The effect of Al/Si ratio on the transport properties of the layered intermetallic compound CaAl(2)Si(2).

We report the results of the electrical resistivity and Seebeck coefficient as well as thermal conductivity measurements on the stoichiometric CaAl(2)Si(2) and non-stoichiometric CaAl(1.75)Si(2.25), CaAl(1.9)Si(2.1), CaAl(2.1)Si(1.9), and CaAl(2.25)Si(1.75) compounds in the temperature range 10-300 K. It has been found that the magnitude of electrical resistivity decreases for the non-stoichiometric samples, attributed to the shift of Fermi energy from the dip of the density of states as a consequence of the changed Si/Al content. In addition, a systematic change in the magnitude of Seebeck coefficient as a function of Al/Si concentration has been observed. The results have been associated with the effect of hole/electron doping on the Fermi level density of states. A detailed analysis of the electrical resistivity and Seebeck coefficient suggests the presence of two types of charge carrier and the temperature dependent changes in their mobility. From the thermal conductivity results, we correlated the extent of disorder and Al/Si ratio with various thermal scattering mechanisms in the investigated temperature range.

Effect of electron/hole doping on the transport properties of lanthanum manganites LaMnO(3).

We report the results of electrical resistivity, magnetic susceptibility and thermoelectric power measurements on the stoichiometric hole-doped La(0.8)Ca(0.2)MnO(3) and electron-doped La(0.8)Ce(0.2)MnO(3) manganites. The resistivity and thermopower data show a metal-insulator phase transition for both samples. Magnetic susceptibility measurement confirms that both the samples undergo a transition from paramagnetic to ferromagnetic phase at defined Curie temperatures T(c) = 240 K (La(0.8)Ca(0.2)MnO(3)) and 263 K (La(0.8)Ce(0.2)MnO(3)). The electrical resistivity data are then theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e. the Bloch-Gruneisen model. Detailed analysis of the electrical resistivity of electron- and hole-doped manganites suggests that besides the electron-phonon effect another possibility for the change in carrier density arises due to the presence of electron correlation in the metallic system. From the thermopower S(T) results, we suggest that its behaviour is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between carrier diffusion and phonon drag contributions in the polycrystalline samples of hole-doped La(0.8)Ca(0.2)MnO(3) and electron-doped La(0.8)Ce(0.2)MnO(3) manganites.