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CaF2

REFERENCES 1173 K

xc y //,,,

CaO

0.2

0.4

,

0.6 Xsio 2

0.8

SiO2 (qz)

Fig. 3 - - A tentative phase diagram for the system CaO-SiO2-CaF2 at 1173 K.

from the measurements of Benz and Wagnert2] should be approximately correct. However, in view of the stability of CasSi2F2Oa, less weight may be attached to the data of Benz and Wagner in future evaluations of the thermodynamic properties of Ca2SiO4. Perhaps more reliance can be placed on the calorimetric enthalpy of formation[l~ and entropy derived from thermal data. till The mechanistic reinterpretation of the measurements of Benz and Wagner[2] presented in this communication supports the conclusions of Hillert et al.t41 based on coupling of thermodynamic data with the phase diagram. The error in the interpretation of emf measurements, overlooked by other eminent researchers, ll,i2-14] prompts some philosophical reflections on the current methodologies of assessment and evaluation. The importance of sensitivity to the literature and awareness of the critical factors in experimentation cannot be overemphasized. In the interpretation of emf measurements, greater attention needs to be focused on the chemistry of the microsystem at the electrode-electrolyte interface, rather than on the bulk phase composition of the electrodes. X-ray diffraction analysis, which is commonly used to characterize the electrodes before and after emf studies, is quite insensitive to changes at the interface. Use of SEM and transmission electron microscopy with composition analysis can provide more localized information. When fluoride electrolytes are used to measure thermodynamic properties of oxides, the possible formation of oxyfluoride phases, facilitated by the close match of ionic radii of oxide and fluoride ions, merits careful investigation. The effect of oxyfluoride phase formation on emf studies of calcium aluminates using cells incorporating CaF2 has been discussed earlier by Allibert et al. [iSJ

The author wishes to thank Mrs. K. Indira for assistance in the preparation of the manuscript. 660--VOLUME 26B, JUNE 1995

1. J.L. Hass, Jr., G.R. Robinson, Jr., and B.S. Hemingway: J. Phys. Chem. Ref. Data, 1981, vol. 10, pp. 575-669. 2. R. Benz and C. Wagner: J. Phys. Chem., 1961, vol. 65, pp. 1308-11. 3. M. Hillert, B. Sundman, and X-Wang: Metall. Trans. B, 1990, vol. 21B, pp. 303-12. 4. M. HilleR, B. Sundman, X-Wang, and T. Barry: CALPHAD, 1991, vol. 15, pp. 53-58. 5. A. Scacchi: Z. Kristallogr. Mineral. 1877, vol. 1, pp. 398-99. 6. C.E. Tilley: Mineral. Mag., 1947, vol. 28, pp. 90-95. 7. W.J. McCaughey, K. Kautz, and R.G. Wells: Am. Mineral., 1948, vol. 33, p. 200. 8. A. Van Valkenberg and G.F. Rynders: Am. Mineral., 1958, vol. 43, pp. 1195-1202. 9. C. Brisi: J. Am. Ceram. Soc., 1957, vol. 40, p. 175. 10. W. K6ther and F. Miiller: Z. Anorg. Allg. Chem., 1978, vol. 444, pp. 77-90. 11. O. Kubaschewski: High Temp.-High Pressures, 1972, vol. 4, p. 1. 12. Y.E. Lee: CALPHAD, 1982, vol. 6, pp. 283-91. 13. H. Gaye and J. Welfringer: in 2nd Int. Symp. on Metallurgical Slags and Fluxe