Gd
2
AlGe
2
: An “Almost-Zintl Phase” and a New Stacking Variant of the
W
2
CoB
2
Type
Wonyoung Choe, Sean McWhorter, and Gordon J. Miller*
Ames, Iowa / USA, Iowa State University, Department of Chemistry and Ames Laboratory, US Department of Energy
Received March 28th, 2002.
Dedicated to Professor Welf Bronger on the Occasion of his 70
th
Birthday
Abstract. The synthesis, structure determination and calculated
electronic structure of the new phase, Gd
2
AlGe
2
, are reported. The
compound crystallizes in a new structure type with space group
C2/c, a ⫽ 10.126(2) A
˚
, b ⫽ 5.6837(12) A
˚
, c ⫽ 7.7683(16) A
˚
,and
β ⫽ 104.729(3)s. Tight-binding linear-muffin-tin orbital (TB-
LMTO-ASA) calculations show a distinct minimum in the total
density of states for this structure at 18 valence electrons per for-
mula unit (Gd
2
AlGe
2
has 17 valence electrons in its formula unit),
Gd
2
AlGe
2
: eine „Fast-Zintl-Phase“ und eine neue Stapelvariante des W
2
CoB
2
-Typs
Inhaltsübersicht. Die Synthese, Strukturbestimmung und berech-
nete Elektronenstruktur der neuen Phase Gd
2
AlGe
2
wird mitge-
teilt. Die Verbindung kristallisiert in einem neuen Strukturtyp in
der Raumgruppe C2/c, a ⫽ 10,126(2) A
˚
, b ⫽ 5,684(1) A
˚
, c ⫽
7,768(2) A
˚
, β ⫽ 104,729(3)°. Rechnungen nach der Tight-binding-
linear-muffin-tin-Näherung (TB-LMTO-ASA) zeigen ein deutli-
ches Minimum der Gesamtzustandsdichte für diese Struktur bei 18
Introduction
Classical Zintl phases involve combinations of active metals
with more electronegative main group elements from
groups 13⫺16. They are electron precise, valence com-
pounds that show semiconducting behavior. Electronic
structure calculations identify that bonding within the net-
work of the electronegative components is optimized and
responsible for the energy gap [1]. Therefore, we can apply
the octet rule or other simple electron counting rules to
rationalize the observed structure. In the past decade, there
has been significant developments toward studying so-
called “metallic Zintl phases” or polar intermetallics, in
which bonding is optimized within the electronegative metal
net, but there is no energy gap [1]. These compounds gener-
ally involve combinations of alkali or alkaline earth metals
with group 12⫺14 elements. Perhaps the next stage of devel-
opment involves the combination of rare earth elements
* Prof. Dr. Gordon J. Miller
Iowa State University
Department of Chemistry
Ames, Iowa 50011 / USA
FAX: 294-0105
E-mail: gmiller@iastate.edu
Z. Anorg. Allg. Chem. 2002, 628, 1575⫺1580 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044⫺2313/02/628/1575⫺1580 $ 20.00⫹.50/0 1575
which arises from polar covalent bonding within the three-dimen-
sional [AlGe
2
] net, Gd-Ge interactions and three-center, two-elec-
tron bonding between Al and Gd. The structure is a new stacking
variant of the W
2
CoB
2
structure type, which is observed for numer-
ous ternary rare-earth silicides and germanides.
Keywords: Germanides; Crystal structure; Rare-earth elements;
Band structure.
Valenzelektronen pro Formeleinheit (Gd
2
AlGe
2
hat 17 Valenzelek-
tronen pro Formeleinheit); das Minimum rührt von polaren kova-
lenten Bindungen im dreidimensionalen [AlGe
2
]-Netz, Gd
¯
Ge-
Wechselwirkungen und Zwei-Elektronen-drei-Zentren-Bindungen
zwischenAl und Gd her. Die Struktur ist eine Stapelvariante des
W
2
CoB
2
-Typs, welches von zahlreichen ternären Lanthanoidsilici-
den und -germaniden bekannt ist.
with post-transition elements from groups 13⫺14. Such
compounds may show interesting chemical and physical
properties, as for example Gd
5
(Si
x
Ge
1⫺x
)
4
, which undergo
structural phase transitions involving breaking/forming Si/
Ge covalent bonds on heating/cooling coupled with a mag-
netic transition that leads to a giant magnetocaloric effect
[2]. This thermal property finds application in magnetic re-
frigeration [3].
As part of a research effort to identify the coupling of
magnetic and crystallographic structures in the
Gd
5
(Si
x
Ge
1⫺x
)
4
system, we have attempted to replace some
of the Gd atoms by nonmagnetic metal atoms, which have
also produced new R
2
MT
2
(R ⫽ rare earth element; M ⫽
nonmagnetic metal; T ⫽ Si or Ge) compounds. Previously
known compounds in the R
2
MT
2
system include two pri-
mary structure types: (1) the orthorhombic W
2
CoB
2
type
for R
2
AlSi
2
(R ⫽ Er, Ho, Y;) [4], and (2) the tetragonal
Mo
2
FeB
2
type for Yb
2
AlSi
2
[1] and R
2
InGe
2
(R ⫽ La, Ce,
Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, Y; Mo
2
FeB
2
type) [5, 6].
Recently, we have reported a new addition to the Mo
2
FeB
2
family, Gd
2
MgGe
2
, involving a nonmagnetic Mg atom,
which produces a Gd-Ge substructure that resembles part
of the structure of Gd
5
Ge
4
[7]. In an attempt to increase
the valence electron concentration, we replaced Mg by Al
in the Gd
2
MGe
2
system, and, surprisingly discovered a new
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