Ionic Conductivity In A Thermochromic Solid Objectives: --to synthesize the ionic conductor Cu2HgI4 --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

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

ZnS Cu2HgI4 Voids and conduction 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. Cu2HgI4 Single (left) and doubled (right) unit cell of ZnS (zinc blende structure) Voids and conduction

Ionic Conduction Two cations of the same size, different charge: which held less tightly by lattice? Smaller charge Two cations of same charge, different sizes: - which moves more easily through lattice? Smaller size

Resistance: Dimension-Dependant h w l Experimental Apparatus: cylinder of Cu2HgI4

Resistivity R = Resistance (experimentally determined), units =   = Dimensionless Resistivity (calculate) units = W cm measure length in cm of sample calculate cross-sectional area of sample Area of a circle = r2 where r = radius of *sample*

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

Experimental --Hg Compounds Safety: Gloves at all times Do not create or breathe dust *Keep benches, balances, clean*! Clean up spills at once. Dispose of gloves in solid waste container.

Summary --Synthesis of an infinite, non-molecular solid in two different phases --Differences in physical properties with phase change: 1) color 2) ionic conductance