Abstract
13-atom platinum nanoclusters have been synthesized quantitatively in the pores of the zeolites NaY and KL. They reveal highly interesting magnetic properties like high-spin states, a blocking temperature, and super-diamagnetism, depending heavily on the loading of chemisorbed hydrogen. Additionally, EPR active states are observed. All of these magnetic properties are understood best if one considers the near-spherical clusters as analogs of transition metal atoms with low-spin and high-spin states, and with delocalized molecular orbitals which have a structure similar to that of atomic orbitals. These clusters are, therefore, called superatoms, and it is their analogy with normal atoms which is in the focus of the present work, but further phenomena, like the observation of a magnetic blocking temperature and the possibility of superconductivity, are discussed.
Highlights
Platinum is not among the elements which exhibit long-range ferromagnetic order in the bulk.it is well known that ferromagnetism is greatly affected when the dimensionality of the material is reduced to thin films or monoatomic chains [1], and in nanoparticles, elements which are typically diamagnetic in the bulk, like Au, Pd, and Pt, develop an increasingly higher magnetic moment with decreasing size [2,3] and show high-spin states in well-defined clusters [4,5]
We focus on the magnetic properties of Pt nanoclusters which can be prepared quantitatively in the pores of zeolites in a size of 13 ± 2 atoms, as derived from Electron Paramagnetic Resonance (EPR) [8,9] and Extended X-ray Absorption Fine Structure (EXAFS) experiments [10,11]
The spectrum can be simulated based on 12 equivalent Pt nuclei which lead to the regular multiplet structure with an isotropic Pt hyperfine coupling constant of 68.1 G (227 MHz at the given g value) [37]
Summary
Platinum is not among the elements which exhibit long-range ferromagnetic order in the bulk. Predicted [6] but observed experimentally for the first time only recently [7], is the fact that the diamagnetic moment per atom increases dramatically in small clusters. An understanding of these size-dependencies is of considerable interest for fundamental reasons. The focus of the present work is on the superatom nature of 13-atom Pt clusters, with chemisorbed hydrogen, based on previously-reported experimental results This reveals a number of new aspects of magnetism which have so far not been dealt with in detail
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