Acta Crystallographica Section B

Volume 65, Part 5 (October 2009)

research papers

Acta Cryst. (2009). B65, 593-599 [ doi:10.1107/S0108768109029760 ]

Superspace description of the modulated structure of the metal-salt-hybrid Bi_7 -Ni₂Br_5 - 2 ( = 1/9)

B. Wahl and M. Ruck

Abstract: The compound Bi_7 -Ni₂Br_5 - 2 = (Bi₃Ni)₂[Bi_1 -Br₄]Br_1 -2 ( [delta] = 1/9) is a sub-bromide of the intermetallic phase Bi₃Ni. Its crystal structure contains metallic rods, $^1_{\infty}$ [Bi₃Ni], which are embedded in a salt-like matrix of bromido-bismuthate(III) and bromide anions. The non-stoichiometry originates from the variation of the number n of trans edge-sharing octahedra in the [Bi_nBr_4n + 2]^(n + 2)- oligomers (3 [less-than or equal to] n 5), as well as from vacancies on the sites of the isolated Br atoms. The simplified structure is described in the orthorhombic space group Cmcm with a = 4.0660 (4), b = 23.305 (3), c = 17.130 (2) Å. It shows a statistical distribution of vacancies and orientational disorder of the concatenated octahedra. By choosing the modulation vector q = a*/9 + b*/2, the additional weak reflections of the diffraction pattern can be indexed. In the [3 + 1]-dimensional superspace group Pmnm( [alpha] ½0)000, an ordered structure model is achieved. The modulated crystal structure bears a strong resemblance to the somewhat higher oxidized sub-bromide Bi_7 -Ni₂Br₅ ( [delta] = 1/9).

Keywords: modulated structure; orientational disorder; one-dimensional matrix.


	Structure factor file (CIF format) (76.9 kbytes) [ doi:10.1107/S0108768109029760/sn5087Bi7Ni2Br5averagesup2.hkl ] Contains datablock Bi7Ni2Br5average

	Structure factor file (CIF format) (369.4 kbytes) [ doi:10.1107/S0108768109029760/sn5087Bi7Ni2Br5modsup3.hkl ] Contains datablock Bi7Ni2Br5mod

	Structure factor file (CIF format) (38.7 kbytes) [ doi:10.1107/S0108768109029760/sn5087Bi7Ni2Br5simplifiedsup4.hkl ] Contains datablock Bi7Ni2Br5simplified

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Acta Crystallographica Section B

Volume 65, Part 5 (October 2009)

research papers

Superspace description of the modulated structure of the metal-salt-hybrid Bi7 - Ni2Br5 - 2 ( = 1/9)

B. Wahl and M. Ruck

Superspace description of the modulated structure of the metal-salt-hybrid Bi_7 -Ni₂Br_5 - 2 ( = 1/9)