Ferrielectricity in an Organic Ferroelectric

TL;DR

This paper reports the discovery of ferrielectricity in a single-phase organic crystal, TSCC, with unique phase transition properties and evidence from multiple experimental techniques, expanding understanding of ferroelectric phenomena in crystalline solids.

Contribution

It demonstrates ferrielectricity in an organic crystalline solid and characterizes its phase transitions and structural properties, a phenomenon previously rare in true crystals.

Findings

Ferrielectricity observed below 64 K with hysteresis loops.

Phase transition near 130 K confirmed by Raman and structural studies.

Antiferroelectric displacement with bias voltage increases as temperature decreases.

Abstract

We report ferrielectricity in a single-phase crystal, TSCC -- tris-sarcosine calcium chloride [(CH3NHCH2COOH)3CaCl2]. Ferrielectricity is well known in smectic liquid crystals but almost unknown in true crystalline solids. Pulvari reported it in 1960 in mixtures of ferroelectrics and antiferroelectrics, but only at high fields. TSCC exhibits a second-order displacive phase transition near Tc = 130 K that can be lowered to a Quantum Critical Point at zero Kelvin via Br- or I-substitution, and phases predicted to be antiferroelectric at high pressure and low temperatures. Unusually, the size of the primitive unit cell does not increase. We measure hysteresis loops and polarization below T = 64 K and clear Raman evidence for this transition, as well of another transition near 47-50 K. X-ray and neutron studies below Tc = 130K show there is an antiferroelectric displacement out of plane of two sarcosine groups; but these are antiparallel displacements are of different magnitude, leading to a bias voltage that grows with decreasing T. A monoclinic subgroup C2 may be possible at the lowest temperatures (T<64K or T<48K), but no direct evidence exists for a crystal class lower than orthorhombic.