Abstract: A liquid crystal compound selected from a group of compounds represented by formula (a): 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 A1, ring A2 and ring A3 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; Z1, Z2 and Z3 are each independently a single bond, —(CH2)2—, —CH?CH—, —C?C—, —CH2O, —OCH2—, —COO—, —OCO—, —OCF2—, —CF2O—, —CH2CH2OCF2— or —CF2OCH2CH2—; l, m and n are each independently 0, 1 or 2, provided that l+m+n is 0, 1, 2 or 3; one of X1 and X2 is fluorine, and the other is chlorine; Q is and atoms comprising the