Strong positive and negative deviations from Vegard's rule: X-ray powder investigations of the three quasi-binary phase systems BiSX_1 - xY_x (X, Y = Cl, Br, I)

Three quasi-binary phase systems, BiSX_1 - xY_x (X, Y = Cl, Br, I; 0 < x < 1), have been investigated by X-ray powder diffraction. All three systems show unlimited solubility. For BiSCl_1 - xBr_x and BiSCl_1 - xI_x, strong positive and negative deviations from Vegard's rule are observed with respect to the lattice parameters a and c. They are qualitatively explained by the asymmetry of the X coordination polyhedron and by anion-anion repulsions. For x < 0.32 [x > 0.32] the BiSCl_1 - xI_x system mimics the behaviour of the BiSCl_1 - yBr_y [BiSBr_1 - yI_y] system (0 < y < 1).