Soft-mode spectroscopy of tris-sarcosine calcium chloride

We have measured soft modes associated with the second-order ferroelectric phase transition in tris-sarcosine calcium chloride, (CH3NHCH2COOH)3 CaCL2, in both paraelectric D2h16 (Pnma) phase, and ferroelectric C2v9 phase. In the paraelectric phase the mode decreases from 11.2 cm-1 at 167K to 3.2 cm-1 at 130 K (TC = 127 K) and is underdamped over most of this range. A Curie constant of 25 ± 2 K is deduced from these infrared data, in fair agreement with that of 58 K measured in the megahertz regime via conventional dielectric studies. In the ferroelectric phase the mode is measured from 30.5 cm-1 at 4 K to 2.3 cm-1 at 126 K. The system is unique inasmuch as the longitudinal optical phonon decreases in energy by 90% between T = 0 and T = TC, in contrast to normal ferroelectrics in which LO energy is nearly independent of temperature; this shows that the inherent instability in the lattice is in the short-range forces and not to any subtle cancellation of short-range and long-range forces, as proposed in Cochran's original soft mode theory. A number of aspects of the phase transition still have no quantitative explanation; particularly interesting is the role of N-H—Cl hydrogen bonds, the shift in TC upon deuteration, and the relatively large entropy of transition (0.38 cal/mol K). The soft mode temperature dependences measured are completely mean-field; the linewidths are unusual in that they show no broadening whatsoever as TC is approached from above or below.