Crystal structure of κ-Ag2Mg5

Article abstract of DOI:10.1016/j.jssc.2017.10.019

The structure of κ-Ag₂Mg₅ has been refined based on X-ray powder diffraction measurements (R_wp = 0.083). The compound has been prepared by combining mechanical alloying techniques and thermal treatments. The intermetallic presents the prototypical structure of Co₂Al₅, an hexagonal crystal with the symmetries of space group P6₃/mmc, and belongs to the family of kappa-phase structure compounds. The unit cell dimensions are a=8.630(1) ? and c=8.914(1) ?. Five crystallographically independent sites are occupied, Wyckoff positions 12k, 6h and 2a are filled with Mg, another 6h site is occupied with Ag, and the 2c site presents mixed Ag/Mg occupancy. The crystal chemistry of the structure and bonding are briefly discussed in the paper.

Full text of DOI:10.1016/j.jssc.2017.10.019

Author’s Accepted Manuscript
Crystal structure of κ-Ag Mg
2
5
Facundo J. Castro, Gastón A. Primo, Guillermina
Urretavizcaya
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https://doi.org/10.1016/j.jssc.2017.10.019
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Journal of Solid State Chemistry
Received date: 4 September 2017
Revised date: 10 October 2017
Accepted date: 15 October 2017
Cite this article as: Facundo J. Castro, Gastón A. Primo and Guillermina
Urretavizcaya, Crystal structure of κ-Ag Mg , Journal of Solid State Chemistry,
2
5
https://doi.org/10.1016/j.jssc.2017.10.019
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Crystal structure of -Ag2Mg5
Facundo J. Castro1 a, b, Gastón A. Primoc, Guillermina Urretavizcayaa, b
aCNEA, CONICET, Centro Atómico Bariloche, S. C. de Bariloche, Río Negro, Argentina.
bUniversidad Nacional de Cuyo, Instituto Balseiro, S. C. de Bariloche, Río Negro, Argentina.
cIMBIV-CONICET, Departamento de Química Orgánica, Facultad de Ciencias Químicas,
Universidad Nacional de Córdoba. Haya de la Torre y Medina Allende. Edificio de Ciencias II.
Ciudad Universitaria, Córdoba, Argentina.
Abstract
The structure of -Ag2Mg5 has been refined based on X-ray powder diffraction measurements (Rwp
= 0.083). The compound has been prepared by combining mechanical alloying techniques and
thermal treatments. The intermetallic presents the prototypical structure of Co2Al5, an hexagonal
crystal with the symmetries of space group P63/mmc, and belongs to the family of kappa-phase
structure compounds. The unit cell dimensions are a=8.630(1) Å and c=8.914(1) Å. Five
crystallographically independent sites are occupied, Wyckoff positions 12k, 6h and 2a are filled
with Mg, another 6h site is occupied with Ag, and the 2c site presents mixed Ag/Mg occupancy.
The crystal chemistry of the structure and bonding are briefly discussed in the paper.
Keywords: Intermetallics, Structure, Crystal chemistry, X-ray diffraction, Mechanical alloying and
milling
1 Introduction
The kappa-phase structure compounds constitute a crystallographic family based on the
structure of W10Co3C3.4 [1]. These compounds crystallize in the hexagonal system, space group
P63/mmc. The metal atom substructure of this prototypical structure is that of Mn3Al10, and the
different kappa-phase structure compounds are obtained by filling the trigonal prismatic (2c) or the
octahedral (6g) interstices of this “host lattice” by p elements or transition metals. If only the
trigonal prismatic interstices are filled, the Co2Al5 prototypical structure is obtained. On the other
hand, if no more than the octahedral interstices are occupied the Mo12Cu3Al11C6 structure is
obtained. Some compounds have both interstices filled [1].
During an experimental study conducted to analyze MgH2 destabilization by the formation
of Ag-Mg alloys we have identified an intermetallic compound denoted for simplicity Ag2Mg5 that
crystallizes with the Co2Al5 structure. Up to now, only eleven compounds with this prototypical
structure have been reported [2-3], together with numerous RE10TMCd3 and RE10TMAl3 (RE: rare-
earth metal, TM: transition metal) ternary compounds with anti-Co2Al5 structure recently identified
[4-5]. Interestingly, four of the Co2Al5 structure compounds contain Mg and show some regularity
in the periodic table of the elements, namely: Ir2Mg5 [6], Rh2Mg5 [7], Pd2Mg5 [3], and the ternary
compound Ir2.096Mg1.980In2.924 [3]. The existence of the intermetallic Ag2Mg5 follows this trend. To
the best of our knowledge this compound has been only previously mentioned in a PhD thesis [8]
and is not included in the Ag-Mg equilibrium phase diagrams [9-11]. We present here the
refinement of its structure, based on X-ray diffraction experiments on powders.
2 Experimental and refinement details
The compound was prepared by combining mechanical alloying techniques and thermal
treatments. A mixture of magnesium and silver with molar ratio Mg:Ag = 5.25:2 was mechanically
alloyed in a planetary mill (Fritsch Monomill Pulverisette 6) under pure argon (99.999 %)
1 Corresponding author. e-mail: fcastro@cab.cnea.gov.ar. Centro Atómico Bariloche, Av. Bustillo km 9.5, S. C. de
Bariloche, Río Negro, Argentina.

