DOUBLE-Q CONDENSATION AT V-POINT ON THE PHASE-TRANSITION OF K3D(SO4)2

Low temperature Weissenberg photograph of K3D(SO4)2 (approximately 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 GAMMA-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 GAMMA-points were secondary order parameters. The unit cell of the low temperature phase was determined as a x 2b x 2c, and the space group was A2.