Resonant x-ray scattering observation of transitional subphases during the electric-field-induced phase transition in a mixture of Se-containing chiral smectic liquid crystals

Using resonant x-ray scattering techniques, transitional subphases during the electric-field-induced phase transition of a mixture of Se-containing chiral liquid crystals, 80% AS657 and 20% AS620, in a planar-aligned cell geometry were investigated, where the prototypical phase sequence $\mathrm{Sm}{C}_A^{*}-\mathrm{Sm}{C}_{\gamma }^{*}-\mathrm{AF}-\mathrm{Sm}{C}^{*}$ was observed; the transitional subphases were formed during the transition from the three-layer periodicity phase to the ferroelectric phase. In the lower-temperature range where the three-layer $\mathrm{Sm}C{\gamma }^{*}$ phase appeared under the low electric field, nine- and six-layer subphases and a “streak” pattern appeared in sequence after the transition from the $\mathrm{Sm}C{\gamma }^{*}$ phase with increasing applied electric field; the ferroelectric phase was realized. In the higher-temperature range where the four-layer AF phase appeared under a low electric field, the AF phase changed to a three-layer phase at the medium electric field. The twelve-, nine-, and six-layer subphases subsequently appeared in sequence, and finally the ferroelectric phase was generated with increasing electric field. The molecular arrangements of the field-induced subphases, especially the newly found nine-layer periodicity phase, was analyzed. The successive field-induced phase transition of the present results was compared with that of our previous results for pure Se-containing and Br-containing liquid crystals, and the relation to the three-layer ferrielectric phase was discussed.

Figure 1

Molecular structures of samples AS620 and AS657.

Figure 2

Series of resonant x-ray scattering (RXS) profiles obtained at applied voltages from ±45.0 to ±52.0 V at 86.3 °C. In (c) and (d), dotted lines indicate magnified profiles. Open circles in (c) and filled circles in (d) indicate positions where reflection peaks due to the $C_{9p}$ and $C_{6p}$ configurations, respectively, are expected to appear. The inset in (a) shows a two-dimensional diffraction pattern from which the one-dimensional RXS profile was extracted. The white circle (B.S.) corresponds to the shadow of the direct beam stopper, the strong arclike spot on the right (Bragg) is the first-order Bragg diffraction peak, and two weak RXS reflections (arrows) are seen between “B.S.” and “Bragg.” cps denotes counts/s.

Figure 3

Sample photographs at applied voltages of (a) ±45.6 V and (b) ±54.5 V at 86.3 °C. The layer normal was approximately horizontal. Black arrows in (a) show the phase boundary between three-layer and nine-layer periodic phases. In (b), most of the field of view is transformed to the $\mathrm{Sm}{C}^{*}$ phase. Also in (b), the white arrow indicates a typical measurement point, and the black scale bar represents 0.1 mm. Blotlike patterns of various sizes and shapes are due to contamination on the outside of the glass plates.

Figure 4

Series of RXS profiles obtained at applied voltages of ±14.0 to ±40.0 V at 88.0 °C. In (d), (e), and (g), dotted lines indicate magnified profiles. Asterisks in (d), open circles in (e), and filled circles in (g) indicate positions where reflection peaks resulting from $C_{12p}, C_{9p}$, and $C_{6p}$ configurations, respectively, are expected to appear.

Figure 5

Sample photographs at applied voltages of (a) ±0.0 V, (b) ±21.5 V, (c) ±30.8 V, and (d) ±33.7 V at 88.0 °C. In (d), the white arrow indicates a typical measurement point, and the black scale bar represents 0.1 mm.