Ferromagnetic Ordering in Superatomic Solids Chul-Ho Lee, †,‡,§ Lian Liu, ‡ Christopher Bejger, † Ari Turkiewicz, † Tatsuo Goko, ‡ Carlos J. Arguello, ‡ Benjamin A. Frandsen, ‡ Sky C. Cheung, ‡ Teresa Medina, ∥ Timothy J. S. Munsie, ∥ Robert D’Ortenzio, ∥ Graeme M. Luke, ∥ Tiglet Besara, ⊥ Roger A. Lalancette, ∇ Theo Siegrist, ⊥,# Peter W. Stephens, ○ Andrew C. Crowther, ◆ Louis E. Brus, † Yutaka Matsuo, ¶ Eiichi Nakamura, ¶ Yasutomo J. Uemura, ‡ Philip Kim, + Colin Nuckolls, † Michael L. Steigerwald, † and Xavier Roy* ,† † Department of Chemistry, Columbia University, New York, New York 10027, United States ‡ Department of Physics, Columbia University, New York, New York 10027, United States § KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea ∥ Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4L8, Canada ⊥ National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States # Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, Florida 32310, United States ∇ Department of Chemistry, Rutgers State University, Newark, New Jersey 07102, United States ○ Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, United States ◆ Department of Chemistry, Barnard College, New York, New York 10027, United States ¶ Department of Chemistry, The University of Tokyo, Tokyo 112-0033, Japan + Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States * S Supporting Information ABSTRACT: In order to realize significant benefits from the assembly of solid-state materials from molecular cluster superatomic building blocks, several criteria must be met. Reproducible syntheses must reliably produce macroscopic amounts of pure material; the cluster-assembled solids must show properties that are more than simply averages of those of the constituent subunits; and rational changes to the chemical structures of the subunits must result in predictable changes in the collective properties of the solid. In this report we show that we can meet these requirements. Using a combination of magnetometry and muon spin relaxation measurements, we demonstrate that crystallographically defined superatomic solids assembled from molecular nickel telluride clusters and fullerenes undergo a ferromagnetic phase transition at low temperatures. Moreover, we show that when we modify the constituent superatoms, the cooperative magnetic properties change in predictable ways. ■ INTRODUCTION Superatomic solids are three-dimensional periodic arrays in which the fundamental individual building blocks are independently prepared, electronically and structurally comple- mentary molecular clusters. 1−3 Being completely tunable, these molecular cluster superatoms have discrete, well-defined structures and exhibit collective properties that are character- istic of and distributed over the entire cluster. The interaction of the individual cluster magnetic moments within the solids can create a long-range cooperative magnetically ordered phase that is distinct from the independent subunits: the binary compounds [Ni 9 Te 6 (PEt 3 ) 8 ][C 60 ], [Ni 9 Te 6 (PMe 3 ) 8 ][C 60 ], and [Ni 9 Te 6 (PEt 3 ) 8 ][C 70 ] all show spontaneous magnetic ordering. Here we verify the bulk origin of the magnetic ordering in these materials by magnetic susceptibility and muon spin relaxation measurements. Furthermore, we show that the onset of the ferromagnetic transition can be adjusted by varying the intercluster interactions resulting from different crystal packing and electronic coupling. Conventional and superatomic solids both have properties that are consequences of the attributes of the individual building blocks. Collective properties such as ferromagnetism, ferroelectricity, and superconductivity emerge as the result of long-range exchange interactions between the constituents. 4−8 In atomic solids, close-contacting atoms can interact to Received: September 25, 2014 Article pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja5098622 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX