Transcript Document 7478305

MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC PEROVSKITES
• Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH  YxGd3-xFe5O12
• Mixed metal hydroxide aqueous precursor synthesis method,
reactants red brown, solid products olive green
• Firing pellets at 900oC, 18-24 hrs, re-grinding, re-pelletizing,
repeated firing, removes REFeO3 Perovskite impurity
• PXRD used to identify garnet phase, detects any crystalline
impurity phase like REFeO3, enables UC dimensions to be
determined as a function of Y: Ga ratio over range 0 < x < 3
PXRD OF SOLID PRODUCTS OF Y(NO3)3 + Gd(NO3)3 +
FeCl3 + NaOH REACTION
HYDROTHERMAL SYNTHESIS AND CRYSTAL
GROWTH OF YTTRIUM GADOLINIUM IRON GARNET
aqueous basic medium, mineralizes,
temperature gradient transports, deposits
reactants on seed crystal to grow product
yttrium gadolinium iron oxide crystal
baffles
T2
T1
Fe2O3
T2
Gd2O3 /Y2O3
Seed crystal to grow YxGd3-xFe5O12 crystal
GARNETS DISPLAY INTERESTING COOPERATIVE
MAGNETIC BEHAVIOR
• Tunable Garnet magnet by varying magnetic sub-lattice
components without disrupting garnet structure
• Similar idea to magnetic Spinel AB2O4 solid solution
behavior - in which one has magnetically tunable Td (A)
and Oh (B) metal sites
• Rare earth garnets R3Fe5O12
• General Formula C3A2D3O12 (8 formula units per cubic
unit cell - total 160 atoms)
ONE OCTANT OF CUBIC UNIT CELL OF Y3Al5O12 (YAG)
Faces 3 dodecahedral Y(3+) sites
Corners and center 2Oh AlO6 sites
Faces 3Td AlO4 sites
One octant of cubic unit cell of garnet
GARNETS DISPLAY INTERESTING COOPERATIVE
MAGNETIC BEHAVIOR
• C3A2D3O12 isomorphous replacement of
Y(3+) for Gd(3+) on dodecahedral C cation sites
(works for all rare earths except La, Ce, Pr, Nd)
• Forms solid solution as similar ionic radii
• R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å
• Complete family accessible, YxGd3-xFe5O12, 0  x  3
• 2Fe(3+) Oh A-sites, 3Fe(3+) D Td sites, 3RE(3+) C
dodecahedral sites
MODELS FOR DETERMINING THE Y(3+)/Gd(3+)
DISTRIBUTION IN YxGd3-xFe5O12
1. Solid solution - random distribution of two
components - EDX mapping
2. Physical mixture of two end members - phase
segregation - PXRD
3. Compositional gradients - STEM imaging - EDX
mapping
4. Core-corona - cherry model - surface free energy
driven - EDX mapping
5. Microphase separated domains smaller than 10
nm - PXRD line broadening
6. Ordered superlattice of two components - ED
MODELS FOR DETERMINING THE Y(3+)/Gd(3+)
DISTRIBUTION IN YxGd3-xFe5O12
• Interesting problem in solid state materials
characterization
• If any measured physical property P of the product
follows linear Vegard law behavior this defines a
solid solution for the Y(3+)/Gd(3+) distribution
• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3-x)/3(Gd3Fe5O12)
• Measured P of product is the atomic/mole fraction
weighted average P of the end-member materials
MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC MATERIALS
• Cubic unit cell parameter a versus x for YxGd3-xFe5O12
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Composition
Y3Fe5O12
Y2.5Gd0.5Fe5O12
Y2Gd1Fe5O12
Y1.5Gd1.5Fe5O12
Y1Gd2Fe5O12
Y0.5Gd2.5Fe5O12
Gd3Fe5O12
Lattice parameter, nm
1.2370
1.2382
1.2402
1.2423
1.2437
1.2450
1.2468
