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A. J. MAJUMDAR AND RUSTUM ROY
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with increasing concentration of the polymer in the high viscosities of these solutions in which the concentrated non-aqueous solutions (Fig. 7). A diffusion of the macroradicals is very low. The very pronounced increase also occurs in aqueous diffusion of species of low molecular weight, howsolutions at high concentrations of the polymer. ever, is not so strongly hindered in viscous soluA plausible explanation is the decrease in the tions. As a consequence, reactions of the macrorate constant of combination (8) of the free macro- radicals with low molecular weight compounds radicals in concentrated solutions. Such a de- become favored over macroradical combinations. crease is well known from the bulk polymerization I n the case of autoacceleration of polymerizations of several vinyl compounds where autoacceleration the intermediate macroradicals do preferentially occurs at high concentrations of the polymer react with molecules of the vinyl monomer. In the irradiation of concentrated polymer solutions 23 This phenomenon is attributed to the deactivation of the macroradicals by low (22) R. G. W. Norrish and R. R. Smith, Nature, 160, 336 (1942). (23) E. Trommsdorff, H. Kohle and P. Lagally, Makromol. Chem., molecular weight radicals R1 and R2 (eq. 4b and 5 ) are favored. 1, 169 (1948).
EXPERIMENTAL STUDY OF THE POLYMORPHISM OF AgI B Y A. J. MAJUMDAR AND RUSTUM ROY Contribution No. 68-110,The College of Mineral Induatries, The Pennsylvania State University, University Park, Pennsylvania Received April 10, 1060
Polymorphism in AgI has been studied in the temperature range between 25 and 200’ and the gressure range between 1 and 1000 atm. Pressure dependence of the temperature of the transition that takes place at 146.5 at atmospheric pressure has been studied up to 1000 atmospheres, and the volume change has been determined by high temperature X-ray diff raction. From these parameters the enthalpy change at the transition temperature has been calculated. No evidence could be obtained for a definite transition temperature for the important cubic hexagonal polytypic transition. It is concluded that there is no range of thermodynamic stability for the 3C polytype, if indeed i t can be prepared at all. i,
Introduction An examination of the l i t e r a t ~ r e l -reveals ~ the picture of the polymorphism of AgI. At least three forms are said to exist: i, a high temperature cubic form (I) which only forms (and is the stable form of AgI) above 146.5’; ii, a hexagonal form (11) with the wurtzite structure which forms reversibly upon cooling the high temperature cubic form below 146.5’; iii, a second cubic form (111) presumably with the sphalerite structure, which it has been claimed may be stable below 137”. The transition between forms I and I1 is well authenticated and understood even though the high temperature structure is a very rare one in which the Ag+ ions appear to be in random motion in the lattice. The transition between forms I1 and I11 is quite a different matter. It is a polytypic change from the hexagonal close-packing of I1 to cubic close-packing in 111. However, a closer look at the experimental data shows that neither is it certain that an endmember sphalerite-structure form I11 exists, nor is there much justification for any equilibrium transition temperature being assigned for the I1 % I11 reaction. Our interest in the applicability of classical thermodynamics to solid phase transitions caused us to examine two phases of this problem. First, in the I F? I1 transition, in which we have an “ordered” arrangement changing to a “random” arrangement accompanied by a very large entropy
change, do the transition parameters fit the Clapeyron relationship. If not, would the equivalent expression for second-order phase transition fit it better? Second, does AgI actually exist in a sphalerite I11 form and if so, what are the stability relations with the wurtzite I1 form. If a stable transition exists, o’ne has a structural mechanism to explain a possible second-order transition, and the thermodynamic relations in such a case should be most interesting. Experimental Procedure
I. Preparation of the Starting Materials.-Verwey, et aZ.,6 pointed out a long time ago that the structural nature of AgI depends to a large extent on its method of precipitation, and this has been the way in which “different” A I structures have been usually made. Reagent grade AgbOa and K I (Fisher), in the present investigation, were used for the preparation of AgI. The following different methods of preparing the AgI starting material were used: ( a )preci itated from solution by the addition of AgNOa to excess $1 with constant stirring; (b) precipitated from solution by the addition of KI to excess AgNOa with constant stirring; recipitated from solution, using aliquot amounts of Ag Osand KI; (d) previously precipitated AgI dissolved in KI, the solution filtered, and subsequently poured into a large excess of water. The precipitate now appearing filtered and washed free of I- by repeated washing with water; (e) previously precipitated AgI melted in a sealed silica tube, quenched and pulverized carefully; ( f ) previously precipitated AgI treated with concentrated NH40H and evaporated to dryness a t 110”; (g) formed by solid state reaction between metallic Ag powder and excess IZa t 150” in a sealed silica glass tube; ( h ) formed by solid state reaction in a sealed tube between 1 2 and excess metallic Ag powder a t (1) L. W. Strook, 2. physik. Chem. B d . , 2 6 , 441 (1934). (2) L. W. Strock and V. A. Brophy, Am. Mineralogist, 40, 94 300.’; (i) previously precipitated AgI vaporized and redeposited on a glass late; ( j ) previously precipitated AgI (1985). boiled with HsO to &ness; (k) AgI powder melted in the (3) M. L. Huggina, Transition i n Siluer Halides, in “Phase Trans-
formation in Solids,” ed. by Smoluchoweki, et at., John Wiley & Sons, New York, N. Y..1951,Ch. 8. (4) J. W. Manaen, TEISJOURNAL, 60, 806 (1956).
