LATTICE INSTABILITIES NEAR THE CRITICAL V-V SEPARATION FOR LOCALIZED VERSUS ITINERANT ELECTRONS IN LIV1-YMYO2 (M = CR OR TI) AND LI-1-XVO2

From the compositional dependences of the lattice parameters in the system LiV1-yCryO2, a critical V-V separation R(c) = 2.90 +/- 0.01 angstrom has been determined for an array of octahedral-site V3+ ions sharing common octahedral-site edges with six like nearest neighbors. A first-order transition on passing from the itinerant-electron regime R < R(c) to the localized-electron regime R > R(c) was also established. In LiVO2, the electron instability associated with an R almost-equal-to R(c) is manifest by V3 trimer formation at temperatures T < 490 K with R < R(c) within a trimer and R > R(c) between trimers. Introduction of a mixed valence in Li1-xVO2 results in a two-phase mixture in the interval 0 < x less-than-or-similar-to 0.12; the metastable Li1-xVO2 phase in the interval 0.12 less-than-or-similar-to x less-than-or-similar-to 0.33 disproportionates above a T(t)' almost-equal-to 550 K into LiVO2 and Li[V2]O4. Comparisons are made between the instabilities associated with a W almost-equal-to U in the copper oxide superconductors and those in the layered and spinel vanadium oxides. In particular, attention is called to evidences for dynamic charge-density fluctuations that appear to be associated with superconductivity in the copper oxides, but not in the vanadium oxides.