Study of the Thickness Effect to Structural Phase Transition Mechnism of Ferroelectrics Thin Films by Far-Infrared Spectroscopy

采用旋涂法在硅衬底上制备了BaTiO_3薄膜。晶体的晶粒尺寸由原子力显微镜观察确定。在厚度大于60nm的薄膜中，晶粒的平均直径约为50nm。在小于30nm的薄膜中，晶粒尺寸减小。用FTIR光谱仪测量了远红外吸收光谱随厚度的变化。在厚度大于30nm的薄膜中观察到的声子频率与体晶体的频率有很好的对应关系。然而，在较薄的10nm薄膜中，吸收谱线的轮廓有很大的不同。180cm^<-1>、250cm^<-1>和510cm^<-1>的低频模强度随薄膜厚度的减小而减小，而375cm^<-1>模强度保持不变。这表明在30nm以下晶粒尺寸的晶体结构发生了变化。在室温下，对六方钛酸钡进行了红外反射测量，以确定A_<2u>对称的极性光学声子（包括软声子）的本征频率和振子强度。通过因式拟合发现，相关的TO-LO分裂总量与钙钛矿BaTiO_3一样大。和SrT: O_3。大TO-LO分裂的事实表明，动态有效电荷一定很大。六合BaTiO_3和钙钛矿BaTiO_3的TO-LO分裂值几乎相同。在钙钛矿型中，每个单元格中含有一个BaTiO_3分子式，而在六角形中，每个单元格中含有6个分子式。其中一个公式的平均体积为67x10^<-30> m^3，而钙钛矿batio3的平均体积为64x10^<-30> m^3。式(3)的平均体积和光侧值几乎相同，因此可以预测，六方batio3的每个离子的平均动态有效电荷应该与钙钛矿batio3的基本相同。结果表明，六方体batio_3的大to - lo分裂是由于动态有效电荷从标称价电荷的大位移导致了价键特征，钛和氧的八面体本质上具有铁电不稳定性，六方体batio_3也是如此。因此，ATiO_2中表现出铁电不稳定性的倾向与tio6八面体的堆积方式无关，而主要是由tio6八面体的存在决定的。少

Thin films of BaTiO_3 are prepared on silicon substrates by spin-coating processing. The grain sizes of crystalline are determined by AFM observation. The average diameter of grains is about 50nm in films thicker than 60nm. In the thinner films than 30nm the grain size decreases. Far-infrared absorption spectra are measured by FTIR spectrometer as a function of thickness. The frequencies of phonons observed in films thicker than 30nm shows good correspondence with those of bulk crystals. However, in thinner films of 10nm the profiles of the absorption lines are rather different. The intensities of the low frequency modes at 180cm^<-1>,250cm^<-1> and 510cm^<-1> decrease as the thickness of film decreases, whereas the component of 375cm^<-1> mode remains its intensity. This suggests the change of crystal structure below the grain size of 30nm.An infrared reflection measurement was performed at room temperature in hexagonal barium titanate to determine the eigen frequencies and oscillator … More strengths of the polar optic phonons of A_<2u> symmetry, including the soft phonon. It is found by means of the factorized form fitting that a total quantity of the relevant TO-LO splitting is as large as those in perovskite BaTiO_3. and SrT : O_3. The fact of the large TO-LO splitting shows that the dynamical effective charge must be large. The values of TO-LO splitting of hex-BaTiO_3and perovskite BaTiO_3 are almost same. In the perovskite type the each unit cell contains one formula of BaTiO_3, whereas in the hexagonal type there exists 6 formulae in the unit cell. The average volume about one formula is 67x10^<-30> m^3 for the hex-BaTiO_3 and 64x10^<-30> m^3 for the perovskite BaTiO_3 The average volume and the value of light side of equation (3) is almost similar, so that it can be predict that the average dynamical effective charges of each ion of hex-BaTiO_3 should be almost same as those of perovskite BaTiO_3. As the result it is concluded that large TO-LO splitting of the hex-BaTiO_3dues to the large shift of the dynamical effective charge from the nominal valence charge resulting the valance bonding character and that the octahedra of Ti and oxygen has the ferroelectric instability essentially ialso in the case of hex-BaTiO_3. Therefor the tendency of exhibiting ferroelectric instability in ATiO_2 is considered not to be respective of the stacking way of TiO_6 octahedra but to be primarily governed by the existence of TiO_6 octahedra. Less