INCOMMENSURATE VERSUS COMMENSURATE DESCRIPTION OF THE A(X)BX(3) HEXAGONAL PEROVSKITE-TYPE STRUCTURE - SR1.2872NIO3 INCOMMENSURATE COMPOSITECOMPOUND EXAMPLE

We report a new structure determination of the Sr1.2872NiO3 incommensu rate composite hexagonal perovskite compound. Three different refineme nt strategies are presented: (i) a 3D supercell approximation with a n onharmonic development of the atomic displacement factor, (ii) an orig inal (3+1)D incommensurate composite description with the use of crene l functions, and (iii) a (3+1)D commensurate composite model. The thre e strategies are discussed and compared to previous refinements carrie d out for the hexagonal perovskites in a classical way or with the sup erspace group formalism. Out of the three methods, the incommensurate composite approach gives slightly better results with a final global R value of 2.89% for 635 independent reflections (at a I/sigma(I) > 2 l evel) and only 60 parameters (R (3) over bar m(00 gamma)Os superspace group;a = 9.5177(7) Angstrom, c = 2.5739(2) Angstrom, q = 0.64359(4)c , V = 201.93(4) Angstrom(3), and Z = 3). The use of crenel functions n otably improves previously reported results. New structural features a re evidenced for Sr1.2872NiO3: (i) an incommensurate character, (ii) a splitting of the trigonal prismatic nickel atoms over 5 positions, (i ii) a definite stoichiometry which induces a perfect charge balance, a nd (iv) a nonharmonic behavior of some Sr atoms. Those characteristics seem general to most hexagonal perovskite compounds and essential for correctly interpreting their interesting magnetic properties. Finally , a new generic formulation is proposed, which explains the various st oichiometries observed and suggests a new compound possibility.