Polaronic Conductions in Transition Metal Oxides and its Application for Condensers.

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In most transition metal oxides, majority carriers are polarons. A polaron is the combination of an electron (or a hole) localized at an ion and the lattice deformation around the electron induced by electron-phonon interactions.The hopping process of polarons between ions with lattice deformations gives rise to conductivities in such oxides. The effective mass of polarons are, therefore, very large in comparison with conduction electrons in semiconductors like Si, Ge and so on and metals because of lattice deformations and, consequently, one can expect very low drift mobilities of polarons. On the other hand, a motion of a polaron with an electronic charge results in a dipolemoment and the dielectric constant is expected to be very high. These characteristics of polarons lead to a possibility that an oxide which has polarons as majority carriers is one of candidates for condensers. The present project is based upon this strategy.This project consists of the experiments measuring diele … More ctric properties and dc-conductivities and theoretical calculations using Anderson's attractive potential for polarons and a shell model. Experiments on Li_xNi_<1-x>O, Li_xCo_<1-x>O, alpha-Li_xMn_2O_3, SrMnO_<3-x> and SrTiO_3 doped with La_2O_3 and MnO_2 are carried out. The experimental results of Li_xNi_<1-x>O, Li_xCo_<1-x>O and alpha-Li_xMn_2O_3 show typical behaviors due to hopping process of polarons and yield several important parameters relevant to polarons which have been unknown. These parameters are indispensable in theoretical investigations of polarons. Theoretical calculation on Li_xNi_<1-x>O, in addition, proves that polarons can be stable in oxides, and also yield the strength of electron-phonon interaction and magnitudes of displacements of ions around the localized electron. These oxides except alpha-Li_xMn_2O_3 have high dielectric constants and low conductivities as expected, but are not insulators at room temperature. However, the method to insulate boundary layrs in ceramics, which has been technically established, must resolve this difficulty. SrMnO_<3-x> and SrTiO_3 doped with La_2O_3 and MnO_2 do not show polaronic conductions but very useful and important knowledges on theoretical studies of dielectrics are available from the experiments on these oxides. Less