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