β-ZrNCl polycrystalline powder prepared by chemical transport method and NH4Cl are mixed in a molar ratio of 1:2. The mixture is encapsulated in a Au capsule (6 mm in inner diameter and 6 mm in depth) of a reaction vessel 2, which is then enclosed in a highly heat-conductivitive sodium chloride block as an electrically insulating pressure medium 6. The mixture held in the sodium chloride block is placed in a carbon tube 8 for serving as a heater. In a cubic-pressing apparatus using a pyrophyllite 12 as a pressure-transmitting medium, the mixture is heated at 900° C. for 2 hours under an applied pressure of 3 GPa. After the mixture is allowed to stand until it is cooled down to room temperature, the Au capsule is taken out and light green β-ZrNCl single crystals are obtained. A large single crystal among them had a hexagonal plate-like habit and is transparent with dimensions about 2 mm in diameter, and 0.3 mm in thickness.