Charge, Orbital and Spin Charge, Orbital and Spin Ordering in RBaFeOrdering in RBaFe22OO5+x5+x

PerovskitesPerovskites

Patrick M. WoodwardDepartment of Chemistry

Ohio State UniversityPavel Karen

Department of ChemistryUniversity of Oslo

Charge, Orbital and Spin OrderingCharge, Orbital and Spin OrderingCharge Ordering (TCO)

– T > TCO → Delocalized Electrons → Single oxidation state – T < TCO → Localized Electrons → Distinct oxidation states

Orbital Ordering (TOO)– Preferential (anisotropic) occupation of given d-orbitals– Cooperative Jahn-Teller distortion

Spin Ordering (TN or TC)– Long range magnetic ordering

Mixed Valency (Robin & Day Classification)– Type III → Metallic e- transport → Single oxidation state– Type II → Activated e- transport → Two oxidation states at

a given instant, but CO pattern is fluctional– Type I → Insulating → Two oxidation states, with a regular

(long range) CO pattern

Ba2(Bi4+)2O6 → Ba2(Bi3+Bi5+)O6Cox & Sleight (1976)

High Tc Superconductivity in Oxides

Fe3+(Fe2.5+)2O4 → Fe3+(Fe2+Fe3+)O4Verwey (1939)

Double Exchange Ferromagnetism

LaCa(Mn3.5+)2O6 → LaCa(Mn3+Mn4+)O6Wollan, Koehler & Goodenough (1955)

Colossal Magnetoresistance (CMR)

Examples of Charge OrderingExamples of Charge Ordering

Charge Ordering in NdCharge Ordering in Nd0.50.5SrSr0.50.5MnOMnO33

0

50

100

150

200

250

300

350

19 21 23 25 27 29 31

2-Theta (Degrees)

Inte

nsity

160 K

Incipient CO Phase

0

50

100

150

200

250

300

350

19 21 23 25 27 29 31

2-Theta (Degrees)

Inte

nsity

50 K

T = 160 K Valence Mixed State

T = 60 K Charge Ordered State

Synchrotron XSynchrotron X--ray Powder Diffraction Data (NSLSray Powder Diffraction Data (NSLS--X7A)X7A)

A series of weak superstructure reflections arise (1% intensity at the strongest) that indicate doubling of the a -axis.

Woodward, Cox, Vogt, Rao, Cheetham, Chem. Mater. 11, 3528-38 (1999).

Examples of Orbital OrderingExamples of Orbital Ordering

22××2.18 2.18 ÅÅ

22××1.91 1.91 ÅÅ

22××1.94 1.94 ÅÅ

22××2.07 2.07 ÅÅ

LaMnO3 (298 K)Rodriguez-Carvajal, et al. Phys.

Rev. B 57, R3189 (1998).

NdSrMn2O6 (50 K)Woodward, et al. Chem.

Mater. 11, 3528-38 (1999).

Mn3+

Mn4+

4×1.90 Å

Orbital Ordering & Cooperative Orbital Ordering & Cooperative JahnJahn--Teller DistortionsTeller Distortions

d(x2-y2)

d(z2)

Mn3+

eg (σ*)

t2g (π*)

RegularOctahedron

Axially ElongatedOctahedron

Examples of Orbital OrderingExamples of Orbital Ordering

22××2.18 2.18 ÅÅ

22××1.91 1.91 ÅÅ

22××1.94 1.94 ÅÅ

22××2.07 2.07 ÅÅ

LaMnO3 (298 K)Rodriguez-Carvajal, et al. Phys.

Rev. B 57, R3189 (1998).

NdSrMn2O6 (50 K)Woodward, et al. Chem.

Mater. 11, 3528-38 (1999).

Mn3+

Mn4+

4×1.90 Å

Orbital Ordering in NdOrbital Ordering in Nd0.50.5SrSr0.50.5MnOMnO33

Upon cooling below 150 K, the a & c-axes expand and the b-axis contracts. This is the signature of orbital ordering

Woodward, Cox, Vogt, Rao, Cheetham, Chem. Mater. 11, 3528-38 (1999).

