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Keywords:

  • Vegard's rule;
  • quasi-binary phase systems;
  • lattice parameters

Three quasi-binary phase systems, BiSX1 − xYx (X, Y = Cl, Br, I; 0 < x < 1), have been investigated by X-ray powder diffraction. All three systems show unlimited solubility. For BiSCl1 − xBrx and BiSCl1 − xIx, 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 BiSCl1 − xIx system mimics the behaviour of the BiSCl1 − yBry [BiSBr1 − yIy] system (0 < y < 1).