THE NEODYMIUM NICKEL SILICIDES ND42NI22-XSI31 AND ND6NI2-XSI3

The title compounds were prepared by reaction of cold-pressed pellets of the elemental components in an are-melting furnace followed by anne aling at 900 degrees C, Their crystal structures were determined from single-crystal X-ray data. Nd42Ni22-xSi31 crystallizes with a new stru cture type: P6(3)/m, a = 3399.2(4)pm, 417.88(8) pm, Z = 2, R = 0.030 f or 2083 structure factors and 192 variable parameters, The neodymium a toms occupy 14 different sites with between 2 and 6 Ni, 3 and 6 Si, an d 8 and 10 Nd neighbors with coordination numbers varying between 15 a nd 20. The nickel atoms occupy 9 sites: 7 of these have coordination n umber 9, with 6 Nd neighbors forming a trigonal prism and 3 Si neighbo rs capping the rectangular faces of that prism; 2 nickel sites on the hexagonal axis have the ideal occupancies of 25% assuming all sterical ly allowed sites to be occupied, resulting in the ideal composition Nd 42Ni21.75Si31 and the Pearson formula hP189.5. One of these nickel ato ms has 9 Nd neighbors forming a tricapped trigonal prism with 2 additi onal Ni neighbors capping the trigonal faces of the prism, while the o ther one has octahedral Nd coordination with 2 additional Ni atoms on the 6, axis. The 11 different silicon sites all have ninefold coordina tion: a trigonal prism of 6 Nd neighbors capped by 3 Ni, 2 Ni, and 1 N d, or 1 Ni and 2 Nd atoms. Nd42Ni22-xSi31 contains building blocks of the CrB and Ce5Ni2Si3 type structures. The structure of Nd6Ni2-xSi3 (P 6(3)/m, a = 1193.4(2) pm, c = 426.8(1) pm, Z = 2, Pearson formula hP21 .5, R = 0.028 for 671 F values and 25 variables) is similar to the pre viously reported structure of Ce,Ni,Si,. It is also closely related to the structure of Nd42Ni22-xSi31. A series of homologous rare earth (R )-transition metal (T)-main group element (or metametal, M) compounds is predicted with the general formula R3(n2+5n+4) T3(n2+n+2+2(1-x))M3( n2+3n+2+2), of which Nd84Ni44-2xSi62 double right arrow Nd42Ni22-xSi31 is the member with n = 3. (C) 1998 Academic Press.