Gold(I) bromide

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Gold(I) bromide
Crystal structure of AuBr-P
Identifiers
CAS number 10294-27-6 YesY
Properties
Molecular formula AuBr
Molar mass 276.87 g/mol
Appearance lemon yellow crystals (AuBr-I), yellow-brown (AuBr-P)
Density 7.9 g/cm3
Melting point 165 °C (329 °F; 438 K) (decomposes)
Hazards
NFPA 704
Flammability (red): no hazard code Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity (yellow): no hazard code Special hazards (white): no codeNFPA 704 four-colored diamond
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Gold(I) bromide can be formed by synthesis from the elements or partial decomposition of gold(III) bromide by careful control of temperatures and pressures. It occurs in two modifications. One (I-AuBr) is isostructural with gold(I) chloride and has a body centered tetragonal unit cell with a=6.734A and c=8.674A and space group I41/amd. The other is isostructural with gold(I) iodide and has a primitive tetragonal cell a=4.296A and c=12.146A and space group P42/ncm. Single crystals of both modifications have been grown by chemical vapor transport.[1] Small amounts of Al,Ga or Fe were used as catalysts for the transport process to obtain the I-AuBr modification.

The two structures both consist of -Br-Au-Br-Au-Br- polymeric zigzag chains, but they are stacked in a different arrangement. In the primitive tetragonal phase the chains form layers (see figure) in contrast to the body centered one, where they are more interwoven. Another difference is that the Au-Br-Au angle is only 77° in the former, but 92.3° in the latter.

Density functional calculations on the monohalides of group 11 (Cu, Ag, Au)[2] have tried to shed light on the question why gold halides form rather different, low symmetry structures rather than the cubic zinc blend or rock salt structures of the silver and copper halides. It was shown that this calculation technique accurately predicts which structure type should be stable. The peculiar structures of the gold halides are mostly a result of the relativistic effects that occur for the elements of the later periods of the periodic table.

References[edit]

  1. ^ E.M.W. Janssen and G.A. Wiegers (1978). "Crystal growth and the crystal structures of two modifications of gold monobromide, I-AuBr and P-AuBr". Journal of the Less-Common Metals 57 (2): P47–P57. doi:10.1016/0022-5088(78)90248-5. 
  2. ^ Tilo Söhnel, Holger Hermann and Peter Schwerdtfeger (2005). "Solid state density functional calculations for the group 11 monohalides". J. Phys. Chem B 109 (1): 526–531. doi:10.1021/jp046085y. PMID 16851044. 

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