TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Ion-exchange, injection, intercalation type synthesis
• Ways of modifying existing solid state structures while
maintaining the integrity of the overall structure
• Precursor structure
• Open structure or porous framework
• Ready diffusion of guest atoms, ions, organic molecules,
polymers, organometallics, coordination compounds,
nanoclusters, bio(macro)molecules into and out of the
structure/crystals
TOPOTAXY: HOST-GUEST INCLUSION
1D- Tunnel Structures
-TiO2
-hWO3
-TiS3
Pivotal topotactic materials
properties for functional utility in
Li solid state battery electodes,
electrochromic mirrors and
windows, fuel and solar cell
electrolytes and electrodes,
chemical sensors, superconductors
2D- Layered Structures
3D-Frameworks
-zeolites
-LiMn2O4
-cWO3
-Graphite
-TiS2
-TiO2(B)
-KxMnO2
-FeOCl
-HxMoO3
-b alumina
-LixCoO2
TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Penetration into interlamellar spaces: 2-D intercalation
• Into 1-D channel voids: 1-D injection
• Into cavity spaces: 3-D injection
• Classic materials for this kind of topotactic chemistry
• Zeolites, TiO2, WO3: channels, cavities
• Graphite, TiS2, NbSe2, MoO3: interlayer spaces
• Beta alumina: interlayer spaces, conduction planes
• Polyacetylene, NbSe3: inter chain channel spaces
TOPOTACTIC SOLID-STATE SYNTHESIS
METHODS: HOST-GUEST INCLUSION CHEMISTRY
• Ion exchange, ion-electron injection, atom, molecule
intercalation and occlusion, achievable by non-aqueous,
aqueous, gas phase, melt techniques
• Chemical, electrochemical synthesis methods
• This type of topotactic solid state chemistry creates new
materials with novel properties, useful functions and
wide ranging applications and myriad technologies
GRAPHITE
out of plane pp orbitals - p/p* delocalized bands
A
B
sp2 in plane s bonding
VDW gap 3.35Å
C-C 1.41Å, BO 1.33
A
ABAB stacked
hexagonal graphite
Pristine graphite - filled p band - empty
p* band - narrow gap - semimetal
GRAPHITE INTERCALATION COMPOUNDS
4x1/4 K = 1
8x1 C = 8
C8K stoichiometry
G (s) + K (melt or vapor)  C8K (bronze)
C8K (vacuum, heat)  C24K  C36K  C48K  C60K
Staging, distinct phases, ordered guests, K  G CT
AAAA sheet stacking sequence
K nesting between parallel eclipsed hexagons,
Typical of many graphite H-G inclusion compounds
GRAPHITE INTERCALATION
ELECTRON DONORS AND ACCEPTORS
SALCAOs of the p-pi-type create the p valence and p*
conduction bands of graphite, very small band gap, essentially
metallic conductivity, single crystal properties in-plane 104
times that of out-of plane conductivity - thermal, electrical
properties tuned by degree of CB band filling or VB emptying
E
C
C8K electron transfer to
C2pp CB – metallic
reductive intercalation
p*
CB
p*
Eg
p
s
VB
p
s
E(F)
C8Br electron depletion
from C2pp VB – metallic
oxidative intercalation
p*
p
E(F)
s
N(E)
INTERCALATION REACTIONS OF GRAPHITE
Oxidative, Reductive or Charge Neutral?
