Structures of CoAl(1 1 1) surface: A first principles study
Research highlights
▶ Al segregation tendency plays dominant role on the CoAl(1 1 1) surface. ▶ There are three stable states in the CoAl(1 1 1) surface phase diagram. ▶ The barriers from the initial surface structure to the stable structure are large.
Introduction
The surface structures of alloys attract great interests because of their importance in technology applications. The surface structures are usually different with the corresponding bulk structures because of the effects induced by surface, such as surface segregation and surface reconstruction. The TM–Al (TM = Fe,Co,Ni) alloys have excellent mechanical properties and their surface structures have been extensively investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. The bulk structures of TM–Al are all B2 structure [1]. However, their surface structures show various morphologies. They can be almost bulk truncated, such as Fe(0 0 1) [2] and NiAl(1 1 0) [3], [4]. There are also completely reconstructed surfaces, such as FeAl(1 1 0) and FeAl(1 1 1) [5], [6], [7]. For the surface of NiAl(0 0 1), experiments show that there are several different surface structures [8], [9].
For CoAl surface, the effect of Co anti-sites plays an important role on the surface structures of slightly Co rich CoAl, even if the amount of Co anti-sites is small [10], [11], [12], [13], [14]. For the stoichiometric CoAl(0 0 1), the surface is terminated by pure Al [1], while for the slightly Co rich one, the surface layer is partially occupied by Co anti-sites. Both experiments and theoretical simulations show a low–high–low behavior of Co concentration on the CoAl(0 0 1) surface with the increase of temperature [10], [11], [12], [15]. For CoAl(1 1 0), the Co anti-sites are in the subsurface layer [13]. For CoAl(1 1 1), in the calculated stable structure, the CoAl(1 1 1) surface is almost bulk truncated with Al termination, except that the third layer under the surface is occupied by Co atoms rather than Al atoms [14]. However, in the experiment [14], the stable structure appears only at temperature higher than 820 K. At low temperatures, another (1 × 1) structure is observed instead.
Above experiments show that for CoAl surface, there are two segregating tendencies. One is the Al segregating tendency. The other is the segregating tendency of the Co anti-site atoms in Co rich alloys. The two tendencies corresponds to two different driving forces in the formation of the surface structures of CoAl, which may cause various structures at different temperatures or Co concentrations. For CoAl(1 1 1) surface, previous theoretical calculations focus on the stable structures with low Co anti-site concentrations [14]. In order to have better understanding on the CoAl(1 1 1) surface structure at different anti-site concentrations and temperatures, we systematically investigated the CoAl(1 1 1) surface structures and calculated the surface phase diagram of ground states in this work. We find a strong segregating tendency of Al atoms in a wide range of chemical potential. We have also analyzed the energy barriers from the surface structure appeared at low temperature to the stable structure and discussed the effect of temperature.
In Section 2 we will describe the model and method used in our calculations. In Section 3, we will show the results of the surface structures with various Co concentrations. In Section 4 we analyze the kinetics on the CoAl(1 1 1) surface. A conclusion will be given in Section 5.
Section snippets
Method
The calculations have been performed using VASP (Vienna ab initio simulation program) [16], [17]. In all the calculations, we use the exchange correlation functional with the generalized gradient approximation of PW-91 [18]. The energy cut-off of plane wave basis is 300 eV. All the calculations are spin-polarized.
The nudged elastic band method [19] is used to calculate the diffusion barriers in the surface region of CoAl(1 1 1). In the calculations of diffusion barriers, we use 10-layer slabs with
The surface structures
We have calculated the structural parameters of the AlCo3 surface and compared the results with the experimental data [14]. The results are shown in Table 1. The calculation results agree well with the experimental data. Besides, the layer distance in the center layer of the 25-layer slab is 0.821 Å, which is very close to the calculated layer distance of 0.826 Å in the bulk. Thus the slabs used in the calculations are suitable for studying the surface structures of CoAl(1 1 1).
For the low Co
The effect of temperature
In the experiment [14], the observed structure at low temperature is different from the calculated stable structure. The experimental CoAl(1 1 1) surface is treated under Ne+ ions. The Ne+ ions take away more Al atoms than Co atoms, which leads to a Co rich surface. A Co rich surface is unstable according to Fig. 1. To evolve to the stable structure, the Al atoms must diffuse from the inner layers to the surface layer.
In the followings, we study the kinetics of the CoAl(1 1 1) surface structure
Conclusions
We have investigated the surface structures of CoAl(1 1 1) for various Co concentrations. Our calculations show a strong tendency for Al atoms to segregate to the topmost layer. The surface layer is always occupied by pure Al for Co concentration values up to 72%. This indicates that the Al segregation tendency plays a dominant role as compared to the segregation tendency of the Co anti-sites and is different with that for the CoAl(0 0 1) surface, where a number of Co anti-sites segregates to the
Acknowledgements
This research is supported by the National Natural Science Foundation of China (Grant No. 10974107 and No. 10721404) and Natural Science Funds of Shandong Province for Distinguished Young Scholar (JQ200802).
References (22)
- et al.
Surf. Sci.
(1993) - et al.
Surf. Sci.
(1995) - et al.
Surf. Sci.
(1998) - et al.
Physica B
(1996) - et al.
Surf. Sci.
(1996) - et al.
Surf. Sci.
(2000) - et al.
Surf. Sci.
(1995) - et al.
Acta Mater.
(1997) J. Phys.: Condens. Matter
(2003)- et al.
Phys. Rev. Lett.
(1985)
J. Phys. Condens. Matter
Cited by (1)
Activity of calcined coal gangue fine aggregate and its effect on the mechanical behavior of cement mortar
2015, Construction and Building Materials