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Date Created:April 2, 1960
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Keywords:high-pressure phase transformations
Context:article reprinted from "Nature"
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(Reprinted from Nature, Vol. 186, No. 4718, pp. 34 and 71, April 2, 1960)
High-pressure Phase Transformations in Laboratory Mechanical Mixers and Mortars*
The mechanical action of simple laboratory grinders, mortars and similar devices has occasionally been used to assist in chemical reactions in addition to performing their primary physical functions. Among these chemical reactions are phase transformations. That the effect is due to some kind of a pressure component in the mechanical action may appear obvious, but the magnitude of the pressures is not readily appreciated, nor can it be calculated or measured. However, in the course of work in this laboratory on the high-pressure polymorphism of lead dioxide (PbOa), the information that a phase identical with the new high-pressure polymorph, which had been shown to be stable only in the region above 10,000 bars, had been formed by simple grinding1 was noted with no little surprise. The observation was promptly confirmed by grinding a small amount (2 gm.) of the common rutile form (I) of lead dioxide in a mechanical mortar and pestle combination of laboratory pattern. Grinding in air for a few hours converted an estimated one-third to the denser orthorhombic form (II). After preliminary work, it transpired that nothing new in principle had been added to some similar results which had been reported by Burns and Bredig2 on the transformation of calcite to aragonite by grinding in a mortar. The high-pressure phase was formed from the low-pressure one, the amount of change was dependent on time, and subsequent heating would form the low-pressure phase. A significant difference is that phase equilibrium and thermochemical studies8 place the calcite-aragonite transformation at about 3,000 bars at room temperature, which is considerably lower than the 10,000 bars necessary for the lead, oxide I ^11 transformation.
The obvious question that arises is whether or not the high-pressure types are actually being formed in their field of stability, or whether the shearing stresses so dominant in such an environment either alter the relative free-energy relations of the two forms or permit metastable nucleation of the high-pressure phase. Therefore, other substances with known p-t relationships between polymorphs (work in this
* Contribution No. 51-59, College of Minora! Industries, The Pennsylvania State University, University Park, Pennsylvania.
laboratory, with L. Azzaria and W. B. White) were chosen for further study : MnFa, BeFa, Si02) PbO, Sb20 3, B203 and BAs04. Their transition pressures at room temperature (extrapolated from our equilibrium data obtained at higher temperatures) are near 9,500, 15,500, 13,500, 5,500, 10,000, 18,500 and 30,000 bars respectively. It was found that grinding for several hours in the mortar assembly under air or nitrogen (where oxidation was a possibility) would produce varying amounts (as determined in every case by X-ray diffraction) of the high-pressure phases of PbO, PbO2, CaCO3, MnFj, Sb203 and BeFa (questionable) in decreasing order. Also of new but related interest is our finding that the 'quenchable' transitions in calcium carbonate and lead dioxide and in many other selected phases could be effected (partially), in a few minutes to 2 hr., by the action of a small rapidly vibrating mixer-grinder. The type used (trade name 'Wig-L-Bug') is common in spectroscopic laboratories and was used with a metal vial and ball and 50-100 mgm. of sample. Furthermore, it was found that the same (vide supra) relative effectiveness in forming detectable amounts of the high-pressure phases was evident in the action of the 'Wig-L-Bug'. Of the highest-pressure phase, BeF2 (coesite), only a trace was formed. Making allowances for the very sluggish nature of the quartz-coesite transition of silica and the more complex relation4 and serious hydration problem in the new high-pressure Ba03 polymorph, it appears that pressures in the region of 10-20,000 bars are being applied intermittently to the fine grains subjected to the 'grinding' actions in each of these types of apparatus.
