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Hydrogen Trapping Interactions with Dislocations in Pd at Low Temperature

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. Low Temp. Hydride Formation?.

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Hydrogen Trapping Interactions with Dislocations in Pd at Low Temperature

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  1. 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 Low Temp. Hydride Formation? Hydrogen Trapping Interactions with Dislocations in Pd at Low Temperature Brent J. HeuserUniversity of Illinois at Urbana-Champaign Goal:Characterization of hydrogen in perturbed environment of a dislocation 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. SANS at NIST dS/dW vs Q Q=(4p/l)sinq/2 Energy Window 1.2 ± 1.1 meV 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 FANS at NIST

  5. Dispersion Flat TO Modes Inelastic Neutron Scattering Hydrogen Vibrational DOS in Polycrystalline PdH0.7 (Incoherent INS) 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. 4K PdH0.63 PdH0.0008 PdH0.015 295K PdH0.0008 Loss of Degeneracy? Vibrational Density of States Incoherent INS Measurements (21g Pd sheet): Deformed PdH0.0008 (0.15 mg H) b-PdH0.63 Well-annealed a-PdH0.015 Comparisions: 4 K: PdH0.0008 similar to b-PdH0.63 295 K: PdH0.0008 similar to a-PdH0.015 Conclusion: a→b phase transformation at dislocations upon cooling from 295 to 4 K.

  7. 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. • 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. OR E0.015-E0.0008 0.63 = 0.14 [H]/[Pd] X 68 meV – 59 meV Peak Shift at 295 K

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

  9. Pd: 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) w/o H Pd site volumetric strain Oct. site volumetric strain compressive compressive tensile tensile w/1 H Pd site volumetric strain Oct. site volumetric strain Dislocation Core: DV/V = 0.089 DV/V = 0.045 (1st NN oct. site) Bulk (not shown): DV/V = 0.046 DV/V = 0.025 (1st NN oct. site) compressive compressive tensile tensile DFT Relaxation of an Edge Dislocation in Pd Local Volumetric Dilatation Circles are relaxed Pd positions

  10. ZFC M(T)@0.5 Oe w/o H w/H net=w/H – w/o H Diamagnetic contribution below 50 K Net M(H)@2 K Curie-Weiss: q = -29 K Diamagnetic response—looks like Type II SC Magnetic Susceptibility Measurements in Deformed PdH0.0004 Pd 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 behavior below 50 K and exhibits irreversible M(H) behavior at 2 K indicative of a Type II superconductor. 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.

  12. 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

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