Ferroelectricity and switching polarization on the C-HMIDLINE HORIZONTAL ELLIPSISπ bond in a pure organic molecular crystal-1,3,5-trimethylnitrobenzene

The novel single component molecular ferroelectric single crystal of 1,3,5-trimethylnitrobenzene (TMNB) has been investigated in terms of its temperature dependent polar properties. Three phases have been found in the solid state of TMNB: phase I above 161 K, phase II (incommensurate) between 161 and 152 K and phase III below 152 K. The TMNB crystal structures were determined at 100 K (phase III, sp. gr. Pca2(1)) and at 250 K (phase I, sp. gr. Pnma). Ferroelectricity of phase III along the c-axis has been confirmed by the pyroelectric measurements and observations of the hysteresis loops, revealing spontaneous polarization as small as similar to 2 x 10(-6) C m(-2). The crystal is the first example of an origin of spontaneous polarization related to flexibility of the C-HMIDLINE HORIZONTAL ELLIPSIS pi hydrogen bonds (molecular joints). The TMNB molecules in the dimer can change the mutual orientation of the dipole moments without breaking the C-HMIDLINE HORIZONTAL ELLIPSIS pi bonds. In this paper, the mechanism of the phase transitions will be discussed, which is governed by the dynamics of the NO2 groups and the flexibility of the C-HMIDLINE HORIZONTAL ELLIPSIS pi dimer.

Automated summary

This study investigates the temperature-dependent polar properties of the novel single-component molecular ferroelectric single crystal of 1,3,5-trimethylnitrobenzene (TMNB). Three solid-state phases of TMNB have been identified, with phase III exhibiting ferroelectricity along the c-axis. The origin of spontaneous polarization in this crystal is attributed to the flexibility of the C-H···π hydrogen bonds in the molecular joints. The phase transitions in TMNB are governed by the dynamics of the NO2 groups and the flexibility of the C-H···π dimer.