# Presentation: 472500

## Slide 1

## Slide 2

## Slide 3

## Slide 4

## Surfaces, Interfaces, Buried  layers

*In situ* UHV chamber

- 6- circle diffractometer
- Surface XRD
- XSW
- XAFS
- Reflectivity- specular
- 	    - diffuse

-              MBE-2
- 3 growth sources
- RHEED
- Auger
- Argon sputtering
- -100C to 800C

## 6- circle (4s + 2d) diffractometer

**6- circle (4s + 2d) diffractometer**
**         **
**4 sample-orienting circles**
-  	****or )
-    	**** (rotate about vertical axis)
-    	****(flip)
****(azimuthal)
**2 detector-orienting circles**
**	******** **(or 2)
- 	

**Surface XRD**
**       **In-plane scattering
-        Out-of-plane (rod scans)
**Bulk  XRD**

**Fig. 1. Haydoo You, J.Appl.Cryst. 32(1999) 614.**

## Bulk and Surface Diffraction from GaAs0.5Sb0.5 in MBE-2

- Bulk and Surface Diffraction from GaAs**0.5**Sb**0.5**** **in MBE-2

- GaAsSb grown on InP by OMVPE (organometallic vapour phase epitaxy) has the potential to produce in-plane CuAu atomic ordering. (Watkins, SFU)

**Double period (100) fringes indicate **
**Ordering**
**Plan-view [001], down-pole high resolution TEM lattice image**
- W.Y. Jiang et al App.Phys.Lett. 85 (2004) 5589

**- Arsenic**
**- Antimony**
**- Gallium**

- 8 nm

- (100)

- (010)

## Slide 8

## Slide 9

## Slide 10

## In-plane structural Anisotropy  of ultra-thin Fe films on GaAs(001)

**In-plane structural Anisotropy  of ultra-thin Fe films on GaAs(001)**
**R.A. Gordon and E.D. Crozier, Physics, Simon Fraser University**
-                                         CAP Congress 2005, Phys. Rev. B (2006) accepted

- Development of spintronics
-    (spin injection through ferromagnetic/semiconductor
-  			            thin-film interface)
- Fe/GaAs
-      Ideal system- Fe has high Curie temperature
- 	           - small mismatch (1.4%) between
-                            cubic GaAs a/2= 2.827Å
- 	             and bcc Fe    a= 2.866 Å
- 	           - controlled epitaxial growth

## Persistent issue: the origins of uniaxial in-plane magnetic 		             anisotropy

- Persistent issue: the origins of uniaxial in-plane magnetic 		             anisotropy
- “Structural and magnetic properties of ultrathin epitaxial Fe films on GaAs(001)”
- G. Wastlbauer and J. A.C. Bland, Adv Phys **54** (2005) 1378-219.

- Magnetic properties of Fe/GaAs depend on
-      - thickness of layers
-      - nature of interface
- 	- reaction at interface?----(Fe2As,  Fe3Ga2-xAsx)
- 	- bonding at interface/out-diffusion from substrate?
- 	- strain at interface?
- Ga-terminated reconstruction: 4 x 6 (Pseudo 4x6- 4x2 & 2x6)
-      - gives good epitaxial films
-      - does not lead to magnetic dead layers ( surface reaction)
**But: **universal effect --UIPMA appears independent  of reconstruction
-                                     with easy axis along (110)
- Stronger effect at lower coverage- effect due to interface itself
- Focus on ultra-thin films- only a few monolayers thick

## The experiment itself...

- First-principles calculations by Mirbt et al.
-  Structural and electronic properties of the interface of Fe on GaAs(001)
- (S. Mirbt, B. Sanyal, C. Isheden and B. Johansson, Phys. Rev. B 67 (2003) 155421.
- VERSUS
- Our earlier XAFS measurements.
- (R. A. Gordon, E. D. Crozier, D. T. Jiang, T. L. Monchesky, B. Heinrich, Phys. Rev. B 62 (3), July, 2151-2157 (2000), R.A. Gordon, E.D. Crozier, D.-T. Jiang, P.S. Budnik, T.L. Monchesky, B. Heinrich, Surf. Sci. 581(1), 47–57 (2005))

- Both agree on distortion to give *c/<a>* = 1.03 near 5ML
- XAFS indicates body-centred tetragonal *c/<a>*  is constant from 5 to 30 ML

