  We report an infrared spectroscopy study on K$_{0.83}$Fe$_{1.53}$Se$_2$, a
semiconducting parent compound of the new iron-selenide system. The major
spectral features are found to be distinctly different from all other Fe-based
superconducting systems. Our measurement revealed two peculiar spectral
structures: a double peak structure between 4000-6000 cm$^{-1}$ and abundant
phonon modes much more than those expected for a 122 structure. We elaborate
that those features could be naturally explained from the blocked
antiferromagnetism due to the presence of Fe vacancy ordering as determined by
recent neutron diffraction experiments. The double peaks reflect the
coexistence of ferromagnetic and antiferromagnetic couplings between the
neighboring Fe sites.
