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Linear Metal Chains in Ca2M2X (M = Pd, Pt; X = Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability

Doverbratt, Isa LU ; Ponou, Simeon LU ; Zhang, Yuemei; Lidin, Sven LU and Miller, Gordon J. (2015) In Chemistry of Materials 27(1). p.304-315
Abstract
A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Delta chi) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C-2/c). All title structures... (More)
A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Delta chi) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C-2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd center dot center dot center dot Pd atoms, whereas the metal chains in II-IV are pairwise distorted, resulting in short connected {Pd-2} or {Pt-2} dumbbells that are separated by longer M center dot center dot center dot M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Delta chi between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal-metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M-M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Chemistry of Materials
volume
27
issue
1
pages
304 - 315
publisher
The American Chemical Society
external identifiers
  • wos:000348085300038
  • scopus:84921342801
ISSN
0897-4756
DOI
10.1021/cm503985h
language
English
LU publication?
yes
id
8c30e671-d3ec-4eeb-82ae-527c94f224f5 (old id 5201273)
date added to LUP
2015-03-26 13:14:42
date last changed
2017-09-10 03:05:25
@article{8c30e671-d3ec-4eeb-82ae-527c94f224f5,
  abstract     = {A series of four new analogue phases Ca2M2X (M = Pd, Pt and X = Al, Ge) were prepared by direct combination of the respective elements in stoichiometric mixtures at high temperature in order to analyze the impact of valence electron count (vec) and electronegativity differences (Delta chi) on the structure selection and stability. Their crystal structures, as determined from single-crystal X-ray diffraction data, correspond to two different but closely related structure types. The first compound, Ca2Pd2Ge (I), is an unprecedented ternary ordered variant of the Zr2Al3-type (orthorhombic, Fdd2). The three other phases, Ca2Pt2Ge (II), Ca2Pd2Al (III) and Ca2Pt2Al (IV), adopt the Gd2Ge2Al-type structure (monoclinic, C-2/c). All title structures feature linear chains of the noble metals (Pd or Pt). The Pd linear chains in I are undistorted with equidistant Pd center dot center dot center dot Pd atoms, whereas the metal chains in II-IV are pairwise distorted, resulting in short connected {Pd-2} or {Pt-2} dumbbells that are separated by longer M center dot center dot center dot M contacts. The occurrence and magnitude of the pairing distortion in these chains are controlled by the vec and the Delta chi between the constituent elements, a result which is supported by analysis of the calculated Bader effective charges. The metal chains act as charge modulation units, critical for the stability and the electronic flexibility of the structures by an adequate adjustment of the metal-metal bond order to both the vec and the degree of charge transfer. Thus, Ca2Pd2Ge (28 ve/f.u) is a Zintl-like, charge optimized phase with formally zerovalent Pd atoms forming the undistorted metal chains; semimetallic properties are predicted by TB-LMTO calculations. In contrast, the isoelectronic Ca2Pt2Ge is predicted to be a good metal with the Fermi level located at a local maximum of the DOS, a fingerprint of potential electronic instability. This is due to greater charge transfer to the more electronegative Pt atoms forming the metal chains and probably to packing frustration in the well packed structure that may prevent a larger distortion of the Pt chains. However, the instability is suppressed in the aliovalent but isostructural phases Ca2M2Al (27 ve/f.u) with an enhancement of the pairing distortion within the metal chains but lower M-M bond order. Further reduction of the vec as in Ca2M2Cd (26 ve/f.u) may induce a transition toward the more geometrically flexible W2CoB2-type with a low dimensional structure, to create more room for a larger distortion of the metal chain as dictated by the shortage of valence electrons.},
  author       = {Doverbratt, Isa and Ponou, Simeon and Zhang, Yuemei and Lidin, Sven and Miller, Gordon J.},
  issn         = {0897-4756},
  language     = {eng},
  number       = {1},
  pages        = {304--315},
  publisher    = {The American Chemical Society},
  series       = {Chemistry of Materials},
  title        = {Linear Metal Chains in Ca2M2X (M = Pd, Pt; X = Al, Ge): Origin of the Pairwise Distortion and Its Role in the Structure Stability},
  url          = {http://dx.doi.org/10.1021/cm503985h},
  volume       = {27},
  year         = {2015},
}