✍ Scribed by Yurii M. Prots'; Wolfgang Jeitschko
No coin nor oath required. For personal study only.
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. Nd 42 Ni 22؊x Si 31 crystallizes with a new structure type: P6 3 /m, a)4(2.9933؍ pm, c)8(88.714؍ pm, Z,2؍ R030.0؍ 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 Nd 42 Ni 21.75 Si 31 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 3 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. Nd 42 Ni 22؊x Si 31 contains building blocks of the CrB and Ce 5 Ni 2 Si 3 type structures. The structure of Nd 6 Ni 2؊x Si 3 (P6 3 /m, a)2(4.3911؍ pm, c)1(8.624؍ pm, Z,2؍ Pearson formula hP21.5, R820.0؍ for 671 F values and 25 variables) is similar to the previously reported structure of Ce 6 Ni 2 Si 3 . It is also closely related to the structure of Nd 42 Ni 22؊x Si 31 . A series of homologous rare earth (R)-transition metal (T)-main group element (or metametal, M) compounds is predicted with the general formula R 3(n 2 ؉5n؉4) T 3(n 2 ؉n؉2)؉2(1؊x) M 3(n 2 ؉3n؉2)؉2 , of which Nd 84 Ni 44؊2x Si 62 N N Nd 42 Ni 22؊x Si 31 is the member with n.3؍
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