• G (HF/F2/25oC)  C3.3F to C40F (white)
• intercalation via HF2- not F- - relative size, charge, ion, dipole,
polarizability effects - less strongly interacting - more facile diffusion
• G (HF/F2/450oC)  CF0.68 to CF (white)
• G (H2SO4 conc.)  C24(HSO4).2H2SO4 + H2
• G (FeCl3 vapor)  CnFeCl3
• G (Br2 vapor)  C8Br
PROPERTIES OF INTERCALATED GRAPHITE
• Structural planarity of layers often unaffected by
intercalation - bending of layers has been observed intercalation often reversible
• Modification of thermal and electrical conductivity behavior
by tuning degree of p*-CB filling or p-VB emptying
• Anisotropic properties of graphite intercalation systems
usually observed
• Layer spacing varies with nature of the guest and loading
• CF: 6.6 Å, C4F: 5.5 Å, C8F: 5.4 Å
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
Composite CFx cathode
with C black particles to
enhance electrical
conductivity and
poly(vinylidenedifluoride)
PVDF binder to provide
mechanical stability
e
FLiF
Li+
Al contact
SS contact
Li anode
CFx/C/PVDF
cathode
Li+/PEO
BUTTON CELLS
LITHIUM-GRAPHITE FLUORIDE BATTERY
• Cell electrochemistry
• xLi + CFx  xLiF + C
• xLi  xLi+ + e• Cx+xF- + xLi+ + xe-  C + xLiF
Nominal cell voltage 2.7 V
• CFx safe storage of fluorine, intercalation of graphite by fluorine
•
Millions of batteries sold yearly, first commercial Li battery, Panasonic
•
Lightweight high energy density battery - cell requires stainless steel
electrode/lithium metal anode/Li+-PEO fast ion conductor/CFx intercalate acetylene black electrical conductor – poly(vinylidenedifluoride) mechanical
support cathode/aluminum charge collector electrode
C60-G INTERCALATING BUCKBALL INTO GRAPHITE
NEW HYDROGEN STORAGE MATERIAL
• Thermally induced 600oC
intercalation of C60 into G
• Hexagonal close packed C60
between graphene sheets
• Sieves H2 from larger N2
• Physisorbed H2 in intralayer
void spaces
• Rapid adsorption-desorption
• Dead capacity because of
volume occupied by C60
• Capacity possibly enhanced by
reducing filling fraction of C60
SYNTHESIS OF BORON AND NITROGEN
GRAPHITES - INTRALAYER DOPING
• New ways of modifying the properties of graphite
• Instead of tuning the degree of CB/VB filling with
electrons and holes using the traditional methods focus
on interlayer doping
• Put B or N into the graphite layers, deficient and rich
in carriers, enables intralayer doping with holes (VB)
and electrons (CB) respectively
• Also provides a new intercalation chemistry
SYNTHESIS OF AND BC3
THEN PROVING IT IS SINGLE PHASE?
• Traditional heat and beat
• xB + yC (2350oC)  BCx
• Maximum 2.35 at% B incorporation in C
• Poor quality not well-defined materials
• New approach, soft chemistry, low T, flow reaction, quartz tube
• 2BCl3 + C6H6 (800oC)  2BC3 (lustrous film on walls) + 6HCl
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• BC3 + 15/2F2  BF3 + 3CF4
• Fluorine burn technique
• BF3 : CF4 = 1 : 3
• Shows BC3 composition – no evidence of precursors or
intermediates
• Electron and Powder X-Ray Diffraction Analysis
• Shows graphite like interlayer reflections (00l)
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• 2BC3 (polycryst) + 3Cl2 (300oC)  6C (amorph) + 2BCl3
• C (cryst graphite) + Cl2 (300oC)  C (cryst graphite)
• This neat experiment proves B is truly a "chemical"
constituent of the graphite sheet and not an amorphous
component of a "physical" mixture with graphite
• Synthesis, analysis, structural findings all indicate a
graphite like structure for BC3 with an ordered B, C
arrangement in the layers
STRUCTURE OF BORON GRAPHITE BC3
Rietfeld PXRD Structure Refinement
4Cx1/4 + 2Cx1/2 + 10Cx1 = 12C
6Bx1/2 + 1Bx1 = 4B
Probable layer atomic arrangement with stoichiometry BC3
CHEMICAL AND PHYSICAL
CHARACTERIZATION OF BC3
• BC3 interlayer spacing similar to graphite
• Also similar to graphite like BN made from thermolysis of
inorganic benzene - borazine B3N3H6 - thinking outside
of the box - F doping by using fluorinated borazine!!!