atmosphere during 10 h. The milling conditions were: 0.5 MPa of argon pressure, ball-to-powder
mass ratio equal to 40:1, rotational speed of 400 rpm, and steps of 10 minutes of milling followed
by 10 minutes of pause. As raw materials Ag nanopowder (< 100 nm, 99.5%, Sigma-Aldrich) and
Mg powder obtained from MgH2 decomposition at 355°C under vacuum were employed. The
hydride used to produce Mg was obtained by milling MgH2 (hydrogen storage degree, Sigma-
Aldrich) under hydrogen (99.999 %) during 10 h (similar ball-to-powder mass ratio and milling
schedule). The only crystalline phase observed in the as-milled mixture was AgMg (ICDD PDF 65-
220) with an approximate composition Ag0.4Mg1.6. This composition exceeds the equilibrium range
of the intermetallic AgMg [9]. However, is usual to obtain non-equilibrium phases by mechanical
milling [12]. After a short thermal treatment of 15h at 350ºC under 0.2 MPa of Ar the compound
Ag2Mg5 was observed together with AgMg of approximate composition Ag0.9Mg1.1, very close to
the equilibrium composition of AgMg at 350ºC, according to the phase diagram. This material was
further homogenized by a heat treatment at 300ºC under 0.2 MPa of Ar atmosphere for 7 days (the
temperature has been chosen taking into account that Ag2Mg5 decomposes around 450ºC [8]). All
the materials handling has been done within an Ar filled glovebox with O2 and H2O amounts below
1ppm.
X-ray diffraction was measured with a Bruker D8 Advance instrument equipped with a PSD
detector (LynxEye) and using CuK radiation. An air scatter screen was attached to the instrument
to reduce background. The sample was mounted on a low-background Si sample holder. Data were
taken in the 2 range 10-140º with a 0.010231º step size and a 9 s collection interval. Rietveld
refinement of the data was carried out with FullProf [13] using pseudo-Voigt peak-shape functions
including an axial divergence asymmetry correction. Sample vertical displacement and micro-
absorption effects have been taken into account during the refinement. As individual atomic
displacement coefficients could not be reliably refined, only global coefficients were considered.
For Ag2Mg5 the starting structural parameters have been taken from Co2Al5 [2]. 3D visualization of
the structure has been done with VESTA [14].
3 Results and discussion
The diffractogram of the homogenized material presents three phases: Ag2Mg5 (main
phase), AgMg and MgO (Fig. 1). This last phase has been formed by reaction of the sample with
gaseous impurities during the thermal treatment. Data have been successfully refined including the
above mentioned phases. Tables 1 and 2 summarize the main results.
No. of data/parameters
Rwp
12708/48
0.083
Phases composition and abundance
Ag1.94Mg5.06
Ag0.93Mg1.07
65 wt.%
27 wt.%
8 wt.%
MgO
Ag2Mg5 Crystal data
Refined composition
Ag1.945Mg5.055
Z
4
Formula weight
Crystal system
332.7 g mol-1
hexagonal
Space group
a
P63/mmc (nº 194)
8.630(1) Å
c
8.914(1) Å
Volume
calc
0.5749 nm3
3.84 g cm-3
Table 1. General results of the refinement and crystal structure data for Ag2Mg5.

