Millimeter wavelength spectroscopy of the ferroelectric phase transition in tris-sarcosine calcium chloride

Abstract

Abstract We have measured the soft mode responsible for the ferroelectric phase transition in tris-sarcosine calcium chloride: (CH3 NHCH2 COOH)3 CaCl2, by both far infrared and Raman techniques. In the ferroelectric C9 2v phase the soft TO varies from 30. 5 cm−1 at 4K to 2. 3 cm−1 at TC - 1K = 126 K; in the paraelectric D16 2h phase the soft TO has B2u symmetry varies from 11. 2 cm−1 at 167 K to 3. 2 cm−1 at TC + 3 K = 130 K. A Curie constant of 25 ± 2 K is deduced, which differs from that of 57 K measured in the MHz regime, indicating significant dispersion. This ferroelectric is unique in that the A1(LO) and B2u(LO) modes also soften substantially; in the C2v phase A1(LO) varies from 31 cm−1 at 4 K to 4 cm−1 at TC - 1K. This shows that ferroelectricity in TSCC does not arise from a subtle cancellation of short-range and long-range forces, as in Cochran's theory, but from an intrinsic instability in the short-range force associated with the N-H-Cl hydrogen bonds.