A. J. MAJUMDAR AND RUSTUM ROY
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Polymorphism in Potassium Sulfate and Thallium Sulfate
by A. J. Majumdar and Rustum Roy Materials Research Laboratory, The Pennsylvania State Universitu, University Park, Pennsylvania (Received December 7, 1964)
The structural behavior of K2S04 and that of TlzS04 with increasing temperature are very similar. Above 500" a hexagonal modification of Tl2S04 is the stable polymorph at atmospheric pressure. p-t dsta on the 583" inversion in KzS04 cannot be reconciled with the volume change attendant on the transition and the thermochemical AH. D.t.a. and high temperature X-ray results suggest that the major structural changes in these two compounds are preceded by other minor changes some of which may be of a second-order nature.
Introduction K2S04has been said to invert from an orthorhombic modification to a hexagonal modification a t 590" according to Bredig' who also determined the structure of the high temperature phase. The transition has been studied by many investigators, and Bernard and Jaffray2 in a recent summary make the statement that the t'emperature of transition has been reported to vary from 569 to 610". These authors, however, claim that the transition temperature is 586 f 2" and in addition provide evidence for three new transitions a t 300 f 2, 350 f 3, and 449 f 3" investigated by means of thermal and dilatometric methods. According to Kelley3of the Bureau of Mines the temperature of inversion is 583". Tl2SO4is jsostructural with &So4 at room temperature. Sahman and Tammann4observed in dilatometric work that T12S04goes through a sharp volume change in the vicinity of 430". Fischmeister5 in a recent summary mentions that the volume change a t 430" is not accompanied by any change in enthalpy but correlated this to the 583" transition in KzS04. Experimental In the present investigation, C.P. grade K2S04 and TlzS04 were used. For the determination of the temperature of phase changes, differential thermal analysis and high temperature X-ray diffraction techniques were used. For the latter a platinum-wound furnace was attached to the goniometer head of a Norelco high angle X-ray diffraction unit. The temperature was controlled by a West proportional conThe Journal of Physical Chemistry
troller. Powdered samples of investigated substances were contained in a platinum sample holder. To prevent the samples from falling out a t elevated temperatures, it was found necessary to start with a sample which had been previously melted in a sealed silica glass tube followed by quenching and grinding. From the variation of Bragg angles for a few known reflections, the changes of lattice parameters with temperature were calculated for both K2S04and TlzS04. The effect of pressure on the temperature a t which the hexagonal form appears in K2S04 was studied as a function of pressure up to about 1000 atm. For this a high pressure, differential thermal analysis apparatus described elsewhere6was used.
Results and Discussion I n the present study of polymorphism in KzS04 the temperature a t which the hexagonal modification makes its appearance at atmospheric pressure was found to be 584" by dzerential thermal analysis. The transformation temperature, however, was found t.0 be more reproducible in the cooling cycle, and this temperature, 583", was accepted as the transition temperature. The a-6 transition in Lake Toxaway quartz' showing a maximum rate of heat absorption ~~
(1) (2) (3) (4) (5) (6)
M. A. Bredig, J. Phys. Chem., 46, 747 (1942). M. Bernard and J. Jaffray, Compt. rend., 240, 1078 (1955). K. K. Kelley, personal communication. M. Sahman and G. Tammann, Ann. Physik, 10, 879 (1903). H. Fischmeister, 2.physik. Chem. (Frankfurt), 1, 91 (1956). A. J. Majumdar and R. Roy, J . A p p l . Phys., t o be published.
POLYMORPHISM IN POTASSIUM SULFATE AND THALLIUM SULFATE
at 573.5" was used as the reference material for temperature calibration. Experimental data showing the variation of this transition temperature with pressure are listed in Table I. Table I1 contains the measured values of the lattice parameters of K2S04 a t different temperatures. When the volumes per molecule are calculated from these measured parameters using the
Table 1: Variation of Transition Temperature with Pressure for K2SOc Substance
Reference
K2S04
Quartz
Press., atm. ( 1 2 atm.)
1
200 500 1000
-Temp., Cooling
583 586 592 600
OC.
