1. Diffusion in Intermetallic Compounds
Studied Using Short-Lived Radioisotopes
M. O. Zacate1, M. Deicher2, K. Johnston2, J. Lehnert2, F. Strauß2, G. S. Collins3
1 Department of Physics and Geology, Northern Kentucky University, Highland Heights, KY 41099, USA
2 Technische Physik, Universität des Saarlandes, Saarbrücken, Germany
3 Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, USA

2. Diffusion
Study of diffusion important for the understanding and controlling of impurities in materials
Control of useful/problematical impurities
Activation energies
Study of fundamental diffusion properties.
Already considerable programme of diffusion studies underway at ISOLDE: IS489.
Use of radiotracers using short and long-lived isotopes.

3. Radiotracer Technique (IS489)
CdxZn1-xTe
Ø = 6 mm
d = 800 µm
Implantation
CdTe
thermal
treatment
550 … 900 K
60 keV
Used isotopes :
111Ag, 67Cu, 24Na, 43K, 56Mn, 59Mn(59Fe), 65Ni, 61Mn(61Co), 101Cd(101Pd), 191Hg(191Pt), 193Hg(193Au)
111Ag+
30nm µm
CdTe:111Ag
Tdiff = 570 K
tdiff = 30 min
DAg = 7.6·10-9 cm2/s

4. What’s different about this work
Use of PAC (perturbed angular correlation) to examine the atomic nature of diffusion processes.
PAC very versatile technique (as evidenced by other proposals in this session)….but not usually used to study diffusion.
Study of intermetallic compounds:
L12 compounds: RIn3, RGa3, RSn3
Gd3P7
Examine on a local scale some of the unusual diffusion behaviour associated with these compounds.
Tests last year during the Ag beamtime were encouraging.

5. example: simple vacancy diffusion
Diffusion in solids
example: simple vacancy diffusion V EA x

6. Diffusion in intermetallic compounds
Only first-neighbor jumps allowed
(?)
example: 6-jump cycle 5 6 4 1 2 3

7. (EFG)
For diffusion experiments, monitor the changes of EFG as probe atoms move in the crystal.

8. Heating increases jump frequency...
Observing diffusion by PAC: Reorientation of EFG tensor
Heating increases jump frequency...
G2(t)
time
...first increases, then reduces relaxation.

9. PAC measurements of Cd jump rates in L12 structured compounds*
RIn3, RGa3, RSn3 with R=rare earth
L12 structure:
EFG
Vzz
Vxx
Vyy
Vzz
Vxx
Vyy
Vzz
Vxx
Vyy In R
*Collins, Jiang, Bevington, Selim, Zacate, PRL 102, (2009) and refs. therein; Jiang, Zacate, Collins, Def. & Diff. Forum , 725 (2009);

10. PAC measurements of Cd jump rates in L12 structured compounds*
RIn3, RGa3, RSn3 with R=rare earth
Jump rates at phase boundaries:
Jump rate larger for R-poorer boundary
Jump rate larger for R-rich boundary
LaIn3, CeIn3, PrIn3
LuIn3, TmIn3, ErIn3, HoIn3, DyIn3, TbIn3, GdIn3 LaSn3, CeSn3, SmSn3, GdSn3
divacancy or
6-jump
mechanism
Simple vacancy mechanism
Collins, Jiang, Bevington, Selim, Zacate, PRL 102, (2009)

11. PAC measurements of Cd jump rates in L12 structured compounds*
RIn3, RGa3, RSn3 with R=rare earth
Jump rates at phase boundaries indicate complex diffusion mechanism in light rare earth tri-indides
Possible explanation: constitutional defects differ in light and heavy R compounds
Light R
Heavy R
VR and RIn
VIn and InR
Aim of proposed experiments is to detect which defects are present across the RIn3 series using 111mCd

12. PAC measurements of Cd jump rates in L12 structured compounds*
Why 111mCd?
Too few defects to be observed using 111In
111mCd is an impurity so that association between defects and Cd could enhance site fractions enough to determine defect configurations
Aim of proposed experiments is to detect which defects are present across the RIn3 series using 111mCd

13. Ga7Pd3 Ga(3) Ga(4) One Pd site Two inequivalent Ga sites (3 & 4)
111In/Cd in Ga7Pd3
In/Cd only on Ga sites
No defects observed

14. 111In/Cd in Pd3Ga7 (high temperature)
Damping of signal is due to loss in coherence among spin states as tracers jump
Diffusion and Defect Data 264, (2007)

15. Cd jump rates in Ga7Pd3 – empirical analysis
Stochastic Model
Seven distinct EFGs
Three EFGs for Ga(3) site parameterized by quadrupole interaction frequency wQ(3)
Four EFGs for Ga(4) site parameterized by quadrupole interaction frequency wQ(4)
Four EFG transition rates: r3→3, r4→4, r3→4, and r4→3
Initial fraction of probes on Ga(3) sites: f3
Spectra calculated using SHIML (Comput. Phys. Commun. 182, 1061 (2011)) following the method of Winkler and Gerdau (Z. Phys. 262, 363 (1973)) l
Why difference in jump rates?
Diffusion and Defect Data 264, (2007)

16. Cd jump rates in Ga7Pd3 Scatter in the fits using stochastic model.
Need to understand this better; Could perform DFT calculations, but experimental solution is more direct
Proposal: repeat experiments using the 111mCd probe and analyze using the stochastic model.
Use of 111mCd allows elimination of f3 in the model because of the principle of detailed balance:
This offers a unique opportunity to measure separately inter- and intra-sublattice jump rates.

17. Beam request Isotope T1/2 Target Yield (ions/µC) Ion source Shipping?
111mCd
48 min Sn
2×108
HP (VADIS) No
10 shifts split over 2-3 years.
Collections of ~ 15 mins allow many experiments to be performed over 3-4 day beamtime.
Easy to accommodate other groups.
Work involved:
Need to determine “good” annealing temperatures for the compounds following implantation.
Ga7Pd3 more focused, priority for 2011.
Large variety of L12 compounds to be examined, great flexibility.

18. Sample preparation Option 1 111In in InCl3 dissolved in HCl
Step 1: high purity metal foils folded together
Step 2: Arc melt at 45 DCA for ~1 sec
Option 2
111mCd implanted

19. Simple vacancy diffusion in L12 compounds
Simple vacancy jumps in L12
EFG
Vzz
Vxx
Vyy
Vzz
Vxx
Vyy
Show the jump sequence with changing EFGs
Point out number of EFGs as V went by (4) and that there are15 different EFGs (3 defect-free) and 12 with near neighbor vacancy
Reiterate simulations verify need large [V] and low jump rates
*Muhammed, Zacate, Evenson, Hyperfine Interact. 177, 45 (2007)

20. Simulating Simple Vacancy Diffusion in L12-Structured Compounds…
6-jump cycle L12

21. Ga sites in Ga7Pd3
Tracer jumps that lead to a change in EFG: Ga(4) → Ga(4) Ga(3) → Ga(3) Ga(3) ↔ Ga(4)
Jump distances: Ga(4)-Ga(4): nm (×3) and nm Ga(3)-Ga(3): nm (×4) Ga(3)-Ga(4): nm (×4)