Atom
Wyckoff x
position
y
z
Occup (%)
Mg1
Ag1
Mg2
Ag2/Mg3 2c
Mg4 2a
12k
6h
6h
0.2009(5) 2x
0.8829(2) 2x
0.0517(4) 100
1/4
1/4
1/4
0
100
100
89(1)/11(1)
100
0.538(1)
2x
2/3
0
1/3
0
Table 2. Atomic coordinates for Ag2Mg5.
The refinement of the occupancy resulted in single atom full occupancy of the 12k (Mg1),
6h (Ag1), 6h (Mg2) and 2a (Mg4) sites, and mixed Ag/Mg occupancy of the 2c Wyckoff position.
The 12k, 6h and 2a sites constitute the metal substructure of the kappa phase structure, whereas the
2c site is the trigonal interstice that can be filled or not, depending on the nature of the atom [1]. In
the Co2Al5 structure this site is fully occupied by Co, but mixed occupancy of this interstice has
been observed in Pd2Mg5 [8].
The coordination polyhedra of the Ag and Mg sites are shown in Fig. 2. The atomic
environment of Mg1 is an irregular 14-vertex polyhedron Ag4Mg10, Ag1 is surrounded by a slightly
distorted icosahedron Ag2Mg10, the coordination polyhedron of Mg2 is a tricapped pentagonal
prism Ag3Mg10, that of the 2c Wyckoff position (Ag2/Mg3) is a tricapped trigonal prism formed by
9 Mg atoms, and finally, Mg4 is surrounded by an icosahedron Mg6Ag6. The complete structure can
be imagined as columns built by stacking Mg4 icosahedra in the [001] direction. These icosahedra
share the triangular faces made of Ag1 atoms and are interconnected by the Ag2/Mg3 tricapped
trigonal prisms formed by Mg1 and Mg2 atoms (Fig. 3).
Atom
Mg1
Coord.
Neighbour Distance (Å)
Ag2/Mg3 2.654(8)
1
1
1
2
2
2
2
2
1
2
2
2
2
4
2
1
4
4
2
Ag1
Mg4
Mg1
Ag1
Mg2
Mg2
Mg1
Mg1
Mg2
Mg4
Mg1
Ag1
Mg1
Ag1
2.967(4)
3.038(3)
3.141(4)
3.154(5)
3.217(5)
3.323(5)
3.429(8)
3.535(6)
2.816(9)
2.834(1)
2.967(4)
3.032(3)
3.154(5)
2.816(9)
Ag1
Mg2
Ag2/Mg3 3.059(5)
Mg1
Mg1
Mg2
Mg1
Mg2
Ag1
Mg1
3.217(5)
3.323(5)
3.331(1)
2.654(8)
3.059(5)
2.834(1)
3.038(3)
Ag2/Mg3 6
3
Mg4
6
6
Table 3. Interatomic distances within the first coordination spheres.

