W0281

The Use of Crenel Functions in the Description of Aperiodic Perovskite-like A_1+xBX₃ Oxides and Sulfides. Olivier Gourdon^a Michel Evain,^b and Vaclav Petricek^c, ^aDept. of Chemistry, Iowa State Univ., Ames, Iowa 50011-3111, ^bLaboratoire de Chimie des Solides, I.M.N., UMR C6502 CNRS - Univ. de Nantes, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France, ^cInstitute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8, Czech Republic.

In the A_1+xBX₃ (A = Sr, Ba, etc., B = Co, Ni, Ti etc. and X= O or S) hexagonal perovskite-like family can be described as intergrowth compounds with two chain subsystems: a [BX₃] chain subsystem with various successions of face sharing octahedra and trigonal prisms (for the oxides) and an [A] chain subsystem.

Through single crystal X-ray structure determinations using superspace group formalism, crenel functions, and occasionally non-harmonic development of the Debye-Waller factor, we show that all A_xMX₃ hexagonal perovskite oxides and sulfides can be uniquely described. Indeed, all phases have R-3m(00_)0s as a common superspace group. Moreover, the use of crenel functions is demonstrated to be a key factor for the resolution of these aperiodic structures. The models developed using those crenel functions revealed interesting unforeseen features for Ni^II and Co^II in the oxides that is their instability in the center of the trigonal prism, and showed the particularity of the sulfur structures, bringing to light new original [MS₆] units as called (PO) units for prism-octahedron, that is, units exactly intermediate between trigonal prisms and octahedra. In addition, the displacive modulations, almost exclusively along the chain axis in the oxides, have large out of the chain axis components in the sulfides. Both differences impose different crenel arrangements and lead to different potential twinnings.