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