5.30

5.35

5.40

5.45

5.50

5.55

0 100 200 300

Temperature (K)

Cel

l Par

amet

er (A

)

(b)

c

a

b/sqrt(2)

Oxygen Deficient Double Perovskites Oxygen Deficient Double Perovskites RBaFeRBaFe22OO5+w5+w

R = Trivalent Rare Earth Ion– Nd, Sm, Tb, Ho, Y– Changing the radius of the R ion,

controls the layer spacingM = 1st row Transition Metal Ion

– V, Mn, Fe, Co, Cu– Changing M, alters the electron count

and covalency of the M-O bonds.0 ≥ w ≥ ~ 0.7

– Excess oxygen resides in R layer– The upper limit of w is dictated by

the ionic radius of R– w=0 → M+2.5, w=0.5 → M+3

– changes the local coordination of the transition metal ion from 5 to 6

Synthesis REBaFeSynthesis REBaFe22OO55

• The sample is dehydrated at ~180 °C.• The sample is then heated at ~400 °C to drive off the organic

content and produce an amorphous precursor.• The precursor is calcined (800-900 °C) and then sintered at

high temperature (1000-1150 °C) in a carefully controlled pO2atmosphere.

• The sintered pellets are heated at a lower temperature (600-860 °C) in a controlled atmosphere to attain the desired oxygen content.

RE2O3 + Citric Acid

Fe2O3 + Nitric Acid BaCO3

AmorphousOrganic-InorganicPrecursor

Heat

& Stir

Powder Diffraction Data CollectionPowder Diffraction Data Collection• Synchrotron X-ray Powder Diffraction

– NSLS - X7A (Dave Cox, Tom Vogt)– NSLS – X3B (Peter Stephens, Sylvina Pagola)– ESRF – BM1B (Swiss-Norwegian Beamline)

• Neutron Powder Diffraction– NIST – BT1 (Brian Toby) - YBaFe2O5– ILL – D2B (Emmanuel Suard) - HoBaFe2O5 & NdBaFe2O5– PSI - HRPT (Peter Fischer) - TbBaFe2O5– ANSTO – MRPD (Andrew Studer) - TbBaFe2O5

• Neutron Thermodiffractometry– ILL – D20 (Emmanuel Suard) - HoBaFe2O5 & NdBaFe2O5

• Mossbauer Spectroscopy– Abo Akademi (Johan Linden)

DSCDSC

1.E+00

1.E+02

1.E+04

1.E+06

150 200 250 300

Temperature (K)

Res

istiv

ity (o

hm.c

m)

SmBaFe2O5+x

TbBaFe2O5+x

x=0.016

x=0.030

x=0.046

x=0.249

x=0.095

Electrical Electrical ResistivityResistivityT>TPM Mössbauer shows one Fe2.5+ signal (Type III MV)TPM >T>TCO Mössbauer signal begins to split Fe2.5+x + Fe2.5-x (Type II MV)TCO > T Mössbauer shows Fe2+ + Fe3+ (Type I MV, Charge Ordered State)

0.15

0.10

0.05

0.00

Heat flow (mW/g)

300250200 T (K)Nd

PmSm

EuGd

TbDy

Ho

RII IIII IIIIII

TTCOCO

TTpmpm

Karen, Woodward, Santhosh, Vogt, Stephens, Pagola, J. Solid State Chem.167, 480 (2002).

TbBaFeTbBaFe22OO55 Type III MV StructureType III MV StructureTemperature 350 KSpace Group Pmmma 3.94453(4) Åb 3.93331(4) Åc 7.58655(8) Å

Bond ValencesBa 1.98Tb 2.88Fe 2.52O(x) 1.94O(y) 1.95O(z) 2.05

Fe-O Distances2×2.0002(6) [Oy]2×2.0046(5) [Ox]1×1.9977(7) [Oz]

Karen, Woodward, Linden, Vogt, Studer, Fisher, Phys. Rev. B 64, 214405 (2001).

TbBaFeTbBaFe22OO55 Synchrotron XSynchrotron X--rayray

0

20

40

60

80

100

120

15 17 19 21 23 25

2-Theta (Degrees)

Inte

nsity

(Arb

. Uni

ts)

Superstructure Reflections indicate a

doubled a-axis

Space Group = PmmaSuperstructure

reflections are stronger when R = Nd

Superstructure reflections are very

difficult to observe in neutron powder patterns.

Karen, Woodward, Linden, Vogt, Studer, Fisher, Phys. Rev. B 64, 214405 (2001).

TbBaFeTbBaFe22OO55 Neutron DiffractionNeutron Diffraction

110 120 130 140 150

2-Theta (Degrees)

Inte

nsity

(Arb

. Uni

ts)

Room Temp. Structure1 Fe Site

Rietveld refinements of neutron powder diffraction data confirm charge ordered structure, TbBaFe3+Fe2+O5.

Mixed Valence Model

110 120 130 140 150

2-Theta (Degrees)

Inte

nsity

(Arb

. Uni

ts)

Doubled a-axis 2 Fe Sites

Charge Ordered Model

Karen, Woodward, Linden, Vogt, Studer, Fisher, Phys. Rev. B 64, 214405 (2001).

