The low-temperature modification of Sr3(O3C3N3)2 was prepared and assigned as α-SCY after the high-temperature phase (now called β-SCY) and its frequency-doubling properties were reported recently. The crystal structure of α-SCY was solved and refined by single-crystal X-ray diffraction. Both modifications of SCY crystallize in noncentrosymmetric space groups, with the low-temperature phase (α-SCY) adopting the lower symmetry structure (Cc). Atomic positions in α-SCY (Cc) reveal only small deviations in comparison to those in the structure of β-SCY (R3c). The reversible phase transition between both modifications of SCY was studied by means of temperature-dependent powder X-ray diffraction. NLO measurements of both SCY modifications are reported in comparison to the commercial frequency-doubling material KTiOPO4 (KTP).