WE CLAIM:
1. A thermotropic liquid crystal smectic A composition exhibiting a smectic type A phase
made up of multiple layers and capable of forming a liquid crystal optical device when
sandwiched between a pair of electrodes, wherein:
under the influence of different electric fields applied between the electrodes, the alignment of the layers of the composition can become more ordered or more disordered, and
the composition has stable states in which the alignment of the layers of the composition are differently ordered including an ordered State, a disordered State and intermediate states, the composition being such that, once switched to a given State by an electric field, it remains substantially in that State when the field is removed,
which composition comprises, in weight %:
(a) 25-75% in total of at least one siloxane of the general formula I:
A is a phenyl or cyclohexyl ring which may be the same or different and are bonded
together in para positions, R = a Ci_3 alkyl group, which may be the same or different, X = a Ci-12 alkyl group, and Z = F, Cl, Br, I, CN, NH2, N02, NMe2, NCS, CH3, or OCH3, CF3, OCF3, CH2F,
CHF2; (b) 0.001 - 1% in total of at least one quaternary ammonium ionisable species of the general formula II:

wherein:
T= a methyl group or a silyl or siloxane group and
v = 1 to 30,
Rl, R2, and R3, which may be the same or different, are C1-4 alkyl,
Q" is an oxidatively stable ion, (c) 20-65% in total of at least one polarisable linear molecule having an alkyl chain, the molecule having the general formula III:
wherein:
D Stands for a CMÖ straight chained alkyl or alkoxy group, optionally containing one or more double bonds;
k = 2or3,
A' is a phenyl, cyclohexyl, pyrimidine, 1,3-dioxane, or l,4-bicyclo[2,2,2]octyl ring, wherein each A' may be the same or different and are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl and
Y is located in the para position of the terminal ring of the group A'k and is selected
from Z (as defined above in connection with Formula I), CMÖ straight chained
alkyl, C1-16 straight chained alkoxy, OCHF2, NMe 2, CH3, OCOCH3, and COCH3;
and
(d) 2 - 20%, in total of at least one side chain liquid crystal polysiloxane of the general
formula IV:
wherein:
a, b and c each independently have a value of 0 to 100 and are such that a+b+c has an average value in the ränge 3 to 200, and a is such that the chain units of the

formula Y-R2SiO-[SiR2-0]a represents 0 to 25 mole percentage of the Compound of the general formula IV, and c is such that the units of the formula chain -[SiHR-0]c-R2SiO-Y represents 0 to 15 mole percentage of the Compound of the general formula IV, and c is such that the units of the formula chain -[SiHR-0]c-R2SiO-Y represents 0 to 15 mole percentage of the Compound of the general formula IV,
m = 3 to 20,
t = 0orl,
k = 2 or 3
A is a phenyl or cyclohexyl ring which may be the same or different and the rings are bonded together in para positions,
R = a Ci-3 alkyl group, each of which may be the same or different, and
Y = a Ci-12 alkyl group, a chromophore or a calamitic liquid crystal group and each of which may be the same or different, and
Z is as defined above in connection with Formula I,
and wherein the amounts and nature of the components are selected such that the composition possesses smectic A layering and siloxane-rich sub-layering, as detected by X-ray diffraction.
2. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the siloxane (a) is a Compound of the formula la:
where X, R, p, q, and t are defined above in connection with Formula I, and g and h each independently stand for 0, 1 or 2, and j Stands for 1, 2, or 3, subject to the requirement that g+h+j is 2 or 3.
3. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the side chain siloxane liquid crystal, component (d), which may be a polymer, copolymer, or terpolymer, is a Compound of the general formula IVa

where a, b, c, m, and t are as defined in connection with Formula IV, g = 0, 1, or 2, h = 0, 1 or 2, j = 1, 2, or 3, subject to the requirement that g+h+j is 2 or 3; each R may be the same or different and is an alkyl group, and Y = a Ci-g alkyl group, a chromophore, or a calamitic liquid crystal group.
4. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the ionisable species (b) of formula II is a Compound of the formula (IIa):
where v, Rl, R2, R3 and Q are as defined in claim 1 in connection with Formula IL
5. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the ionisable species (b) of formula II is a Compound of the formula IIb:
wherein v, Rl, R2, R3 and Q are as defined in claim 1 in connection with Formula II and T' is a silyl or siloxane group.

6. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the molecule having the general formula (III) in component (c) comprises an organic calamitic mesogen which exhibits either a nematic or a Smectic A liquid crystal phase.
7. The thermotropic liquid crystal smectic A composition as claimed in claim 1, wherein the at least one polarisable linear molecule, component (c), includes a Compound of the formula lila and/or a Compound of the formula Illb.
where a = 1 to 15 and b = 1 to 13; f = 0 or 1 , j = 1 , 2, or 3; g = 0,1 ,or 2, and h = 0, 1, or 2, subject to the requirement that g+h+j does not exceed 3.
8. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which
further includes:
(e) up to 10% by weight in total of at least one positive or negative dichroic dye,
optionally a cyan, yellow, magenta, red, green or blue dye or an emissive dye, the dye
being aligned with neighbouring mesogenic components of the composition.
9. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which
further includes:
(f) up to 10% by weight of one or more viscosity-reducing solvents or diluents.
10. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which
further includes:
(g) up to 10 wt% of at least one molecule that is not a liquid crystal, but which can be
incorporated into the formulation, without degrading the smectic A layer quality of
the composition.

