Double q Condensation at V-Point on the Phase Transition of K3D(SO4)2

Low temperature Weissenberg photograph of K 3 D(SO 4 ) 2 (∼ 92% D) was taken at 15 K to investigate the symmetry change associated with the phase transition. Three type superlattice reflections were found below T c (77.5 K), ones at (0 ± 1/2 ± 1/2) V-point and others at lattice points (Z and Γ-points) violating the extinction rule of the space group A2/a of the high temperature phase. The diffuse scattering was observed above T c around (0 1/2 1/2) type reflection. It was revealed that the double q condensation occurred at the V-point and ones at the Z and Γ-points were secondary order parameters. The unit cell of the low temperature phase was determined as a ×2 b ×2 c , and the space group was A2.