( 5 ) E. J. W. Verwey and H. R. Kruyt. 2. physik. Chem., 8161, Spez. S. 142 ff. (1933).
. I
Nov., 1959
EXPERIMENTAL STUDY OF
THE
POLYMORPHISM OF SILVER IODIDE
1850
presence of slight excess of Iz in a sealed tube, quenched and powdered. 11. Apparatus and Technique.-For studying the sphalerite type & wurtzite type transition, samples of AgI with or without other reagents were placed in silica glass tubes and sealed. These sealed tubes were then heated to different temperatures for periods of time ranging from a few hours to a few months. The transformation a t 146.5’ was studied in two different ways. (1)The pressure dependence of the transition temperature was determined by means of differential thermal analysis a t high pressures. Nitrogen was used as the pressure transmitting medium. The pressure was generated by a laboratory pump described by Roy and Osborn.6 The sample holder was contained in a special “test-tube” bomb6 with four thermocouple leads “packed” in by means of a commercial “Conax” head. (2) The crystallographic behavior of AgI was followed by means of X-ray diffraction on powdered samples a t elevated temperatures. A high angle Norelco diffractometer was used throughout. By measuring precisely (at 1/8’ 28 per min. scanning speed) the changes in the positions of a few reflections in the X-ray diffraction spectra as a function of temperature, the lattice parameters for the crystalline forms below and above the transformation temperature could be determined. The temperature of transition determined incidentally served as a check on the temperature of transformation obtained by thermal analysis.
Results and Discussion None of the methods of preparation described above yielded a “pure” sphalerite structure, although methods a, d, and e appear to give what has been called, on the basis of peak intensities, a pure wurtzite structure. Verwey,6 trying various methods came to the conclusion that methods b and e give much more sphaleritic than wurtzitic structure. The main criterion used to distinguish between the two forms is that while the hexagonal wurtzite structure gives three X-ray reflections in the region of 22” 20 for Cu K a (dloo = 3.98 A.; do02 = 3.73 8.; dlol = 3.50 8.), the cubic sphalerite structure gives oiily one reflection in the same region (dloo = 3.72 A). The results of the present investigation are summarized in a schematic diagram (Fig. 1). The letters a, b, c, etc., of the diagram refer to the methods of preparation referred to in the text with the corresponding letter. Results of all experiments have not been shown; the methods giving the most sphaleritic forms were repeated several times. The first general and surprising observation is, therefore, that AgI cannot be synthesized in the pure cubic sphalerite structure in any reproducible manner by methods previously suggested. Attention was then directed to the possibility that the direction of equilibrium could be indicated by converting one form to the other. Experiments were performed where samples of AgI made by different methods were placed on a glass slide for examination by X-rays. After having been examined a t room temperature by X-ray the same slides were heated to 100 and 132”, respectively, and the results are also shown schematically jn Fig. 1. As can be seen from the diagram (Fig. 1) a few of the many different preparations of AgI do appear to become more sphaleritic when heated to 110”. Attention is drawn to cases d and e. The claim of Mansen4 that in the presence of 1, vapor AgI goes to the hexagonal form readily was not borne out by all the samples, (6)
R. Roy and E. F. Osborn, Eoon. C d . , 47
[7] 717 (1952).
