We have shown previously that the origin of the layer expansion and block contraction is the removal of second-order Jahn-Teller distortions in the transition metal-oxygen octahedra. More recent computational analysis suggests that this even leads to metal-metal bond formation between niobium and/or titanium atoms in edge-sharing (Ti,Nb)O6 octahedra at the Wadsley-Roth block edges and corners in Nb2O5 and TiNb2O7. 17 ).