# Presentation: 079949

## Total Scattering The Key to Understanding the Local- and Medium-Range Structure of MaterialsThomas Proffen tproffen@lanl.gov

- LA-UR 05-1010
- LA-UR 06-6075

## The challenge : Knowing the local structure

- Traditional crystallographic approach to structure determination is insufficient or fails for
**Non crystalline materials**
**Disordered materials**: The interesting properties are often governed by the defects or local structure !
**Nanostructures**: Well defined local structure, but long-range order limited to few nanometers (-> poorly defined Bragg peaks)
- A new approach to determine local and nano-scale structures is needed.

## Total scattering ?

**Cross section of 50x50x50 u.c. model crystal consisting of 70% black atoms and 30% *****vacancies***** !**
**Properties might depend on vacancy ordering !! **

## Bragg peaks are blind ..

**Bragg peaks are blind ..**

**Bragg scattering****: Information about the *****average***** structure, *****e.g.***** average positions, displacement parameters and occupancies.**

## Diffuse scattering to the rescue ..

**Diffuse scattering to the rescue ..**

**Diffuse scattering****: Information about *****two-body correlations*****, *****i.e.***** chemical short-range order or local distortions.**

## See http://www.totalscattering.org/teaching/ 

**See **_**[http://www.totalscattering.org/teaching/](http://www.totalscattering.org/teaching/)**__** **_

## How about powder diffraction ?

**How about powder diffraction ?**

## Finally the Pair Distribution Function (PDF)

**The PDF is the ****Fourier transform ****of the ****total scattering ****diffraction pattern !**

- Proffen, *Z. Krist*, **215**, 661 (2000)

## What is a PDF?

**What is a PDF?**

**Pair distribution function (PDF) gives the probability of finding an atom at a distance “r” from a given atom.**

## PDF allows r-dependent refinements

**PDF allows r-dependent refinements**

- In both cases:
- ‘Real’ NN distance = 1.0Ǻ
**Average structure: **
- Ordered: 	NN distance = 1.00Ǻ
- Disordered: 	NN distance = 0.98Ǻ

## Instruments

**and**
**Software**

## What is required to obtain high quality PDFs ?

- The PDF (similar to the Patterson) is obtained via Fourier transform of the normalized total scattering S(Q):

***Requirements to obtain ‘good’ PDF:***
- High maximum momentum transfer, Qmax.
- High Q-resolution.
- Good counting statistics @ high Q.
- Low instrument background** **

## NPDF: the key to high quality, high r PDFs

- The PDF (similar to the Patterson) is obtained via Fourier transform of the normalized total scattering S(Q):

*Requirements to obtain ‘good’ PDF:*
-  High maximum momentum transfer, Qmax.
-  High Q-resolution.
-  Good counting statistics @ high Q.
-  Low and stable instrument background.

## NPDF: Overview

- Specifications
- Upgrade finished Sep. 2002
- L1: 32m, Qmax=50Å-1, d/d=0.15%
- Typical PDF measurement 1 - 4 hrs
- Sample amounts down to 200 mg
- Ancillary: 10K-1500K, soon: 0.5K, 11T
- Science
- 95% PDF studies, hard matter
- Many users *new* to PDF
- Oversubscription in 2006: ~1.6
- Software
- Web based instrument interface
- Automatic creation of PDF
- Integration in SNS data portal (soon)
- _[http://www.lansce.lanl.gov](http://www.lansce.lanl.gov/)_

## X-ray PDF: The fast way

- Image plate
- detector

- P.J. Chupas, X. Qiu, J.C. Hanson, P.L. Lee, C.P. Grey and S.J.L. Billinge, **Rapid-acquisition pair distribution function (RA-PDF) analysis**, *J. Appl. Cryst.* **36**, 1342-1347 (2003).

- Exposure: 25 sec !

## X-ray PDF: In house measurements

- Huber Gunier diffractometer
*Q*max = 13.5Å-1

***fci-*****Ho-Mg-Zn**

- Brühne et al., *Z. Kristallogr.* **219**  (2004) 245-258

**X-ray PDF: In house measurements**

## Software: Data reduction

- X-rays
- IDL based.
- Laboratory and synchrotron data
- _[http://www.pa.msu.edu/cmp/billinge-group/programs/PDFgetX2/](http://www.pa.msu.edu/cmp/billinge-group/programs/PDFgetX2/)_

- Neutrons
- GLASS package / Perl/TK frontend
- Automatic data reduction on NPDF
- _[http://pdfgetn.sourceforge.net/](http://pdfgetn.sourceforge.net/)_

## Software: Data modeling 

- PDFgui
- Part of DANSE project.
- PDF refinement of ‘small’ models.
- _[http://www.diffpy.org/](http://www.pa.msu.edu/cmp/billinge-group/programs/PDFgetX2/)_