R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å
MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC MATERIALS
• Isomorphous random replacement of Y3+ for Gd3+on dodecahedral sites
of cubic lattice
• Linear Vegard law behavior
• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3-x)/3(Gd3Fe5O12)
• Any property of a solid-solution member is the atom/mole fraction
weighted average of the end-members - distinguishes statistical from
other types of mixtures (core-corona, phase separation, domains,
gradients, superlattices)
• Cubic lattice parameter a shows linear Vegard law behavior with x
TUNABLE MAGNETIC PROPERTIES BY VARYING x IN
THE BINARY GARNET YxGd3-xFe5O12
• Counting e and unpaired e-spins – book keeping
• x dodec Y(3+) sites
4d0, 4f0
0 UPEs
• (3-x) dodec Gd(3+) sites HS 4f7
7 UPEs
• 3 Td Fe(3+) sites
HS 3d5
5 UPEs
• 2 Oh Fe(3+) sites
HS 3d5
5UPEs
TUNABLE MAGNETIC PROPERTIES BY VARYING x
IN THE BINARY GARNET YxGd3-xFe5O12
• Ferrimagnetically coupled material, oppositely
aligned electron spins on Td and Oh Fe(3+)
magnetic sub-lattices
• Counting spins Y3Fe5O12
• 3 x 5 - 2 x 5 = 5UPEs
ferrimagnetic at low T
• Counting spins Gd3Fe5O12 ferrimagnetic at low T
• 3 x 7 -3 x 5 + 2 x 5 = 16UPEs
• Tunable magnetic garnet: 16 to 5 UPEs
VEGARD LAW AT THE NANOSCALE
SYNTHESIS OF COMPOSITION TUNABLE MONODISPERSE CAPPED
ZnxCd1-xSe ALLOY NANOCRYSTALS
SYNTHESIS OF COMPOSITION TUNABLE ZnxCd1-xSe
ALLOY NANOCRYSTALS
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Sequential synthesis of small Eg core and large Eg shell precursor nanoclusters
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Cd(stearate)2 + (octyl)3PO + high temperature solvent octadecylamine
Reaction temperature 310-330°C
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Se + (octyl)3P
Mixing temperature 270-300°C
• Provides core nanocluster precursor (CdSe)n(TOPO)m
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Add ZnEt2 + (octyl)3P in controlled stoichiometry increments
Mixing temperature 290-320°C
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Monitor photoluminescence until constant wavelength emission
• Desired core-corona nanocluster product
(ZnxCd1-xSe)n(TOPO)m
TEM OF COMPOSITION TUNABLE ZnxCd1-xSe
ALLOY NANOCRYSTALS
SHOWS MONOTONIC INCREASE IN DIAMETER OF NANOCRYSTALS
WITH ADDITION OF ZnSe CORONA TO CdSe CORE
SPATIALLY
RESOLVED EDX
SHOWS
NANOCRYSTAL
COMPOSITIONAL
HOMOGENIETY
ABSORPTION-EMISSION SPECTRA OF COMPOSITION
TUNABLE ZnxCd1-xSe ALLOY NANOCRYSTALS
EXPECTED BLUE SHIFT OF ABSORPTION AND EMISSION WITH
INCREASING AMOUNTS OF WIDE BAND GAP ZnSe COMPONENT IN
NARROW BAND GAP CdSe NANOCRYSTALS
PXRD PATTERNS OF COMPOSITION TUNABLE
ZnxCd1-xSe ALLOY NANOCRYSTALS
EXPECTED LINEAR VEGARD LAW DECREASE IN UNIT CELL
DIMENSIONS WITH INCREASING AMOUNTS OF SMALLER UNIT CELL
ZnSe COMPONENT IN LARGER UNIT CELL CdSe NANOCRYSTALS
MODE OF FORMATION OF COMPOSITION TUNABLE
ZnxCd1-xSe ALLOY NANOCRYSTALS
Effect of Different Reaction Temperatures
SYNTHESIS OF COMPOSITION TUNABLE ZnxCd1-xSe
ALLOY NANOCRYSTALS
• High structural and optical quality ZnxCd1-xSe semiconductor alloy
nanocrystals successfully prepared using core-corona precursor made by
growing stoichiometric amounts of Zn and Se on surface of preprepared CdSe nanocrystal seeds and thermally inducing alloy
