A computational investigation of boron-doped chromium and chromium clusters by density functional theory

Abstract

The geometries, stabilities and electronic properties of Cr n and Cr n B (n=2-9) clusters have been systematically investigated by density functional theory. The results suggest that the lowest energy structures for Cr n B clusters can be obtained by substituting one Cr atom in Cr n+1 clusters with B atom. The geometries of Cr n B clusters are similar to that of Cr n+1 clusters except for local structural distortion. The second-order difference and fragmentation energy show Cr 4 , Cr 6 , Cr 8 , Cr 3 B, Cr 5 B and Cr 8 B cluster are the most stable among these studied clusters. The impurity B increases the stabilities of chromium cluster. When B is doped on the Cr n clusters, cluster geometry does dominate positive role in enhancing their stability. The doped B atom does not change the coupling way of the Cr site in Cr n clusters, but breaks the symmetry and the Cr atoms are no longer equivalent. The doped B atom increases the total magnetic moments of Cr n in most cases. Cr n and Cr n B clusters, geometries, electronic properties PACS: 31.15.A-, 36.40.Cg, 31.15.EwUnraveling the structural evolution and physicochemical properties of transition metal clusters are technological important in cluster science. Chromium is unique among transition metals. Its 3d 5 4s 1 electronic configuration results in strong d-d bonding in dimer with short bond length (1.68 Å) [1−4], while bulk chromium is antiferromagnetic with a body-centered-cubic (bcc) structure (nearest Cr-Cr distance: 2.50 Å) [5]. In last decade, the bonding nature and electronic structure in small chromium clusters have been studied by experimental and theoretical methods [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The geometric and magnetic properties of chromium clusters have been performed by density functional theory (DFT) [20,21]. Bloomfield et al. [22] investigated Cr 8 -Cr 156 and found that magnetic moments range from 0.5μ B to 1.0μ B per atom; small chromium clusters possess ferromagnetically coupled spins. Wang et al.[23] studied Cr n N (n<5) clusters, and found that N is bonded to only three Cr atoms; the coupling of N to the nearest-neighbor Cr is antiferromagnetic. Hence all Cr atoms nearest neighbor to N are coupled ferromagnetically. It is well known that the electronic structure of boron is characterized by three-center bonding. What is the situation for B impurities in chromium clusters? What magnetic moments properties appear? No systematic studies on this subject have been reported to our knowledge. In this paper, we will study this interesting question using density functional theory and provide an ab initio structural and electronic investigation for Cr n B clusters (n ≤ 9). In order to examine the effect of B atom to chromium clusters, geometry optimizations of pure chromium clusters were also calculated using identical method.