||Selenium substitution effect on crystal structure of stibnite (Sb2 S3 )
Kyono, Atsushi ,
Hayakawa, AkinobuHoriki, Mayumi
Physics and chemistry of minerals
490 , 2015-06 , Springer Berlin Heidelberg
Composition dependence of the crystal structure between stibnite and antimonselite is investigated by using the single-crystal X-ray diffraction and the ab initio calculation methods to clarify the Se substitution effect on the crystal structure, especially focusing on the stereochemical behavior of Sb 5s 2 lone-pair electrons. The single-crystal X-ray diffraction measurements indicate no phase change throughout the solid solution range. The lattice parameters of a, b, c and unit cell volume are linearly increased as Se content increases. The lattice parameter variations normalized show an anisotropic expansion that the largest expansion is observed along the a-axis, followed by the c- and b-axes. The large Se atom exhibits a strong site preference for the X(1) and X(3) sites, while the small S atom prefers to occupy the X(2) site. The intra-ribbon Sb(1)–X and Sb(2)–X distances (X = S, Se) are continuously increased with the Se content. The three Sb(1)–X bond distances in the trigonal pyramids are changed within the similar range between 2.52 and 2.68 Å, while the five Sb(2)–X distances in the tetragonal pyramids vary from 2.45 to 2.59 Å, from 2.68 to 2.80 Å, and from 2.86 to 3.00 Å, respectively. With increasing Se content in the solid solution, the inter-ribbon distances where the Sb 5s 2 LPEs are located monotonously increase as well. However, the variations between the ribbons are considerably smaller than those of intra-ribbon distances. The polyhedral volumes of the Sb(1)X7 and Sb(2)X7 in which the Sb 5s 2 LPEs are accommodated constantly increase from 35.9 to 40.0 Å3 and from 34.1 to 38.8 Å3, respectively, and these eccentricity parameters decrease from 0.66 to 0.62 and from 0.57 to 0.55. As a result of ab initio calculation, the Sb 5s orbitals on the Sb(1) atoms remain almost unchanged throughout the solid solution. On the other hand, those on the Sb(2) atoms become smaller with the incorporation of Se. The result gives a more reasonable interpretation that the stereochemistry of Sb 5s 2 LPEs and the stereochemistry of the coordination polyhedra around the Sb atoms are affected by the Se substitution in the structure.