# Presentation: 657643

## Charge, Orbital and Spin Ordering in RBaFe2O5+x Perovskites  

**Patrick M. Woodward**
*Department of Chemistry*
*Ohio State University*
**Pavel Karen**
*Department of Chemistry*
*University of Oslo*

## Charge, Orbital and Spin Ordering

**Charge Ordering (T****CO****)**
- T > TCO  Delocalized Electrons  Single oxidation state
- T < TCO  Localized Electrons  Distinct oxidation states
**Orbital Ordering (T****OO****)**
- Preferential (anisotropic) occupation of given d-orbitals
- Cooperative Jahn-Teller distortion
**Spin Ordering (T****N**** or T****C****)**
- 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+)O6 
Cox & Sleight (1976)
High Tc Superconductivity in Oxides

*Cox & Sleight (1976)*
- High Tc Superconductivity in Oxides

**Fe****3+****(Fe****2.5+****)****2****O****4**** ******** Fe****3+****(Fe****2+****Fe****3+****)O****4**
*Verwey (1939)*
- Double Exchange Ferromagnetism

**LaCa(Mn****3.5+****)****2****O****6 ******** LaCa(Mn****3+****Mn****4+****)O****6**
*Wollan, Koehler & Goodenough (1955)*
- Colossal Magnetoresistance (CMR)

**Examples of Charge Ordering**

## Charge Ordering in Nd0.5Sr0.5MnO3

**T = 160 K **
**Valence Mixed State**

**T = 60 K **
**Charge Ordered State**

**Synchrotron X-ray Powder Diffraction Data (NSLS-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 Ordering

**Examples of Orbital Ordering**

- 22.18 Å

- 21.91 Å

- 21.94 Å

- 22.07 Å

**LaMnO****3**** (298 K)**
- Rodriguez-Carvajal, *et al*. *Phys. Rev. B* **57**, R3189 (1998).

**NdSrMn****2****O****6**** (50 K)**
- Woodward, *et al*. *Chem. Mater.* **11**, 3528-38 (1999).

**Mn****3+**

**Mn****4+**

- 41.90 Å

## Orbital Ordering & Cooperative Jahn-Teller Distortions

- d(x2-y2)

- d(z2)

**Mn****3+**

- eg (*****)

- t2g (*****)

- Regular
- Octahedron

- Axially Elongated
- Octahedron

## Examples of Orbital Ordering

**Examples of Orbital Ordering**

- 22.18 Å

- 21.91 Å

- 21.94 Å

- 22.07 Å

**LaMnO****3**** (298 K)**
- Rodriguez-Carvajal, *et al*. *Phys. Rev. B* **57**, R3189 (1998).

**NdSrMn****2****O****6**** (50 K)**
- Woodward, *et al*. *Chem. Mater.* **11**, 3528-38 (1999).

**Mn****3+**

**Mn****4+**

- 41.90 Å

## Orbital Ordering in Nd0.5Sr0.5MnO3

- 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).

## Oxygen Deficient Double Perovskites RBaFe2O5+w

**R = Trivalent Rare Earth Ion**
- Nd, Sm, Tb, Ho, Y
- Changing the radius of the R ion, controls the layer spacing
- M = **1****st**** 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 REBaFe2O5

- 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 pO2 atmosphere.
- 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

**BaCO****3**

- Amorphous
- Organic-
- Inorganic
- Precursor

- Heat

- & Stir

## Powder 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)

## DSC

**Electrical Resistivity**

**T>T****PM**** **Mössbauer shows one Fe2.5+ signal (Type III MV)
**T****PM **>**T>T****CO** Mössbauer signal begins to split Fe2.5+x + Fe2.5-x (Type II MV)
**T****CO **> **T **Mössbauer shows Fe2+ + Fe3+ (Type I MV, Charge Ordered State)

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

## TbBaFe2O5 Type III MV Structure

**Temperature****	****    350 K**
**Space Group****	****    Pmmm**
**a****	****	****	****3.94453(4) Å**
**b****	****	****	****3.93331(4) Å**
**c****	****	****	****7.58655(8) Å**
**Bond Valences**
**	****Ba****	****	****   1.98**
**	****Tb****	****	****   2.88**
**	****Fe****	****	****   2.52**
**	****O(x)****	****	****   1.94**
**	****O(y)****	****	****   1.95**
**	****O(z)****	****	****   2.05**

**Fe-O Distances**
**2********2.0002(6) [O****y****]**
**2********2.0046(5) [O****x****]**
**1********1.9977(7) [O****z****]**

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

## TbBaFe2O5 Synchrotron X-ray

- Superstructure Reflections indicate a _doubled a-axis_
**Space Group = *****Pmma***
- Superstructure 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).