DEGREES 2 8 (CUW).
Fig. 1.-Schematic representation of diffractomcter patterns of starting materials prepared by various methods (see text) shown in left-hand column. The effect of heat treatment also is illustrated for two temperatures 110 and 132”. No consistent pattern of conversion toward the sphalerite form can be detected.
i.e., there is no clear evidence that the sphaleritic material always converts to wurtzite a t about 120”, although in cases b, d and h this seems to be the trend. All start’ing materials except d were heated to temperature ranges between 128-142” dry, as well as in the presence of HzO, and ”,OH for periods ranging from two weeks to three months. In no case was a pure sphalerite structure formed, while in most cases good wurtzite structures were formed. Moreover, in ZnS Smith and Hill’ have shown that the effect of powdering sphalerite and wurtzite is to introduce disorder, making especially the hexagonal material look “more cubic” on cursory examination. AgI powder is very plastic and intensity measurements using “powdered” or ground sztmples are not likely to be reliable. Thus, although there was plenty of evidence for several other complex polytypes of AgI, it must be concluded that AgI does not exist stably in the sphalerite form. Hence, the 120” transition of Mansen4 and 137” reported by others as the transition temperature between sphalerite and wurtzite forms of Agl is without significance. (7) F. G. Smith and V. G. Hill, Art5 Crust., 9, 821 (l95l2.
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A. J. MAJUMDAR AND RUSTUM ROY
Vol. 63 TABLEI
PRESSURE-TEMPERATURE DATAON 146.5' TRANSITION IN AgI 6E
z
67
3
0 W -1 0
I
a
66
W
s
$
65
64
HeatTemp., i l °CoolC.
*2 atm.)
1 w
ing
Ad
NaCl
1 200 400 800 1000
146.5 143 140 135 132
146 143 140 135 134
The thermodynamic quantities-enthalpy change (AH), volume change (AV)-of a substance undergoing transformation a t a temperature T are related to the pressure dependence of the transition temperature (dp/dT) by the well known Clapeyron relationship as dp/dt = AHt,/TAV. Taking dp/dT as 1000,'- 14.5 atm./'C. and A V as - 1.727 cc./mole, the value of AM comes out to be 1208 cal./mole. Taking the maximum limits of error in the measurement of the experimentally determined quantities, the total uncertainty in the value of AMt, is f l 4 0 cal./mole. Kelley8 quotea a thermochemically measured value of 1470 cal./ mole, which in our experience may be considered as very good agreement in such work. Secondorder behavior is ruled out by the markedly sharp volume change. This example also demonstrates that in cases of transitions where the volume change undergone by the substance a t the transition temperature, and dp/dT are fairly large, the Clapeyron relationship can yield an accurate value of the other parameters associated with the transition.
W J
'
-
Ref.
(Iodyrite) m mu
Pressure (atm.
Substance
I
50
I
I
100
TEMPERATURE
200 OC,
Fig. 2.-High temperature X-ray data on cell volume (converted to per "molecule" baais) of AgI as a function of temperature. Note negative thermal expansion of wurtzite and sharpness of volume change with no "second-order" behanor.
The volume of AgI per molecule as a function of temperature, as obtained by high temperature X-ray measurements, has been plotted in Fig. 2. It is interesting to note that the thermal expansion of the wurtzite form below 146.5" and the volume change for the transition both turned out to be negative. The temperature of transition was found to decrease with pressure. The data showing the dependence of the transition temperature for the 146.5' transformation on pressure are listed in Table I.
Conclusions There is no experimental proof of the existence of a true sphalerite form of AgI. Hence, the 132" transition temperature between the sphalerite and wurtzite form is meaningless and should be striken from the records. Wurtzite is the stable form in the range 146.5-120" and probably from 146.5 to room temperature. The transition at 146.5' is clearly first order and takes place with a negative volume change (the transition temperature going down with pressure). The transition-enthalpy is 1208 f140 cal./mole. Acknowledgment.-This work forms part of a study on the phase rule and polymorphism supported by the National Science Foundation under Grants G-1000 and G-4648. ( 8 ) K. K. Kelley, Bur. of Mines Bull. 476, 1949.
f
