Type-II Na24Si136 clathrate octahedral single crystals surrounded by {111} facets were grown by evaporating Na from a molten mixture of Na4Si4 and Na9Sn4 at 823 K for 12 h. One of the obtained single crystals was used as a seed for the following single crystal growth of the type-II clathrate using the same method. The single crystal grown on the seed maintained the octahedral shape. The weight of the crystal grown with the seed was increased from 0.6 to 30.4 mg by repeating the seeded growth and was proportional to the surface area of the seed crystal.