## Calculations suggest...

- An in-plane distortion in addition to the out-of-plane
- Calculated distortions along (110) and (-110) indicate changes in nearest-neighbour distances
- Film	NN-(110)/Å		NN-(-110)/Å
- Bulk Fe	2.482		2.482
- 2ML	2.471		2.531	**Δ**** ****~0.06****Å**
- 5ML	2.472		2.510             **Δ**** ~0.04Å**
- Use Surface X-ray Diffraction?
-  need in-situ capability (MBE-2)
-  require out-of-plane component to isolate film but the films are very thin or islands
- Use XAFS?
-  need kmax~ 36 Å-1 to resolve ΔR ~ 0.04Å but typical kmax~ 15 Å-1

## How to see a splitting ~ 0.04Å with XAFS?

- Polarization-dependent XAFS

- Unpolarized

- X-ray electric field out-of-plane along (001)

- X-ray electric field in-plane along (110) [or (-110)]

- We can use this to isolate sets of nearest-neighbours!

- Absorption co-efficient

## Sample preparation

- Substrate prepared, 2ML and 5ML films deposited and monitored by RHEED
- See differences in RHEED images of reconstruction
- (x6 spots better defined – larger terraces along this direction)

## X-ray Absorption Fine Structure Data

- XANES

- EXAFS

## Fit results

- Key fit parameters: R, σ2 for each shell, j, with N atoms scattering

- Orientation    R1/Å   		σ12/10-4Å2		Calculation
- 2ML
- (001)  	2.470(10)		109(5)		<2.501>
- (110)	2.461(10)		112(5)		  2.471
- (-110)	2.478(10)		112(5)		  2.531
- R(-110)-R(110)
-         ΔR   0.017				  0.06
- 5ML
- (001)	2.485(11)		66(4)		<2.467>
- (110)	2.490(10)		79(3)		  2.472
- (-110)	2.484(10)		68(3)		  2.510
-        ΔR  ~0.0				  0.04
- Fe foil	2.482		52(4)

- In island-growth regime:
- see small splitting

- In layer regime:
- splitting not detected, but more disorder along (110)

## In-plane Structural Anisotropy of Ultra-thin Fe Films on GaAs(001)-4x6

-                                                                                   Phys. Rev. B (2006) accepted

**Conclusions**:
- Some evidence for a distortion in-plane before islands coalesce into layers.
- Smaller distortion at 2ML than calculations predicted, but direction agrees.
- No strong evidence for distortion at 5ML, but anisotropy in disorder present:
-  can disorder affect uniaxial in-plane magnetic anisotropy?
-  how does layer formation relieve strain in distorted islands?
- Are the anisotropy and disorder independent of reconstruction?
**Next experiments** – *in situ *SXRD with MBE-2.

## Molecular-scale Characterization of Arsenic in Mining and Metallurgical WastesDogan PaktuncCANMET, Mining and Mineral Sciences Laboratories, Canada

**Molecular-scale Characterization of Arsenic**** in Mining and Metallurgical Wastes**Dogan PaktuncCANMET, Mining and Mineral Sciences Laboratories, Canada

**Complex, heterogeneous bulk samples **
**should be analyzed with micro-XAFS**

## scorodite

- FeAsO4·2H2O

- arseniosiderite
- Ca2Fe3(AsO4)3O2·3H2O

- arsenopyrite
- FeAsS

**Notes:**
With 46 % As, arsenopyrite is one of the most important As sources in mining and metallurgical wastes. Shown on the top left is a photomicrograph of an arsenopyrite grain measuring about 60 um as islands or relicts (white) in secondary scorodite (grey) and arseniosiderite (light grey). Oxidative dissolution of arsenopyrite often results in the formation of scorodite, a ferric arsenate composed of isolated arsenate tetrahedra (purple) corner linked to isolated iron oxide octahedra (yellow). In arsenopyrite, three arsenic atoms (purple balls) occur at the corners of Fe octahedra (brown) along with three S atoms (yellow balls). Transformation from As1- to As5+ occurs in several steps. It initially forms arsenite 3+ in solution and further oxidized to As5+ and precipitated with Fe as a ferric arsenate. Scorodite is relatively stable at low pH but transforms to Ca-ferric arsenates (arseniosiderite, yukonite) or goethite as the pH increases. In the arseniosiderite structure, arsenate is corner linked to Fe oxide octahedra by sharing its three basal oxygens. The apical oxygen is linked to Ca polyhedra which is coordinated to 8 oxygens. Ca polyhedra link As-Fe-As layers in the structure. Based on bond valences and experimental evidence, Ca-O bonds are weak and when Ca-O bonds are broken, the structure disintegrates by releasing As-Fe-As layers. Without micro-XAFS, determination of the local structures wouldn’t have been possible and their bearings on the release of As.

