1. Hydrogen Trapping Interactions with
Dislocations in Pd at Low Temperature
Brent J. Heuser University of Illinois at Urbana-Champaign
Goal: Characterization of hydrogen in perturbed environment of a dislocation
Dislocation in Pd + trapped H
Experimental
Incoherent Inelastic Neutron Scattering
Small-Angle Neutron Scattering
Magnetic Susceptibility
Ab Initio Computations
Relaxed Dislocation + H
Binding Energetics
Vibrational DOS
Hydride Formation?
Low Temp.
Supported by the NSF, the ACS-PRF, and the Univ. Illinois

2. Collaborators and Facilities
J. King (U. Michigan), G. Summerfield (U. Michigan)
E. Epperson (ANL), F. Boue (CEA Saclay)
W.C. Chen (U. Illinois, NIST), H. Ju (U. Illinois)
T. Udovic (NIST), J. Barker (NIST), C. Glinka (NIST)
D. Trinkle (U. Illinois), A. Lipson (U. Illinois, Russian Acad. Sci.)
NCNR at NIST, Lujan Center at LANL, LLB at Saclay, IPNS at ANL
MRL at U. Illinois

3. Background Pd-H phase diagrams Relevant Properties of H (D) in Pd:
Pd good catalyst for H2 ↔ H + H.
H is an octahedral interstitial in fcc lattice.
elastic response due to 1s-4d hybridization.
DV/V=0.11 (100% a’ at RT).
hydride formation accompanied
by dislocation generation.
0.2 eV activation energy for diffusion.
~0.7 eV trapping energy at dislocation cores.
stoichiometric hydride phase difficult.
b-Pd superconducting (Tc~1-8 K); Pd is
paramagnetic.

4. Neutron Scattering Instruments
SANS at NIST
dS/dW vs Q
Q=(4p/l)sinq/2
Small-angle neutron scattering:
no neutron energy loss
measurements in Q domain
length scales ~10 to 2000 Å
good for H (D) in metals
“clean” single crystals
Energy Window
1.2 ± 1.1 meV
Incoherent inelastic neutron scattering:
neutron energy loss or gain
measurements in time domain
vibrational density of states
good for H in metals
FANS at NIST

5. Inelastic Neutron Scattering Hydrogen Vibrational DOS in
Polycrystalline PdH0.7
(Incoherent INS)
Flat TO
Modes
Dispersion
w vs. k Phonon Dispersion Curves in Single Crystal
PdD0.63
(Coherent INS) LO TO
Hunt and Ross
J. Less-C. Metals (1976) 169.
Rowe et al., PRL 33 (1974) 1297.

6. Incoherent INS Measurements (21g Pd sheet):
Vibrational Density of States
Incoherent INS Measurements (21g Pd sheet):
Deformed PdH (0.15 mg H)
b-PdH0.63
Well-annealed a-PdH0.015
PdH0.63
PdH0.0008 4K
PdH0.015
295K
Comparisions:
4 K: PdH similar to b-PdH0.63
295 K: PdH similar to a-PdH0.015
Conclusion:
a→b phase transformation at dislocations
upon cooling from 295 to 4 K.
Loss of
Degeneracy?

7. Peak Shift at 295 K OR 0.63 = 0.14 [H]/[Pd]
Near-core trapping sites more open:
softer optic modes and shift to lower energy.
lack of symmetry of trapping sites should result
loss of degeneracy and broader peak.
H trapped further away for core (T=295 K):
strain perturbation weaker and sites
still degenerate.
peak shift due to local expansion due to
presence of H atom. OR
E0.015-E0.0008
68 meV – 59 meV
0.63 = 0.14 [H]/[Pd] X

8. Cylinder of trapped solute w/radius Ro and length Lo
SANS Measurements of Deformed Single Crystal PdD0.0055
Cross Section Model:
Cylinder of trapped solute
w/radius Ro and length Lo
w/D
w/o D
Deformed PdD at RT
(trapped D in equil. w/bulk D)
J. Alloys Compd. 261 (1997) 225.
local trapped concentration ~0.15 [D]/[Pd]

9. DFT Relaxation of an Edge Dislocation in Pd
w/1 H
w/o H
Pd site volumetric strain
Oct. site volumetric strain
compressive
tensile
Dislocation Core:
DV/V = 0.089
DV/V = (1st NN oct. site)
Bulk (not shown):
DV/V = 0.046
DV/V = (1st NN oct. site)
Pd: ao= Å ( Å exp.)
H-H=0.766 Å (0.74 Å)
C11=324 GPa (315 GPa)
C12=196 GPA (257 GPa)
C44=86 GPa (71 GPa)
Local Volumetric Dilatation
Circles are relaxed Pd positions

10. Magnetic Susceptibility Measurements in Deformed PdH0.0004
Pd is paramagnetic—low T tail due to Fe impurities:
fit of K to paramagnetic Langevin function yields CFe<10appm.
Deformed PdH has a diamagnetic behavior
below 50 K and exhibits irreversible M(H) behavior
at 2 K indicative of a Type II superconductor.
w/o H
w/H
net=w/H – w/o H
Net K
ZFC Oe
Diamagnetic
response—looks
like Type II SC
Curie-Weiss:
q = -29 K
Diamagnetic contribution below 50 K
Phys. Lett. A, 339 (2005) 414.

11. Conclusions
a→b phase transformation upon cooling 295→4 K based on incoherent INS.
peak shape and peak location can serve as a probe of local disorder of trapping site.
RT local concentration from SANS sufficient for phase transformation upon cooling.
DFT calculations demonstrate large local dilatation with addition of one H atom.

13. Elastic Neutron Scattering
Filling dislocation with H or D
Scattering response: I  Dr2
Scattering length density: r = Natomb r
bulk Pd
No H/D Dr
bulk Pd
w/H Dr
bulk Pd Dr
w/D x