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(ND3)6Cl2w

Magnus H. Sørby,*aOle Martin Løvvik,

abMasami Tsubota,

cTakayuki Ichikawa,

c

Yoshitsugu Kojimacand Bjørn C. Hauback

a

Received 12th August 2010, Accepted 7th December 2010

DOI: 10.1039/c0cp01479f

The crystal structure and dynamics of Mg(ND3)6Cl2 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 ND3 molecule. Molecular dynamics show highly

correlated rotational and translational motion of the ND3 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 ND3.

Introduction

Metal ammine halides M(NH3)nXm are produced by absorption

of ammonia into a metal halide MXm.1 Such solid/ammonia

systems have been investigated for ammonia separation purposes,2

as chemical heat pumps3 and for energy storage. Christensen

et al. suggested to use Mg(NH3)6Cl2 in combination with an

ammonia decomposition catalyst as a solid hydrogen storage

material4 with 9.1 mass% hydrogen. Other possibilities to use

Mg(NH3)6Cl2 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(NH3)6Cl2 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(NH3)6Cl2 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 K2PtCl6-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 MgN6 octa-

hedron (Fig. 1b).

Full crystal structure determination and Fourier density

maps of several Ni hexammines, Ni(NH3)6X2 (X = Br�, I�,

NO3� and PF6

�; H = natural hydrogen or deuterium), with

the K2PtCl6-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 NH3 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 hydrogen12,13 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 NH3 complexes are orientationally

disordered. Quasielastic neutron scattering data on

Ni(NH3)6Br2 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(NH3)6I2,

Cr(NH3)6I2, Mn(NH3)6Cl2, Fe(NH3)6Cl2, Fe(NH3)6Br2,

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: magnuss@ife.no;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.

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of an edge in the H density square so that two of the square

corners can be occupied by H simultaneously. Thus, 2/3 of the

H will most of the time be close to the square corners, which

explain the clear H density maxima there.

In the present paper, powder neutron diffraction (PND)

and molecular dynamics (MD) are used to investigate the

deuterium distribution in Mg(ND3)6Cl2.

Methodology

Mg(ND3)6Cl2 was synthesized in a direct solid-state–gas reac-

tion between MgCl2 (99.99%, Aldrich Co. Ltd.) and deuterated

ammonia, ND3 (99%, Euriso-top CEA group). Deuterated

ammonia was used due to the high incoherent neutron scattering

cross-section for natural hydrogen. 0.4 g MgCl2 was transferred

to a stainless steel reaction chamber with volume about 8 cm3

in a glove box with purified argon (o1 ppm O2,o1 ppm H2O).

The reaction chamber was evacuated and connected to a

60 cm3 reservoir volume where 5 bar ND3 was introduced at

ambient temperature. The temperature immediately increased

to about 50 1C due to the exothermic formation of the ammine

complex. The reaction was allowed to proceed for 2 hours but

appeared to be mostly completed after about 30 minutes as no

more heat was evolved. The final pressure was around 3 bar.

The reaction product was a snow white, fine powder.

Powder neutron diffraction (PND) data were collected at

the PUS instrument at the reactor JEEP II (Kjeller, Norway).18

The sample was contained in a rotating vanadium container

with 6 mm inner diameter. Neutrons with the wavelength

l = 1.555 A were provided by a vertically focusing Ge

(5 1 1) monochromator. The instrument features two detector

banks, each with 7 vertically stacked 3He-filled position sensitive

detector tubes that cover a 201 range in scattering angle. The

2y range from 101 to 1301 was thus covered by moving each

detector bank to 3 different positions. The data were analyzed

with the Rietveld method implemented in the software

package GSAS19 with the graphical user interface EXPGUI.20

The Bragg profiles were described by the Thompson–Cox–

Hastings pseudo-Voigt function21 with 3 free parameters. The

background was fitted with a 12 term Chebyshev polynomial.

Different atoms of the same element were constraint to have

the same isotropic displacement factor.

First-principles molecular dynamics (MD) was performed

using the Vienna ab initio Simulation Package (VASP).22,23

The electronic structure was represented by plane-waves using

the projector augmented wave method24 and the generalized

gradient approximation.25 The plane-wave cut-off was 500 eV

for the MD runs, and 780 eV for the initial relaxation. The

k-space integration grid had a maximum distance of 0.2 A�1

between the k-points. Smearing of the partial occupancies was

performed using the linear tetrahedron method, and the

convergence criterion for the electronic density relaxation

was 10�6 eV.

Since partial occupancy is not possible to implement in

atomistic studies like this one, the NH3 units were directly

represented, starting from the gas phase structure of NH3,

with the hydrogen atoms pointing in the same direction as in

the experimental structure with partial occupancy. The struc-

ture resulting from this construction was then relaxed, using

the quasi-Newton method with the RMM-DIIS algorithm.

The force relaxation criterion was 0.05 eV A�1. This relaxed

structure was used as the input for the MD calculations. The

MD temperature was 300 K, with initial velocities randomly

generated by VASP. The time step was 1.3 fs, and the total

number of MD steps was 1400.

Results and discussion

The PND data could be fully indexed according to the cubic

unit cell suggested by Hwang et al. (a = 10.19 A with

systematic absence according to F-centring).9 Rietveld refine-

ment was first performed using the simple model described

above, with four 75% occupied D sites arranged in a square

for each N (space group Fm�3m). The refined model was in

Fig. 1 (a) The K2PtCl6-type structure of Mg(NH3)6Cl2 without hydrogen atoms. (b) Cubic configuration of Cl atoms around the

MgN6-octahedra. The 4-fold hydrogen configuration often reported in isostructural hexammines is indicated. Small, dark spheres: magnesium;

large, light spheres: nitrogen; large, dark spheres: chlorine; small, white spheres: typical hydrogen sites, 75% occupied (see text for discussion).

