7644 Phys. Chem. Chem. Phys., 2011, 13, 7644–7648 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 7644–7648 Crystal structure and dynamics of Mg(ND 3 ) 6 Cl 2 w Magnus H. Sørby,* a Ole Martin Løvvik, ab Masami Tsubota, c Takayuki Ichikawa, c Yoshitsugu Kojima c and Bjørn C. Hauback a Received 12th August 2010, Accepted 7th December 2010 DOI: 10.1039/c0cp01479f The crystal structure and dynamics of Mg(ND 3 ) 6 Cl 2 have been investigated by powder neutron diffraction and molecular dynamics. The powder diffraction data can be well described by 4 partly occupied deuterium sites in a square arrangement around the N atoms, which is seemingly inconsistent with the 3-fold symmetry of the ND 3 molecule. Molecular dynamics show highly correlated rotational and translational motion of the ND 3 molecules which explains the apparent 4-fold symmetry of the deuterium arrangement. A more disordered structure model based on the molecular dynamics results gives a better fit to the experimental data and is in agreement with the 3-fold symmetry of ND 3 . Introduction Metal ammine halides M(NH 3 ) n X m are produced by absorption of ammonia into a metal halide MX m . 1 Such solid/ammonia systems have been investigated for ammonia separation purposes, 2 as chemical heat pumps 3 and for energy storage. Christensen et al. suggested to use Mg(NH 3 ) 6 Cl 2 in combination with an ammonia decomposition catalyst as a solid hydrogen storage material 4 with 9.1 mass% hydrogen. Other possibilities to use Mg(NH 3 ) 6 Cl 2 in ammonia-mediated energy storage systems are for example in combination with direct ammonia fuel cells, 5 or reaction of ammonia with metal hydrides to generate hydrogen reversibly. 6,7 The solid Mg(NH 3 ) 6 Cl 2 is easy to handle, and can store ammonia safely with almost the same volumetric density as liquid ammonia. 8 The atomic positions of Mg, N and Cl in Mg(NH 3 ) 6 Cl 2 have been determined from single crystal X-ray diffraction by Hwang et al. 9 No attempts were done to localize the H atoms. The phase takes a cubic K 2 PtCl 6 -type structure (space group Fm 3m, a = 10.19 A ˚ ), which implies a face-centred cubic packing of Mg with Cl in all tetrahedral interstices (Fig. 1a). This can alternatively be described as a primitive cubic packing of Cl with Mg in the body centre of every second cube. Each Mg atom is octahedrally coordinated by six N atoms with the Mg–N bonds parallel to the unit cell axes. The N atoms are close to the face centres, but slightly out-of-plane, of the Cl cube that surrounds the MgN 6 octa- hedron (Fig. 1b). Full crystal structure determination and Fourier density maps of several Ni hexammines, Ni(NH 3 ) 6 X 2 (X =Br,I, NO 3 and PF 6 ; H = natural hydrogen or deuterium), with the K 2 PtCl 6 -type structure have been performed by single crystal neutron diffraction. 10–13 A common feature in the density maps is four clear hydrogen density maxima in a square planar configuration for each NH 3 unit. The hydrogen density maxima are between the N atom and the four X anions in the closest face of the surrounding cube of X atoms. Such a configuration is indicated in Fig. 1b. Calculations of the crystal potential energy have shown that these positions are favourable for hydrogen 12,13 in agreement with the experi- mental results, which is not surprising considering the positive charge of H in ammonia and the negative charge of the X anions. The higher number of H density maxima than H atoms means that the NH 3 complexes are orientationally disordered. Quasielastic neutron scattering data on Ni(NH 3 ) 6 Br 2 have shown that the disorder is dynamic. 14 Despite the apparent inconsistency between the 3-fold symmetry of the ammonia molecule and the 4-fold symmetry of maxima in the H density, several hexammine crystal structures have been reported with four 75% occupied hydrogen positions per ammonia molecule, in e.g. V(NH 3 ) 6 I 2 , Cr(NH 3 ) 6 I 2 , Mn(NH 3 ) 6 Cl 2 , Fe(NH 3 ) 6 Cl 2 , Fe(NH 3 ) 6 Br 2 , Co(NH 3 ) 6 Br 2 , Ni(NH 3 ) 6 Cl 2 , 15 Mn(NH 3 ) 6 I 2 and Fe(NH 3 ) 6 I 2 . 