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Phase stability in the systems AeAl2-xMgx (Ae = Ca, Sr, Ba) : Electron concentration and size controlled variations on the laves phase structural theme

Amerioun, Shahrad ; Yokosawa, Tadahiro ; Lidin, Sven LU and Häussermann, Ulrich (2004) In Inorganic Chemistry 43(15). p.4751-4760
Abstract

The systems AeAl2-xMgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 → MgNi2 → MgZn 2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl2-xMgx (0 < x ≤ 2) and BaAl2-xMgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 → MgNi2 → MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary... (More)

The systems AeAl2-xMgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 → MgNi2 → MgZn 2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl2-xMgx (0 < x ≤ 2) and BaAl2-xMgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 → MgNi2 → MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary compound BaAl2 does not exist, but more Ba-rich Ba 7Al13 forms. The reinvestigation of the crystal structure of Ba7Al13 by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba21Al40 (space group P31m, Z = 1, a = 10.568(1) Å, c = 17.205(6) Å). In Ba 21Al40 a size match problem between Ba and Al present in Ba7Al13 is resolved. The structure of Ba 7Al13 (Ba21Al40) can be considered as a Ba excess variant of the hexagonal MgNi2 Laves phase type structure. An incommensurately modulated variant of the MgNi2 structure is obtained for phases BaAl2-xMgx with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn2 type structure takes place.

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type
Contribution to journal
publication status
published
in
Inorganic Chemistry
volume
43
issue
15
pages
10 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:3242795042
ISSN
0020-1669
DOI
10.1021/ic0400235
language
English
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no
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fa5d2fbb-7746-481a-995a-2c9c87fd52af
date added to LUP
2019-04-08 15:24:39
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2022-04-18 03:53:35
@article{fa5d2fbb-7746-481a-995a-2c9c87fd52af,
  abstract     = {{<p>The systems AeAl<sub>2-x</sub>Mg<sub>x</sub> (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu<sub>2</sub> → MgNi<sub>2</sub> → MgZn <sub>2</sub> with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl<sub>2-x</sub>Mg<sub>x</sub> (0 &lt; x ≤ 2) and BaAl<sub>2-x</sub>Mg<sub>x</sub> (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu<sub>2</sub> → MgNi<sub>2</sub> → MgZn<sub>2</sub> occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu<sub>2</sub> structure at low x. For Ae = Ba a binary compound BaAl<sub>2</sub> does not exist, but more Ba-rich Ba <sub>7</sub>Al<sub>13</sub> forms. The reinvestigation of the crystal structure of Ba<sub>7</sub>Al<sub>13</sub> by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba<sub>21</sub>Al<sub>40</sub> (space group P31m, Z = 1, a = 10.568(1) Å, c = 17.205(6) Å). In Ba <sub>21</sub>Al<sub>40</sub> a size match problem between Ba and Al present in Ba<sub>7</sub>Al<sub>13</sub> is resolved. The structure of Ba <sub>7</sub>Al<sub>13</sub> (Ba<sub>21</sub>Al<sub>40</sub>) can be considered as a Ba excess variant of the hexagonal MgNi<sub>2</sub> Laves phase type structure. An incommensurately modulated variant of the MgNi<sub>2</sub> structure is obtained for phases BaAl<sub>2-x</sub>Mg<sub>x</sub> with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn<sub>2</sub> type structure takes place.</p>}},
  author       = {{Amerioun, Shahrad and Yokosawa, Tadahiro and Lidin, Sven and Häussermann, Ulrich}},
  issn         = {{0020-1669}},
  language     = {{eng}},
  month        = {{07}},
  number       = {{15}},
  pages        = {{4751--4760}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Inorganic Chemistry}},
  title        = {{Phase stability in the systems AeAl<sub>2-x</sub>Mg<sub>x</sub> (Ae = Ca, Sr, Ba) : Electron concentration and size controlled variations on the laves phase structural theme}},
  url          = {{http://dx.doi.org/10.1021/ic0400235}},
  doi          = {{10.1021/ic0400235}},
  volume       = {{43}},
  year         = {{2004}},
}