• Four probe basal plane resistivity on BC3 flakes
 s(BC3)AB ~ 1.1 s(G)AB, (greater than 2 x 104 ohm-1cm-1)
• Implies B effect is not the unfilling of VB to give a metal
but rather the creation of localized states in electronic band
gap making boron graphite behave like a substitutionlly
doped graphite maybe with a larger band gap – recall BN is
a wide band gap insulator!!!
4-PROBE CONDUCTIVITY MEASUREMENTS
L
A
I = V1/R1
I
Rsample = V2/I
Rsample = (V2R1)/V1
r = Rsample (A/L)
V2
Constant
current
source
R1
s = 1/r
V1
Ohmeter
REPRESENTATIVE BC3 INTERCALATION CHEMISTRY
• BC3 + S2O6F2  (BC3)2SO3F Oxidative Intercalation
•
Note: O2FSO--OSO2F, peroxydisulfuryl fluoride strong oxidizing agent, weak
peroxy-linkage easily reductively cleaved to stable fluorosulfonate anion 2SO3F-
•
(BC3)2SO3F
Ic = 8.1 Å,
•
BC3
Ic = 3-4 Å ,
(C7)SO3F
C
Ic = 7.73 Å,
Ic = 3.35 Å,
(BN)3SO3F
BN
Ic = 8.06 Å
Ic = 3.33 Å
• More Juicy Redox Intercalation Chemistry for BC3
•
BC3 + Na+Naphthalide-/THF  (BC3)xNa (bronze, first stage, Ic ~ 4.3 Å)
•
BC3 + Br2(l)  (BC3)15/4Br (deep blue)
ATTEMPT TO INCORPORATE NITROGEN INTO
THE GRAPHITE SHEETS, EVIDENCE FOR C5N
• Pyridine + Cl2 (800oC, flow, quartz tube)  silvery
deposit (PXRD Ic ~ 3.42 Å)
• Fluorine burning of silver deposit  CF4/NF3/N2
• No signs of HF, ClF1,3,5 in F2 burning reaction
• Superior conductivity wrt graphite?
• Try to balance the fluorine burning reaction to give the
nitrogen graphite stoichiometry of C5N - a challenge for
your senses!!! 4C5N + 43F2  20CF4 + 2NF3 + N2
Soft Synthesis of Single-Crystal Silicon Monolayer Sheets
Intercalation Facilitated Exfoliation
Structural model of CaSi2
SYNTHESIS OF SILICON NANOSHEETS
• Chemical exfoliation of calcium disilicide, CaSi2
• CaSi2 synthesized from stoichiometric amounts
CaSi, Si, Mg, Cu crucible, RF heating, Ar
atmosphere, cool to RT, product plate-like crystals
• Hexagonal layered structure (a) consisting of
alternating Ca layers and corrugated Si (111) planes
in which the Si6 rings are interconnected
• To exfoliate precursor-layered crystals into their
elementary layers must adjust the charge on the Si
layer.
• Because CaSi2 is ionic (i.e. Ca2+(Si)2) the
electrostatic interaction between the Ca2+ and Si
layers is strong so key is to reduce charge on the
negatively charged silicon layers.
SYNTHESIS OF SILICON NANOSHEETS
• Mg-doped CaSi2 prepared CaSi1.85Mg0.15 in
which Mg was doped by ion exchange into the
CaSi2 or direct synthesis
• Si monolayer sheets (b, c) prepared through
chemical exfoliation of CaSi1.85Mg0.15 by
immersion in a solution of propylamine
hydrochloride (PA·HCl),
• Ca(2+) ions are de-intercalated and converted into
a dispersion of silicon sheets charge balanced by
PAH(+)
• The composition of monolayer silicon sheets was
determined by XPS to be Si:Mg:O=7.0:1.3:7.5,
structure by XRD, ED, TEM, AFM
CHARACTERIZATION OF SILICON NANOSHEETS
TEM, ED, XRD, AFM
OPTICAL PROPERTIES OF SILICON NANOSHEETS
RT optical properties of Si
nanosheets
a) UV/Vis spectra of
suspensions of Si
Nanosheets at various
concentrations. Inset: the
absorbance at 268 nm is
plotted against concentration
of sheets.
b) PL spectra of Si Nanosheets
dispersed in water with an
excitation wavelength of 350
nm (indicated by an arrow).
