CHE-30043
Materials Chemistry & Catalysis :
Solid State Chemistry lecture 4
Rob Jackson
LJ1.16, 01782 733042
r.a.jackson@keele.ac.uk
www.facebook.com/robjteaching
@robajackson
Plan of lecture
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Defects in crystalline materials
Classification of point defects
Intrinsic and extrinsic defects
Ionic conductivity in crystalline materials
Solid state electrolytes
Fast ion conductors
Lithium ion batteries
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Defects in crystalline materials
• Crystalline materials are not perfect above
0K, since at finite temperatures atoms can
move from their lattice positions.
• Also, impurities are nearly always introduced
during crystal growth (intentionally or not!)
• We will be concentrating on point defects,
which affect isolated atom sites, and have a
significant effect on the chemistry of the
crystal.
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Classification of point defects
• First, distinguish between intrinsic and
extrinsic defects.
• Intrinsic defects do not affect the chemical
composition of the crystal, and include:
vacancies – atoms missing from the lattice
interstitials – atoms at non-lattice positions
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Classification of intrinsic defects
• Frenkel defects occur when an atom moves
from a lattice site to an interstitial site.
(this will be illustrated)
• Schottky defects occur when a formula unit
of vacancies is created. Note that this is still
a neutral defect since both cation and anion
vacancies are created (e.g. Na+ and Clvacancies in NaCl).
(this will be illustrated)
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Which intrinsic defects form? - 1
• Frenkel defects occur on one or other
sublattice in the crystal (i.e. in MX, either the
M+ or X- ions are involved).
• A Frenkel defect is called a ‘Frenkel pair’ – it
is the combination of a vacancy and an
interstitial.
• Frenkel pair formation depends on the
availability of space in the lattice for
interstitial formation.
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Which intrinsic defects form? - 2
• Schottky defects will normally form when
there is insufficient space in the lattice for
interstitials.
e.g. the rock salt structure – very little space, so
Schottky defects predominate.
• However, in the fluorite structure there are
alternate unoccupied cube centres and
Frenkel defects can form.
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Extrinsic defects
• Impurity atoms present in crystals are called
extrinsic defects.
• They may be there deliberately, or as a
consequence of the preparation process.
– Examples are K+ ions in NaCl
• Charge must always be balanced – e.g. if
Ca2+ substitutes for Na+ in NaCl, a Na+
vacancy or Cl- interstitial must be formed.
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Ionic Conduction Mechanisms
• The presence of point defects makes it
possible for ions to move through a
structure.
• There are two common mechanisms that
are adopted, the vacancy mechanism and
the interstitial mechanism.
• These mechanisms will be illustrated – but
see the diagrams in Smart and Moore for
an alternative view!
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How is ionic conductivity defined?
• Ionic conductivity () is defined as follows:
 = n Ze 
where n is the number of charge carriers
(defects per unit volume), Ze is their charge
(e is the electron charge), and  is the
mobility of the ions – a measure of the speed
with which they move through the lattice.
• So  depends on (i) the number of available
defects (ii) the charge of the defect species,
and (iii) how easily the ion can move through
the lattice.
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Some conductivity values
conduction type
material
conductivity
(ohm -1 m -1)
ionic conductors
ionic crystals
solid electrolytes
liquid electrolytes
electronic conductors
metals
semiconductors
insulators
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10-16 – 10-3
10-1 – 103
10-1 – 103
103 – 107
10-3 – 104
< 10-10
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Solid Electrolytes
• Solid electrolytes are ionic crystals with a
sufficient volume of defects to be able to act
like a liquid electrolyte.
• Example – LiI (rock salt structure, Schottky
defects) has sufficient vacancies in its crystal
structure to enable Li+ ions to move freely.
• It was one of the first solid electrolytes to be
used in the design of lithium batteries.
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Lithium batteries: how they work - 1
• Lithium batteries are made up of:
– a lithium anode, ‘iodine’ cathode (iodine
embedded in a polymer, poly-2-vinyl-pyridine).
– a lithium iodide electrolyte.
• Li+ ions can pass through the electrolyte by
the vacancy mechanism, and the valence
electrons go round the external circuit.
• A diagram will be drawn in the lecture.
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Lithium batteries: how they work - 2
• The following cell reaction occurs:
anode:
2Li(s)  2Li+ + 2ecathode: I2(s) + 2e-  2Ioverall:
2Li(s) + I2(s)  2LiI(s)
• Note – the same principal applies to different
Li salts (e.g. Li2CO3).
