MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X ATOMS ARE
          CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES
 
.

Mo2Cl10

                  click here to see a key to this data bar                                  

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.
.      
 

Mo2Cl10.gif

 
          Figure 104 -  Mo2Cl10 lattice

click image to enlarge

                         structure
   
 
.      
Polyhedral framework
.      

linking surfaces

Mo2Cl10.gif

 

      Figure 105 -  Mo2Cl10 polyhedral
                   framework model
                  

1 edge

                          click image to enlarge
.      
     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

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