magnetic garnets, y x gd 3-x fe 5 o 12 tunable magnetic perovskites y(no 3 ) 3 + gd(no 3 ) 3 + fecl...

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

Fe2O3

Seed crystal to grow YxGd3-xFe5O12 crystal

Gd2O3 /Y2O3

baffles

aqueous basic medium, mineralizes, temperature gradient transports, deposits reactants on seed crystal to grow product yttrium gadolinium iron oxide crystal

T2 T1 T2

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)

One octant of cubic unit cell of garnet

Faces 3 dodecahedral Y(3+) sites

Corners and center 2Oh AlO6 sites

Faces 3Td AlO4 sites

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


TUNABLE MAGNETIC MATERIALS

• Cubic unit cell parameter a versus x for YxGd3-xFe5O12

• Composition Lattice parameter, nm

• Y3Fe5O12 1.2370

• Y2.5Gd0.5Fe5O12 1.2382

• Y2Gd1Fe5O12 1.2402

• Y1.5Gd1.5Fe5O12 1.2423

• Y1Gd2Fe5O12 1.2437

• Y0.5Gd2.5Fe5O12 1.2450

• Gd3Fe5O12 1.2468R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å


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 ferrimagnetic at low T

• 3 x 5 - 2 x 5 = 5UPEs

• 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

• Sequential synthesis of small Eg core and large Eg shell precursor nanoclusters

• Cd(stearate)2 + (octyl)3PO + high temperature solvent octadecylamine • Reaction temperature 310-330°C

• Se + (octyl)3P• Mixing temperature 270-300°C

• Provides core nanocluster precursor (CdSe)n(TOPO)m

• Add ZnEt2 + (octyl)3P in controlled stoichiometry increments • Mixing temperature 290-320°C

• 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 pre-prepared 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

Additionof reagent

nucleation

aggregation

BASICS OF NANOCLUSTER NUCLEATION, GROWTH, CRYSTALLIZATION AND CAPPING STABILIZATION

capping and stabilization


Gb > Gs

supersaturation

Spatial and quantum confinement and dimensionality

CAPPED MONODISPERSED SEMICONDUCTOR NANOCLUSTERS


EgC = Eg

B + (h2/8R2)(1/me* + 1/mh*) - 1.8e2/R

Coulomb interaction between e-h

Quantum localization term

TUNING CHEMICAL AND PHYSICAL PROPERTIES OF MATERIALS WITH SIZE AS WELL AS COMPOSITION AND STRUCTURE

THINK SMALL DO BIG THINGS!!!

EgC = Eg

B + (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.

• 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

• 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 MoS2 Particle Size nm

• 1:0 45

• 1:4 18

• 1:16 8

• NaCl washed away after reaction

• High quality anion solid solutions such as MoS1-xSex can be made using the precursor Na2S1-xSex formed by co-precipitation 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 REACTANT B

• Na2(S,Se) GaCl3

• Na3(P,As) MoCl5

• WCl6

• PRODUCT

• Ga(P,As)

• Mo(S,Se)2

• W(S,Se)2

• (Mo,W)S2

• 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

• 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


• 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

RAPID SS PRECURSOR SYNTHESIS OF MATERIALS: LixQy + MClx MQy + xLiCl METATHESIS METAL EXCHANGE REACTION


SUMMARIZING KEY FEATURES OF RAPID SOLID STATE SYNTHESIS OF MATERIALS

• 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 (C2

2-) IN CaC2!!!

• Inert, hard, refractory, electrically conducting ceramics

• Cutting tools, crucibles, catalysts, hard steel manufacture

magnetic garnets, y x gd 3-x fe 5 o 12 tunable magnetic perovskites y(no 3 ) 3 + gd(no 3 ) 3 + fecl...

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