11. The thermotropic liquid crystal smectic A composition as claimed in claim 10, wherein the at least one molecule that is not a liquid crystal comprises a Compound of the formula
12. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which also furtherincludes:
(h) up to 50% by weight in total of at least one birefringence-altering additive selected from:
where R = CMO alkyl, n = 0 or 1, L is selected from hydrogen or C1-3 alkyl and X = CN, F, NCS, CF3, OCF3 or alkyl, or

where R = a CMO alkyl group.
13. The thermotropic liquid crystal smectic A composition as claimed in claim 12, wherein the total amount of the birefringence-altering additive component (h) and the total amount of component (c) is in the ränge of 35 - 73 wt%.
14. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which has a birefringence in the ränge 0.15 to 0.3 at 20°C and 589nm and is opaque in the disordered State and clear in the ordered State.
15. The thermotropic liquid crystal smectic A composition as claimed in claim 14, which further includes up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue or a black dye, or an emissive dye, the dye being aligned with neighbouring mesogenic components of the composition.

16. The thermotropic liquid crystal smectic A composition as claimed in claim 1, which (i) has a birefringence in the ränge 0.07 to 0.15 at 20°C and 589nm, (ii) is translucent in the disordered State and clear in the ordered State and (iii) includes up to 10% by weight in total of at least one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, the dye being aligned with neighbouring mesogenic components of the composition.
17. A cell comprising a pair of opposed spaced-apart electrodes at least one of which is light transmitting, and a thermotropic liquid crystal smectic A composition as claimed in claim 1 located between the electrodes.
18. The cell as claimed in claim 17, which forms a Single optical element or multiple pixelated optical elements.
19. The cell as claimed in claim 17, which forms multiple pixelated optical elements that are individually addressable, thereby allowing the cell to display Information, which may be graphic or in the form of data characters.
20. The cell as claimed in claim 17, wherein the spacing between the electrodes is in the ränge of 2 - 50 microns.
21. The cell as claimed in claim 17, that has a front and a back, wherein
the pair of electrodes includes (a) a light-transmitting electrode located at the front through which the thermotropic liquid crystal smectic A composition of the cell can be observed by a user and (b) a rear electrode located at the back.
22. The cell as claimed in claim 21, wherein the rear electrode is reflective and reflects radiation incident on its back through the cell.
23. The cell as claimed in claim 21, wherein the rear electrode is transparent and the rear of the cell includes a surface that can transmit light through the cell, or the surface is light emissive, the reflective or emissive surface optionally being coloured and/or bearing Information.

24. The cell as claimed in claim 17, wherein each of the electrodes is supported on a Substrate, which may be rigid or flexible.
25. An optical device comprising at least one cell as claimed in claim 17.
26. The optical device as claimed in claim 25, that includes multiple pixelated optical elements that are addressed individually.
27. The optical device as claimed in claim 25, which comprises a Stack composed of two or more cells stacked on top of each other, and wherein the thermotropic liquid crystal smectic A composition in each cell (i) contains up to 10% by weight in total of at least, one positive or negative dichroic dye, optionally a cyan, yellow, magenta, red, green or blue dye, or a black dye or an emissive dye, the dye being aligned with neighbouring mesogenic components of the composition and (ii) is selected to exhibit a low birefringence in the ränge of 0.08 to 0.15, at 20°C and 589nm.
28. A method of switching a cell containing a thermotropic liquid crystal smectic A composition as claimed in claim 1 sandwiched between a pair of electrodes, the switching being from one State to a more ordered State, which comprises applying an alternating electric field between the electrodes having a frequency of at least 1000Hz.
29. A method of switching a cell containing a thermotropic liquid crystal smectic A composition as claimed in claim 1 sandwiched between a pair of electrodes, the switching being from one State to a more disordered State, which method comprises applying an alternating electric field having a frequency of less than 500Hz or a non-alternating electric field between the electrodes.
30. A thermotropic liquid crystal composition capable of forming a liquid crystal display when sandwiched between a pair of electrodes, the composition comprising
a composition exhibiting a smectic type A phase; and
a dopant in the form of a quaternary ammonium ionisable species of the general formula
VI:

wherein:
v = 1 to 30,
Rl, R2 and R3 are C1-4 alkyl, which may be the same or different,
T = a silyl or siloxane group, and
Q" is a CICV ion.
31. A cell, which may form a Single optical element or multiple pixelated optical elements, comprising a pair of opposed spaced-apart electrodes at least one of which is light transmitting, and a thermotropic liquid crystal composition as claimed in claim 30 located between the electrodes.
32. An optical device comprising at least one cell as claimed in claim 31.