Liquid crystalline isothiocyanates with dioxane ring and liquid crystalline admixtures containing same

New compounds of the general formula: I wherein the symbols X stand for carbon atom bonded with two hydrogen atoms /-CH2- group/ and the symbols Z stand for oxygen atom or the symbols X stand for oxygen atom and the symbols Z stand for carbon atom bonded with two hydrogen atoms /-CH2- group/, m is an integer of 1 to 2, R represents an alkyl /H2n+1Cn/ or an 4-alkylphenyl /H2n+1Cn-C6H4/ or an alkylbiphenyl /H2n+1Cn-C6H4-C6H4/ or an alkylcyclohexylphenyl group /H2n+1Cn-C6H10-C6H4/, the term alkyl group signifies an alkyl chain containing 1 to 15 carbon atoms or it also signifies a branched-chain alkyl group C2H5-CH/CH3/-/CH2/k where k in an integer of 1 to 3, have utility as liquid crystals. A process for manufacture of compounds of the general formula I. Liquid crystalline admixtures containing those compounds and a way of selection of their compositions to obtain the admixtures which exhibit smectic, smectic and nematic or only smectic properties as well as their use in electrooptical devices is described.