Thermochromism in an Ionic Conductor

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Thermochromism in an Ionic Conductor
Objectives --to synthesize the ionic conductor
Cu2HgI4 --to observe thermochromism and ionic
conductivity in different phases of a
non-molecular inorganic solid
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Close-Packed Ions
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Sites (Voids,Vacancies, Holes) in Close-Packed
Structures
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Face-Centered Cubic Unit Cell
--smallest repeating volume element --one ion on
every face of the cube
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ZnS
Single (left) and doubled (right) unit cell of
ZnS (zinc blende structure)
Voids and conduction
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Ionic conduction

• For two cations of the same size but different
  charge, which will be held less tightly by the
  lattice?
• For two cations of the same charge but different
  sizes, which will move more easily through the
  lattice?

Smaller charge
Smaller size
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Resistance Dimension-Dependant
h
w
l
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Resistivity
R Resistance (experimentally determined), units
?
? Dimensionless Resistivity (calculate)
units W cm

• measure length in cm of sample
• calculate area of sample
• Surface area of a cylinder 2?r(r h) where
  h height (length)
• of sample
• r radius of
  sample

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Conductivity
? Conductivity (calculate)
units S cm-1 Siemens 1/W
--Show that ionic conductivity varies for same
compound in different phase --Determine how
changing phase changes conductivity

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Experimental
--Hg Compounds Safety Gloves at all times Do
not create or breathe dust Keep benches,
balances, clean! Clean up spills at
once. Dispose in proper waste container.
--Apparatus for resistance measurements