Abstract: Disclosed is a process for preparing crystals of anhydrous sodium sulfide, the process being characterized in that the process comprises the steps of heating crystals of polyhydrate of sodium sulfide under a pressure of about 500 torr or less at a temperature varying not more than .+-.about 10.degree. C. from the phase transition point between polyhydrate crystals and monohydrate crystals of sodium sulfide for about 2 hours or longer and further heating the monohydrate under atmospheric or reduced pressure at about 90.degree. to about 200.degree. C. for about 2 hours or longer.

Abstract: Disclosed is a process for preparing crystals of anhydrous sodium sulfide, the process being characterized in that the process comprises the steps of heating a water-containing non-crystalline sodium sulfide at a temperature of about 30.degree. to about 89.degree. C. under a pressure of about 500 torr or less for about 2 hours or longer in a first-stage heating step and further heating the sodium sulfide at a temperature of about 90.degree. to about 200.degree. C. under atmospheric pressure or reduced pressure for about 2 hours or longer in a second-stage heating step.

Abstract: Disclosed are single crystals of anhydrous sodium sulfide, and a method of producing single crystals of anhydrous sodium sulfide, characterized in that the method comprises concentrating an aqueous solution of sodium sulfide at a temperature of 97.degree. C. or higher, the solution containing about 2 to about 15% by weight of NaOH based on the aqueous solution to precipitate single crystals of anhydrous sodium sulfide from the aqueous solution.