Phenanthridine derivatives, and their use in liquid-crystalline mixtures A ferroelectric liquid-crystal mixture containing one or more phenanthridine derivatives of the formula (I) ##STR1## E.sup.1 and E.sup.2 are identical or different and are --N--, --CF-- or --CH--; R.sup.1 and R.sup.2 are identical or different and are, for example, a straight-chain or branched alkyl radical having 1 to 20 carbon atoms; Q is --CH.sub.2 --O--, --CO--O-- or a single bond; M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 --, --CH.sub.2 --CH.sub.2 --, --C.tbd.C--, or a single bond; A.sup.1 and A.sup.2 are identical or different and are 1,4-phenylene, in which one or more H atoms may be replaced by F, trans-1,4-cyclohexylene, pyrimidine-2,5-diyl, pyridine-2,5-diyl, where one H atom may be replaced by F, 1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl, 1,3-dithiane-2,5-diyl, 1,3-thiazole-2,4-diyl, 1,3-thiazole-2,5-diyl, thiophene-2,4-diyl or thiophene-2,5-diyl; m and n are zero or one, but their sum is at most one. Even addition of only small amounts of phenanthridine derivatives of the formula (I) gives ferroelectric liquid-crystal mixtures having high negative values for the dielectric anisotropy.