A liquid crystal compound selected from a group of compounds represented by formula (a): ##STR00001## wherein Ra and Rb are each independently alkyl having 1 to 10 carbons, alkoxy having 1 to 9 carbons, alkoxyalkyl having 2 to 9 carbons, alkenyl having 2 to 10 carbons, alkenyloxy having 2 to 9 carbons, fluoroalkyl having 1 to 10 carbons or fluoroalkoxy having 1 to 9 carbons; ring A.sup.1, ring A.sup.2 and ring A.sup.3 are each independently trans-1,4-cyclohexylene, trans-1,3-dioxane-2,5-diyl, trans-tetrahydropyran-2,5-diyl, 1,4-phenylene, 2-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene, 2,5-difluoro-1,4-phenylene, 2,6-difluoro-1,4-phenylene, pyridine-2,5-diyl, 6-fluoropyridine-2,5-diyl or pyridazine-2,5-diyl; Z.sup.1, Z.sup.2 and Z.sup.3 are each independently a single bond, --(CH.sub.2).sub.2--, --CH.dbd.CH--, --C.ident.C--, --CH.sub.2O, --OCH.sub.2--, --COO--, --OCO--, --OCF.sub.2--, --CF.sub.2O--, --CH.sub.2CH.sub.2OCF.sub.2-- or --CF.sub.2OCH.sub.2CH.sub.2--; l, m and n are each independently 0, 1 or 2, provided that l+m+n is 0, 1, 2 or 3; one of X.sup.1 and X.sup.2 is fluorine, and the other is chlorine; Q is ##STR00002## and atoms comprising the compound each may be an isotope thereof.