Operating fluid for a steam cycle device and a method for its operation

The invention relates to a working fluid for a steam cycle, which in an apparatus comprising a steam generator, an expander, a condenser and a reservoir for the operating fluid is carried out, comprising - a working medium is vaporized by heat supply in the steam generator, in the vapor phase performs mechanical work by expansion in the expander, and condensed in the condenser; - an ionic liquid which undergoes a mixture with the working medium, wherein the melting point of the mixture below the freezing point of pure working medium is located.