RAMAN-SCATTERING STUDIES OF THE IMPURITY-INDUCED FERROELECTRIC PHASE-TRANSITION IN KTAO3-LI
RAMAN-SCATTERING STUDIES OF THE IMPURITY-INDUCED FERROELECTRIC PHASE-TRANSITION IN KTAO3-LI
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DOI：
10.1103/physrevb.23.5904
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发表时间：
1981-01-01
影响因子：
3.7
通讯作者：
BOATNER, LA
中科院分区：
文献类型：
--
作者：
PRATER, RL;CHASE, LL;BOATNER, LA
利用拉曼散射和光学退偏测量研究了KTa O 3中锂取代钾形成K1 − x Li x Ta O 3所引起的相变。研究了高达5.4摩尔%的锂浓度。在低浓度（x<~ 0.004）下，纯KTa O3的无序诱导散射特征强度随Li含量的增加而增加。一个低频的功能，这是以前与锂共振模式，似乎是散射从耦合TA和TO分支由于无序引入的锂。对于x ≤ 0.01，在光学退偏中观察到一个明确定义的步骤，在一个温度Tc迅速增加与锂浓度，并达到一个值Tc = 70 K在x= 0.054。对入射光偏振方向的依赖性表明，形成了具有对称轴的四方晶或正交晶低温相< 100>。新的拉曼活性声子观察附近和低于Tc。在非极化样品中没有观察到极模所期望的非寻常声子能量的各向异性。极化可以通过用外加电场冷却晶体通过Tc来实现，之后观察这些声子的极性特征。非极化样品中各向异性的缺乏是直径小于光学波长的铁电畴的存在的结果。此外，去极化测量意味着至少几千μ m的域大小。这些结果显然不同意先前的说法，即当x< 0.24时，K 1− x Li x Ta O 3不是铁电体，并且Li中心在低温下形成极性玻璃。最后，锂杂质的存在使区域中心处的TO分支变硬，并且铁电相由偏离中心的锂离子的有序-无序转变产生。
Raman scattering and optical depolarization measurements have been employed in investigations of the phase transition induced by the substitution of lithium for potassium in KTa O 3 to form K 1− x Li x Ta O 3. Lithium concentrations as high as 5.4 mol% were studied. At low concentrations, x<~ 0.004, the intensities of the disorder-induced scattering features observed for pure KTa O 3 increase as the Li fraction increases. A low-frequency feature, which was previously associated with a Li resonance mode, appears to be scattering from the coupled TA and TO branches due to disorder introduced by the lithium. For x≳ 0.01, a clearly defined step in the optical depolarization is observed at a temperature T c which increases rapidly with lithium concentration and reaches a value of T c= 70 K at x= 0.054. The dependence on the polarization direction of the incident light indicates that a tetragonal or orthorhombic low-temperature phase is formed with< 100> symmetry axes. New Raman-active phonons are observed near and below T c. The anisotropy of the energy of the extraordinary phonon expected from polar modes is not observed in the unpoled samples. Poling can be accomplished by cooling the crystals through T c with an applied electric field, after which the polar character of these phonons is observed. The lack of anisotropy in unpoled samples is the result of the presence of ferroelectric domains with diameters smaller than optical wavelengths. Additionally, the depolarization measurements imply domain sizes of at least a few thousand Å. These results clearly disagree with the previous claims that K 1− x Li x Ta O 3 is not ferroelectric for x< 0.24 and that the Li centers form a polar glass at low temperatures. Finally, the presence of the lithium impurities stiffens the TO branch at the zone center, and the ferroelectric phase results from an order-disorder transition of the off-center lithium ions.