International Journal of Hydrogen Energy 27 (2002) 403–412 www.elsevier.com/locate/ijhydene The adsorption of hydrogen on B2 TiFe surfaces G.Lee a ,J.S.Kim a ,Y.M.Koo a ,S.E.Kulkova b; ∗ a Department of Physics, Pohang University of Science and Technology, Pohang, 790-784, South Korea b InstituteofStrengthPhysicsandMaterialScienceoftheRussianAcademyofSciences,pr.Academichesky2=1,Tomsk,634021,Russia Abstract We investigate the interaction of hydrogen with the B2 TiFe (001) and (110) surfaces using the full-potential linearized augmented plane wave (FLAPW) method. The changes in the electronic structures in the dierent B2 TiFe surfaces in comparisonwiththebulkgroundstateareanalyzed.FerromagneticorderisfoundintheFe-terminated(001)surfacewiththe magneticmoment2:27B,whichquicklydiminishesinsidethelm.TheabsorptionofhydrogenontotheFe= TiFe(001)surface results in a decrease in the magnetic moment. For the fully relaxed surfaces interacting with hydrogen, the driving bonding mechanismsfordierentadsorptionsitesarediscussed.Amicroscopicexplanationofthelocalsurfacereactivityisgiven.Itis foundthatthehydrogenatomsformstrongerchemicalbondswiththeironatomsthanwiththetitaniumatomsintheB2TiFe surfaces. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen storage alloy; TiFe; Hydrogen adsorption; Electronic structure 1. Introduction Theelectronicstructure(ES)ofthinlms,interfaces,and the two-dimensional magnetism of transition metal (TM) surfaces, is the subject of intense investigation due to in- creasinginterestfrombothafundamentalandatechnologi- calviewpoint.Manysurfacephenomenaaredirectlyrelated to the electronic structure of the surface. It is well known thattheadsorptionofatomsontransitionmetalsurfacescan strongly inuence their structural, electronic, catalytic and magnetic properties. The surface ES has mainly been in- vestigated for pure transition metals or a monolayer of TM on an inert substrate, while the study of transition metal alloy surfaces using ab initio methods has received less attention[1–7].Oneofthemostimportantproblemsinsur- face physics is elucidating the mechanism of gas (hydro- gen, oxygen, nitrogen, etc.) adsorption onto metal surfaces [8–14].Forexample,adsorptionofhydrogenoroxygenmay ∗ Corresponding author. Tel.: +7-3822-286848; fax: +7-3822- 259576. E-mail address: kulkova@ispms.tsc.ru (S.E. Kulkova). modify the properties of the TM interface growth. Surface oxygencanconsiderablyimprovesurfaceorderandenhance themagneticpropertiesofthetopmostFelayersintheTM compounds.Hydrogencanalsoinuencethemagneticprop- erties and phase transformations on the TM surface. Fur- thermore, the introduction of hydrogen into the V layers in multilayers of Fe and V provides a new approach for the manipulationofinterlayerexchangecoupling,whichcanbe switchedfromantiferromagnetictoferromagnetic[15].Ex- periments on TM alloys and their compounds have shown unusual adsorption properties, dierent from those of the pure metal surfaces [12,13]. Very few theoretical investi- gations of gas adsorption onto intermetallic alloy surfaces have been carried out [11–15]. To understand this problem at the microscopic level it is rst necessary to know the electronic structure of the pure alloy surface. Ab initio cal- culationsoftheelectronicstructureofthesurfacesofsome intermetallicB2alloyswererecentlyperformed[1–6].The appearanceofsurfacemagnetismin3dTMalloyswasstud- ied in [1,2,4]. The change of ES in the surface layer was considered in more detail [3,5,6]. Thus, the knowledge of theelectronicstructureofTMalloysurfacesisofparticular interest. 0360-3199/02/$20.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII:S0360-3199(01)00139-2