Incommensurately modulated phase of Rb2CoBr4 at 295 and 200 K.

The crystal structure of Rb2CoBr4 at 295 and 200 K has been determined. At these temperatures Rb2CoBr4 exhibits an incommensurately modulated structure with wavevector q = (1/3 + delta)a*. At room temperature only the average structure was refined. Lattice parameters are a = 9.732 (3), b = 13.328 (4), c = 7.654 (3) A, space group Pnam. The R(F) value was 0.0414 for 286 observed reflections (0.0778 for all 477 reflections). At 200 K the lattice parameters are a = 9.691 (4), b = 13.278 (5), c = 7.630 (6) A, superspace group P:Pnam:1ss. Main reflections and satellite reflections of first order were measured. The refinement converged at R(F) = 0.052 for 309 observed reflections (255 main reflections and 54 satellites) and 0.2971 for all reflections (1849; 695 main reflections and 1154 satellites). Amplitudes and phases of the modulation function as well as bond distances show close relationships to those observed in the incommensurately modulated phase of Rb2ZnBr4.