Precipitates in semi‐insulating single crystal doped with 0.03 w/o Fe have been investigated by optical microscopic and x‐ray diffraction techniques. The precipitates with a hollow center were identified as orthorhombic iron‐diphosphide from lattice constants measurements. The precipitation mechanism is discussed from the viewpoint of crystallization morphology, and it is concluded that the precipitates are ones of primary phase in eutectic solidification.