Single crystals of sodium containing silicon clathrate compounds Na8Si46 (type 1) and NaxSi136 (type II) were prepared from the mixtures of NaSi and Si under high-pressure and high-temperature conditions of 5 GPa at 600-1000 degrees C. The type 11 crystals were obtained at relatively low-temperature conditions of 700-800 degrees C, which were found to have a Na excess composition Na30.5Si136 in comparison with the compounds NaxSi136 (x <= 24) obtained by a thermal decomposition of NaSi under vacuum. The single crystal study revealed that the Na excess type 11 compound crystallizes in space group Fd (3) over barm with a lattice parameter of a = 14.796(1) angstrom, slightly larger than that of the ambient phase (Na24S136), and the large silicon hexakaidecahedral cages (@Si-28) are occupied by two sodium atoms disordered in the two 32e sites around the center of the @ Si-28 cages. At temperatures <90 K, the crystal symmetry of the compound changes from the face-centered to the primitive cell with space group P2(1)3, and the Na atoms in the @Si-28 cages are aligned as Na-2 pairs. The temperature dependence of the magnetic susceptibility of Na30.5Si136 suggests that the two Na ions (2 Na+) in the cage are changed to a Na-2 molecule. The Na atoms of Na30.5Si136 can be deintercalated from the cages topochemically by evacuation at elevated temperatures. The single crystal study of the deintercalated phases NaxSi136 (x = 25.5 and 5.5) revealed that only excess Na atoms have disordered arrangements.