Section 3.1.5.2.9. Tris-sarcosine calcium chloride (TSCC)

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.

Figure 3.1.5.14 | top | pdf | Structure of tris-sarcosine calcium chloride, (CH3NHCH2COOH)3CaCl2. The hydrogen ion (proton) on the COOH group is relocated in the crystal onto the N atom to form a zwitter ion, forming an H—N—H group that hydrogen bonds to adjacent chlorine ions. Each nitrogen forms two such hydrogen bonds, whereas each chlorine has three, forming a very complex network of hydrogen bonding. The phase transition is actually displacive, involving a rather rigid rolling of whole sarcosine molecules, which stretches the N—H bonds; it is not order–disorder of hydrogen ions in a Cl⋯H—N double well. (The Cl⋯H—N wells are apparently too asymmetric for that.)

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).

Figure 3.1.5.15 | top | pdf | `Soft' optical phonon frequencies versus temperature in both ferroelectric and paraelectric phases of tris-sarcosine calcium chloride.