That a real pressure component is operative in the action of the mortar assembly was shown by increasing or reducing the weight of the pestle-head assembly by means of a suitable weight or counterpoise to twice or one-sixth the normal weight. Thus, although about 20 per cent conversion of calcite to aragonite was achieved by only 13 hr. of grinding under normal weight, and about 5 per cent in the same time under half the normal weight, more than 100 hr. under the one-sixth weight failed to produce detectable amounts of aragonite. Presumably the greatly reduced weight of pestle is not sufficient to generate pressures greater than 3,000 bars on the calcite, leaving it still in its equilibrium field. Another example is the litharge-massicot transition of lead oxide in which litharge is the low-pressure (and also low-temperature) form. The action of the light-weight pestle (under nitrogen) will drive massicot readily to the stable litharge (this being merely the effect on kinetics due to bond breakage), but if litharge is the starting material no
change to massicot is observed. To accomplish the latter the heavier pestle is necessary, presumably since only then is enough pressure available to reach the massicot field. On the other hand, the heavy pestle will produce some litharge when acting on massicot, because all values of pressure up to the maximum must be encountered, since the apparatus of necessity produces a whole 'spectrum' of pressures. A similar example was found on grinding a mixture of the senarmontite and valentinite forms of SbaO?.
Thus it is clear that the stresses causing fracture generally operate to accelerate reactions, although the effect is larger in some pairs of structures than in others. Superimposed on this kinetic effect is a hydrostatic pressure effect. Detailed verification of the separability of the 'stress' and pressure effects has been effected5 in an uniaxial high-pressure device6 so modified that a slow continuous oscillating displacing shear action by the movement of one piston through 2° of arc is applied to samples (of the above compounds) simultaneously subjected to high pressures and temperatures. It is sufficient to mention here that it has been established that stresses in the form of displacing shear do not alter equilibrium pressures of transitions (within the experimental limits), although they do contribute enormously to the rates of such reactions.
Burns and Bredig2 have already pointed out that the possibility of phase transformations taking place on grinding must be taken into account in explaining discrepancies in the behaviour of calcium carbonate. Our results show that : (1) 'grinding' or mixing, in both 'automatic mortars' and 'Wig-L-Bug'-type shakers not only can introduce appreciable amounts of surface and strain energy but also can quite generally cause the formation of high-pressure phases ; (2) the effective maximum in the 'spectrum' of hydrostatic pressures obtained in such instruments is not less than about 15,000 bars ; (3) in addition to the quasi-hydrostatic pressures exerted, the shearing stresses causing the breakage of bonds and/or the storage of strain energy result in a very marked acceleration on the kinetics of such transformations.
This work was done as part of the high-pressure studies under contract with the Office of Naval Research, Metallurgy Branch, NONR-656(20). We are also indebted to S. Merrin and D. Corrigan, who carried out some of the experiments reported.
Frank Dachille
Rustum Roy Department of Geophysics and Geochemistry, Pennsylvania State University, University Park, Pa. Feb. 10.
1 Zaslavsky, A. I., Kondrasliov, Y. A., and Tolkachev, S. S., Doklady Akad. Nauk., 75, 559 (1950). Bode, H., and Voss, E., Z. Elektro-chem., 60, 1053 (1956). * Burns, J. H., and Bredig, M. A., J. Chem. Phys., 25, 1281 (1956). At the time of oral presentation (see ref. 5) of this work, Prof. J. Goldsmith directed our attention to his work with J. C. Jamieson on the grinding of magnesian caleites, etc., reported orally at the American Orystallographic Association, Milwaukee meeting, 1958.
3	MacDonaid, G. J. F., Amer. Min., 41, 744 (1956). Jamieson, J. 0.,
J. Chem. Phys., 21, 1385 (1953). Clark, S. P., Amer. Min., 42 564 (1957).
4	Dachille, F., and Roy, B„ J. Amer. Ceram. Soc., 41, 78 (1959).
' Dachille, F., and Roy, R., oral presentation, Geol. Soc. Amer.,
Pittsburgh meeting, November 3 (1959). 'Dachille, F., and Roy, R., Z. Krist. (in the press).
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