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Lab. of Cryst, UniGe 8 March 200 5

Yaroslav Filinchuk. Directional Metal-Hydrogen Bonding in Interstitial Hydrides III - Structural Study of Ce 2 Ni 7 H 4. Lab. of Cryst, UniGe 8 March 200 5. Ce 2 Ni 7. CeNi 3 D 2.8. CeNi 3. CeNi 5. Cromer, 1959. Yartys et al., 2003. CeNi 3 D 2.8. Ce 2 Ni 7 D 4. Van Essen et al., 1980.

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Lab. of Cryst, UniGe 8 March 200 5

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  1. Yaroslav Filinchuk Directional Metal-Hydrogen Bondingin Interstitial HydridesIII - Structural Study of Ce2Ni7H4 Lab. of Cryst, UniGe 8March 2005 Ce2Ni7 CeNi3D2.8 CeNi3 CeNi5 Cromer, 1959 Yartys et al., 2003

  2. CeNi3D2.8 Ce2Ni7D4 Van Essen et al., 1980 Van Essen et al., 1980 • big plateau at 0.1 bar, 50°C • expansion solely along c (30%) • big plateau at 0.2 bar, 50°C • expansion solely along c (21%) Yartys et al., 2003 This work • orthorhombic distortion of the parent hexagonal lattice • deuterium atoms enter into the CeNi2 slabs • D-atoms do not fill existing interstices, but form new D-occupied sites(big atomic shifts) • the same as for the CeNi3D2.8 • [NiD4] complexes form in the CeNi2 slabs • different ordering of [NiD4] complexes leads to a different degree of the orthorhombic distortion Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  3. Ce2Ni7D4 Analysis of group-subgroup trees: P63/mmc  Cmcm  Pmcn or Pmnm SNBL, λ~ 0.52 Å Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  4. Ce2Ni7D4 Crystal chemistry of the metallic matrix from synchrotron diffraction data Pmcn is consistent with a full occupancy of D-atoms (NPD) 50% occ. Ni Ni-Ni ~2 Å full occ. Ni accounts for weak prim. reflections Cmcm Pmcn 33 coordinates 4 Ce + 11 Ni Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  5. SNBL, λ~ 0.52 Å In-situ synchrotron powder diffraction Ce2Ni7Hx freshly charged with 30 bar of H2 Modelled by two Ce2Ni7Hx phases, having different degrees of the orthorhombic distortion Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  6. a, Å b, Å c, Å V, Å3 * 1 4.92041(13) 8.4650(2) 29.7033(7) 1237.18(5) -0.67% 2 4.91840(15) 8.4618(2) 29.6753(7) 1235.04(6) -0.67% 3 4.88077(14) 8.5219(2) 29.6523(6) 1233.34(5) +0.81% 4 4.87688(10) 8.52590(17) 29.6322(6) 1232.10(4) +0.93% The two Ce2Ni7Hx phases have deviations of the b/a ratio from the ideal pseudo-hexagonal value 3 with opposite signs Deuterium positions from NPD must be considered ! * The degree of the orthorthombic distortion is defined as  = (b/3 - a)/a Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  7. NPD: DMC, HRPT Limited resolution orthorhombic distortion in Ce2Ni7D4 does not show up as individual split peaks, but as complex peak profiles. Intensities can be partially resolved using profile information (Rietveld refinement). Remaining strong correlations complicate elucidation of details introduced by the orthorhombic deformation, i.e. those features that deviate from the hexagonal average. Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  8. T, K a, Å c, Å V, Å3 1.5 4.8917(3) 29.612(4) 613.63(10) 50 4.8918(3) 29.614(4) 613.72(10) 100 4.8927(3) 29.622(4) 614.09(10) 150 4.8948(3) 29.632(4) 614.84(10) V = f (T) DMC λ~ 2.56 Å No magnetic or structural transitions down to 1.5K Orth. distortion does not increase on lowering T Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  9. HRPT, λ~ 1.49 Å Ce2Ni7D4 • D-atom positions were found by FOX • and from difference nuclear-density maps • numbering is the same as for CeNi3D2.8 • positions D1-D6 – fully occupied, • the same as inCeNi3D2.8 • positions D7 and D8 – partially occupied, • differ from thoseinCeNi3D2.8 Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  10. Tetrahedral [NiD4] “complexes” in Ce2Ni7D4 and CeNi3D2.8 Ce2Ni7D4 Authors did not mention in 2003 Tetrahedral [NiD4]moieties CeNi3D2.8 Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  11. Ce2Ni7D4 b > a3 Different ordering of [NiD4] tetrahedra is the origin of two phases with  > 0 and  < 0 Assumption ! Cause: different orientation of the [NiD4] tetrahedra Means of influence: Ce…D interactions Consequence: positive (a) and negative (b) orthorhombic distortion . b < a3 Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

  12. Conclusions, remaining problems… • Achievements: • complete analysis of the • group-subgroup sequences 2 CeNi2D4+ 2 CeNi5 2 Ce2Ni7D4 2 CeNi2D4.2 + CeNi5 3 CeNi3D2.8 • [NiD4] complexes form in the CeNi2 slabs • the structure is stable down to 1.5K • different ordering of [NiD4] complexes leads to a different degree of the orthorhombic distortion • structure with  < 0 is more stable at higher D-content • for the phase with  > 0 (more stable) orientation of the [NiD4] tetrahedra is the same as in CeNi3D2.8 • Problems: • strong intensity correlations in the NPD pattern complicate elucidation of those features that deviate from the hexagonal average (for CeNi3D2.8  is larger, ~1.5%) • partially occupied sites D7 and D8 differ from thoseinCeNi3D2.8 (D8 is also partially occupied) • not perfect geometry of the [NiD4] tetrahedron High-res. in-situ NDP Dream Y. FilinchukSeminar, Lab. of Cryst., UniGe, 8 Mar 2005

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