## Micro-EXAFS goethite α-FeO(OH)

**Micro-EXAFS goethite ****α****-****FeO(OH)**

**As-O**

_**As-O**_** **
**R=1.69****Å **
**CN =4.0-4.8**

_**As-Fe:**_** **
**R=3.24-3.32****Å  CN =0.7-2.6**

_**As-Ca:**_
**R=4.14-4.17Å**
**CN =2.6-5.7**

**bidentate-binuclear**

**Notes:**
These are micro-XAFS spectra from a series of goethite grains with a range of As concentrations from low to high. Shown at the top left is a typical goethite grain showing colloform type texture with variable As concentrations. The goethite containing 2 wt % As2O5 is represented by the spectra on the bottom and goethite with 26 % As2O5 is at the top. Among the three possible arsenate sorption geometries, xafs data indicate that arsenate tetrahedra are corner linked to Fe-oxide octahedra as bidentate-binuclear complexes. 
In addition, xafs spectra revealed the presence of a third shell which is modeled to be Ca. The presence of Ca shell in goethite was a surprise finding of this study. Initially, based on bulk XAFS experiments, I thought that the Ca was arising from the presence of Ca-Fe arsenates in the bulk samples. But micro-XAFS spectra obtained from about 5 um spots in goethite indicated that it was not the case. 

## (001)

- (010)

-  10 dioctahedral chains
-  28 singly-coordinated
-   surface sites
-  max. AsO4 adsorption:
-   3.3 AsO4 per nm2
-   (bidentate-binuclear)
-  molar Fe/As=1.4

**Notes:**
The As vs Fe plot on the upper right shows the composition of goethite grains identified by electron microprobe analysis and Debye-Sherarer X-ray diffraction. 
The highest As concentrations corresponding to molar Fe/As ratio of ~ 1.4 are only possible by a small unit of goethite such as shown here, composed of 10 dioctahedral chains. 
There are 20 singly-coordinated surface sites that are oxo sites and 8 singly-coordinated surface sites that are hydroxo sites. These reactive surface sites can accommodate up to 14 inner-sphere bidentate-binuclear arsenates. The maximum AsO4 adsorption density in this case is 3.3 arsenate per nm2. This corresponds to molar Fe/As ratio of 1.4 and it is the highest possible arsenate adsorption. With the attachment of 14 arsenate ions, all the reactive surface sites become used inhibiting particle growth in all direction and the particle remains as a nanoparticle.

## Molecular-scale Characterization of Arsenic in Mining and Metallurgical WastesDogan PaktuncCANMET, Mining and Mineral Sciences Laboratories, Canada

**Molecular-scale Characterization of Arsenic**** in Mining and Metallurgical Wastes**Dogan PaktuncCANMET, Mining and Mineral Sciences Laboratories, Canada

**Results****  **
-  Micro-XAFS of individual mineral grains determined the speciation of
-    As and resolved complex bulk XAFS data in a gold mine tailings.
-   In Fe(III) oxyhydroxides, arsenate tetrahedra are corner linked to
-    edge-sharing iron octahedra as bidentate-binuclear complexes.
-   Extremely high As concentrations in goethite (i.e. Fe/As=1.4) are only
-    possible by small units of goethite composed of 10 dioctahedra.
-   Ca-Fe arsenates coprecipitated with Fe(III) oxyhydroxides occur as
-    arseniosiderite-like nanoclusters.

## Environmental Sciences GroupKen J. Reimer,  Royal Military College

***Interests:*** Basic/applied environmental sciences & engineering

**Arsenite/Arsenate**

**Arsenic(III)-sulphur**

**Arsenite**

- Arsenic speciation in plants, solid tissue,  and soil matrices

**Notes:**
The Environmental Sciences Group is a multidisciplinary organization involved in basic and applied environmental science and engineering.  They are based out of Kingston ON, and located on the campus of the Royal Military College.

