Anderson transitions in Ln 1-x Sr x CoO 3 (Ln=Pr or Nd)

Electrical conductivity measurements show that Ln 1−x Sr x CoO 3 , (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0<x<0.l obeys the T −1/4 law up to 400 K and the a.c. conductivity data follow the ω δ law with δ in the range 0.6-0.8. Surprisingly, however, δ seems to increase with increasing temperature. Thermopower measurements show clear evidence for a change in the mechanism of conduction (around 350 K) from variable range hopping to transport at E c . All these transport properties suggest that Co 4+ ions in Ln 1-x Sr x CoO 3 (0<x<0.1), considered as moving in a lattice of Co 3+ , form a degenerate gas and that the states at the Fermi energy are Anderson localized.