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

在钛酸钡(BaTiO_3-6)上，存在两种晶型：立方相(c-BaTio；3-6)和六方相(h-Batio&lt；3-6&gt；)。立方型是典型的铁电体，六角型在低温下也表现出铁电性。这项研究的主要目的是通过掺杂电子或在两种钛酸钡上施加静水压力来阐明其电学性质的变化。在这项资助的帮助下，我们成功地构建了一套低温(10K-300K)和高压(1.5 Gpa)下的电阻率和/或介电常数的电测量系统。对于1.5 Gpa以上的压力，我们使用的是ISSP的高压测量系统。，并与北海道大学的RIES合作进行了软模的拉曼散射研究。得到如下结果：1)在1大气压下具有半导体性质的还原h-BaTio_&lt；3-6&gt；和c-BaTio_&lt；3-6&gt；分别在高于2 Gpa和6.5 Gpa的高压下在整个温度范围内表现出金属行为。2)在低于100K的温度范围内，这两种钛酸钡的压力感生金属相的电阻率p随温度的变化为p^*T^3，这表明金属相中的电子与金属相中的电子密切相关。4)简并h-BaTio；3-6&gt；软模的临界折射率β随氧缺位增量连续变化：β=0处的β=1/3变为β=1/2处的β=0.02.5)在名义上纯的c-Batio_3-6&gt；上，我们发现在100K左右的低温下有一个弛豫型介电弛豫。

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.