The perfect soft mode: giant phonon instability in a ferroelectric

• Published in: Journal of physics. Condensed matter Vol. 25; no. 21; p. 212201
• Main Authors: Mackeviciute, R, Ivanov, M, Banys, J, Novak, Nikola, Kutnjak, Zdravko, Wencka, Magdalena, Scott, J F
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 29.05.2013; Institute of Physics
• Subjects: Calorimetry; Condensed matter; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Dielectrics, piezoelectrics, and ferroelectrics and their properties; Electronics; Exact sciences and technology; Ferroelectric materials; Ferroelectricity; Ferroelectricity and antiferroelectricity; Lattice dynamics; Phase transitions and curie point; Phased arrays; Phonon states and bands, normal modes, and phonon dispersion; Phonons; Phonons and vibrations in crystal lattices; Physics; Presses; Antiferroelectricity; Amino acids; Domain wall motion; Calcium chloride; High pressure; Soft modes; Optical phonons; Specific heat; Ferroelectricity; Phonon mode; Sarcosine; Waveguides; Instability; Rectangular plate; Complex permittivity
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/25/21/212201

Previous studies of unstable ('soft') optical modes in ferroelectrics have reported minimum frequencies of 1 cm−1 (30 GHz) for underdamped phonons. In this work we fabricate a cylindrical coaxial specimen and rectangular plate waveguide specimens of tris-sarcosine calcium chloride (TSCC) and follow its soft mode several orders of magnitude lower to 1 GHz. Below 30 GHz the relaxation time is probably characteristic of domain wall motion; the new theory of Pakhomov et al (2013 Ferroelectrics at press) predicts 0.5 THz far from TC and a (T − TC) TC dependence, in agreement with our experimental values. This discovery has implications for GHz electronics such as phased array radar or other voltage-tunable low-loss components. The mean-field frequency description of the soft mode response f(T) is supported via precision calorimetry on TSCC with and without Br-doping. The ferroelectric-antiferroelectric phase transition, previously suggested from high-pressure data, is confirmed at 45 K at 1 atm.