The composite structure of Cu2.33-xV4O11

The copper vanadium oxide bronze Cu2.33-xV4O11 exhibits a three part composite structure refined on the basis of XRD low-temperature studies. It crystallizes in the triclinic system with the non-centric superspace group X1 and cell parameters a=15.280(3) A; b 1=3.616(1) A; c=14.674(3) A; α=90.0 deg. ; β=101.95(3) deg. ; γ=90.0 deg. with a modulation q-vector equal to (0,0.11,0). The three different parts of this composite structure differ by their b-unit cell repeat defined as b 1 ; b 2=2.964(1) A (b 2*=b 1*+2q) and b 3=3.257(1) A (b 3*=b 1*+1q). These parts are respectively associated to the V4O11 substructure and to each of the two different copper sites. Such refinement allows us to describe the structure using only one and fully occupied crystallographic site for each of the Cu ions. The maximum composition (x=0) is then achieved. Bond valence sum calculations on the basis of such composite structure is in agreement with electronic structure calculation made using the average one and allows us to attribute the proper valence state to each Cu ions. Then, the calculated ratio appears, contrary to the average structure, in prefect agreement with the one deduced from XPS experiment