Artigo Revisado por pares

The most populous of all crystal structure types—the tetragonal BaAl4 structure

1985; Elsevier BV; Volume: 56; Issue: 3 Linguagem: Inglês

10.1016/0022-4596(85)90177-x

ISSN

1095-726X

Autores

W. B. Pearson,

Tópico(s)

Inorganic Chemistry and Materials

Resumo

The BaAl4 (ThCr2Si2) tI10 structure, MN2X2, is not only the most populous of all known structure types, being adopted by some 400 phases, but is representative of a new group of metallurgically important structures that do not adhere to classifications of Cu-type close packings or tetrahedral close packings. The structure can be described by a square pyramidal framework of N and X atoms which by edge-sharing from slabs in (001) planes, the slabs being interconnected by XX bonds along [001] (in perhaps 10% of the phases no [001] XX bonds exist). This framework creates tetragonal hexagonal prismatic voids, that enclose the large M atoms. The average observed NX and [001] XX distances are interpreted in a model that shows that the framework, which is responsible for structural stability, accounts for almost all of the valency electrons provided by N and X. The M atoms (diameters 3.1 to 5.1 Å) are accommodated in the voids of the framework according to the principles of geometrical packing, apparently without size restriction. In some cases MN or MX interactions are shown to control the variable a, c, and zX structural parameters, but they only very modestly influence the NX distances of the framework. Some comparison with the Cu2Sb tP6 structure is made.

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