Gd2AlGe2：新型‘准-Zintl相’与W2CoB2结构堆叠变体

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"这篇学术论文详细介绍了Gd2AlGe2这一新型材料的合成、结构确定及计算电子结构。Gd2AlGe2被描述为一种'几乎-Zintl相'，并呈现W2CoB2类型的新型堆叠变体。作者Wonyoung Choe、Sean McWhorter和Gordon J. Miller来自美国爱荷华州立大学化学系和阿莫斯实验室，该研究是在Welf Bronger教授70岁生日之际献给他的。" 在化学领域，Zintl相是指一类具有特殊电子配置的无机化合物，通常由主族金属元素和碱土金属或过渡金属组成，其电子结构接近于简单的金属-非金属键合，表现出类似分子晶体的特性。Gd2AlGe2被称作“几乎-Zintl相”，意味着它在某种程度上表现出了Zintl相的特点，但可能在电子结构或化学键合上存在微妙的差异。论文报道了Gd2AlGe2的晶体结构，它属于C2/c空间群，具有独特的晶格参数：a = 10.126(2)Å，b = 5.6837(12)Å，c = 7.7683(16)Å，以及β = 104.729(3)°。这一结构表明它与传统的Zintl相和W2CoB2类型有所不同，后者通常具有不同的原子堆积方式和电子分布。利用紧束缚线性 muffin-tin 球轨道（TB-LMTO-ASA）计算方法，研究人员分析了Gd2AlGe2的电子结构。这些计算揭示了该材料的总态密度在一个特定值（18价电子/公式单元）处有一个明显的最小值，而Gd2AlGe2的每个公式单元实际上含有17个价电子。这表明Gd2AlGe2的电子结构可能具有一种特殊的稳定性或者电子分布模式，这可能是其被称为“几乎-Zintl相”的原因。此外，论文中还提到的W2CoB2型结构，是一种已知的材料结构类型，而Gd2AlGe2是这一类别的新成员，具有独特的堆叠变体。这种新的堆叠方式可能影响材料的物理和化学性质，例如磁性、电导率和热稳定性等。 Gd2AlGe2是化学和材料科学领域的一项重要发现，其独特的“几乎-Zintl相”特征和W2CoB2型结构变体为理解和设计新型功能材料提供了新的视角。这一研究对于深入探索电子结构对材料性能的影响，以及开发新的电子和磁性应用具有重要意义。

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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