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Thermochromism in an Ionic Conductor

Thermochromism in an Ionic Conductor. Objectives: --to synthesize the ionic conductor Cu 2 HgI 4 --to observe thermochromism and ionic conductivity in different phases of a non-molecular inorganic solid. Close-Packed Ions. Sites (Voids,Vacancies, Holes) in Close-Packed Structures.

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Thermochromism in an Ionic Conductor

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  1. 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

  2. Close-Packed Ions

  3. Sites (Voids,Vacancies, Holes) in Close-Packed Structures

  4. Face-Centered Cubic Unit Cell --smallest repeating volume element --one ion on every face of the cube

  5. Cu2HgI4 Disordered high-T (right) and ordered low-T (left) unit cell of Cu2HgI4. In the disordered phase, there are on average two Cu+ ions and one Hg2+ ion for every four Iˉ anions. However, the positions of these cations are not fixed – in any given unit cell, it is expected that these three cations will occupy a random subset of the four potential tetrahedral Cu/Hg sites. ZnS Single (left) and doubled (right) unit cell of ZnS (zinc blende structure) Voids and conduction

  6. 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

  7. Resistance: Dimension-Dependant h w l

  8. 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*

  9. 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

  10. Experimental --Hg CompoundsSafety: 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:

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