16 Other investigators have used a higher number of less occupied sites to model the hydrogen distribution in e.g. Co(NH 3 ) 6 Cl 2 . 17 However, only very few publications have discussed the relationship between the local, instantaneous orientations of NH 3 and the four observed H density maxima. Schiebel et al. 13 proposed that the rotational motion of the ammonia molecules in Ni(NH 3 ) 6 I 2 are strongly coupled with translational motion. The N atom is shifted towards the centre a Institute for Energy Technology, Physics Department, P.O. Box 40, 2027 Kjeller, Norway. E-mail: [email protected]; Fax: +47 6381 09 20; Tel: +47 6380 6000 b SINTEF Materials and Chemistry, P.O. Box 124 Blindern, 0314 Oslo, Norway c Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan w This article was submitted following the 1st workshop on Energy Materials, organised by The Thomas Young Centre, and held on 7–9 September 2010 at University College London. PCCP Dynamic Article Links www.rsc.org/pccp PAPER
5
Embed
Citethis:hys. Chem. Chem. Phys .,2011,13 ,76447648 PAPERfolk.uio.no/olem/papers/sorby2011.pdfThis can alternatively be described as a primitive cubic packing of Cl with Mg in the body
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
7644 Phys. Chem. Chem. Phys., 2011, 13, 7644–7648 This journal is c the Owner Societies 2011
Co(NH3)6Br2, Ni(NH3)6Cl2,15 Mn(NH3)6I2 and Fe(NH3)6I2.
16
Other investigators have used a higher number of less
occupied sites to model the hydrogen distribution in e.g.
Co(NH3)6Cl2.17 However, only very few publications have
discussed the relationship between the local, instantaneous
orientations of NH3 and the four observed H density maxima.
Schiebel et al.13 proposed that the rotational motion of the
ammonia molecules in Ni(NH3)6I2 are strongly coupled with
translational motion. The N atom is shifted towards the centre
a Institute for Energy Technology, Physics Department, P.O. Box 40,2027 Kjeller, Norway. E-mail: [email protected];Fax: +47 6381 09 20; Tel: +47 6380 6000
b SINTEF Materials and Chemistry, P.O. Box 124 Blindern,0314 Oslo, Norway
c Institute for Advanced Materials Research, Hiroshima University,Higashi-Hiroshima 739-8530, Japanw This article was submitted following the 1st workshop on EnergyMaterials, organised by The Thomas Young Centre, and held on 7–9September 2010 at University College London.
Bragg peak positions are marked with vertical ticks. Rwp = 5.10%.
Table 1 Results from Rietveld refinement of powder neutron diffrac-tion data for Mg(ND3)6Cl2 at 298 K using Model I. Space groupFm�3m, a = 10.199(1) A. Calculated standard deviations are given inparentheses
Fig. 3 Track of two ND3 units from the MD simulation. One unit is
directly below the other in the viewing direction. Each configuration is
represented by grey circles for the deuterium positions and black
circles for the nitrogen positions. The MD simulation was performed
at 300 K, with a time step of 1.3 fs. The first 1000 steps are shown in
this figure.
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 7644–7648 7647
structural parameters (two for D2 and one for N). The refined
structural parameters are given in Table 2.
A ND3 unit from the converged structure (Model II) is
shown in Fig. 5 and the similarities with the MD model are
striking. The arrangement of partly occupied sites can easily be
regarded as a weighted superposition of the 4 different ND3
positions and orientations shown in Fig. 5. The N–D distances
within the outlined molecules are 0.966(6) A (N–D1) and
1.06(1) A (N–D2) which are in good agreement with the
reference value of 1.02 A for free ammonia molecules. The
D–N–D angles are 102.8(3)1 (D1–N–D1) and 110.1(3)1
(D1–N–D2) which are in fair agreement with the experi-
mentally determined gas phase value of 107.81. It should be noted
that the refinement was performed without any geometrical
constrains.