7646 Phys. Chem. Chem. Phys., 2011, 13, 7644–7648 This journal is c the Owner Societies 2011

good agreement with the data (Fig. 2) despite the inconsistency

with the 3-fold ND3 geometry (Rwp = 5.10%). The refined

structure data are shown in Table 1. The refined N–D

distances are 0.992(4) A which is in good agreement with that

in gaseous ammonia (1.02 A26). The D–N–D angles are on the

other hand too low as expected due to the 4-fold symmetry

(84.41 vs. 107.81 for gaseous ammonia). For comparison,

a model with 12 D sites (each 25% occupied) evenly distri-

buted on a circle for each N-atom gave a much poorer fit

(Rwp = 7.49%). The latter would be a better description of the

D distribution if ND3 was rotating as a stiff unit between

four different positions with one D atom always pointing

toward a Cl.

To clarify this apparent contradiction, first-principles molecular

dynamics (MD) calculations were performed. The initial

structure model was generated from the refined model above,

but with the 0.75 occupied D4 groups replaced by fully

occupied D3 groups, giving molecular units ND3 resembling

the experimental structure of gaseous ND3. Fig. 3 shows the

trace of two ND3 units, viewed along their common 3-fold

symmetry axis, after 1000 MD steps. It is evident that the

average positions of the deuterium atoms are consistent with

the 4-fold symmetry demonstrated by diffraction. At the

same time, the local geometry of the ND3 units is quite stable

during the MD simulation (apart from various phonon

modes that should be expected at room temperature). This

means that the ammonia units may be regarded as bodies

with limited flexibility and molecular structure similar to that

of gaseous ND3.

Furthermore, the movement of the ammonia molecules was

inspected from the MD movie. It was seen that the ND3 units

for most of the time stayed with D atoms in two of the four

density maxima seen in Fig. 3, with occasional rotations

between such structures. This implies that the nitrogen atoms

most of the time are shifted from their average position

towards one of the edges of the surrounding square. Opposite

ND3 molecules were seen to form 4-fold symmetry with four

of their six D atoms most of the time, except during the

rotations, which were clearly correlated.

This led us to propose an alternative structure model

(Model II) for the structural refinement. This is based on rigid

ND3 units with two of the D atoms placed in corners of the

square defined by the D positions of Model I. This leads to a

third D atom near the opposing square edge and a shift of the

N atom away from the centre of the square.

Rietveld refinement was used to check the consistency

between the PND data and Model II. The simple model

in Fig. 1b (Model I) was thus modified by reducing the

occupancy of the original D site from 75% to 50% (D1) and

introducing new, 25% occupied D sites (D2) between them. To

reduce the bias towards the suspected atomic arrangement, D2

was initially positioned so that the distances N–D1 and N–D2

were equal. This configuration yielded a poorer fit to the

experimental data (Rwp = 6.61%) as long as the atomic

positions were fixed. Refinement of the D positions did,

however, give a slightly better fit (Rwp = 5.01%) than for

Model I. The N–D1 distance was elongated while N–D2 was

shortened during the refinement in accordance with the

MD-model. In the last stage of the refinement, N was moved

slightly away from its 24e (x 0 0) position into a 96j position

(x dy 0) while reducing the occupancy to 25%. The N atoms

moved distinctly away from the 24e site on refinement. The fit

to the experimental data is improved with a final Rwp = 4.82

(Fig. 4). A Hamilton test27 shows that the decrease in Rwp is

significant within a 25% confidence level with the 3 additional

Fig. 2 Rietveld fit to PND data for Mg(ND3)6Cl2 using four 75%

occupied deuterium positions pr. ND3 unit (Model I). Open circles—

experimental data, solid line—calculated data, below—difference plot.

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

Atom Site x y z Biso21/A Occupancy

Mg 4a (fixed) 0 0 0 3.4(3) 1Cl 8c (fixed) 1

414

14

3.4(1) 1N 24e (0 0 z) 0 0 0.2195(3) 4.70(9) 1D 96k (x x z) 0.0654(2) 0.0654(2) 0.2498(4) 6.4(1) 3

4

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

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Fig. 4 Rietveld fit to PND data for Mg(ND3)6Cl2 using a super-

position of four different ND3 orientations (Model II). Open circles—

experimental data, solid line—calculated data, below—difference plot.

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

Atom Site x y z Biso21/A Occupancy

Mg 4a (fixed) 0 0 0 3.4(3) 1Cl 8c (fixed) 1

414

14

3.0(1) 1N 96j (0 y z) 0 0.021(1) 0.2161(3) 4.70(9) 1

4D1 96k (x x z) 0.0739(3) 0.0739(3) 0.2427(5) 4.3(1) 1

2D2 96j (0 y z) 0 0.0690(8) 0.2670(9) 4.3(1) 1

4

Fig. 5 Refined ND3 arrangement in Model II regarded as a weighted

superposition of four ND3 orientations. Small sphere: nitrogen; large

sphere: deuterium. The average nitrogen position from Model I is

found in the middle of the square. The deuterium atoms on the square

corners are denoted D1 and those slightly off the square edges are D2

in Table 2. The size of the N atoms is reduced for clarity.

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