nanocluster formation by interdiffusion of element components within
nanocluster - diffusion length control of reaction between two solid
reagents
• With increasing Zn content, a composition-tunable photoemission across
most of the visible spectrum has been demonstrated by a systematic
blue-shift in emission wavelength (QSE) demonstrating alloy
nanocluster formation and not phase separation
• A rapid alloying process is observed at the “alloying point” as the core
and corona components mix to provide a homogeneous linear Vegard
law type distribution of elements in the nanoclusters
ARRESTED GROWTH
OF MONODISPERSED
NANOCLUSTERS
CRYSTALS, FILMS
AND LITHOGRAPHIC
PATTERNS
nMe2Cd + nnOct3PSe + mnOct3PO  (nOct3PO)m(CdSe)n + n/2C2H6
BASICS OF NANOCLUSTER NUCLEATION,
GROWTH, CRYSTALLIZATION AND CAPPING
STABILIZATION
Gb > Gs
supersaturation
nucleation
Addition
of reagent
aggregation
capping and
stabilization
nMe2Cd + nnOct3PSe + mnOct3PO  (nOct3PO)m(CdSe)n + n/2C2H6
Spatial and quantum confinement and
dimensionality
EgC = EgB + (h2/8R2)(1/me* + 1/mh*) - 1.8e2/R
Quantum
localization term
Coulomb interaction
between e-h
CAPPED MONODISPERSED
SEMICONDUCTOR
NANOCLUSTERS
TUNING CHEMICAL AND PHYSICAL
PROPERTIES OF MATERIALS WITH SIZE AS
WELL AS COMPOSITION AND STRUCTURE
nMe2Cd + nnOct3PSe + mnOct3PO  (nOct3PO)m(CdSe)n + n/2C2H6
THINK SMALL DO BIG THINGS!!!
EgC = EgB + (h2/8R2)(1/me* + 1/mh*) - 1.8e2/R
tuning chemical and physical
properties of materials with size as
well as composition and structure
2 MoCl5 + 5 Na2S  2 MoS2 + 10 NaCl + S
Richard Kaner: Rapid Solid State Synthesis of Materials
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy + MClx  MQy + xLiCl METATHESIS METAL EXCHANGE REACTION
Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)
• Many useful materials, such as ceramics, are most often produced from
high temperature reactions (500-3000°C) which often take many days
due to the slow nature of solid-solid diffusion.
• Rapid SS new method enables high quality refractory materials to be
synthesized in seconds from appropriate solid state precursors.
• Basic idea is to react stable high oxidation state metal halides with
alkali or alkaline earth compounds in a metathesis metal exchange
reaction to produce the desired product plus an alkali(ne) halide salt
which can simply be washed away.
• Since alkali(ne) salt formation is very favorable
many of these reactions are thermodynamically
downhill by 100-200 kcal/mol or more.
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy + MClx  MQy + xLiCl METATHESIS METAL EXCHANGE REACTION
Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)
• MoS2, a layered material with VDW interlayer forces used as a
lubricant in low T,P aerospace applications, as a cathode for
rechargeable LSSB and as a hydrodesulfurization catalyst for
removing S from organosulfur compounds, is normally prepared by
heating the elements Mo/S to 1000°C for several days
• New SSS gives pure crystalline MoS2 from a self-initiated reaction
between the solids MoCl5 and Na2S in seconds!!!
• 2 MoCl5 + 5 Na2S  2 MoS2 + 10 NaCl + S
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NaCl byproduct is simply washed away.