The interatomic distances within the first coordination spheres are given in Table 3. In the
following we compare these distances with those reported for -AgMg4 [15] and for the
isostructural Pd2Mg5 [3]. The only Ag–Ag pairs in Ag2Mg5 are Ag1–Ag1, with an interatomic
distance of 3.032 Å, similar to the Ag–Ag distance of 2.96 Å reported for -AgMg4 and comparable
to the Ag–Ag distance of 2.889 Å for fcc Ag. This suggests significant Ag–Ag interactions in
Ag2Mg5. On the contrary, the Pd–Pd distance of 3.200 Å for Pd2Mg5 is considerably greater than
Pd–Pd distances in fcc Pd (2.751 Å), indicating weak TM–TM interactions in this compound. Mg–
Ag distances in Ag2Mg5 are in the range 2.65–3.15 Å, a slightly smaller range than 2.70–3.40 Å
reported for -AgMg4. Comparing with Mg–Pd distances in Pd2Mg5, 2.60–3.11 Å, the values appear
very similar, although Pd is smaller than Ag. Mg–Mg distances in Ag2Mg5 extend from 3.04 to 3.54
Å, close to the interatomic distances 3.00–3.70 Å for -AgMg4, and covering a wider range than
Mg–Mg distances in Pd2Mg5, 2.93–3.25 Å. If we compare these values with the Mg–Mg distance of
3.209 Å in hcp Mg we note that 62% of the Mg–Mg pairs within the first coordination sphere in
Ag2Mg5 have distances greater than 3.209 Å. On the contrary, only 32% of Mg-Mg pairs exceed
this value in Pd2Mg5 suggesting that Mg–Mg bonding is not as important in Ag2Mg5 as it is in
Pd2Mg5. Therefore predominant bonding in Ag2Mg5 is Ag–Ag and Ag–Mg, whereas in Pd2Mg5 is
Pd–Mg and Mg–Mg [3].
An interesting consequence of the characteristics of bonding in Ag2Mg5 is the greater c/a
ratio of this compound compared with that of the isostructural compounds Ir2Mg5, Rh2Mg5 and
Pd2Mg5 (Table 4). The difference is due to the greater c lattice parameter of Ag2Mg5. From the
geometry of the structure (Figs. 2 and 3) it can be seen that the magnitude of c depends on the Ag1–
Mg4 and Ag1–Ag1 distances. Longer TM1–Mg4 distances and shorter TM1–TM1 distances
contribute to greater c lattice parameters. This particular combination takes place in Ag2Mg5 (Table
4). TM1-Mg4 distances are in all the cases 7–9% shorter than the sum of the corresponding atomic
radii but the greater Ag atomic radius produces a longer Ag1–Mg4 distance. On the other hand, the
Ag1-Ag1 distance is shorter than the other TM1-TM1distances and, interestingly, it is only 5%
greater than Ag–Ag distance in fcc Ag, whereas Rh1–Rh1 and Pd1–Pd1 distances are 14% and 16%
greater than the corresponding TM1-TM1 distances in the pure TM fcc structure. This suggests
again a greater TM–TM interaction in Ag2Mg5.
Compound
a (Å)
c (Å)
c/a
TM1-Mg4
distance (Å)
-
2.678 (2.949) 3.073 (2.689) [7]
2.753 (2.980) 3.200 (2.751) [3]
2.834 (3.057) 3.032 (2.889) This work
TM1-TM1
distance (Å)
-
Ref.
[6]
Ir2Mg5
8.601
8.536
8.671
8.630
8.145
8.025
8.164
8.914
0.947
0.940
0.942
1.033
Rh2Mg5
Pd2Mg5
Ag2Mg5
Table 4. Lattice parameters of Mg-containing Co2Al5-type compounds and TM1–Mg4 and TM1–
TM1 distances. The numbers between parentheses are the sum of the atomic radii of the elements,
estimated from the fcc (TM) or hcp (Mg) structures of the pure metals.
Concerning the stability of -Ag2Mg5, complementary experiments suggest that this phase is
a stable low-temperature phase of the Ag-Mg system. In a DSC experiment (not shown here) it was
observed that -Ag2Mg5 decomposes into AgMg and -AgMg3 around 460ºC, but interestingly, if
this AgMg and -AgMg3 mixture is kept at 250ºC under Ar atmosphere for 6 days, -Ag2Mg5
reappears, indicating that the intermetallic is an equilibrium compound. A similar result was
reported by Kudla in his PhD dissertation [8]. Detailed thermodynamic studies of the composition
of -Ag2Mg5 and its stability region in the Ag-Mg phase diagram are currently under way.

Acknowledgements
The authors gratefully acknowledge fruitful discussions with Dr. J. J. Andrade Gamboa. G. A.
Primo also acknowledges the Summer School 2014 from Instituto Balseiro.
Funding: This work was supported by CONICET [PIP 112 201501 00610]; and Universidad
Nacional de Cuyo [06/C486].
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Ir2.096Mg1.980In2.924 with hexagonal Co2Al5 type structure, Intermetallics, 12 (2004) 533-537.
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Dalton Trans., 46 (2017) 1083–1092.
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ternären Phasen der Systeme Ag–Mg und Ag–Ga–Mg, PhD dissertation, Technischen
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Figure captions
Fig. 1. Observed, calculated and difference X-ray diffraction patterns of the homogenized material.
Fig. 2. Coordination polyhedra of Mg1 (orange sphere labelled 1), Ag1 (grey sphere labelled 1),
Mg2 (orange sphere labelled 2), Ag2/Mg3 (bicolour sphere labelled 2/3) and Mg4 (orange sphere
labelled 4).
Fig. 3. Two views of the complete structure. In orange the coordination polyhedra of Mg4 and in
grey the coordination polyehdra of Ag2/Mg3. Black lines represent the unit cell.

Highlights
 The structure of -Ag2Mg5 is refined based on X-ray powder diffraction data.
 The compound has the prototypical Co2Al5 structure.
 The intermetallic belongs to the family of kappa-phase structure compounds.
 Predominant bonding in Ag2Mg5 is Ag–Ag and Ag–Mg.