1 Directional Metal-Hydrogen Bonding in Interstitial Hydrides III - Structural Study of Ce 2 Ni 7 H 4 Lab. of Cryst, UniGe 8 March 2005 Yaroslav Filinchuk CeNi 3 D 2.8 CeNi 3 Ce 2 Ni 7 CeNi 5 Yartys et al., 2003Cromer, 1959

2 orthorhombic distortion of the parent hexagonal lattice deuterium atoms enter into the CeNi 2 slabs D-atoms do not fill existing interstices, but form new D- occupied sites (big atomic shifts) Yartys et al., 2003 CeNi 3 D 2.8 big plateau at 0.1 bar, 50°C expansion solely along c (30%) Van Essen et al., 1980 Ce 2 Ni 7 D 4 big plateau at 0.2 bar, 50°C expansion solely along c (21%) Van Essen et al., 1980 This work the same as for the CeNi 3 D 2.8 [NiD 4 ] complexes form in the CeNi 2 slabs different ordering of [NiD 4 ] complexes leads to a different degree of the orthorhombic distortion Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

3 SNBL,  ~ 0.52 Å Analysis of group- subgroup trees: P6 3 /mmc  Cmcm  Pmcn or Pmnm Ce 2 Ni 7 D 4 Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

4 Crystal chemistry of the metallic matrix from synchrotron diffraction data Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar % occ. Ni Ni-Ni ~2 Å Cmcm full occ. Ni accounts for weak prim. reflections Pmcn 33 coordinates 4 Ce + 11 Ni Ce 2 Ni 7 D 4 Pmcn is consistent with a full occupancy of D-atoms (NPD)

5 In-situ synchrotron powder diffraction Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005 Ce 2 Ni 7 H x freshly charged with 30 bar of H 2 SNBL,  ~ 0.52 Å Modelled by two Ce 2 Ni 7 H x phases, having different degrees of the orthorhombic distortion

6 The two Ce 2 Ni 7 H x phases have deviations of the b/a ratio from the ideal pseudo-hexagonal value  3 with opposite signs a, Åb, Åc, ÅV, Å 3 ** (13)8.4650(2) (7) (5)-0.67% (15)8.4618(2) (7) (6)-0.67% (14)8.5219(2) (6) (5)+0.81% (10) (17) (6) (4)+0.93% * The degree of the orthorthombic distortion is defined as  = (b/  3 - a)/a Deuterium positions from NPD must be considered ! Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

7 DMC, HRPT NPD: DMC, HRPT Limited resolution  orthorhombic distortion in Ce 2 Ni 7 D 4 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. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

8 T, Ka, Åc, ÅV, Å (3)29.612(4)613.63(10) (3)29.614(4)613.72(10) (3)29.622(4)614.09(10) (3)29.632(4)614.84(10) DMC  ~ 2.56 Å No magnetic or structural transitions down to 1.5K Orth. distortion does not increase on lowering T Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005 V = f (T)

9 Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005 HRPT,  ~ 1.49 Å D-atom positions were found by FOX and from difference nuclear-density maps numbering is the same as for CeNi 3 D 2.8 positions D1-D6 – fully occupied, the same as in CeNi 3 D 2.8 positions D7 and D8 – partially occupied, differ from those in CeNi 3 D 2.8 Ce 2 Ni 7 D 4

10 Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005 Tetrahedral [NiD 4 ] “complexes” in Ce 2 Ni 7 D 4 and CeNi 3 D 2.8 CeNi 3 D 2.8 Ce 2 Ni 7 D 4 Authors did not mention in 2003 Tetrahedral [NiD 4 ] moieties

Different ordering of [NiD 4 ] tetrahedra is the origin of two phases with  > 0 and  < 0 Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005 Cause: different orientation of the [NiD 4 ] tetrahedra Means of influence: Ce…D interactions Consequence: positive (a) and negative (b) orthorhombic distortion . b > a  3 b < a  3 Ce 2 Ni 7 D 4 Assumption !

12 Conclusions, remaining problems… Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar CeNi 2 D CeNi 5  2 Ce 2 Ni 7 D 4 2 CeNi 2 D CeNi 5  3 CeNi 3 D 2.8 [NiD 4 ] complexes form in the CeNi 2 slabs the structure is stable down to 1.5K different ordering of [NiD 4 ] 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 [NiD 4 ] tetrahedra is the same as in CeNi 3 D 2.8 Achievements: complete analysis of the group-subgroup sequences Problems: strong intensity correlations in the NPD pattern complicate elucidation of those features that deviate from the hexagonal average (for CeNi 3 D 2.8  is larger, ~1.5%) partially occupied sites D7 and D8 differ from those in CeNi 3 D 2.8 (D8 is also partially occupied) not perfect geometry of the [NiD 4 ] tetrahedron High-res. in-situ NDP Dream