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The crystal structure of the new compound Er3Si5-х (x = 0.24) obtained at 600ºС was studied by X-ray single-crystal diffraction. This orthorhombic structure can be derived from the structure type AlB2 by considering an ordered distribution of vacancies on the small-atom sites and appeared to be modulated along one crystallographic direction: superspace group Amm2(α00), a = 0.40872(2), b = 1.13516(5), c = 0.65739(3) nm, q = 0.2223(1)a*. In contrast to the hexagonal structure type Th3Pd5 with empty triangular channels, in the structure of Er3Si5-x Si atoms move from one trigonal prism to a neighboring one according to the modulation wave, the direction of the modulation vector being parallel to the prism axes.