Other layered transition MS2 can be produced in analogous rapid solid-solid reactions:
M = W, Nb, Ta, Rh
Na2Se used for MSe2 syntheses
PARTICLE SIZE CONTROL
USE AN INERT DILUENT LIKE NaCl TO AMELIORATE THE HEAT OF REACTION,
CONTROL NUCLEATION AND LIMIT THE GROWTH OF PARTICLES
• MoCl5/NaCl
• 1:0
• 1:4
• 1:16
MoS2 Particle Size nm
45
18
8
• NaCl washed away after reaction
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS
LixQy + MClx  MQy + xLiCl METATHESIS METAL EXCHANGE REACTION
Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)
• High quality anion solid solutions such as MoS1-xSex can be
made using the precursor Na2S1-xSex formed by coprecipitation of Na2S/Na2Se mixtures from liquid ammonia
• High quality cation solid solutions such as Mo1-xWxS2 can
be made by melting together the metal halides MoCl5 and
WCl6, followed by reaction with Na2S
• The solid-solution products can be analyzed by studying
the MoW alloys formed after reduction in hydrogen ASSUMING NO SEGREGATION!!!
SOLID SOLUTION PRECURSORS
• REACTANT A
• Na2(S,Se)
• Na3(P,As)
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PRODUCT
Ga(P,As)
Mo(S,Se)2
W(S,Se)2
(Mo,W)S2
REACTANT B
GaCl3
MoCl5
WCl6
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl METATHESIS METAL EXCHANGE REACTION
Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)
• These SS metathesis reactions are becoming a general
process for synthesizing important materials.
• For example, refractory ceramics such as ZrN (m.p. ~
3000°C) can be produced in seconds from ZrCl4 and Li3N
• ZrCl4 + 4/3Li3N  ZrN + 4LiCl + 1/6N2
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NOTE CHANGE IN OXIDATION STATE Zr(IV) REDUCED TO Zr(III)
WITH OXIDATION OF N(-III) TO N(0)
• MoSi2, a material used in high temperature furnace
elements, can be made from MoCl5 and Mg2Si
RAPID SS PRECURSOR SYNTHESIS OF MATERIALS:
LixQy + MClx  MQy + xLiCl METATHESIS METAL EXCHANGE REACTION
Q = N, P, As (PNICTIDES), S, Se, Te (CHALCOGENIDES), C, Si (CARBIDES, SILICIDES)
• The III-V SCs GaP and GaAs can be made in seconds from
the solid precursors GaCl3 and Na3P or Na3As
• Recently, high pressure methods have been employed to
allow the use of metathesis to synthesize gallium nitride
(GaN) using Li3N and GaCl3
• Very important blue laser diode material, a synthesis which
was not possible using the methods for GaP or GaAs
SUMMARIZING KEY FEATURES OF RAPID SOLID
STATE SYNTHESIS OF MATERIALS
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Metathesis – metal exchange pathway
Access to large number of materials
Extremely rapid about 1 second!!!
Initiated at or near RT – rapid rise in reaction temperature
Self-initiated self-propagating
Thermodynamic driving force of Go alkali(ne) halides
Control of particle size with inert alkali(ne) halide matrix
Solid solution materials synthesis feasible
Most recent addition to metathesis zoo are carbides
METAL CARBIDES - TRY TO BALANCE THESE
EQUATIONS - OXIDATION STATE CHALLENGE
• 3ZrCl4 + Al4C3  3ZrC + 4AlCl3
• 2WCl4 + 4CaC2  2WC + 4CaCl2 + 6C
• 2TiCl3 + 3CaC2  2TiC + 3CaCl2 + 4C
• DO NOT CONFUSE CARBIDE C4- IN Al4C3 FROM
ACETYLIDE (C22-) IN CaC2!!!
• Inert, hard, refractory, electrically conducting ceramics
• Cutting tools, crucibles, catalysts, hard steel manufacture