We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe$_2$ from transport and scanning tunneling microscopy (STM) studies. Below the transition ($T_{rm C} approx 275$ K on cooling) a $q = 1/5$ charge modulation was observed, which is consistent with previous studies. Additional modulations [$q_n = (3n+2)^{-1}$] appear below a second transition at $T_{rm S}approx 180$ K on cooling. The coexistence of various modulations persist up to $T_{rm C}$ on warming. The atomic structures of charge modulations and the temperature dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice which partially melts below $T_{rm S}$ on cooling. Our results provide compelling evidence that the ground state of IrTe$_2$ is a commensurate 1/6 charge modulation, which originates from periodic dimerization of Te atoms visualized by atomically resolved STM images.
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