Structural, Electronic And Magnetic Properties Of Gadolinium Oxide Clusters:the Density Functional Theory Investigation

The rare earth oxides nanoparticles are drawing considerable attentions because of their unique magnetic, optical, and electrical properties, which are promising for wide applications in high-density storage, magnetic refrigeration, optical devices, and biomedical. A typical exemplification is the gadolinium oxide nanoparticles with excellent structure and magnetic properties, which have been extensively coated by various organic strategies and biofunctionalized as novel contrast agents for medical magnetic resonance imaging (MRI). Since small Gd2O3nanoparticles play a decisive role in synthesizing cluster assembled large nanoparticles, the knowledge of their detailed magnetic, electronic, and geometrical structure is essential for further improving their performances in various applications. In this essay, the geometries, stabilities, electronic and magnetic properties of GdnOm (2≤n+m≤13) and (Gd2O3)n(n=1-10) clusters have been systematically studied by using density functional theory with the generalized gradient approximation.(1) For GdnOm (2≤n+m≤6) clusters, all possible structures of various compositions are thoroughly examined to search for the ground state structures. The results show that the clusters with compositions close to the stoichiometry of bulk Gd2O3are often being the lowest energy clusters. In the lowest energy structures the Gd and O atoms are prefer three and two coordinations, respectively, owing to the origin from the electronic configurations of Gd (4f75d16s2) and O (2p42s2) atoms. The local moments of Gd atoms are ranged from6.8μB to7.5μB together with the ferromagnetic couplings among the adjacent Gd atoms; the local moments of O atoms are ranged from-0.4to-0.1μB together with the antiferromagnetic coupling among the adjacent Gd and O atoms.(2) For GdnOm (7≤n+m≤13) clusters, only two compositions that are close to (or exactly matching) the bulk stoichiometry are investigated. The bond-length of Gd-O changes from2.15A to2.17A, and the energy difference between the ferromagnetic and antiferromagnetic coupling is very small (less than0.01eV/atom) in the same configuration. Moreover, Gd2O3and Gd4O6clusters can be regarded as the magic sizes because of their high binding energies (Eb) and HOMO-LUMO energy gaps (Eg), which indicate their physical and chemical bistabilities.(3) The structural evolution of (Gd2O3)n (n=1-10) clusters are favored to be the cage-like growth pattern. Particularly, the regular decahedral spherical-like structure with a giant moment of7.12μB/Gd atom is favored for Gd20O30cluster, which can be regarded as a good magnetic encapsulation structure. The binding energies of the (Gd2O3)n clusters increase monotonically as the cluster size become larger. The curves of second-order energy difference exhibit strong odd-even alternations, indicating that the even-atom sizes (n=2,4,6,8,) are relatively stable in (Gd2O3)n clusters. The HOMO-LUMO energy gap presents a decrease trend, especially at the sizes of n=2,4,6,10, suggesting that clusters metallicity increases gradually.(4) The Mulliken population analyses show that two6s electrons of Gd atoms are almost transferred to its5d and6p orbitals or to2p orbital of adjacent O atoms, thus forming strong ionic bonds between Gd and O atoms. The Gd-4f electrons have near no contributions of the formation of Gd-O bond, which together with the O-2p electrons mainly contribute to the magnetic moment of clusters. There are strong sp-d hybridizations between Gd (5d,6s) and O (2p) atoms from the analyses of the PDOS of the clusters. Moreover, total (MT) and local (MGd, Mo) magnetic moments are weakly dependent on cluster size, structural configuration, and compositions. For different GdO clusters, the energies of their ferromagnetic states are nearly degenerate with that of the anti-ferromagnetic states. The magnetic couplings that contributed from the localized Gd-4f are mainly conducted by the delocalized s and p electrons of O atoms.