Development of the Lead-free Relaxor Ferroelectrics and Giant Piezoelectric Materials, and Evaluation of the Physical Properties

开发新型无铅弛豫铁电材料是环境保护领域亟待解决的问题。我们进行了一项寻找无铅弛豫铁电材料的研究项目。(1)在Ba-Ti氧化物体系中，我们发现了一种新的铁电材料BaTi_2O_5，其居里温度Tc为700 ~ 750 K，室温下自发极化Ps为7 μCcm^<-2>（晶系为单斜晶系，空间群C_2）。在空气中制备的晶体在铁电居里温度为Tc=752 K时具有明显的介电异常，ε′_b=30,000，即使在高温下也具有较低的介电损耗：在800 K时tanδ<0.1。相比之下，在还原气氛中制备的晶体在Tc= 703k附近呈现出弥漫性相变。因此，该晶体是非常理想的高温应用。此外，我们还进行了x射线衍射测量和拉曼散射测量。在300 ~ 1000 K的温度范围内，对粉碎后的单晶进行了XRD分析；估计了晶格参数的热膨胀系数。在Tc附近，晶格常数vs.T曲线出现了Δa/a=0.10%、Δb/b=-0.11%和Δc/c=0.13%的微小跳跃。用球形样品在室温和773 K下对单晶进行了x射线衍射测量。结果揭示了BaTi_2O_5铁电性的原子位移。到目前为止，通过拉曼散射测量还没有观测到软光模式。因此，我们认为BaTi_2O_5的铁电相变机制为有序-无序型。(2)在（Sr_<1-x>Ba_x）_2Nb_2O_7体系中，当x=0.32时，在465 K以下发现了新的铁电相。(3)在K-Nb氧化物体系中，我们发现了一种新的材料K_5Nb_9O_<25>，该材料在ca.100K以下表现出铁电活性。这些不含铅离子的新型铁电材料，特别是BaTi_2O_5，在电容器、电光开关、FeRAM等领域具有广阔的应用前景。少

Development of new relaxor ferroelectric materials with lead-free compositions is an urgent problem in an environmental aspect. We have performed a research project to search for lead-free relaxor ferroelectric materials. (1)In the system of Ba-Ti oxides, we found a new ferroelectric material of BaTi_2O_5, that has a high Curie temperature Tc of 700-750 K and has a spontaneous polarization Ps of 7 μCcm^<-2> at room temperature (the crystal system is monoclinic, space group C_2). The crystal, prepared in air, has a sharp dielectric anomaly reaching ε'_b=30,000 at the ferroelectric Curie temperature of Tc=752 K and has a low dielectric loss even at high temperatures : tanδ<0.1 at 800 K. By contrast, the crystal prepared in a reducing atmosphere shows a diffuse phase transition near Tc=703 K. Thus the crystal is very desirable for high temperature application. Furthermore, we carried out X-ray diffraction measurement and Raman scattering measurement. XRD patterns of the pulverized single … More crystals were measured in the temperature range of 300-1000 K ; the thermal expansion coefficients of lattice-parameters were estimated. Tiny jumps were observed in the lattice constants vs.T curves around Tc by Δa/a=0.10%, Δb/b=-0.11% and Δc/c=0.13%. X-ray diffraction measurements for the single crystal were carried out at room temperature and at 773 K using a spherical sample. The result revealed atomic displacements responsible for the ferroelectricity of BaTi_2O_5. Up to now, no soft optic mode was observed by the Raman scattering measurements. Thus, we consider the mechanism of the ferroelectric phase transition of BaTi_2O_5 as the order-disorder type. (2)In the system of (Sr_<1-x>Ba_x)_2Nb_2O_7, we found a new ferroelectric phase at temperatures below 465 K for x=0.32. (3)In the system of K-Nb oxides, we found a new material K_5Nb_9O_<25>, which shows ferroelectric activity at temperatures below ca.100K. These new ferroelectric materials without lead ions, especially BaTi_2O_5, are promising for a variety of application such as condenser, electro-optic switch and FeRAM. Less