INTERCALATION SYNTHESIS OF TRANSITION
METAL DICHALCOGENIDES
• Group IV, V, VI MS2 and MSe2 Compounds
• Layered structures
• Most studied is TiS2
• hcp S2• Ti4+ in Oh sites
• Van der Waals gap
INTERCALATION SYNTHESIS OF TRANSITION
METAL DICHALCOGENIDES
•
Li+ intercalated between the layers
•
Li+ resides in well-defined Td S4 interlayer sites
•
Electrons injected into Ti4+ t2g CB states
•
LixTiS2 with tunable band filling and unfilling
•
Localized xTi(III)-(1-x) Ti(IV) vs delocalized Ti(IV-x)
electronic bonding models???
• VDW gap prized apart by 10%
SEEING INTERCALATION - DIRECT
VISUALIZATION OPTICAL MICROSCOPY
Intercalating lithium - see the layers spread apart
ELECTROCHEMICAL SYNTHESIS OF LixTiS2
TiS2 + xLi+ + xe-  LixTiS2 AN ATTRACTIVE ENERGY STORAGE SYSTEM???
2.5V open circuit = (EF(Li)-EF(TiS2) - no
current drawn - energy density 4 x
Pb/H2SO4 battery of same weight
Controlled potential coulometry, voltage
controlled Li+ intercalation where x is
number of equivalents of charge passed
e-
Li metal anode: Li  Li+ +ePEO/Li(CF3SO3) polymer-salt electrolyte or
propylene carbonate/LiClO4 non aqueous
electrolyte
Li+
PVDF(filler)/C(conductor)/TiS2/Pt(contact)
composite cathode:
TiS2 + xLi+ +xe-  LixTiS2
CHEMICAL SYNTHESIS OF LixTiS2
• xC4H9Li + TiS2 (hexane, N2/RT)  LixTiS2 + x/2C8H18
E
•
E
t2g Ti(IV) delocalized
Filter, hexane wash
• 0x1
t2g Ti(III) localized
S(-II) 3pp VB
N(E)
• Electronic description LixTix(III)Ti(1-x) (IV)S2 mixed valence
localized t2g states (hopping semiconductor - Day and
Robin Class II) or LixTi (IV-x)S2 delocalized partially filled
t2g band (metal - Day and Robin Class III)
Li/TiS2 AN ATTRACTIVE ENERGY SOURCE BUT
MANY TECHNICAL OBSTACLES TO OVERCOME
• Technical problems need to be overcome with both the
Li anode, intercalation cathode and polymer-salt
electrolyte
• Battery cycling causes Li dendritic growth at anode need other Li-based anode materials, Li-C composites,
Li-Sn, Li-Si alloys - also rocking chair LixMO2
configuration
• Mechanical deterioration at the cathode due to multiple
intercalation-deintercalation lattice expansioncontraction cycles
• Cause lifetime, corrosion, reactivity, and kaboom safety
hazards
LiCoO2
LixC6
ROCKING CHAIR LSSB
LiCoO2
Li
OTHER INTERCALATION SYNTHESES WITH TiS2
• Cu+, Ag+, H+, NH3, RNH2, Cp2Co, chemical, electrochemical
• Cobaltacene Cp2Co(II) especially interesting 19e guest
• [Cp2Co(III)]x+Tix3+Ti1-x4+S2 chemical-electronic description
consistent with structure, hopping SC, spectroscopy
Synthesis, Cp2CoCH3CN (solution)TiS2(s)
Co
Co
• Temperature dependent solid state NMR shows two forms of Cp ring
wizzing (lower T) and molecule tumbling dynamics (higher T) with
Cp2Co+ molecular axis orthogonal and parallel to layers, dynamics yields
activation energies for the different rotational processes
EXPLAINING THE MAXIMUM 3Ti: 1Co
STOICHIOMETRY IN (Cp2Co)0.3TiS2
Interleaved Cp2Co(+)
cations
Matching trigonal
symmetry of hcp
chalcogenide sheet
Third of interlayer
space filled
Geometrical and steric
requirements of packing
transverse oriented
metallocene in VDV gap
Inhibition of Energy Transfer between Conjugated Polymer
Chains in Host-Guest Nanocomposites Generates White
Photo- and Electroluminescence
PXRD DIAGNOSTICS
• Chemical structures of
blue-emitting PFO,
green-emitting F8BT,
and red-emitting
MEH-PPV
• XRD patterns of a
restacked SnS2 film
(no polymer), and a
blend-intercalatedSnS2 nanocomposite
film.
