Hydrogen Trapping Interactions with Dislocations in Pd at Low Temperature

Supported by the NSF, the ACS-PRF, and the Univ. Illinois

Brent J. Heuser University of Illinois at Urbana-Champaign

Goal: Characterization of hydrogen in perturbed environment of a dislocation

Dislocation in Pd + trapped H

ExperimentalIncoherent Inelastic Neutron Scattering

Small-Angle Neutron ScatteringMagnetic Susceptibility

Ab Initio ComputationsRelaxed Dislocation + H

Binding EnergeticsVibrational DOS

Hydride Formation?

Low Temp.

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 ANLMRL at U. Illinois

Background

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.• V/V=0.11 (100% ’ 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.• -Pd superconducting (Tc~1-8 K); Pd is paramagnetic.

Pd-H phase diagrams

Neutron Scattering Instruments

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

Incoherent inelastic neutron scattering:•neutron energy loss or gain•measurements in time domain•vibrational density of states•good for H in metals

SANS at NIST

d/d vs QQ=(4/)sin/2

Energy Window1.2 ± 1.1 meV

FANS at NIST

Inelastic NeutronScattering

Rowe et al., PRL 33 (1974) 1297.

vs. k Phonon Dispersion Curves in

Single CrystalPdD0.63

(Coherent INS)

Hydrogen Vibrational DOS in Polycrystalline PdH0.7

(Incoherent INS)

Hunt and RossJ. Less-C. Metals (1976) 169.

TO

LO

Flat TOModes

Dispersion

Incoherent INS Measurements (21g Pd sheet):

Deformed PdH0.0008 (0.15 mg H)

-PdH0.63

Well-annealed -PdH0.015

PdH0.63

PdH0.0008

4K

PdH0.015

PdH0.0008

295K

Comparisions:

4 K: PdH0.0008 similar to -PdH0.63

295 K: PdH0.0008 similar to -PdH0.015

Conclusion:

→ phase transformation at dislocations

upon cooling from 295 to 4 K.

Lossof

Degeneracy?

Vibrational Density of States

Peak Shift at 295 K

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 meV0.63 = 0.14 [H]/[Pd]X

SANS Measurements of Deformed Single Crystal PdD0.0055

w/D

w/o D

Deformed PdD0.0055 at RT(trapped D in equil. w/bulk D)

Cross Section Model:Cylinder of trapped solutew/radius Ro and length Lo

local trapped concentration ~0.15 [D]/[Pd]

J. Alloys Compd. 261 (1997) 225.

DFT Relaxation of an Edge Dislocation in PdPd: ao=3.8528 Å (3.8718 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)

Circles are relaxed Pd positions

w/1 H

w/o HPd site volumetric strain Oct. site volumetric strain

Pd site volumetric strain Oct. site volumetric strain

compressive

tensile

compressive

tensile

tensiletensile

compressivecompressive Dislocation Core:V/V = 0.089V/V = 0.045 (1st NN oct. site)Bulk (not shown):V/V = 0.046V/V = 0.025 (1st NN oct. site)

Local Volumetric Dilatation

Magnetic Susceptibility Measurements in Deformed PdH0.0004Pd is paramagnetic—low T tail due to Fe

impurities:fit of M(H)@2 K to paramagnetic Langevin function

yields CFe<10appm.

Deformed PdH0.0004 has a diamagnetic behaviorbelow 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 M(H)@2 K

ZFC M(T)@0.5 Oe

Diamagneticresponse—looks

like Type II SC

Curie-Weiss:= 29 K

Diamagnetic contribution below 50 K

Phys. Lett. A, 339 (2005) 414.

Conclusions

• → 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.

Elastic Neutron Scattering

No H/D

w/H

w/D

x

Filling dislocation with H or D

bulk Pd

bulk Pd

bulk Pd

Scattering response: I 2

Scattering length density: = Natomb