Previous work by this group focused on using XAS to “fill in gaps” in their arsenic speciation research which result from the limitations (namely extraction and sample prep) of the more conventional methods at their disposal.  ESG has been working in the area of arsenic speciation for over 20 years and has, in collaboration with others, developed some of the key methods for arsenic characterization.  They have published a paper highlighting XANES spectra for environmentally significant arsenic compounds, and past work has focused on collecting XAS data for a variety of environmentally interesting samples including contaminated soils, bioreactor solids (remaining after arsenic is removed from water), feathers, rodent fur and foodstuffs such as the radish we see here.  These results led to the question of where in the plant these arsenic compounds might be located, and our group (PNC/XOR) suggested trying microprobe experiments.
 <CLICK>
ESG is interested in determining the spatial distribution of arsenic compounds within these samples because the information would be useful in elucidating mechanisms by which arsenic compounds are produced, transformed and transported in the environment. Here we can see maps of the arsenic distribution in radish leaves, also collected on the BM line.  Data was collected at energies corresponding to the arsenite and arsenate K-edges.  
<CLICK>
When the data collected are compared by subtraction, the resulting maps suggests a segregation of arsenic compounds in the  transport vessels of the plant.  This same segregation was observed in stem sections studied.  

## Environmental Sciences Group

***Interests:*** Basic/applied environmental sciences & engineering

- Current extraction methods for removing arsenic from fur/hair are unable to distinguish the distribution of the arsenic within the sample, synchrotron techniques are essential for this type of study.

**Notes:**
In addition to studying plants, ESG is looking at rodents and birds as part of a studies on arsenic in terrestrial and marine food chains.  Both fur and feathers have been found to have low extraction efficiencies for arsenic resulting in a large amount of unidentified arsenic remaining in the solid sample even after extraction.  In such cases synchrotron may be the only method to analyze these solid samples.

Red squirrel fur (two toned) was collected from animals living in an arsenic contaminated Yellowknife site.  A horizontal scan of the fur was conducted on the ID line in an effort to determine the distribution of arsenic within the rodent fur.
<CLICK>The peak in the center of each scan indicates that the higher concentration of arsenic is in the center of the hair. This seems to correlate well with the medulla (a “fatty region”  that collects contaminants and may not be visible if not containing contaminants). We find this type of information valuable when trying to elucidate the mechanisms by which arsenic compounds are produced, transformed and transported in the environment, and ESG is not aware of another method available that can deliver these results.
<CLICK> 
XANES scans on the ID line found As5, As3 and AsS compounds present in this sample.  The As3 and As-S compounds are not unexpected, but the presence of an As(V) compound apparently inside of the hair was an unexpected finding as it is usually associated with surface contamination, and will require further study. 

Random Notes:
The fur was washed with 70% ethanol and ddH2O.

## Trace Metals in Human Cementum

**Trace Metals in Human Cementum**

- 	The roots of human teeth are surrounded by a series of concentric mineralized rings, the cementum rings. These rings are deposited annually and thus may represent a chronological record of metal exposure. Since the ring system is approximately 100 microns wide and individual rings are of the order of 5 microns across Synchrotron radiation analysis is ideal for studying this system.** **

- Ronald R. Martin, Chemistry, University of Western Ontario

## annual growth rings

- 100 m

## Intensities for selected elements.

**Intensities for selected elements**.
- -  Tooth extracted in 2006.
-   The peaks in Pb indicate lead
-     exposure events early in the life
-     of the 40 year old man.

**Conclusions**
- Analysis of annual growth rings permits distinguishing endogenous (before death) from diagenetic (after death) deposition of trace metals1,2.
- Provides method of studying diet.