TbBaFeTbBaFe22OO55 Type I CO StructureType I CO Structure

2.134(4)Å

2.046(5)Å

1.965(4)Å

Fe2+ BVS=2.37

x2-y2

z2

xy

yzxz

x2-y2

z2

xz

xyyz

2.134(4)Å

1.965(4)Å

1.975(1)Å

1.895(6)Å

Fe3+ BVS=2.76

Temperature 70 KSpace Group Pmmaa 8.0575(2) Åb 3.85032(6) Åc 7.5526(2) Å

1.962(1)Å

TbBaFeTbBaFe22OO55 Magnetic ScatteringMagnetic Scattering

10 20 30 40 50

2-Theta (Degrees)

Inte

nsity

(Arb

. Uni

ts)

350 K

10 20 30 40 50

2-Theta (Degrees)

Inte

nsity

(Arb

. Uni

ts)

70 K

T > Tco Magnetic Cell2a × 2b × 2c

7.88Å × 7.87Å × 15.17Å

T < Tco Magnetic Cell2a × 2b × c

8.05Å × 7.70Å × 7.55ÅThe charge ordering induces a rearrangement of the

antiferromagnetic structure!

TbBaFeTbBaFe22OO55 MV State (T>TMV State (T>TCOCO) ) Magnetic StructureMagnetic Structure

AFM Coupling in AFM Coupling in abab PlanePlane

Isostructural with YBaFeCuO5

Fe-O-Fe Superexchange AFMFe-Fe Direct Exchange FM

FMFM

AFMAFM

AFMAFM

TbBaFeTbBaFe22OO55 CO State (T<TCO State (T<TCOCO) ) Magnetic StructureMagnetic Structure

AFM Coupling in AFM Coupling in abab PlanePlane

G-Type AFM StructureFe-O-Fe Superexchange AFMFe-Fe Direct Exchange AFM

AFMAFM

AFMAFM

AFMAFM

Charge & Spin Ordering in YBaMCharge & Spin Ordering in YBaM22OO55

YBaMn2O5TCO > 300 K (CB)

P4/nmmTC = 165 K (Ferri)

Millange, et al. Mater. Res. Bull. 1999, 34, 1.

YBaFe2O5TCO > 308 K (ST)

PmmaTN = 430, 308 K

Woodward, Karen Inorg. Chem. 2003, 42, 1121.

YBaCo2O5TCO > 220 K (ST)

PmmaTN = 330 K

Vogt, et al. PRL 2003, 84, 2969.

Orbital Ordering in YBaMOrbital Ordering in YBaM22OO55

YBaMn2O5P4/nmm

xz yzxy

z2

x2-y2

xz yzxy

z2

x2-y2

Mn3+

Mn2+

YBaFe2O5Pmma

xz yzxy

z2

x2-y2

xz yzxy

z2

x2-y2

Fe3+

Fe2+

YBaMn2O5Pmma

xz yzxy

z2

x2-y2

xz yzxy

z2

x2-y2

Co3+

Co2+

Phase Transitions Phase Transitions –– RBaFeRBaFe22OO55• Paramagnetic to Antiferromagnetic (430-450 K)

– YBaFeCuO5 Type AFM Order– Small Magnetostrictive coupling leads to a subtle Tetragonal

to Orthorhombic Distortion• Premonitory Charge Ordering (290-330 K)

– Subtle charge localization can be seen in DSC & Mossbauer, but not in diffraction measurements

– Mixed valency changes from Type I to Type II • Long Range Charge Ordering (240-290 K)

– Induces a large orbital ordering transition– Orbital ordering stabilizes a stripe CO pattern – Stabilizes G-type Antiferromagnetic order (changes the sign

of the Fe-Fe direct exchange)

Changing the size of the RE ion modifies the Fe-Fe distance.

3.55

3.57

3.59

3.61

3.63

3.65

3.67

3.69

3.71

115 117 119 121 123 125

R Radius (CN=8, pm)

Fe-F

e D

ista

nce

(Ang

stro

ms)

Ho

NdFe-Fe Dist.

1.96

1.97

1.98

1.99

2.00

2.01

2.02

2.03

2.04

115 117 119 121 123 125

R Radius (CN=8, pm)

Fe-O

Dis

tanc

e (A

ngst

rom

s)

Ho

Nd

Ave. Fe-O

156

157

158

159

160

161

162

163

115 120 125

R Radius (CN=8, pm)

Fe-O

Dis

tanc

e (A

ngst

rom

s)

Ho Nd

Ave. Fe-O-Fe

Fe-Fe Dist. Ave. Fe-O Dist.

