Tóm tắt
Abstract
In the scope of providing new chemical insight of the oxidation states of copper and vanadium, crystal structures of VOB's β′-Cu
x
V2O5 with x = 0.29, 0.47 and 0.59, have been carefully investigated. The structures are monoclinic, space group C2/m, with a = 15.233(3) Å, 15.218(1) Å, 15.2007(7) Å, b = 3.6133(9) Å, 3.6271(6) Å, 3.6375(3) Å, c = 10.0927(9) Å, 10.087(8) Å, 10.0925(4) Å, β = 107.30(1) Å, 106.55(4) Å, 106.13(2)°, R = 0.0467, 0.0290, 0.0315.
A thorough examination of bond distances and angles allows to give a precise description of the structural evolution versus increasing values of copper insertion x in the tunnels of the [V2O5]
n
framework. The three independent vanadium atoms exhibit distorted oxygen coordination polyhedra, octahedra for V(1) and V(2), square pyramid for V(3). The copper, as revealed by successive Fourier map differences, is spreaded over a sort of toroidal section inserted in a huge oxygen trigonal bipyramid; more precisely the copper coordination changes, in its asymmetric site, from CN4 for Cu(1) (in the mirror plane) to CN3 for Cu(2) (apart the mirror plane, Δy ∼± 0.12) which rarely occurs in sulphides but, up to now never in oxides. The multiplication of Cu sites as well as their various coordination let us think that copper oxidation states could be not unique; such hypothesis is confirmed by ESCA measurements, which clearly indicate that simultaneously Cu(I) and Cu(II) are present. Such results establish that β′′-CuxV2O5 phases are doubly mixed valence vanadium oxide bronzes and can be formally written:
Cu
xt
+Cu
x(1−t)
2+V2−x(2−t)
5+V
x(2−t)
4+O5 (t stands for Cu(I) rate).
