Double Non-Stoichiometry: The Lithium Vanadium Oxide Bronzes βLi_xV₂O_5-y

Abstract

The β (or β') Li_xV₂O₅ phases lose oxygen at high temperature, the approximate formula Li_xV₂O_5-y, leading to the formation of phases Li_xV₆O₁₅ Li_xV₉O₂₂ and Li_xV₁₂O₂₉. A general formula is proposed to relate this series, Li_xV_3(1+n)O_8+7n, to the corresponding n values of 1, 2 and 3. In fact, n, as an integer, can be formally varied in the range 0 ≤ n → ∞. Their crystal structures are derived by a structural mechanism based on extended Wadsley defects, denoted 1/10[051](100), affecting the Li_xV₆O₁₅ structure. Numerous syntheses performed in the Li₂O-V₂O₅-V₂O₄ ystem have revealed that no intergrowth structure occurs between the n values 1-2 and 2-3, while, for n > 3, a defect structure has been evidenced by high resolution electron microscopic investigations. The Li_xV₁₂O₂₉-є phase is depicted as an intergrowth of the structure of the n = 4 member, i.e. Li_xV₁₅O₃₆, in the sequence of the Li_xV₁₂O₂₉ crystal structure. In a general discussion, the structural relationships between these phases are particularly emphasized, extended defects crossing tunnel structures being rather unusual.