Structural, And Magnetic Properties Of Terbium And Terbium Oxide Clusters:a Density Functional Theory Investigation

Rare Earth (RE) and Rare Earth clusters are very attractive due to their unique electronic, optical and magnetic properties. Terbium and Terbium oxide clusters where the Terbium atom has a same large orbital and spin moment offer the opportunity to further understand the growth and magnetic mechanism of these materials, and to obtain the unique structural, electronic and magnetic properties that the RE clusters including vanishing orbital moment have not, leading to the raise of application in the electronics, catalysis, magnetic materials and so on. In this paper, we perform a systemically research on the structural trend, electronic and magnetic properties of Terbium and Terbium oxide clusters by using the density functional theory under both frameworks of the local density approximation (LDA) and the generalized gradient approximation (GGA). The results show:(1) Using the density functional theory (DFT) under both frameworks of the local density approximation (LDA) and the generalized gradient approximation (GGA), we show that the anomalous magnetic moment of Tbn (n= 2-20,22,33) clusters found in recent experiment is due to the antiferromagnetic or ferromagnetic couplings between Tb atoms in clusters. The local spin magnetic moment of Tb atoms is in the range 5.1-5.7μB, which is faintly influenced by the cluster sizes, geometrical structures, and spin arrangements. Geometrical evolution shows that the square-antiprism motif with one centered atom is competitive with the icosahedral motif for small Tb clusters n=9-12, whereas the close packed icosahedral growth is clearly favored for large clusters n=13-20,22,33. The clusters containing 4,7,10,13, and 19 atoms are found to be more stable than their neighboring sizes, in agreement with the early mass spectral measurements. The electronic properties including the HOMO-LUMO energy gaps, ionization potentials, electron affinities, and electric dipole moments are calculated, and more importantly, the variational trends of the magnetic moment and electric dipole moment qualitatively fit well with the experimental observations.(2) To investigate the effect of oxygen atom on the geometrical structures, electronic and magnetic properties of small terbium clusters, we carried out the first-principles calculations on TbnO (n=1-14) clusters. The capping of an oxygen atom on one trigonal-facet of Tbn structures is always favored energetically, which can significantly improve the structural stability. The far-infrared (IR) vibrational spectroscopies are found to be different from those of corresponding bare clusters, providing a distinct signal to detect the characteristic structures of TbnO clusters. The primary effect of oxygen atom on magnetic properties is to change the magnetic orderings among Tb atoms and to reduce small of local magnetic moments of the O-coordinated Tb atoms, both of which serve as the key reasons for the experimental magnetic evolution of an oscillating behavior. These calculations are consistent with, and help to account for, the experimentally observed magnetic properties of monoxide TbnO clusters. The ionization potentials (IP) and electron affinities (EA) of TbnO clusters are slightly lower than that of Tbn clusters. The dissociation energies of Tbn clusters are slightly lower than that of TbnO clustes, indicating that the dopant oxygen atom does not alter the "magic size" of Tbn clusters. However, the electric dipole moments (EDMs) of Tbn clusters are obviously lower than that of TbnO. Those indicate that the dopant oxygen atom enormously decrease the symmetries of Tbn clusters.