Phonon analysis and crystal chemical simulation of dielectrics based on sub-millimeter wave ellipsometry

采用亚毫米波椭偏光谱技术研究了钛酸钡的尺寸效应和钛酸钡基弛豫剂的相变行为。椭偏仪是通过测量反射光偏振态的变化来测定材料复介电常数的仪器。验证了所研制的椭偏仪的精度，并直接测定了钛酸钡单晶的复介电常数。在尺寸效应方面，钛酸钡陶瓷的介电常数在晶粒尺寸大于1.1μm时随着晶粒尺寸的减小而增大，在晶粒尺寸小于1μm时介电常数减小。BaTiO_3晶粒中大部分畴为简单的片层结构，90°畴宽随晶粒尺寸的减小而减小。基于宽频带介电光谱，研究了介电常数的域贡献。在晶粒尺寸大于1μm的BaTiO_3陶瓷中，高畴密度增强了偶极子极化率和离子极化率。相比之下，晶粒尺寸小于1μm的BaTiO_3陶瓷的极化率有所减弱。对于钛酸钡基弛豫器的相变，测量了BaTiO_3 （BT）、Ba_<0.6>Sr_<0.4>TiO_3 （BST-0.6）和BaZr_<0.25>Ti_<0.75>O_3 （BZT-0.25）陶瓷在kHz到THz范围内的宽带介电谱。在BST-0.6和BZT-0.25中介电最高温度（T_m）附近的偶极极化行为可以用极性纳米区（pnr）的行为来解释，而在BT中铁电畴的行为有助于偶极极化，BT中的弛豫可以解释为从正常铁电体到弛豫铁电体的极化行为的连续变化。

Sub-millimeter wave spectroscopic ellipsometry has been developed to study the size effect of barium titanate and the phase transition behaviors of barium titanate-based relaxors. Ellipsometer is equipment which measures the change of polarizing state of reflected light to determine the complex dielectric permittivity of materials. Accuracy of the ellipsometer developed in this study was verified and the complex permittivity of a barium titanate single crystal was directly determined using it. S for the size effect, the permittivity of barium titanate ceramics increased with decreasing grain size when the grain size was over 1.1μm, while that decreased when the grain size was below 1μm. Most of the domains in BaTiO_3 grains had simple lamellar structures, and 90° domain width decreased with decreasing grain size. Based on wide band dielectric spectroscopy, the domain contribution to the permittivity was investigated. In BaTiO_3 ceramics with grain sizes over 1μm, the dipole and ionic polarizabilities were enhanced by the high domain density. In contrast, for the BaTiO_3 ceramics with grain sizes below 1μm, these polarizabilities were weakened. As for the phase transition of barium titanate-based relaxors, wideband dielectric spectra from kHz to THz were measured for ceramics of BaTiO_3 (BT), Ba_<0.6>Sr_<0.4>TiO_3 (BST-0.6) and BaZr_<0.25>Ti_<0.75>O_3 (BZT-0.25). The behavior in the dipole polarization near the dielectric maximum temperature (T_m) in BST-0.6 and BZT-0.25 could be explained by the behavior in polar nanoregions (PNRs), while the ferroelectric domains contribute to dipole polarization in BT. Relaxation in BT could be interpreted as successive change in polarization behavior from normal ferroelectrics to relaxor ferroelectric via DPT ferroelectrics.