- DISCUS
- Disordered materials simulations
- Refinement via DIFFEV / RMC
- _[http://discus.sourceforge.net/](http://pdfgetn.sourceforge.net/)_

## Disorder in Fe3(CO)12

**Thomas Proffen (LANL)**

**Disorder in Fe****3****(CO)****12**

## Diffuse scattering single crystal diffractometer

**Diffuse scattering single crystal diffractometer**

## Fe3(CO)12 : AMC refinement

**Model**

**Data**

- [100]

- [101]

**Fe****3****(CO)****12 ****:**** AMC refinement**

## Fe3(CO)12 : Resulting structure

**Two modes of libration not distinguished by**
**Bragg scattering**

**Fe****3****(CO)****12 ****: Resulting ****structure**

- Positions of Fe averaged over unit cells containing either (+ +) or (- -)
- combinations on nearest-neighbor vector 1.

## Example:

- Simon Billinge (Columbia)
- Thomas Proffen (LANL)
- Peter Peterson (SNS)

**Example:**
**Local atomic strain in ZnSe****1-x****Te****x**

## ZnSe1-xTex : Structure

- Zinc blend structure (F43m)
- Technological important : Electronic band gap can be tuned by the composition *x*.
- Bond length difference *Zn-Se* and *Zn-Te*  strain.
- Local structural probe required !

## ZnSe1-xTex : Total scattering

**ZnSe****1-x****Te****x**** : Total scattering**

- Peterson et al., *Phys. Rev. *B**63, **165211 (2001)

## ZnSe1-xTex : Nearest neighbors and Z-plots ..

**ZnSe****1-x****Te****x**** : Nearest neighbors and Z-plots ..**

## Example:

- Emil Bozin
- Xiangyn Qiu
- Thomas Proffen

**Example:**
**Local structure in La****x****Ca****1-x****MnO****3**

## LaMnO3: Jahn-Teller distortion

- Mn-O bond lengths are invariant with temperature, right up into the R-phase
- JT distortions persist locally in the pseudocubic phase
- Agrees with XAFS result:  M. C. Sanchez et al., PRL (2003).

- Average structure

- Local structure

- Jahn Teller
- Long Mn-O bond

## Refinement as function of

- X. Qiu, Th. Proffen, J.F. Mitchell and S.J.L. Billinge, **Orbital correlations in the pseudo-cubic O and rhombohedral R phases of LaMnO3**, *Phys. Rev. Lett.* **94**, 177203 (2005).

- atom-atom distance *r* !

## LaMnO3 : T-dependence of orbital clusters from PDF

- Diameter of orbitally ordered domains above TJT is 16Ǻ
- Appears to diverge close to TJT
- Red lines are a guide to the eye (don’t take the fits too seriously!)

## La1-xCaxMnO3: Structural phase diagram

- Phase diagram draws itself from the parameters.
- Unexpected detail emerges and demands interpretation.

**Atomic displacement parameter (ADP) for Oxygen**
- (measure for thermal and static deviations from site)

## La1-xCaxMnO3: Structural phase diagram

- Mn-O
- long bond

- ADP
- Oxygen

**La****1-x****Ca****x****MnO****3****: Structural phase diagram**

## Example:

**“****Complete” structure of**
**Gold Nanoparticless**

- Katharine Page
- Ram Seshadri
- Tony Cheetham
- Thomas Proffen

## Gold nanoparticles

- Nanoparticles often show different properties compared to the bulk.
- Difficult to study via Bragg diffraction (broadening of peaks).
- PDF reveals “complete” structural picture – core and surface.
- This study:
- 5nm monodisperse Au nanoparticles
- 1.5 grams of material
- Neutron measurements on NPDF

## Gold nanoparticles: First NPDF data

- Bulk gold

- Gold nanoparticles

- Average diameter ~3.6nm

- K.L. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri and A.K. Cheetham, **Direct Observation of the Structure of Gold Nanoparticles by Total Scattering Powder Neutron Diffraction**, *Chem. Phys. Lett.* **393**, 385-388 (2004).

## Au nanoparticles: Particle size

- We’re dealing with a length scale that can be simulated on an atom by atom basis, perhaps opening the door to extremely detailed refinements.

**Au nanoparticles: Particle size**

## Summary

- Analysis of total scattering gives valuable insight in structure  properties relationship
- High-resolution instruments open the door to medium-range order investigations
- Obtain structural information from disordered crystalline, amorphous of composite materials
- Fast powder measurements allow systematic exploration of local structure as function of *T, x, P*

_**http://www.totalscattering.org**_

## Thank you

***LANSCE***