The Neodymium Nickel Silicides Nd42Ni22−xSi31and Nd6Ni2−xSi3

Abstract

The title compounds were prepared by reaction of cold-pressed pellets of the elemental components in an arc-melting furnace followed by annealing at 900°C. Their crystal structures were determined from single-crystal X-ray data. Nd42Ni22−xSi31crystallizes with a new structure type:P63/m,a=3399.2(4) pm,c=417.88(8) pm,Z=2,R=0.030 for 2083 structure factors and 192 variable parameters. The neodymium atoms occupy 14 different sites with between 2 and 6 Ni, 3 and 6 Si, and 8 and 10 Nd neighbors with coordination numbers varying between 15 and 20. The nickel atoms occupy 9 sites: 7 of these have coordination number 9, with 6 Nd neighbors forming a trigonal prism and 3 Si neighbors capping the rectangular faces of that prism; 2 nickel sites on the hexagonal axis have the ideal occupancies of 25% assuming all sterically allowed sites to be occupied, resulting in the ideal composition Nd42Ni21.75Si31and the Pearson formulahP189.5. One of these nickel atoms has 9 Nd neighbors forming a tricapped trigonal prism with 2 additional Ni neighbors capping the trigonal faces of the prism, while the other one has octahedral Nd coordination with 2 additional Ni atoms on the 63axis. The 11 different silicon sites all have ninefold coordination: a trigonal prism of 6 Nd neighbors capped by 3 Ni, 2 Ni, and 1 Nd, or 1 Ni and 2 Nd atoms. Nd42Ni22−xSi31contains building blocks of the CrB and Ce5Ni2Si3type structures. The structure of Nd6Ni2−xSi3(P63/m,a=1193.4(2) pm,c=426.8(1) pm,Z=2, Pearson formulahP21.5,R=0.028 for 671Fvalues and 25 variables) is similar to the previously reported structure of Ce6Ni2Si3. 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 formulaR3(n2+5n+4)T3(n2+n+2)+2(1−x)M3(n2+3n+2)+2, of which Nd84Ni44−2xSi62⇒Nd42Ni22−xSi31is the member withn=3.