Strong positive and negative deviations from Vegard's rule: X-ray powder investigations of the three quasi-binary phase systems BiSX(1 - x)Y(x) (X, Y = Cl, Br, I).

Three quasi-binary phase systems, BiSX(1 - x)Y(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 - x)Br(x) and BiSCl(1 - x)I(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 - x)I(x) system mimics the behaviour of the BiSCl(1 - y)Br(y) [BiSBr(1 - y)I(y)] system (0 < y < 1).