(*la)Heating
584 588 594 604
presently accepted structural assignment and the same plotted as a function of temperature, certain anomalies are observed. Instead of showing a smooth increase up to the transition temperature (583"), the volume shows certain irregularities a t lower temperatures. The b and c parameters (orthorhombic room temperature indexing) expand smoothly over the entire temperature range. The a parameter, on the other hand, goes through peculiar breaks. It appears to have a low positive expansion separated by temperatures over which it is essentially constant, i.e., when the expansion coe5cient goes to zero. In the present investigation, a new transition has been discovered in TlzS04 at 500 f 2", where a hexagonal structure analogous to that of the high temperature modification of K2S04makes its appearance. The powder data of this form are given in Table 111. The measured values of the lattice parameters a t different temperatures based on the orthorhombic structure stable a t room temperature are listed in Table 11. It can be seen that there is an anomaly in the volume data. Thermal and differential thermal analyses were run on T12SO4 many times. The transition a t 500 =k 2" was recorded reproducibly on heating and on cooling, but no conclusive evidence was observed for the transition at 430" as reported by Sahman and Tammann.4 Only a slight change of slope in differential thermograms was noticed in the neighborhood of that temperature. The present investigation of polymorphism in K2SO4 and TlzS04 has also brought out certain aspects which had been overlooked by past investigators. I n the case of K2SO4 the change in volume (using Bredig's indexing of the orthorhombic and hexagonal cells)
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a t 583" was found to be 1.01 cc./mole. The p-t slope for this transition as determined in this study is reliable if not precise. When these values are combined by means of the well-known Clapeyron equation, a value of 1230 cal./mole is obtained for the heat of transition. According to K e l l e ~ this , ~ value is 2140 cal./mole, and this inversion is among the very few transitions for which the thermochemical heat data are most dependable. One may be tempted to ascribe this situation either to the incorrectness of the thermochemical AH or to the difficulty in obtaining AV owing to the steep rise of AV just before the transition. However, there is good reason for believing that Kelley's AH value is of the correct order and 1230 cal. is incorrect. Simple comparison of d.t.a. patterns with AgI, CsC1, etc., and direct quantitative comparison with cryolite show that the 2140 value is reasonable. Moreover, the difficulties with the AV term are believed to be due to a much more fundamental cause, namely, the incorrect indexing of the pattern just below 583". There are also indications that there may be at least one and possibly three other lower temperature transitions in K2S04.2 The high temperature X-ray works was done three times on two separate samples, with the hope of getting greater precision for the 583" AB term. I n the lower temperature region the abnormal behavior of the a axis was confirmed, though not enough low temperature points were obtained to use this to fix any transition temperatures. High temperature patterns were also obtained for Tl2S04 which is isostructural with K2SO4a t room temperature, and an analogous anomaly was found in the volume-temperature plot. Furthermore, the disappearance of certain reflections and appearance of other minor ones, which were attributed at first merely to intensity changes with the temperature factor and displaced sample positions, may be ascribed in retrospect to new structures. Since no obvious simple changes such as the transformation of the orthorhombic to a tetragonal cell by coincidence of a and b could be found, no attempt was made actually to attempt new unit cell assignment. Instead, for a quantitative check on Bernard's suggestions, d.t.a. patterns were run at atmospheric pressure a t the maximum amplification. There is evidence of a very small peak at 290-300" and possibly one at 350". Under these conditions, beginning near 390 and 445", one observes also a characteristic reproducible wide oscillation pattern in the differential (7) 0. F. Tuttle and M. L. Keith, Am. J . Sci., Bowen Volume, 203 (1952). (8) Graphical data and more extensive numerical data are t o be found in the Ph.D. dissertation listed in the acknowledgments.
Volume 69, Number 6
M a y 1966
A. J. MAJUMDAR AND RUSTUM ROY
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Table 11: Volume Data on KzS04 and TLSOt at Different Temperatures Temp., Substance
oc.
&so4
30 238 325 440 504 542 580 600 630 670 30 250 342 394 423 443 468 494 514 580
TlzS04
r
-Unit a
5.82 5.86 5.88 5.91 5.92 5.92 5.92 5.922 5.928 5.932 5.932 5.990 6.050 6.092 6.092 6.092 6.106 6.148 6.156 6.160
cell psrsmeters, b
10.06 10.22 10.24 10.30 10.32 10.36 10.38
10.648 10.680 10.724 10.738 10.710 10.680 10.660 10.610
A.+ C
7.48 7.60 7.64 7.72 7.78 7.86 7.90 8.12 8.16 8.20 7.85 8.00 8.00 7.94 8.08 8.16 8.16 8.27 8.28 8.29
Table 111 : X-Ray Powder Data for High Temperature Form of TlzSOt (at 508")
a, I .
I/Io
5.37 4.51 3.267 3.100 2.556 2.243 2.058 2.014 1.961 1.810 1.784 1.712 1.580
80 100 80 20 15 40 20 20 30 40 40 30 10
e.m.f. This lasts for about 10" and then disappears. This behavior has been observed by the writers in the case of certain other transitions which have been called second order by some authors. The ferroelectric transition in KNb03 and the ferromagnetic one in Fe304
The Journal of Physical Chemistry
V/cell,
%.a
437.95 455.16 460.14 471.765 574.31 482.06 485.45 246.610 248.327 249.881 495.84 511.79 519.04 519.40 527.18 520.91 531.34 539.452 271.90 272.48
V/molecule,
109.24 113.54 115.00 117.94 118.83 120.52 121.36 123.31 124.16 124.94 123.96 127.95 129.75 129.85 131.80 132.73 132.78 134.86 135.90 136.24
%.a
AV/molecule,
AV/mole,
00.
1.66 at 583"
1.01 a t 583"
0.82 at 500"
0.50 at 500"
appear to do the same thing. I n summary, therefore, the present work has developed unexpected support for the proposed new transitions in K2S04 a t 300 f lo", in agreement with Bernard and Jaffray,2 and suggests another at 390". As a result of this work, the indexing of the powder pattern of KzSO~below 583" is suspect since it is unlikely that these transitions would all take place without crystallographic changes. Hence, in connection with the Clapeyron relation, the AT' term is seen to be quite unreliable. I n T12S04,likewise, the irregular volume expansion below the temperature where the hexagonal form appears suggests a change of a fundamental nature. In spite of quite different chemical properties, and T12S04seem to behave in a very similar fashion (at least structurally) toward increasing temperature.
Acknowledgments. The work on this project was supported by the National Science Foundation, Grant 4648. Details on the data may be obtained in the Ph.D. dissertation (by A. J. M.), in Geochemistry, The Pennsylvania State University, 1958.
