Space-Group Determination of the Orthorhombic Form of NaV₆O₁₁

The unit cell and space group of orthorhombic NaV₆O₁₁ (low-temperature form) are found by an X-ray single-crystal diffraction study at 300 K (hexagonal, P6₃/mmc), 100 K (hexagonal, P6₃mc) and 20 K (orthorhombic). The orthorhombic form (o) shows no superstructure and its unit cell is related to the hexagonal one (h): a_o ≃ a_h + b_h, a_o ≃ −a_h + b_h and c_o ≃ c_h. Bijvoet-pair examination confirms that it crystallizes in the noncentrosymmetric space group Cmc2₁. The hexagonal (P6₃mc)-orthorhombic (Cmc2₁) structural phase transition is proved to be of second order.