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, P6(3)/mmc), 100 K (hexagonal, P6(3)mc) and 20 K (orthorho mbic). The orthorhombic form to) shows no superstructure and its unit cell is related to the hexagonal one (h): a(o) similar or equal to a(h ) + b(h), b(o) similar or equal to -a(h) + b(h) and c(o) similar or eq ual to c(h). Bijvoet-pair examination confirms that it crystallizes in the noncentrosymmetric space group Cmc2(1). The hexagonal (P6(3)mc)-o rthorhombic (Cmc2(1)) structural phase transition is proved to be of s econd order.