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, pp. 367-368
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Tris-sarcosine calcium chloride has the structure shown in Fig. 3.1.5.14. It consists of sarcosine molecules of formula CH3NHCH2COOH in which the hydrogen ion comes off the COOH group and is used to hydrogen bond the nitrogen ion to a nearby chlorine, forming a zwitter ion. As is illustrated in this figure, this results in a relatively complex network of N—H⋯Cl bonds. The COO− ion that results at the end group of each sarcosine is ionically bonded to adjacent calcium ions. The resulting structure is highly ionic in character and not at all that of a `molecular crystal'. The structure at ambient temperatures is () with ; below 127 K it distorts to () with Z still 4.
It had been supposed for some years on the basis of NMR studies of the Cl ions, as well as the conventional wisdom that `hydrogen-bonded crystals exhibit order–disorder phase transitions', that the kinetics of ferroelectricity at the Curie temperature of 127 K in TSCC involved disorder in the proton positions along the N—H⋯Cl hydrogen bonds. In fact that is not correct; even the NMR data of Windsch & Volkel (1980), originally interpreted as order–disorder, actually show (Blinc et al., 1970) a continuous, displacive evolution of the H-atom position along the H⋯Cl bond with temperature, rather than a statistical averaging of two positions, which would characterize order–disorder dynamics. In addition, as shown in Fig. 3.1.5.15, there is (Kozlov et al., 1983) a lightly damped `soft' phonon branch in both the paraelectric and ferroelectric phases. TSCC is in fact a textbook example of a displacive ferroelectric phase transition. The hydrogen bonds do not exhibit disorder in the paraelectric phase. Rather, the transition approximates a rigid rotation of the sarcosine molecules, which stretches the N—H⋯Cl bond somewhat (Prokhorova et al., 1980).
References
Blinc, R., Jamsek-Vilfan, M., Lahajnar, G. & Hajdukovic, G. (1970). Nuclear magnetic resonance study of the ferroelectric transition in diglycine nitrate and tris-sarcosine calcium chloride. J. Chem. Phys. 52, 6407–6411.Google ScholarKozlov, G. V.., Volkov, A. A., Scott, J. F. & Petzelt, J. (1983). Millimeter wavelength spectroscopy of the ferroelectric phase transition in tris-sarcosine calcium chloride. Phys. Rev. B, 28, 255–261.Google Scholar
Prokhorova, S. D., Smolensky, G. A., Siny, I. G., Kuzminov, E. G., Mikvabia, V. D. & Arndt, H. (1980). Light scattering study of the phase transition in tris-sarcosine calcium chloride. Ferroelectrics, 25, 629–632.Google Scholar
Windsch, W. & Volkel, G. (1980). EPR investigation of the dynamics of ferroelectric tris-sarcosine calcium chloride. Ferroelectrics, 24, 195–202.Google Scholar