Pergaimon Nanc.Sm& Mataials. Vol. 10, No. 4. pp. 635-647.1998 Elsevia Science Ltd PII SOP659773(98)00091-9 0 1998 Acta Metallurgica Inc. Printed in the USA. All ri&ts -cd 096s9773m $19.00 + .OO ELECTRONIC AND STRUCTURAL TRENDS IN SMALL GaAs CLUSTERS P. Piquini’, S. Canutd and A. Fazzio2 ‘Departamento de Fisica, Universidade Federal de Santa Maria, 97119-900, Santa Maria, RS, Brazil 21nstituto de Fisica, Universidade de S%o Paulo, CxP 60318,05315-970, Sb Paulo, SP, Brazil zyxwvutsrqponmlkjihgfedcbaZYXWVUTS (Accepted June 17,1998) Abstract-Using the Hartree-Fock methodfollowed by second-order perturbation theory, the structural and electronic properties of the GarAs,,, (n + m I 8) clusters and its positive and negative ions are examined. The ground state structures are obtained through geometry optimization calculations without spatial symmetry constraints. Electronic and structural properties are then calculated. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED In particular it is verified an alternating behavior in the ionization potential beyond three-atom clusters. A structural pattern is noticed where the embryonic forms of these clusters are observed to present highly symmetrical configurations formed by the central As atoms. The Ga atoms appear at positions that enhance a hybridization and chemical ordering which tends to thatpresented by the bulk. The changefor a lay er- ty pe structure is observed to occur already in the eight-atom stoichiometric cluster. A theoretical explanation ofpreviously obtained muss spectrometry results is suggested. 01998 Acta Metallurgica Inc. I. INTRODUCTION The idea that small atomic and molecular clusters could be the precursors of the macroscopic structures is not new. Although it could at first seem obvious, the verification of this hypothesis is not a simple task, even with the theoretical and experimental capabilities at our disposal today. In their route from diatomic molecules tocrystals, the growing clusters will change their electronic and structural properties. It is well known today that the characteristic bulk properties do not appear all at the samle time. The understanding of the process of variation of physical and chemical properties with the number of atoms in a cluster is a great challenge. Actually, each different type of cluster may have their own route. The specificity of these processes depends crucially on the nature of the chemical bonding between the components of the clusters. In the last decade a great advance in the understanding of these systems was attained. This has occurred in part due to the development of new experimental techniques such as the supersonic cluster beam (l), which allows the measurements of some properties of selected isolated clusters. From these observations it could be stated that clusters of monovalent atoms like Na, K, Cs, present 635