## TbBaFe2O5 Neutron Diffraction

**Rietveld refinements of neutron powder diffraction data confirm charge ordered structure, TbBaFe****3+****Fe****2+****O****5****.**** **

**Mixed Valence Model**

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

## TbBaFe2O5 Type I CO Structure

**TbBaFe****2****O****5**** Type I CO Structure**

- 2.134(4)Å

- 2.046(5)Å

- 1.965(4)Å

**Fe****2+**** BVS=2.37**

- 2.134(4)Å

- 1.965(4)Å

- 1.975(1)Å

- 1.895(6)Å

**Fe****3+**** BVS=2.76**

- Temperature	    70 K
- Space Group	   Pmma
- a		8.0575(2) Å
- b		3.85032(6) Å
- c		7.5526(2) Å

- 1.962(1)Å

## TbBaFe2O5 Magnetic Scattering

- T > Tco Magnetic Cell
- 2a  2b  2c
- 7.88Å  7.87Å  15.17Å

- T < Tco Magnetic Cell
- 2a  2b  c
- 8.05Å  7.70Å  7.55Å

**The charge ordering induces a rearrangement of the antiferromagnetic structure!**

## TbBaFe2O5 MV State (T>TCO) Magnetic Structure

- AFM Coupling in *ab* Plane

**Isostructural with YBaFeCuO****5**
- Fe-O-Fe Superexchange AFM
- Fe-Fe Direct Exchange FM

## TbBaFe2O5 CO State (T<TCO) Magnetic Structure

**TbBaFe****2****O****5**** CO State (T<T****CO****) Magnetic Structure**

- AFM Coupling in *ab* Plane

**G-Type AFM Structure**
- Fe-O-Fe Superexchange AFM
- Fe-Fe Direct Exchange AFM

## Charge & Spin Ordering in YBaM2O5

- YBaMn2O5
- TCO > 300 K (CB)
- P4/nmm
- TC = 165 K (Ferri)
- Millange, *et al.* *Mater. Res. Bull.* **1999**, *34*, 1.

- YBaFe2O5
- TCO > 308 K (ST)
- Pmma
- TN = 430, 308 K
- Woodward, Karen *Inorg. Chem.* **2003**, *42*, 1121.

- YBaCo2O5
- TCO > 220 K (ST)
- Pmma
- TN = 330 K
- Vogt, et al. *PRL* **2003**, *84*, 2969.

## Orbital Ordering in YBaM2O5

- YBaMn2O5
- P4/nmm

- xz

- yz

- xy

- z2

- x2-y2

- xz

- yz

- xy

- z2

- x2-y2

- Mn3+

- Mn2+

- YBaFe2O5
- Pmma

- xz

- yz

- xy

- z2

- x2-y2

- xz

- yz

- xy

- z2

- x2-y2

- Fe3+

- Fe2+

- YBaMn2O5
- Pmma

- xz

- yz

- xy

- z2

- x2-y2

- xz

- yz

- xy

- z2

- x2-y2

- Co3+

- Co2+

## Phase Transitions – RBaFe2O5

- 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)

## Structural Evolution: REBaFe2O5

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

**Fe-Fe Dist.**

**Ave. Fe-O Dist.**

**Fe-O-Fe ******

**Ho**

**Y**

**Tb**

**Sm**

**Nd**

**Ho**

**Y**

**Tb**

**Sm**

**Nd**

**Ho**

**Y**

**Tb**

**Sm**

**Nd**

**Structural Evolution: REBaFe****2****O****5**

## Phase Transitions vs. R size

- 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 I
- Charge Ordered

- Type II

## Volume Change at TCO

**Volume Change at T****CO**

- Volume change at TCO (MV II  MV I)

*Pmma*

*P2**1**ma*

## Thermodiffractometry (ILL-D20)

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

## HoBaFe2O5 Unit Cell Evolution1 K Temperature grid, 1 min collection time

**HoBaFe****2****O****5**** Unit Cell Evolution****1 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

## NdBaFe2O5 Unit Cell Evolution1 K Temperature grid, 1 min collection time

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

**NdBaFe****2****O****5**** Unit Cell Evolution****1 K Temperature grid, 1 min collection time**

## Orbital Ordering (RE = Nd, Ho)

- 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 Structure
Fe-Fe Coupling Antiferromagnetic

NdBaFe2O5
YBaFeCuO5 AFM Structure
Fe-Fe Coupling Ferromagnetic
@ TCO
YBaFeCuO5 AFM Structure
Fe-Fe Coupling Ferromagnetic

- YBaFeCuO5 AFM Structure
- Fe-Fe Coupling _**Ferromagnetic**_
-    @ TCO
- G-Type AFM Structure
- Fe-Fe Coupling _**Antiferromagnetic**_
**NdBaFe****2****O****5**
- YBaFeCuO5 AFM Structure
- Fe-Fe Coupling _**Ferromagnetic**_
- @ TCO
- YBaFeCuO5 AFM Structure
- Fe-Fe Coupling _**Ferromagnetic**_

**Magnetism vs. Temperature**

**T****CO**

**T****CO**

## Nd Magnetism in NdBaFe2O5

**Nd Magnetism in NdBaFe****2****O****5**

- Nd moment is ~1.2 B at 2 K, TN (Nd) ~ 30 K
- No rare-earth magnetic order for R = Ho.
**Nd magnetism is induced by Fe magnetism.**

## Structural Tuning in RBaFe2O5

**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
**NdBaFe****2****O****5**** The large size of Nd has several effects**
- Disrupts the ideal pattern of orbital ordering
- The CO structure has *P2**1**ma* 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