WHITE LIGHT LED DIAGNOSTICS
• PL spectra of separate
SnS2/conjugated-polymerintercalated nanocomposites,
• Blend of only the three polymers
(excitation 380 nm),
• PL (excitation 380 nm) and EL of
a blend-intercalated/SnS2
nanocomposite film.
• Inset: excitation spectra for
emission at 580 nm of a blend of
only the three polymers and the
blend-intercalated/SnS2
nanocomposite.
INTERCALATION ZOO
• Channel, layer and framework materials
• 1-D chains: TiO2 channels, (TiS3 [Ti(IV)S(2-)S2(2-)], NbSe3
[Nb(IV)Se(2-)Se2(2-)]), contain disulfide and diselenide units in
Oh building blocks to form chain
• 2-D layers: MS2, MSe2, NiPS3 [Ni2(P2S6), ABA CdI2 type packing,
alternating layers of octahedral NiS6 and trigonal P2S6 groupings
with S…S Van der Waals gap], FeOCl, V2O5.nH2O, MoO3, TiO2
(layered polymorph B – see Chimie Douce later)
• 3D framework: zeolites, WO3, Mo6S8, Mo6Se8 (Chevrel phases)
FACE BRIDGING OCTAHEDRAL TITANIUM
TRISULFIDE AND NIOBIUM TRISELENIDE
BUILDING BLOCKS FORM 1-D CHAINS
TiS3 = Ti(IV)S(2-)S2(2-)
intercalated cations like
Li(+) in channels
between chains to form
LixTiS3
Ti(IV) = S2(2-) =
S(2-) = Li(+) =
3-D OPEN FRAMEWORK TUNGSTEN OXIDE AND
TUNGSTEN OXIDE BRONZES MxWO3
W
O
c-WO3 = c-ReO3 structure type with
injected cation M(q+) center of cube
and charge balancing qe- in CB,
MxWO3 Perovskite structure type
M(q+) O CN = 12, O(2-) W CN = 2,
W(VI) O CN = 6
M
Unique 2-D layered structure of
MoO3
Chains of corner sharing
octahedral building blocks sharing
edges with two similar chains,
Creates corrugated MoO3 layers,
stacked to create interlayer VDW
space,
Three crystallographically distinct
oxygen sites, sheet stoichiometry
3x1/3 ( ) +2x1/2 ( )+1 ( )
ELECTROCHEMICAL OR CHEMICAL
SYNTHESIS OF MxWO3
• xNa+ + xe- + WO3  NaxWx5+W1-x6+O3
• xH+ + xe- + WO3  HxWx5+W1-x6+O3
• Injection of alkali metal cations generates Perovskite
structure types
• M+ oxygen coordination number 12, resides at center of cube
• H+ protonates oxygen framework, exists as MOH groups
SYNTHESIS DETAILS FOR Mx’MO3
WHERE M = Mo, W AND M’ = INJECTED PROTON
OR ALKALI OR ALKALINE EARTH CATION
•
•
•
•
•
•
n BuLi/hexane
CHEMICAL
LiI/CH3CN
Zn/HCl/aqueous
Na2S2O4 aqueous sodium dithionate
Pt/H2
Topotactic ion-exchange of Mx’MO3 with M” cation
• Li/LiClO4/MO3
ELECTROCHEMICAL
• Cathodic reduction in aqueous acid electrolyte
• MO3 + H2SO4 (0.1M)  HxMO3
VPT GROWTH OF LARGE SINGLE CRYSTALS OF
MOLYBDENUM AND TUNGSTEN TRIOXIDE AND
CVD GROWTH OF LARGE AREA THIN FILMS
• VPT CRYSTAL GROWTH
• MO3 + 2Cl2 (700°C)  (800°C) MO2Cl2 + Cl2O
• CVD THIN FILM GROWTH
• M(CO)6 + 9/2O2 (500°C)  MO3 + 6CO2
MANY APPLICATIONS OF THIS M’xMO3
CHEMISTRY AND MATERIALS
• Electrochemical devices like chemical sensors, pH
