The crystal structure of antimony selenoiodide, SbSeI

Abstract

The crystal structure of SbSeI has been determined with the intensities of 821 independent reflections, measured on a computer-controlled Philips PW 1100 single-crystal diffractometer. The cell constants, obtained by least-squares calculations from direct ∂ -value measurements on the diffractometer, are: a = 8.6862(9), b = 10.3927(9), c = 4.1452(3) Å, Z = 4; space group Pnam . The positional and thermal parameters, with anisotropic temperature factors, were refined by full-matrix, least-squares calculations to a final R = 0.054. The compound is isotypic with BiSCl (Voutsas and Rentzeperis, 1980). Each Sb atom is seven-coordinated by three Se atoms at distances 2.601 to 2.796 Å and four iodine atoms at distances 3.150 to 3.822 Å. The four I atoms form a square parallel to c , while the three Se atoms form an isosceles triangle perpendicular to (001), with its base parallel to c . The I atoms and two of the Se atoms are at the vertices of a triangular prism parallel to c and of height equal to c . The third Se atom is at the apex of a pyramid whose base is one of the Se 2 I 2 faces of the triangular prism. The composite coordination polyhedron (prism + pyramid) is linked to a symmetry-equivalent polyhedron by a common Se, Se edge of the corresponding pyramids. This characteristic double-polyhedron system is repeated along the c axis, thus forming an infinite double chain of composition {Sb 2 Se 4 I 8 } n . The chain is further linked to each of four similar chains by a common (I,I) n row parallel to c . This system of chains explains very well the needle-like growth of the crystals along c .