Fluorine-containing five-membered ring compounds, and a liquid-crystaline medium containing them

An achiral liquid crystalline mixture having at least two liquid crystalline components, wherein at least one component is a compound of formula I

• R.sup.1 -(A.sup.1 Z.sup.1).sub.m -A.sup.2 -Z.sup.2 -W I
• in which
• W is ##STR70## ##STR71## is a cyclohexane, cyclohexene or phenyl, Y.sup.1 is O or S,
• Y.sup.2 is (a) O, C.dbd.O or CH.sub.n F.sub.2-n, or (b) if Q is C.dbd.O, CHF or CF.sub.2, Y.sup.2 may additionally be CHR.sup.2 or CFR.sup.2,
• n is 0, 1 or 2,
• Y.sup.3 is (a) N, CH or CF, or (b) of Q' is CF, Y.sup.3 may alternatively be CR.sup.2,
• Q is C.dbd.O, CHF, CF.sub.2, CHR.sup.2 or CFR.sup.2,
• Q' is CF or CR.sup.2,
• R.sup.2 is C.sub.1-15 -alkyl, optionally at least mono-substituted by F,
• X is H, F or Cl,
• R.sup.1 is H, alkyl or alkenyl having 1-15 carbon atoms which is optionally monosubstituted by CN or CF.sub.3 or at least monosubstituted by halogen, at lease one CH.sub.2 group optionally independently being replaced by --O--, --S--, --CO--, ##STR72## --CO--O--, --O--CO-- or --O--CO--O-- in such a manner that heteroatoms are not linked directly to one another,
• A.sup.1 and A.sup.2 are each independently
• a) a trans-1,4-cyclohexylene radical in which one or more non-adjacent CH.sub.2 groups are optionally replaced by --O-- and/or --S--,
• b) a 1,4-phenylene radical in which one or two CH groups are optionally replaced by N,
• c) 1,4-cyclohexyenylene, 1,4-bicyclo(2,2,2)-octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, and which the radicals a) and b) are optionally substituted by CN, Cl or F,
• Z.sup.1 and Z.sup.2 are each independently --CO--O, --O--CO--, --CH.sub.2 O--, --O--CH.sub.2 --, --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --C.ident.C-- or a single bond, and
• m is 0, 1 or 2, with the provisos that at least one of the groups Y.sup.2, Y.sup.3, Q and Q' is a radical containing at least one F atom, and that Y.sup.2 is different from Q.