responsive microelectrochemical chips and
electrochromic displays, smart windows, advanced
batteries
• Behave as low dopant mixed valance hopping
semiconductors
• Behave as high dopant metals
• Electrical and optical properties best understood by
reference to simple DOS picture of M’xMx5+M1-x6+O3
COLORING MOLYBDENUM TRIOXIDE WITH
PROTONS, MAKING IT ELECTRONICALLY, IONICALLY
CONDUCTIVE AND A SOLID BRNSTED ACID
Electronic band structure in HxMoO3 molybdenum oxide bronze, tuning color,
electronic conductivity, acidity with x
COLOR OF TUNGSTEN BRONZES, MxWO3
INTERVALENCE W(V) TO W(VI) CHARGE TRANSFER
IVCT
ELECTRONIC AND COLOR CHANGES BEST
UNDERSTOOD BY REFERENCE TO SIMPLE BAND
PICTURE OF NaxMox5+Mo1-x6+O3
• SEMICONDUCTOR TO METAL
TRANSITION ON DOPING MxMoO3
• MoO3: Band gap excitation from O2(2pp) VB to Mo6+ (5d) CB, LMCT in UV
region, wide band gap insulator
• NaxMox5+Mo1-x6+O3: Low doping level,
narrow band gap hopping
semiconductor, narrow localized Mo5+
(d1) VB, visible absorption, essentially
IVCT Mo5+ to Mo6+ absorption, mixed
valence hopping semiconductor
• NaxMox5+Mo1-x6+O3: High doping level,
partially filled valence band, narrow
delocalized Mo5+ (d1) VB, visible
absorption, IVCT Mo5+ to Mo6+ and
shows metallic reflectivity (optical
plasmon) and conductivity
HxMoO3 TOPOTACTIC PROTON INSERTION
• Range of compositions: 0 < x < 2, MoO3 structure largely unaltered by
reaction, four phases
•
•
•
•
0.23 < x < 0.4
0.85 < x < 1.04
1.55 < x < 1.72
2.00 = x
orthorhombic
monoclinic
monoclinic
monoclinic
• Similar lattice parameters by XRD, ND of HxMoO3 cf MoO3
•
•
•
•
MoO3 high resistivity semiconductor
HxMoO3 insertion material SC to M transition
HxMoO3 strong Brnsted acid – Mo-O(H)-Mo
HxMoO3 fast proton conductor
• See what happens when single crystal immersed in Zn/HCl/H2O
HxMoO3 TOPOTACTIC PROTON INSERTION
INTRALAYER PROTON DIFFUSION
1-D proton conduction along chains
Yellow transparent
Protons begin in basal plane
Moves from two edges along c-axis
INTERLAYER PROTON DIFFUSION
b-axis adjoining layers react
Orange transparent
PROTON FILLING
Eventually proton diffusion complete and
entire crystal transformed Blue bronze
Consistent with structural, electrical and
optical data
PROTON CONDUCTION PATHWAY IN HxMoO3
c-axis
PROTON CONDUCTION
PATHWAY IN HxMoO3
•
•
•
•
•
•
•
•
•
•
Part of a HxMoO3 layer
Showing initial 1-D proton conduction pathway
Apical to triply bridging oxygen proton migration first
1H wide line NMR, PGSE NMR probes of structure and diffusion
Doubly to triply bridging oxygen proton migration pathway
Initial proton mobility along c-axis intralayer direction for x = 0.3
Subsequently along b-axis interlayer direction
Single protonation at x = 0.36, double protonation x = 1.7
