Polyphilic compounds were previously reported [F. G. Tournilhac, L. M. Blinov, J. Simon, and S. V. Yablonski, Nature 359, 621 (1992)] to form a longitudinal ferroelectric liquid crystal phase in the mixture M70 (70% compound 1, 30% compound II). The ferroelectric nature was confirmed by measurements of acoustically induced piezoelectric response and hysteretic behavior in the smectic-X' phase. We carefully examined the structure of the smectic-X' phase with high-resolution x-ray diffraction in an aligned sample and discovered bilayer structure, suggesting that molecules actually adopt antiferroelectric arrangement.