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M. Riccò, T. Shiroka, S. Carretta Dipartimento di Fisica , Università di Parma e I NFM, PARMA C . Femoni, M . C .

Magnetic Properties of the Carbonyl Cluster [Ni 16 Pd 16 (CO) 40 ] 4-. M. Riccò, T. Shiroka, S. Carretta Dipartimento di Fisica , Università di Parma e I NFM, PARMA C . Femoni, M . C . Iapalucci, G . Longoni Dipartimento di Chimica Fisica ed Inorganica,

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M. Riccò, T. Shiroka, S. Carretta Dipartimento di Fisica , Università di Parma e I NFM, PARMA C . Femoni, M . C .

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  1. Magnetic Properties of the Carbonyl Cluster [Ni16Pd16(CO)40]4- M. Riccò, T. Shiroka, S. Carretta Dipartimento di Fisica, Università di Parma e INFM, PARMA C. Femoni, M.C. Iapalucci, G. Longoni Dipartimento di ChimicaFisica ed Inorganica, Università di Bologna, BOLOGNA F. Bolzoni IMEM-CNR, PARMA

  2. Introduction • Only carbonyl clusters with an odd number of valence electrons display a significant magnetic behaviour arising from the presence of a single unpaired electron. • The bimetallic compound [NBu4]4[Ni16Pd16(CO)40] represents an example of a metal-carbonyl cluster which, in spite of an even number of unpaired electrons, shows clear magnetic properties. • In this work we present the results of investigation of magnetism in this cluster using SQUID magnetometry and muon spin spectroscopy (µSR).

  3. Crystalline structure: triclinic a=15.93 Å, b=15.94 Å, c=17.23 Å =74.69, =73.65, =79.09 EHMO* Cluster symmetry: Ci * - Extended Hückel Molecular Orbital Large distances => Not-interacting clusters Clusters and Crystalline Structure

  4. T0= - 3.9 K, p = 4.71 J=2 => p=4.9 Ea= 27.5  0.3 meV DC Susceptibility Measurements Paramagnetic behaviour up to 150 K and activated for higher temperatures.

  5. J=23 ! Fit with one Brillouin function J=2 T=5K • HYPOTHESIS: The spin Hamiltonian could include other terms (beside Zeeman) arising from the interaction with the crystal field (due to cluster shape, ligand, etc.) Magnetization Curves

  6. Fitting the magnetization data with the model given by the modified Hamiltonian gives a much better agreement. D  3 meV E  0.7 meV g = 1.8 (isotropic) Crystal-field multiplets (with J=2) Stevens Operators

  7. H = 50 G Predicted Molar Susceptibility The assumed HJ=2 Hamiltonian can reproduce also the magnetic suscep-tibility data:  = J=2(T) + 0

  8. H J = 2 g = 1.8 • - angle between H and the hard magnetic axis (unknown) Interesting level crossings appear at high fields (~ 28T) Extended Hamiltonian Energy Levels

  9. Schematic LF (ZF) µSR experiment The µSR Technique (ZF/LF) • Muons (I = ½, = 2.2 µs), localise at an interstitial site and precess at the local field, with  = ·Bloc. • ZF - method of detecting weak internal magnetism, that arises due to ordered magnetic moments, or random fields that are static or fluctuating with time. • Applying an external LF field one can “freeze” the muon spin direction, and detect the time evolution of the signal.

  10. LF-µSR results with different applied magnetic fields (re-polarisation) µSR Re-polarization Measurements • The LF re-polarisation shows a bi-exponential recovery => two contributes: from muon trapped near the clusters and counter -ions resp. (two magnetically distinct species, see inset). • The high final saturation value does not agree with a simple paramagnetic behaviour.

  11. Conclusions • The molecular [Ni16Pd16(CO) 40]4- cluster shows a high number of unpaired electrons (4 => J = 2). • The magnetometry measurement results, as well as µSR data, cannot be interpreted only on the basis of a simple Zeeman Hamiltonian. • The introduction of an effective crystal field (arising from the cluster’s shape, the ligand, etc.) allows a consistent interpretation of the experimental data and also predicts the cluster behaviour at higher applied fields.

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