RAPID COMMUNICATIONS PHYSICAL REVIEW B 89, 161406(R) (2014) Chemically ordered decahedral FePt nanocrystals observed by electron microscopy Zi-An Li, 1 M. Spasova, 1 Q. M. Ramasse, 2 M. E. Gruner, 1 C. Kisielowski, 3 and M. Farle 1 , * 1 Faculty of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), University Duisburg-Essen, D-47048 Duisburg, Germany 2 SuperSTEM Laboratory, STFC Daresbury Campus, Keckwick Lane, Daresbury WA44AD, United Kingdom 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, USA (Received 5 August 2013; revised manuscript received 31 March 2014; published 24 April 2014) The crystal structure of FePt nanoparticles of mean size of 6 nm produced by gas-phase condensation is characterized using a combination of high-resolution transmission electron microscopy (HRTEM) and high- angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM). These FePt nanoparticles are found to be chemically ordered, decahedral shaped, and Pt enriched at the surfaces. The experimentally determined crystallographic lattice constants and distribution of Fe and Pt atoms are compared with first-principles calculations of ordered decahedral FePt nanoparticles to confirm the discovery of a unique decahedral structure with Fe/Pt ordering and Pt surface segregation. DOI: 10.1103/PhysRevB.89.161406 PACS number(s): 61.46.Df , 64.75.Jk, 68.37.Og, 75.75.Fk Understanding the equilibrium shape and morphology of nanoparticles has been a challenge for more than a century since the first analysis of morphology and facet formation by Wulff [1]. The experimentally demonstrated coexistence of various structural motifs implies the presence of a complex energy landscape with different local minima for the nucleation of small clusters and the subsequent shellwise growth into nanoparticles (NPs). For example, in the same size range and for identical synthesis conditions, it is possible to find both regular crystal NPs and multiply twinned particles (MTPs), as first observed by Ino [2] and Allpress [3]. Decahedra and icosahedra are specific forms of MTPs which have been the subject of intensive studies, and substantial information about their properties, structure, defects, and elastic deformations has been gathered to date through both experiment and theory, as seen in recent reviews [4,5] and references therein. While the atomistic structure of elementary metallic MTPs such as Au NPs [4] has been fully characterized, the under- standing of compound and multicomponent alloy MTPs is still challenging owing to additional energetic complexities imposed by the homogeneity or local inhomogeneity of their compositions, by their distinct chemistry, and by order- disorder phenomena. Little is known [6] as to whether and how (i) alloying affects the formation of MTPs, (ii) element segregation affects the MTPs formation, and (iii) structurally or chemically driven transitions, such as the disorder-order transition in an alloy, can occur in nanoparticles, which are sus- ceptible to energetically large, non-bulk-like surface effects. We have been investigating a prototype system consisting of gas-phase-synthesized Fe-Pt nanoparticles that exhibit rich structural and magnetic properties [7,8]. Depending on the kinetic synthesis conditions, the FePt alloy crystallizes either in the chemically disordered face-centered cubic structure (A1 phase) or in a chemically ordered structure with face-centered tetragonal coordination (L1 0 phase). Regardless of the synthe- sis route, FePt MTPs often coexist with regular single crystals [9,10]. Recent experiments [11–13] and calculations [14,15] suggest that FePt NPs show Pt segregation towards their surfaces (and possibly also at internal interfaces [16]). Such * Corresponding author: michael.farle@uni-due.de “self-assembled” ferromagnetic particles with a Pt-enriched surface are thus environmentally stable against oxidation [12]. Additionally, the surface Pt acts as a catalyst yielding the possibility to manipulate noninvasively the magnetically active and catalytic particles using magnetic gradient fields [13,17,18]. Theoretical studies have been undertaken to look into the energetics of ordering, twinning, and element segregation in bimetallic 3d -5d transition metal alloys [19,20]. Large scale first-principles calculations predict that small FePt NPs (3 nm or less) thermodynamically favor the formation of chemically ordered FePt MTPs [19]. This is attributed to the efficient strain release due to the formation of twin boundaries in alloyed MTPs. Recent results on single-crystalline 3 nm NPs produced by organometallic synthesis confirm this theoretical finding [21]. Also, the synthesis of ordered decahedral clusters of CoPt and FePt (2 to 5 nm) showing no preferential surface segregation of one element was recently demonstrated using a cluster source technique with subsequent annealing in a carbon matrix [22]. In this Rapid Communication we confirm the theoretically predicted and experimentally observed formation of chem- ically ordered 6 nm FePt decahedra using high-resolution electron microscopy techniques. Detailed lattice parameters and local chemical compositions of the 6 nm particles are evaluated from HRTEM and HAADF lattice images with atomic resolution. This grants direct experimental insight into the formation of alloyed nanoparticle systems and reveals a complex interplay of element ordering, alloying, segregation, and strain. The atomistic structure of single-element decahedral par- ticles [Fig. 1(a)] has been debated mainly in terms of two competing models [23]: (i) the so called homogenous strain model, wherein a body-centered orthorhombic (BCO) unit cell can be constructed within each structural subunit [24]; and (ii) the inhomogeneous strain model, in which a slightly distorted BCO unit cell due to wedge disclination [25] is formed. The orthorhombic lattice constants of each subunit are defined for both models in Figs. 1(b) and 1(c), respectively. Note that these two models are based on purely geometrical considerations, packing identical spheres without taking into account any element specificity or the effects of twinning 1098-0121/2014/89(16)/161406(5) 161406-1 ©2014 American Physical Society