More mobile protons higher loading D(300K) ~ 10-11 vs 10-9 cm2s-1
Proton-proton repulsion
ION EXCHANGE SOLID STATE SYNTHESIS
• Requirements: anionic open channel, layer or
framework structure
• Replacement of some or all of charge balancing
cations by protons or simple or complex cations
• Classic cation exchangers are zeolites, clays,
beta-alumina, molybdenum and tungsten oxide
bronzes
BETA ALUMINA
• High T synthesis of beta-alumina:
• (1+x)/2Na2O + 5.5Al2O3  Na1+xAl11O17+x/2
• Structural reminders:
• Na2O: Antifluorite ccp Na+, O2- in Td sites
• Al2O3: Corundum hcp O2-, Al3+ in 2/3 Oh sites
• Na1+xAl11O17+x/2 defect Spinel, O2- vacancies in conduction
plane, controlled by x ~ 0.2, Spinel blocks 9Å, bridging
oxygen columns, mobile Na+ cations in conduction plane, 2-D
fast-ion conductor
Rigid Al-O-Al
column spacers
Na(+) conduction
plane
0.9 nm
Na1+xAl11O17+x/2
defect spinel
blocks
3/4 O(2-) missing in
conduction plane
Spinel blocks, ccp layers of O(2-)
Every 5th. layer has 3/4 O(2-) vacant, defect spinel
4 ccp layers have 1/2Oh, 1/8Td Al( 3+) cation sites
Blocks cemented by rigid Al-O-Al spacers
Na(+) mobile in 5th open conduction plane
Centrosymmetric layer sequence in Na1+xAl11O17+x/2
(ABCA)B(ACBA)C(ABCA)B(ACBA)
GETTING BETWEEN THE SHEETS OF THE BETA
ALUMINA FAST SODIUM CATION FAST ION
CONDUCTOR: LIVING IN THE FAST LANE
0.9 nm Spinel block
Al-O-Al column
spacers in conduction
plane
Oxide wall of
conduction plane
Mobile sodium cations
ION EXCHANGE IN Na1+xAl11O17+x/2
Thermodynamic and
kinetic considerations
Mass, size and charge
considerations
Lattice energy controls
stability of ionexchanged materials
Cation diffusion,
polarizability effects
control rate of ionexchange
MELT ION-EXCHANGE OF CRYSTALS
• Equilibria between beta-alumina and MNO3 and MCl
melts, 300-350oC
• Extent of exchange depends on time, T, melt composition
• Monovalents: Li+, K+, Rb+, Ag+, Cu+, Tl+, NH4+, In+, Ga+,
NO+, H3O+
• Higher valent cations: Ca2+, Eu3+, Pb2+
• Higher T melts required for exhigher valent cations,
strong cation binding, slower cation diffusion, 600-800oC
typical
MELT ION-EXCHANGE OF CRYSTALS
• Charge-balance requirements:
• 2Na+ for 1Ca2+, 3Na+ for 1La3+
• Controlled partial exchange by control of melt
composition:
• qNaNO3 : (1-q)AgNO3
• Na1+x-yAgyAl11O17+x/2
KINETICS AND THERMODYNAMICS OF SOLID
STATE ION EXCHANGE
• Kinetics of Ion-Exchange
•
•
Controlled by ionic mobility of the cation
Mass, charge, radius, temperature, solvent, solid state structural properties
• Thermodynamics, Extent of Ion-Exchange
•
•
•
•
•
Ion-exchange equilibrium for cations
Binding activities between melt and crystal phases
Site preferences
Binding energetics, lattice energies
Charge : radius ratios