THE CRYSTAL-STRUCTURES OF ZR3AL3C5, SCAL3C3, AND UAL3C3 AND THEIR RELATION TO THE STRUCTURES OF U2AL3C4 AND AL4C3

Single crystals of Zr3Al3C5-a carbide previously reported with the for mula ZrAlC2-x-were isolated from a sample prepared by reaction of ZrC with an excess of aluminum. The carbides ScAl3C3 and UAl3C3 were synth esized from the elemental components by are-melting. The crystal struc tures of these three compounds were redetermined from four-circle X-ra y diffractomter data. In the original structure determination of ZrAlC 2-x, the metal positions were found to form close-packed layers in the space group P6(3)/mmc, while the carbon atoms were assumed to occupy 5/6 of the octahedral voids at random. The present structure determina tion in the space group P6(3)/mc (R = 0.024 for 519 structure factors and 23 variable parameters) shows that all carbon positions are fully occupied and one has a trigonal bipyramidal aluminum coordination. The structures of ScAl3C3 and UAl3C3 also have originally been determined in the space group P6(3)/mmc. The present structure refinements in th e space group P6(3)mc (ScAl3C3: R = 0.031 for 282 F values and 16 vari ables; UAl3C3: R = 0.029 for 217 F values and 16 variables) essentiall y confirms the structures with the exception of one aluminum site. In all of these structures the metal atoms are arranged in close-packed l ayers and together with the previously reported structure of U2Al3C4 t hey form a homologous series with the general formula T1+nAl3C3+n wher e n = 0, 1, 2 for ScAl3C3, U2Al3C4, and Zr3Al3C5, respectively. The pa cking of the metal atoms is represented by the Zhdanov symbols (4)(2), (5)(2), and (6)(2). The arrangement of the aluminum atoms is very sim ilar to that of the binary carbide Al4C3, while the other metal atoms form a cubic stacking sequence, as it is found in the binary carbides TC with NaCl type structure. (C) 1998 Academic Press.