The neodymium nickel silicides Nd42Ni22-xSi31 and Nd6Ni2-xSi3

摘要

The title compounds were prepared by reaction of cold-pressed pellets of the elemental components in an are-melting furnace followed by annealing at 900 degrees C, Their crystal structures were determined from single-crystal X-ray data. Nd42Ni22-xSi31 crystallizes with a new structure type: P6(3)/m, a = 3399.2(4)pm, 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 Nd42Ni21.75Si31 and the Pearson formula hP189.5. 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 6, axis. 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-xSi31 contains building blocks of the CrB and Ce5Ni2Si3 type structures. The structure of Nd6Ni2-xSi3 (P6(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 previously 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. [References: 44]