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.

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Directional Metal-Hydrogen BondingDirectional Metal-Hydrogen Bondingin Interstitial Hydridesin Interstitial Hydrides

III - Structural Study of CeIII - Structural Study of Ce22NiNi

77HH44 Lab. of Cryst, UniGe

8 March 2005

Yaroslav Filinchuk

D 2

CeNi3D2.8

CeNi3

Ce2Ni7

CeNi5

Yartys et al., 2003Cromer, 1959

2

• 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)

Yartys et al., 2003

CeNi3D2.8

• big plateau at 0.1 bar, 50°C

• expansion solely along c (30%)

Van Essen et al., 1980Ce2Ni7D4

• 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 CeNi3D2.8

• [NiD4] complexes form in the CeNi2 slabs

• different ordering of [NiD4] complexes leads to a different degree of the orthorhombic distortion

Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

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SNBLSNBL, , λλ ~ 0.52 Å~ 0.52 Å Analysis of group-subgroup trees:

P63/mmc

Cmcm

Pmcn or Pmnm

Ce2Ni7D4

Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

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Crystal chemistry of the metallic matrix from synchrotron

diffraction data

Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

50% occ. NiNi-Ni ~2 Å

Cmcm

full occ. Ni

accounts forweak prim.reflections

Pmcn33 coordinates4 Ce + 11 Ni

Ce2Ni7D4

Pmcn isconsistent with a full occupancy of D-atoms (NPD)

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In-situ synchrotron powder diffraction

Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

Ce2Ni7Hx

freshly charged

with 30 bar of H2

SNBLSNBL, , λλ ~ 0.52 Å~ 0.52 Å

Modelled by two Ce2Ni7Hx phases,

having different degrees of the orthorhombic distortion

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The two Ce2Ni7Hx phases have deviations of the b/a ratio from the ideal pseudo-hexagonal value 3

with opposite signs

  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 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

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NPD: DMC, HRPTDMC, 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. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

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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)

DMCDMCλλ ~ 2.56 Å~ 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)

9Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

HRPTHRPT, , λλ ~ 1.49 Å~ 1.49 Å

• 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 in CeNi3D2.8

• positions D7 and D8 – partially

occupied, differ from those in CeNi3D2.8

Ce2Ni7D4

10Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

Tetrahedral [NiD4] “complexes” in

Ce2Ni7D4 and CeNi3D2.8

CeNi3D2.8

Ce2Ni7D4

Authors did not mention in 2003 Tetrahedral [NiD4]

moieties

Different ordering of [NiD4] 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 [NiD4] tetrahedra

Means of influence:Ce…D interactions

Consequence: positive (a) and negative (b) orthorhombic distortion .

b > a3

b < a3

Ce2Ni7D4

Assumption !

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Conclusions, remaining problems…

Y. Filinchuk Seminar, Lab. of Cryst., UniGe, 8 Mar 2005

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

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 CeNi3D2.8 is larger, ~1.5%)

• partially occupied sites D7 and D8 differ from those in CeNi3D2.8 (D8 is also

partially occupied)

• not perfect geometry of the [NiD4] tetrahedron High-res. High-res. in-situin-situ NDP NDPDreamDream