• Lithium batteries have been replaced by
lithium ion batteries, considered later.
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Fast ion conductors
• Fast ion conductors are particular examples
of solid electrolytes.
• We will consider some examples and show
how they work.
• There are two mechanisms:
(i) structures which have unoccupied lattice
sites without defects being present.
(ii) structures whose conductivity occurs
because of defects.
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Example 1 - -AgI - 1
• The structure of -AgI is such that there are
more possible sites for the Ag+ ions than
needed (see Smart and Moore for
diagrams). Structure is BCC with I- anions
at the BCC sites.
• Conductivity occurs because the Ag+ ions
can easily move through the lattice via
unoccupied lattice sites.
• Conductivity is 131 ohm-1 m-1 (high).
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Example 1 - -AgI - 2
• We can relate the high conductivity to
properties of the structure:
– Low charge on the Ag+ ions
– Low coordination of the Ag+ ions
– Many vacant Ag+ ion sites
• However the -phase of AgI only exists
above 146C, limiting its use.
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More examples of fast ion conductors
• -AgI can be modified to lower the
temperature at which fast ion conduction
occurs.
– Partial replacement of Ag by Rb results in
RbAg4I5, which has an ionic conductivity of 25
ohm-1 m-1 at room temperature.
• Materials with the fluorite structure should
have good potential as fast ion conductors,
since there are many interstitial sites.
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Zirconia and stabilised zirconia
• Zirconia, ZrO2 exists naturally as the mineral
baddeleyite, with a monoclinic structure.
• On heating, the structure transforms first to a
tetragonal phase, and then, at 1600 C, to a
cubic fluorite phase, where it would be
expected to have good ionic conduction
properties.
• It is possible to stabilise the structure in the
cubic phase at room temperature by adding,
e.g. 15 mol % CaO.
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Stabilised zirconia
• Addition of CaO (or other oxides, including
Y2O3), produces a cubic fluorite phase.
• The structure is a good ion conductor
because of the interstitial sites, but addition
of the stabiliser ions improves this further.
• If Ca2+ is substituted for Zr4+, the charge has
to be compensated.
• This can be achieved by creating O2vacancies – so oxygen ion conduction can
occur. Stabilised zirconia is widely used in
fuel cells.
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Oxygen ion conduction in CaO
stabilised zirconia - summary
• The CaO can be added up to 28 mol%.
• Addition of the CaO stabilises the structure in
the cubic phase at room temperature.
• The interstitial sites mean oxygen ion
conduction will be possible via the interstitial
mechanism.
• Further oxygen vacancy sites are created to
compensate for the charge imbalance caused
by Ca2+ substituting for Zr4+. This enhances
conduction further.
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Lithium ion batteries
• Lithium batteries were not always ideal to
work with, mainly because of the reactivity of
the lithium metal!
• Lithium ion batteries were developed in the
1990s, mainly by Sony, to provide reliable
rechargeable batteries. They are now used
routinely in laptop computers, mobile phones,
mp3 players etc.
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How lithium ion batteries work – (i)
• Their design is based on intercalation
compounds (compounds formed by the
reversible addition of ‘guest’ ions to a
host lattice).
• The electrolyte is a conducting
polymer such as polyacetylene:
n (H-CC-H)
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How lithium ion batteries work – (ii)
• The anode is composed of Li
embedded in graphitic carbon, forming
LixC6.
• The cathode is made from Li combined
in an intercalation compound with a
transition metal oxide like CoO2,
forming LixCoO2.
• The electrolyte is a conducting polymer.
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How lithium ion batteries work – (iii)
• The Li+ ions move between
intercalation compounds.
the
two
– At the anode, LixC6 = xLi+ + 6C + xe– At the cathode:
xLi+ + CoO2(S) + xe- = LixCoO2
• The Li+ ions move through the electrolyte
while the electrons go around the external
circuit.
• The process is reversed on charging.
• See diagram on the next slide.
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Lithium ion battery diagram
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Improvements to the lithium ion
battery
• There is much current research on
improving the performance of lithium
batteries.
• These have focussed on using
nanostructured
materials
for
the
cathode and anode.
– The rationale is that the ‘hopping distance’
for the Li+ ions is reduced.
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Improvements to anode materials
Journal of New Materials for Electrochemical Systems, 15(4) 2012, 233-236
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Nanoporous cathode materials
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Summary of lecture
• Defects in solids have been introduced
and classified
• Ion migration mechanism have been
discussed.
• Applications to fast ion conductors and
battery materials have been described.
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