First-order Phase Transition Characteristic Of The High Temperature Metal-semiconductor Transition In [ca_2coo_3]_(0.62)[coo_2]

摘要

Transportation and thermodynamic properties of misfit-layered polycrystalline [Ca_2CoO_3]_(0.62)[CoO_2] were measured in order to clarify the nature of metal-semiconductor transition (MST) at T_(MS) ≈ 400 K, above which the simultaneous decrease of resistivity and increase of thermopower with temperature give rise to a great enhancement of thermoelectric power factor up to 1000 K. A first-order phase transition characteristic around T_(MS) was revealed by anomalies of resistivity, differential scanning calorimetry, and thermal expansion. The first-order characteristic of the MST can be rationalized from the Virial theorem at an itinerant to localized electron transition in the narrow e~T band within the [CoO_2] plane. Above T_(MS), the reduction of the retained delocalized states within the matrix of localized states and the enhancement of charge carrier effective mass with increasing temperature might account for the considerable enhancement of the thermopower.