Ionic Conductivity In A Thermochromic Solid Objectives: --to synthesize the ionic conductor Cu 2 HgI...
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Transcript of Ionic Conductivity In A Thermochromic Solid Objectives: --to synthesize the ionic conductor Cu 2 HgI...
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
Zn+2
S-2
a
b
c
Zn+2
S-2
Zn+2
S-2
Single (left) and doubled (right) unit cell of ZnS (zinc blende structure)
CuHg
I
Hg
Cu
I
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
Hg
Cu
I
Voids and conduction
Ionic Conduction• Two cations of the same size, different charge:
– which held less tightly by lattice?
Smaller charge
Smaller size
•Two cations of same charge, different sizes:
- which moves more easily through lattice?
Resistance: Dimension-Dependant
lw
h
Experimental Apparatus: cylinder of Cu2HgI4
length
areaR×=ρ
Resistivity
ρ = Dimensionless Resistivity (calculate)
R = Resistance (experimentally determined), units =
units = cm
1) measure length in cm of sample2) calculate cross-sectional area of sample
Area of a circle = r2 where r = radius of *sample*
ρσ 1=
Conductivity
σ = Conductivity (calculate)
units = S cm-1
Siemens = 1/
--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