International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. D. ch. 3.1, p. 365

Section 3.1.5.2.5. Potassium nitrate, KNO3

J. F. Scottc*

3.1.5.2.5. Potassium nitrate, KNO3

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Potassium nitrate has a rather simple phase diagram, reproduced in Fig. 3.1.5.9[link]. Two different structures and space groups were proposed for the ambient temperature phase I: Shinnaka (1962[link]) proposed [D^6_{3d}] ([R{\bar 3}2/c]) with two formula groups per primitive cell ([Z =2]), whereas Tahvonen (1947[link]) proposed [D^5_{3d}] ([R{\bar 3}m]) with one formula group per primitive cell. In fact, both are wrong. The correct space group is that of Nimmo & Lucas (1973[link]): [D^6_{3d}] ([R{\bar 3}2/c]) with one formula group per primitive cell. Again, Raman spectroscopy of phonons shows that the Tahvonen structure predicts approximately twice as many spectral lines as can be observed. Balkanski et al. (1969[link]) tried creatively but unsuccessfully to account for their spectra in terms of Tahvonen's space-group symmetry assignment for this crystal; later Scott & Pouligny (1988[link]) showed that all spectra were compatible with the symmetry assigned by Nimmo and Lucas. In this case, in contrast to the perovskites strontium titanate and lanthanum aluminate, the confusion regarding space-group symmetry arose from the large degree of structural disorder found in phase I of KNO3. The structures of phases II and III are unambiguous and are, respectively, aragonite [D^{16}_{2h}] ([Pnma]) with [Z = 4] and [C^{5}_{3v}] ([R3m]) with Z = 1.

[Figure 3.1.5.9]

Figure 3.1.5.9 | top | pdf |

Phase diagram of potassium nitrate, KNO3.

References

First citation Balkanski, M., Teng, M. K. & Nusimovici, M. (1969). Lattice dynamics in KNO3. Phases I, II and III. In Light scattering spectra of solids, edited by G. B. Wright, pp. 731–746. Paris: Flammarion.Google Scholar
 First citation Nimmo, J. K. & Lucas, D. W. (1973). The crystal structures of [\gamma]- and [\beta]-KNO3 and the [\alpha\leftarrow\gamma\leftarrow\beta] phase transformations. Acta Cryst. B32, 1968–1971.Google Scholar
 First citation Scott, J. F. & Pouligny, B. (1988). Raman spectroscopic study of submicron KNO3 films. J. Appl. Phys. 64, 1547–1551.Google Scholar
 First citation Shinnaka, Y. (1962). X-ray study on the disordered structure above the ferroelectric Curie point in KNO3. J. Phys. Soc. Jpn, 17, 820–828.Google Scholar
 First citation Tahvonen, P. E. (1947). X-ray structure of potassium nitrate. Ann. Acad. Sci. Fenn. Ser. A, 44–51.Google Scholar








































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