Conclusions
Two crystal structure models for Mg(ND3)6Cl2 are proposed
and refined against PND data. In Model I, four partly
occupied deuterium sites arranged in a square are associated
with each nitrogen atom (Model I). The model is in good
agreement with the available PND data, but it is incompatible
with the 3-fold symmetry of ammonia. Model II was developed
based on MD simulations that showed that the nitrogen atom
is most of the time displaced from its ‘‘average’’ position, thus
allowing two deuterium atoms to be close to the D positions in
Model I simultaneously. It yields a better fit to the experi-
mental data and is, more importantly, in good agreement with
the expected geometry of ammonia. Model II is thus preferred
over Model I.
Notes and references
1 W. Biltz and G. F. Hutting, Z. Anorg. Allg. Chem., 1921, 119,115–131.
2 C. Y. Liu and K. Aika, Bull. Chem. Soc. Jpn., 2004, 77, 123–131.3 W. Wongsuwan, S. Kumar, P. Neveu and F. Meunier, Appl.Therm. Eng., 2001, 21, 1489–1519.
4 C. H. Christensen, R. Z. Sorensen, T. Johannessen, U. J. Quaade,K. Honkala, T. D. Elmoe, R. Kohler and J. K. Norskov, J. Mater.Chem., 2005, 15, 4106–4108.
5 J. C. Ganley, J. Power Sources, 2008, 178, 44–47.6 Y. Kojima, S. Hino, K. Tange and T. Ichikawa, Mater. Res. Soc.Symp. Proc., 2008, 1042, S06-01.
7 H. Yamamoto, H. Miyaoka, S. Hino, H. Nakanishi, T. Ichikawaand Y. Kojima, Int. J. Hydrogen Energy, 2009, 34, 9760–9764.
8 C. H. Christensen, T. Johannessen, R. Z. Sorensen andJ. K. Norskov, Catal. Today, 2006, 111, 140–144.
9 I. C. Hwang, T. Drews and K. Seppelt, J. Am. Chem. Soc., 2000,122, 8486–8489.
10 A. Hoser, W. Joswig, W. Prandl and K. Vogt,Mol. Phys., 1985, 56,853–869.
11 A. Hoser, W. Prandl, P. Schiebel and G. Heger, Z. Phys. B:Condens. Matter, 1990, 81, 259–263.
12 P. Schiebel, A. Hoser, W. Prandl, G. Heger, W. Paulus andP. Schweiss, J. Phys.: Condens. Matter, 1994, 6, 10989–11005.
13 P. Schiebel, A. Hoser, W. Prandl, G. Heger and P. Schweiss,J. Phys. I, 1993, 3, 987–1006.
14 J. A. Janik, J. M. Janik, A. Migdal-Mikuli and E. Mikuli,Acta Phys. Pol., A, 1988, 74, 423–431.
15 R. Essmann, G. Kreiner, A. Niemann, D. Rechenbach,A. Schmieding, T. Sichla, U. Zachwieja and H. Jacobs, Z. Anorg.Allg. Chem., 1996, 622, 1161–1166.
16 H. Jacobs, J. Bock and C. Stuve, J. Less Common Met., 1987, 134,207–214.
17 J. M. Newman, M. Binns, T. W. Hambley and H. C. Freeman,Inorg. Chem., 1991, 30, 3499–3502.
18 B. C. Hauback, H. Fjellvag, O. Steinsvoll, K. Johansson,O. T. Buset and J. Jørgensen, J. Neutron Res., 2000, 8, 215.
Fig. 4 Rietveld fit to PND data for Mg(ND3)6Cl2 using a super-
position of four different ND3 orientations (Model II). Open circles—
Bragg peak positions are marked with vertical ticks. Rwp = 4.82%.
Table 2 Results from Rietveld refinement of powder neutron diffractiondata for Mg(ND3)6Cl2 at 298 K using Model II. Space group Fm�3m,a= 10.199(2) A. Calculated standard deviations are given in parentheses.Biso are constrained to have the same value for the same elements