- R.R. Martin  *et al*, Journal of Archaeological Science (2006) accepted
- Metal concentrations in cementum rings as an aid to bioarchaeology, R.R. Martin *et al* APS Science 2004, ANL-05/04  (May 2005)page 101

## Slide 30

## 25 oC

- 25 oC

- 191 oC

- 600 oC

- 632 oC

- Granitic Glass
- +1 M Na2CO3
- + Vapor

- Liq+Vapor Liq
- Homogenization

- Silicate Melt +
- Alkaline
- Hydrothermal
-  Fluid

- Total
- Homogenization
- (P ~ 700 MPa)

- Br map

- Nb map

- Nb map at 632  oC

## XRF Spectra of Aqueous Fluid Phase (Coexisting With Melt)

## Nb K-edge XANES Spectra

- Nb *K*-edge XANES Spectra

- Pre-edge
- feature

## Mapping Elemental Distribution 

- Mapping Elemental Distribution
- in the Diablo Meteorite

-  Blue area –
-  kamacite
- (7 % Ni in Fe)

- (approx 600 x 600 micron area,
-  	X-ray spot 5 microns)

- R G Cavell et al
- Univ of Alberta

- N.B. Ga (80 ppm) and Ge  (500 ppm) readily mapped. 	EXAFS analysis done for these elements.

- Black area - Carbide phase –
- has  almost no Ni - all Fe and C

- (Cavell et al, *Amer  Mineralogist, *(2004) 89, 519)

- Phosphide phase (which has  Ni > Fe)

- Low Fe
- (Phosphide)

- (E-microprobe backscatter)

## C = cohenite

- 	(carbide)

- K = kamacite
- 	(7% Ni in Fe)

- S = schreibersite
- 	(phosphide)

- Relative Elemental Distribution across diagonal (C-D) traverse

- Canyon Diablo Meteorite Specimen

- C      Traverse       D

- Relative Intensity

- (Cavell et al,
*	**Amer Mineralogist, *
* **	*(2004) 89, 519)

- Notes:
- Ge and Ga
- are high in
- Kamacite.

- No Ge in
- phosphide.

- Phosphide
- has high Ni
- conc and
- some Ga.

- Ni

- Ge

- Ga

- Fe

- S   C   K  C   S   C  K

- R G Cavell et al
- Univ of Alberta

## (Model - 8 BCC cubes assembled into unit, one unique atom, X, in center of assembly. Fit with FEFF) 

- (Model - 8 BCC cubes assembled into unit, one unique atom, X, in center of assembly. Fit with FEFF)

- Ge – in kamacite

-       Ge ref matl
- (2.5 mol%  Ge in Fe)

- Ga signal from
- kamacite

- EXAFS  Analysis of Ga and Ge in Canyon Diablo  (kamacite has BCC structure)

- experimental

- calculated

- Fourier Transform
-  Magnitude

- First and second
- shells

- Third shell

- (Cavell et al, *Amer  Mineralogist,  *
- (2004) 89, 519)

- R G Cavell et al
- Univ of Alberta

- Key conclusion: Ga and Ge are inherent parts of kamacite phase (“mineral”)!

## Slide 37

## Slide 38

## Slide 39

## Slide 40

## Slide 41

## Slide 42

## Slide 43

## Slide 44

## (001)

-  78 dioctahedral chains
-  max. AsO4 adsorption:
-   2.8 AsO4 per nm2
-   (bidentate-binuclear)
-  molar Fe/As=3.5

- (010)

**Notes:**
If we double the crystal size of goethite to about 20 angst and 78 dioctahedral chains of Fe-O octahedra, maximum arsenate adsorption would become 3 arsenate per nm2 for bidentate-binuclear geometry. The molar Fe/As ratio in this case is 3.

## Molecular-scale Characterization of Arsenic in Mining and Metallurgical Wastes

**Molecular-scale Characterization of Arsenic**** in Mining and Metallurgical Wastes**

-   Presence of Ca in process solutions has a detrimental effect on
-    the stability of the Fe(III) oxyhydroxide precipitates.
-   Ferric arsenates, Ca-Fe arsenates and Fe(III) oxyhydroxides
-    with Fe/As<4 are the sources of As releases.
-  There are no apparent differences in terms of interatomic
-    distances and coordination numbers among ferrihydrites and
-    ferric arsenates with variable Fe/As ratios from 1 to 10.
-   Ferric arsenate precipitated at 70oC and pH 4 appears to be
-    composed of five dioctahedra linked at corners in a single layer.
-   Maghemite-rich iron oxides resulting from pyrometallurgy
-    operations have both As3+ and As5+ species, whereas
-    hematite-rich iron oxides have much less As3+ species.
-   Incomplete oxidation of As during its volatilization from pyrite
-    structure reduces gold recovery.

**Results, cont’d**