[en] The infrared reflectance spectra associated with lattice vibrations in the epitaxial Al_xGa_1-xAs/GaAs(100) heterostructures with different Al content in the cation sublattice are studied. The structures are grown by metal-organic chemical vapor deposition. In the spectrum of the structure with x ∼ 0.50, the vibration modes corresponding to the superstructurally ordered AlGaAs₂ phase are detected. The atomic force microscopy of the surface of the sample with x ∼ 0.50 reveals areas of ordered nano-scaled profile, with a period of ∼ 115 nm. The ordered domains involve the AlGaAs₂ structured phase