Polymorf - Knowhwere- Al2Br6

MINERALS AND SYNTHETIC COMPOUNDS, A_mX_n, IN WHICH X ATOMS ARE

CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES

Al₂Br₆

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tetra	1/6	hcp	molecular	1 edge	A- 4c, X- fcc

structure

The Al₂Br₆ structure is a layered molecular crystal comprised of A₂X₆ dimer

molecules. The X atoms pack together HCP. Only 1/3 of the tetrahedral voids of

every other layer are occupied by A atoms making it a very dispersed composition.

Two AX₄ groups are joined at one edge to form each molecule.

Ex. Al₂Br₆ ( Al - A , Br - X )

Crystal lattice

Coloring the AX₄ groups in each occupied layer of the HCP lattice clearly

reveals the dimer molecules and the 1/3 occupancy rate. A careful examination of

the lattice structure reveals that, despite the dispersed arrangement of the AX₄

groups, they pack together so as to provide all of the necessary close packed

bromine atoms necessary to build the structure, including the unoccupied layers.

This demonstrates how such molecular crystals achieve the structural stability

necessary to fill space.

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		Figure 69 - Al₂Br₆ lattice structure
click image to enlarge


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Polyhedral framework

The polyhedral framework model demonstrates intuitively why Al₂Br₆ vaporizes to

discrete dimer molecules when heated to its sublimation point.

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Page 47 - Structure matters - Al₂Br₆