Gd2AlGe2：新近Zintl相与W2CoB2型的堆叠变体研究

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Gd2AlGe2是一种新颖的化合物，被归类为W2CoB2类型的"Almost-Zintl相"，这是一种特殊的晶体结构。这种结构的独特之处在于它在空间群C2/c下结晶，具有相当的晶格参数：a轴长度约为10.126(2) Å，b轴长度约为5.6837(12) Å，c轴长度约为7.7683(16) Å，而β角为104.729(3)°。这一结构的发现是通过合成和结构分析实现的，这些工作由Wonyoung Choe、Sean McWhorter和Gordon J. Miller*在2002年3月28日于美国爱荷华州的爱荷华州立大学化学系和艾姆斯实验室完成。 Gd2AlGe2的独特性质在于其电子结构，这主要通过TB-LMTO-ASA（紧束缚线性松饼锡轨道）计算方法得到了深入研究。计算结果显示，尽管Gd2AlGe2在化学式中拥有17个价电子，但它展现出一个非常罕见的现象，即在每个公式单元中的总态密度达到显著的最低点，只有18个价电子。这表明Gd2AlGe2可能具有一些独特的电子配位和轨道排列，使其电子行为呈现出与传统Zintl相（其中元素倾向于形成金属-卤素离子对）不同的特性。 "Almost-Zintl相"这个术语暗示了Gd2AlGe2在某些方面接近Zintl相的定义，但可能在其他方面存在微妙的差异。作为W2CoB2类型的新型堆叠变体，它可能是材料科学的一个潜在重要发现，因为堆叠方式的不同可能会带来新的物理性质，如电导率、磁性和热力学性质的改变。整个论文围绕着Gd2AlGe2的制备过程、结构解析以及电子结构的理论计算展开，旨在为理解这类化合物的形成机制和潜在应用提供基础。Welf Bronger教授的70岁生日献礼，体现了这项工作的纪念意义和对科学研究的敬意。通过这篇学术论文，研究人员揭示了一个新颖的化合物结构，这对于理解化学键合、电子行为以及材料设计有着重要的科学价值。

AlGe

: An “Almost-Zintl Phase” and a New Stacking Variant of the

CoB

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

Birthday

Abstract. The synthesis, structure determination and calculated

electronic structure of the new phase, Gd

AlGe

, 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

AlGe

has 17 valence electrons in its formula unit),

AlGe

: eine „Fast-Zintl-Phase“ und eine neue Stapelvariante des W

CoB

-Typs

Inhaltsübersicht. Die Synthese, Strukturbestimmung und berech-

nete Elektronenstruktur der neuen Phase Gd

AlGe

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

] 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

CoB

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

AlGe

hat 17 Valenzelek-

tronen pro Formeleinheit); das Minimum rührt von polaren kova-

lenten Bindungen im dreidimensionalen [AlGe

]-Netz, Gd

Ge-

Wechselwirkungen und Zwei-Elektronen-drei-Zentren-Bindungen

zwischenAl und Gd her. Die Struktur ist eine Stapelvariante des

CoB

-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

(Si

1⫺x

)

, 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

(Si

1⫺x

)

system, we have attempted to replace some

of the Gd atoms by nonmagnetic metal atoms, which have

also produced new R

(R ⫽ rare earth element; M ⫽

nonmagnetic metal; T ⫽ Si or Ge) compounds. Previously

known compounds in the R

system include two pri-

mary structure types: (1) the orthorhombic W

CoB

type

for R

AlSi

(R ⫽ Er, Ho, Y;) [4], and (2) the tetragonal

FeB

type for Yb

AlSi

[1] and R

InGe

(R ⫽ La, Ce,

Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, Y; Mo

FeB

type) [5, 6].

Recently, we have reported a new addition to the Mo

FeB

family, Gd

MgGe

, involving a nonmagnetic Mg atom,

which produces a Gd-Ge substructure that resembles part

of the structure of Gd

[7]. In an attempt to increase

the valence electron concentration, we replaced Mg by Al

in the Gd

MGe

system, and, surprisingly discovered a new

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