Polymorf - Knowhwere- Mo2Cl10

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

CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES

Mo₂Cl₁₀

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octa	20%	fcc	molecular	1 edge	A- 6c, X- 2c/fcc

structure

Mo₂Cl₁₀ is a molecular crystal in which the molecules pack together so that their

X atoms are FCC. Forty percent of the octahedral voids of every other layer are

occupied with A atoms for an overall occupancy rate of 20%. Two AX₆ groups are

linked by one edge to form the dimer molecule.

Ex. Mo₂Cl₁₀ ( Mo - A , Cl - X )

Crystal lattice

The lattice model shows the dispersion of the dimer molecules in the crystal. The

staggered arrangement of the molecules in every other layer results in every

molecule being surrounded by octahedral voids that are devoid of cations. This

explains why the Mo₂Cl₁₀ crystal vaporizes to discrete dimer molecules when heated

to the sublimation point since there are no intervening cations to bind the molecules

together. The model also shows that despite the dispersed distribution of the

molecules throughout the crystal they provide all of the close packed X atoms

necessary to build the structure including the unoccupied layers.

Polyhedral framework

The model shows the dispersion of the discrete molecules in an occupied layer.

Blue triangles are used to position the molecules correctly.

Isostructural minerals/compounds

Nb₂Cl₁₀ is a molecular crystal with an identical occupied layer structure. However

the alternating occupied layers stack together in the HCP sequence. It vaporizes to

discrete NbCl₅ molecules.

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Page 67 - Structure matters - Mo₂Cl₁₀