Chiral smectic transition phases appearing near the electric-field-induced phase transition observed by resonant microbeam x-ray scattering

The electric-field-induced phase transition of a chiral liquid crystal containing Br revealed a transition phase between the three-layer periodicity ferrielectric phase and the synclinic ferroelectric phase in the electric field versus temperature phase diagram. Resonant x-ray scattering from the transition phase showed a diffuse streak or spotty weak reflections, which were composed of strong $m$/3-order (where $m$ = 1 and 2) reflections and other weak peaks. The spotty reflections were found to be related to a 12-layer periodicity phase with a weak contribution from the 15-layer periodicity. An x-ray intensity analysis based on the Ising model suggested that the 12-layer periodicity phase was composed of two three-layer ferrielectric blocks and six synclinic layers. This model indicated that, in the transition phase, the three-layer ferrielectric molecular configuration gradually changed to the synclinic configuration. The diffuse streak appearing around $m$/3-order reflections near the field-induced transition from the four-layer periodicity phase to the synclinic ferroelectric phase is also discussed.

Figure 1

A series of RXS and ω-angular profiles observed during the experiment at applied fields of (a) ±20 V, (b) ±27 V, (c) ±40 V, and (d) ±44 V at 140 °C. The RXS intensity pattern reveals the large white circles from the direct beam stop at the center, strong first-order and weak second-order layer reflections on both sides, and weak RXS satellites between the white circle and the first-order layer reflection. A few leftmost small spots in (d1) are due to an electronic noise.

Figure 2

Sample photographs obtained at fields of (a) ±27 V and (d) ±44 V at 140 °C. The white arrows indicate the measurement point. In (a), the upper region and lower regions correspond to the three-layer and two-layer periodicity phases, respectively, and the zigzag phase boundary in between. In (b), the region appearing in the topmost part of the photograph corresponds to the ferroelectric phase accompanied by the zigzag phase boundary, while other region appears to be the three-layer periodicity phase. The bar in (b) indicates a scale mark of 0.1 mm.

Figure 3

RXS patterns at applied fields of (a) 0 V and (b) ±26.5 V at 144 °C. Note that only the left parts of the diffraction patterns are shown: The strong first-order and weak second-order layer reflections are located around the center and the left edge, respectively. (c) The ω-angular profile and (d) a sample photograph at ±26.5 V. A zigzag phase boundary with a weak contrast is seen at the left topmost region. The bar in (d) indicates a scale of 0.1 mm.

Figure 4

RXS patterns at applied fields of (a) ±3 V, (b) ±6 V, (c) ±20 V, and (d) ±22 V at 145.1 °C.

Figure 5

RXS patterns at applied fields of (a) ±1.5 V and (b) ±6 V at 146 °C.

Figure 6

Phase diagram of temperature and applied voltage. The vertical lines show the temperatures at which the experiments were performed and the black squares denote the field-induced phase transition voltages. The integer values indicate the layer periodicity determined by RXS. “Streak” indicates diffuse scattering and the spotty patterns along the layer normal and “ferro” marks the region where no RXS satellite appears. The temperature was corrected according to polarized microscope images between different samples and different experimental runs. The phase boundaries are marked by dashed lines as a guide to the eyes.

Figure 7

The intensity distribution along the layer normal as a function of the normalized scattering vector ($q_Z/q_0$, where $q_0=2\pi /d$ and $d$ is a layer spacing) from the diffraction pattern obtained at 140 °C and ±44 V. The background (red) was subtracted from the original data (black squares and line). The black solid and pink dashed bars at the bottom correspond to the satellite position for the 12- and 15-layer periodicities, respectively. The inset photograph is horizontally reversed to agree with the graph.

Figure 8

Calculated RXS satellite intensities based on two Ising models for the 12-layer periodicities shown in the lower panels, where $m$ denotes a satellite order. The arrows indicate molecules which flip the tilt direction.