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

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

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.

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

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.

Incoherent INS Measurements (21g Pd sheet): 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 PdH0.63 PdH0.0008 4K PdH0.015 295K 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. Loss of Degeneracy?

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

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 PdD0.0055 at RT (trapped D in equil. w/bulk D) J. Alloys Compd. 261 (1997) 225. local trapped concentration ~0.15 [D]/[Pd]

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 = 0.045 (1st NN oct. site) Bulk (not shown): DV/V = 0.046 DV/V = 0.025 (1st NN oct. site) 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) Local Volumetric Dilatation Circles are relaxed Pd positions

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. w/o H w/H net=w/H – w/o H Net M(H)@2 K ZFC M(T)@0.5 Oe Diamagnetic response—looks like Type II SC Curie-Weiss: q = -29 K Diamagnetic contribution below 50 K Phys. Lett. A, 339 (2005) 414.

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.

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