Abstract
We report the resistivity and thermoelectric power of , for various stoichiometries, , and under pressures up to . The pristine system exhibits a semiconductor-insulator transition at , which is evidenced in both resistivity and thermopower and is probably induced by charge ordering. We observe a pronounced change in the nature of the phase transition under pressure and we attribute it to the tuning of the nearest neighbor Coulomb interaction . At ambient pressure, as the system moves away from stoichiometry to , disorder is introduced into the strontium sublattice and the phase transition is immediately suppressed. The temperature dependence of the thermoelectric power gradually weakens as the system moves away from , indicating the importance of disorder. While for compound thermoelectric power shows evidence of a localized contribution to the conduction, which may involve polaronic effects, the activation energies speak against small polarons in the pristine compound. We explain our results in a model of conduction through localized states in the off-stoichiometric systems and of thermally activated conduction in the pristine system.
2 More- Received 11 July 2007
DOI:https://doi.org/10.1103/PhysRevB.76.235111
©2007 American Physical Society