We have synthesized mixed-valence derivatives of the layered oxide HLa Nb2O7, structurally related to perovskite, by chemical reduction, either at room temperature or at 650°C. The thermal behavior of this compound, its structure and change in transport properties upon high temperature doping are discussed. High temperature reduction turns the white insulating precursor into a mixed-valence blue bronze with a layered structure and semiconducting-like properties.

This research was supported by grants from the Spanish DGICYT (PB93-0122), CICYT (MAT93-0240-C04-01) and the MIDAS Program (92-1592). We acknowledge predoctoral fellowships awarded to M.R.P. (Generalitat de Catalunya) and M.L. (MEC-AECI).

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