STUDIES OF PRESSURE INDUCED METAL-INSULATOR TRANSITION ON FERROELECTRIC BARIUM TITANATE

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On barium titanate (BaTiO_<3-6>), two polymorphic forms of cubic (c-BaTiO_<3-6>) and hexagonal (h-BaTiO_<3-6>) exist. The cubic-type is representative ferroelectrics, and the hexagonal-type also shows ferroelectricity at low temperatures. The main purpose of this research is to elucidate change of electrical properties by doping electrons or by applying hydrostatic pressures on both barium titanates.We succeeded to construct an electrical measurement system for an electrical resistivity and/or a dielectric constant at low temperatures (10K-300K) and at high pressures up to 1.5GPa on the help of this grant-in-aid. For above 1.5GPa, we used a high-pressure measurement system in ISSP, Univ. of Tokyo, and we performed investigations about soft-modes by Raman scattering in cooperation with RIES, Hokkaido Univ. Following results are obtained.1) Both of reduced h-BaTiO_<3-6> and c-BaTiO_<3-6> with semiconducting properties at 1 atm becomes to show metallic behavior in a whole temperature range under high pressures above 2GPa and 6.5GPa, respectively.2) In the pressure induced metallic phase on both barium titanates, the resistivity p changes with temperatures as p^* T^3 within the temperature range below 100K, which indicates that electrons in the metallic phase are strongly correlated.3) From results of Raman scattering measurements on reduced h-BaTiO_<3-6>, an anomaly at around To=200K on an electrical resistivity vs. T curve is proved to be due to a structural phasetransition associated with softning of an optic phonon.4) The critical index beta for the soft mode on reduced h-BaTiO_<3-6> changes continuously with oxygendeficiency delta : beta=1/3 at delta=0 changes to beta=1/2 at delta=0.02.5) On nominally pure c-BaTiO_<3-6>, we found a dielectric relaxation of relaxor type at lowtemperatures around 100K.