Composition dependent semiconductor-metal transition in new mixed valent Ti-oxides

Article abstract of DOI:10.1016/S0022-3697(97)00099-1

A composition dependent semiconductor-metal transition is observed in a new series of Ti oxides of the type LnBa₂Ti₃Oy (Ln = La, Gd and Y) in which Ti is in lower and/or mixed valent state. The new phases have been synthesised by the dc arc melting technique and characterised by powder X-ray diffraction, thermal analysis, electrical and magnetic measurements. Single phase compounds are found to form only when the nominal (average) Ti valence (V_Ti) is between 3.67 to 2.33. The crystal structure is simple cubic in which Ln/Ba and various Ti ions occupy respectively the A- and B-sites of a disordered ABO₃ perovskite type lattice. The semiconductor-metal transition is also found to depend on the V_Ti. The compounds show semiconducting behaviour for 3.67 < V_Ti > 2.67 and are metallic when V_Ti = 2.33 which is the lowest possible value of V_Ti in the system. No evidence for superconductivity has been observed in any of the metallic compositions down to 15 K.