  Rare earth perovskite cobaltites are increasingly recognized as materials of
importance due to rich physics and chemistry in their ordered-disordered
structure for the same composition. Apart from colossal magnetoresistance
effect, like manganites, the different forms of cobaltites exhibit interesting
phenomena including spin, charge and orbital ordering, electronic phase
separation, insulator-metal transition, large thermoelectric power at low
temperature. Moreover, the cobaltites which display colossal magnetoresistance
effect could be used as read heads in magnetic data storage and also in other
applications depending upon their particular properties. The A-site
ordereddisordered cobaltites exhibit ferromagnetism and metal-insulator
transitions as well as other properties depending on the composition, size of
A-site cations and various external factors such as pressure, temperature,
magnetic field etc. Ordered cobaltites, having a 112-type layered structure,
are also reported to have an effectively stronger electron coupling due to
layered A-site cationic ordering. Most importantly for the present article we
focus on La-Ba-Co-O based ordered-disordered perovskite phases, which exhibit
interesting magnetic and electron transport properties with ferromagnetic
transition, TC ~ 177K, and it being the first member of lanthanide series.
Zener double exchange mechanism considered to be crucial for understanding
basic physics of the ferromagneticmetallic phase, yet does not explain clearly
the insulating-type phase. In terms of electron transport the
ferromagnetic-metallic or insulating/semiconducting states have been discussed
in the present article with different types of hopping model.
