A Surfaces that present expansion instead of relaxation?

[voila]

Since there are no bonds at the other side of the surface, external layers of solids are usually closer to the next layer. This process is called relaxation. (Example in picture a here).

However, at a lecture I attended the other day it was mentioned that some surfaces present expansion, instead of the more common relaxation. I haven't been able to find anything on the topic. Anyone knows about it?

 

[Vagn]

Aluminium (111) is claimed to exhibit surface expansion here and here

 

[Useful nucleus]

Theoretically , I recall that Lennard -Jones potential leads to outer-surface relaxation (i.e. expansion). This is one reason why it was considered a bad interaction potential for metals.