Nature of the structural distortion and of the chemical bonding in Sn M3Rh 4Sn 12 ( M = LaGd, Yb, Ca, Sr, and Th)

Abstract

The crystal structure of all the Sn(1) M 3Rh 4Sn(2) 12 compounds ( M = LaGd, Yb, Ca, Sr, and Tb) have been refined from single crystal X-ray diffraction data. Although the compounds with M = La, Ce, Pr, Nd, Sm and Gd have the phase I′ structure the refinements have been carried out by the use of the phase I unit cell. The only significant difference between the two sets of structures lies in the thermal ellipsoid of the Sn(2) atoms. With the exception of the Th compound the major axis of the Sn(2) thermal ellipsoid is on the average 0.06 Å longer for the structures of phase I′ than for those of phase I. The structure of the Th compound exhibits the longest major axis for the Sn(2) thermal ellipsoid, it must contain thus the phase I′ distortion. The absence of the superstructure spots can be explained by a structural disorder. This anomalously long axis has been interpreted as due to a static displacement of the Sn(2) atoms along the Sn(1)Sn(2) bonds. The distortion from phase I to phase I′ consists thus in a loss of point symmetry of the Sn(1)Sn(2) 12 polyhedra. A detailed analysis of the variation of the interatomic distances across the series shows that the chemical bonds in these compounds have a covalent/metallic-metallic character as the second-nearest-neighbor interactions are rather strong. However, an electron transfer takes place in these compounds indicating that the bonds have also an appreciable ionic character. The loss of symmetry which takes place when going from phase I to phase I′ is accompanied by a loss of ionic character of the Sn(1)Sn(2) 12 polyhedra. The Eu 2+ and Yb 2+ compounds contain some appreciable amount of Eu 3+ and Yb 3+ cations, respectively.