZrF₄, and between KZrF₅ and ZrF₄ at 440° and 60% ZrF₄.
Fig. 2. Line diagrams of compounds in the KF—ZrF₄ system
Several modifications were found for the fluorozirconate K₂ZrF₆ (Fig. 3). The transformations of K₂ZrF₆ may be represented by the scheme:
\[ \alpha\text{-}\mathrm{K_2ZrF_6} \ \xrightarrow{240^\circ}\ \beta\text{-}\mathrm{K_2ZrF_6} \ \xleftarrow[130^\circ]{}\ \gamma\text{-}\mathrm{K_2ZrF_6} \ \xrightarrow{298^\circ} \]
\[ \gamma\text{-}\mathrm{K_2ZrF_6} \ \underset{}{\stackrel{445^\circ}{\rightleftarrows}}\ \delta\text{-}\mathrm{K_2ZrF_6} \ \underset{}{\stackrel{585^\circ}{\rightleftarrows}}\ \text{liquid} + \mathrm{K_3ZrF_7}. \]
The fluorozirconate KZrF₅ undergoes polymorphic transformations at 400 and 424°. The fluorozirconate of composition K₃Zr₂F₁₁ undergoes a polymorphic transformation at 313°. K₃ZrF₇ has one modification, crystallizing in a face-centered cubic lattice
Fig. 3. Line diagrams of the α, β, γ modifications of K₂ZrF₆
\[ a = 8.966 \pm 0.003 \text{ kX} \]
and forms a solid solution with K₂ZrF₆ (in the region 73—75% KF).
Moscow State University
named after M. V. Lomonosov
Received
18 III 1961
CITED LITERATURE
- C. J. Barton, W. R. Grimer et al., J. Phys. Chem., 62, 665 (1958).
- Materials of the U.S. Atomic Energy Commission, Nuclear Reactors, 2, Nuclear Reactor Technology, IL, 1957, p. 584.
- Abstracts of Reports, All-Union Conference on the Physical Chemistry of Molten Salts and Slags, Sverdlovsk, 1960, p. 12.
- G. Creten, Gordo, C. R., 246, No. 15, 2266 (1958).
- E. R. Schulze, Zs. Kristallogr., 84, 477 (1934).
- R. A. Sense, M. J. Snyder, R. B. Tilley, J. Phys. Chem., 58, 995 (1954).
- H. M. Haendler, C. M. Wheder, D. M. Robinson, J. Am. Chem. Soc., 74, 2352 (1952).
- H. Lipson, A. Wilson, J. Sci. Instrum., 18, 144 (1941).
- G. A. Yagodin, V. I. Tarasov, ZhNKh, 5, issue 9, 1987 (1960).