Fe-O-Fe ∠

Ho

Y

Tb

Sm

Nd

Ho

Y Tb

Sm Nd

HoY

Tb

SmNd

Structural Evolution: REBaFeStructural Evolution: REBaFe22OO55

Phase Transitions vs. R sizePhase Transitions vs. R size

200

220

240

260

280

300

320

340

114 116 118 120 122 124 126

R Radius (CN=8, pm)

∆V (C

ubic

Ang

stro

ms)

Tpm

TCO

Ho

Nd

TCO

As the radius of the R ion increases

–TCO (MV II→MV I) decreases significantly.–TPM (MV III→MV II) decreases more gradually.–TN1 changes very little.

Type III Mixed Valent

Type ICharge Ordered

Type II

0.15

0.17

0.19

0.21

0.23

0.25

0.27

0.29

0.31

0.33

114 116 118 120 122 124 126

R Radius (CN=8, pm)

∆V

(Cub

ic A

ngst

roms)

HoNd

Sm

Pmma P21ma

Volume Change at TVolume Change at TCOCO

Volume change at TCO (MV II → MV I)

Pmma P21ma

Thermodiffractometry Thermodiffractometry (ILL(ILL--D20)D20)

Woodward, Karen, Suard, J. Amer. Chem. Soc. 125, 8889 (2003).

Pmma

PmmmHoBaFe2O5

400

020

200/

0 20

T(K

)

100

200

300

400

G GG

NdBaFe2O5

P21ma

Pmmm

400

020

200/

0 20

20 40 60 802-theta

M M M M

T(K

)

100

200

300

400

3.75

3.80

3.85

3.90

3.95

4.00

4.05

0 100 200 300 400 500Temperature (K)

Cell

Edge

(Ang

stro

ms)

a/2

c/2

b TN

Tpm

Tco

HoBaFe2O5 Unit Cell Evolution1 K Temperature grid, 1 min collection time

No noticeable change at TPM

Woodward, Karen, Suard, J. Amer. Chem. Soc. 125, 8889 (2003).

P4/mmm to Pmmm at TN

3.80

3.85

3.90

3.95

4.00

0 100 200 300

Temperature (K)

Cell

Edge

(Ang

stro

ms) a/2

c/2

bTpmTco

Woodward, Karen, Suard, J. Amer. Chem. Soc. 125, 8889 (2003).

NdBaFe2O5 Unit Cell Evolution1 K Temperature grid, 1 min collection time

Orbital Ordering (RE = Orbital Ordering (RE = NdNd, Ho), Ho)

0.00

0.05

0.10

0.15

0.20

0 200 400 600

Temperature (K)

(a/2

)-b

(Ang

stro

ms)

R=Nd

R=Ho

0.010

0.012

0.014

0.016

0.018

0.020

250 300 350

Temperature (K)

(a/2

)-b

(Ang

stro

ms)

R=Nd

R=Ho

Tpm

Tpm

Orthorhombic distortion saturates at Tpm for NdBaFe2O5. Similar

behavior is not observed for HoBaFe2O5

Orthorhombic distortion increase at TCO is much larger in HoBaFe2O5.

RBaFe2O5 (R=Tb, Y, Ho)YBaFeCuO5 AFM Structure

Fe-Fe Coupling Ferromagnetic

@ TCO

G-Type AFM StructureFe-Fe Coupling Antiferromagnetic

NdBaFe2O5YBaFeCuO5 AFM Structure

Fe-Fe Coupling Ferromagnetic

@ TCO

YBaFeCuO5 AFM StructureFe-Fe Coupling Ferromagnetic

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 100 200 300 400 500

Temperature (K)

Fe M

omen

t (B

ohr

Mag

neto

ns)

R=NdR=Ho

Magnetism vs. Temperature

TCOTCO

Nd Nd Magnetism in NdBaFeMagnetism in NdBaFe22OO55

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 20 40 60

Temperature (K)

Nd

Mom

ent

(Boh

r M

agne

tons

)Nd moment is ~1.2 µB at 2 K, TN (Nd) ~ 30 KNo rare-earth magnetic order for R = Ho.

Nd magnetism is induced by Fe magnetism.

Structural Tuning in RBaFeStructural Tuning in RBaFe22OO55

As the radius of the R ion increases (R = Ho-Sm)– The spacing across the R-layer increases– TCO decreases significantly, TPM decreases more gradually– TN1 changes very little– Patterns of charge, orbital and spin order remain constant

NdBaFe2O5 The large size of Nd has several effects– Disrupts the ideal pattern of orbital ordering

• The CO structure has P21ma symmetry rather than Pmma• The volume change at TCO is anomalous• The orthorhombic distortion parameter saturates at TPM

– Decouples the magnetic and charge order• There is no rearrangement of the magnetic structure at TCO

– Destabilizes the long range charge order• TCO is much lower than other members of the series