THE CRYSTAL-STRUCTURE OF 2 NOVEL COMPOUNDS - CEALSI2 AND CE3AL4SI6

In a reinvestigation of the phase relations within the section CeSi2-A l at 600 and 500 degrees C, two new compounds were found: CeAlSi2 and Ce3Al4Si6. The crystal structures were solved from X-ray powder diffra ction data employing Patterson and difference Fourier syntheses follow ed by full-matrix full-profile Rietveld refinement. Both compounds cry stallize with new trigonal structure types as follows. CeAlSi2: <P(3)o ver bar>m1, Z = 2, a = 0.41707(1) nm, c = 1.12147(2) nm, R(I) = 0.077, R(F) = 0.050. Ce occupies the 29 site with z = 0.1855(1), Si is in th e 2d sites with z(1) = 0 and z(2) = 0.3525(5), and Al is in the 2d sit e with z Ce3Al4Si6: <P(3)over bar>m1, Z = 1, a = 0.41887(1) nm, c = 1. 81202(6) nm, R(I) = 0.065, R(F) = 0.035. Ce occupies the 1b site and t he 2c site with z = 0.1149(1); Si is in the 2d sites with z(1) = 0, z( 2) = 0.2198(9), and z(3) = 0.6096(6); Al is in the 2d sites with z(1) = 0.3564(8) and z(2) = 0.752(1). Both structures combine structural un its of the AlB2 type, CeSi2 (m), and of the La2O3 type, CeAl2Si2 (n), and form a structural series RE(m+n)Al(2n)Si(2m+2n) with CeAlSi2 (m = 1, n = 1) and Ce3Al3Si6 (m = 1, n = 2) as two early representatives of the series. (C) 1996 Academic Press.