钽酸锂铁电相变的喇曼光谱研究

本文测定了钽酸锂晶体从室温到居里温度以上(20—800℃)的喇曼光谱。没有观察到软模现象。我们发现在居里点附近,铁电相和顺电相的喇曼光谱十分一致,经过居里点喇曼光谱不发生变化。根据这个新的实验结果和文献上关于用中子衍射测定的晶体结构资料,我们做出了钽酸锂的铁电相变为有序无序型的推论。 关键词：     

Luminescence properties of Eu- and Mg-doped LiTaO3 obtained via the polymeric precursor method

This work reports the pure lithium tantalate (LiTaO₃), europium (III)-doped LiTaO₃ and magnesium (II)-europium (III)-doped LiTaO₃ preparation by the polymeric precursor method, using four different powered samples of Eu³⁺ ion concentrations 0.1 and 1 at%, observing their effect on the luminescent property of the material. Results indicated LiTaO₃ phase free of secondary phases at 650 °C and the photoluminescence (PL) emission spectra showed the characteristic sharp emission bands given by Eu³⁺ ions when they are excited at a wavelength of 399 nm. An increase of dopants led us to a non-homogeneous broadening and showed a slightly larger one when Mg was added. A displacement of the transition ⁵D₀→⁷F₀ to shorter wavelengths as function of Eu³⁺concentration was also noticed.     

Metastable phases in HxLi1-xTaO3 waveguide layers and pure LiTaO3

The formation of high-temperature phases in low-doped H:LiTaO₃ waveguide layers in Z-cut LiTaO₃ has been observed both by refractive-index and IR-spectra measurements. This permits us to correlate the index jumps to the changes of the OH bonds in the crystal lattice. Reversible phase transitions were detected in the temperature interval T=50–200 °C over a wide range of hydrogen content including as-grown LiTaO₃. The high-temperature phases are metastable close to room temperature. This was demonstrated by tracing the time evolution of the refractive-index change. It was shown that the high-temperature phases are responsible for the long-term refractive-index instabilities in both H:LiTaO₃ waveguides and virgin LiTaO₃. Received: 7 May 2001 / Revised version: 10 August 2001 / Published online: 30 October 2001     

An optical spectroscopy study of defects in lithium tantalate single crystals

The congruent, stoichiometric, and Mg doped stoichiometric LiTaO₃ single crystals have been successfully grown by the Czochralski technique. The evolution of defect structures caused by varying composition and post-growth processing has been evaluated from the optical absorption and photoluminescence measurements. Optical absorption studies showed that the UV absorption edge is very sensitive to the composition of LiTaO₃ crystals. Photoluminescence of various LiTaO₃ single crystals at room temperature was observed. The emission bands centered at 360, 430, and 530 nm were assigned to different defects, which can well show the defect information in LiTaO₃ crystals.     

Growth and optical properties of Mg, Fe Co-doped LiTaO3 crystal

Mg, Fe double-doped LiTaO₃ and LiNbO₃ crystals have been grown by Czochralski method. The optical properties were measured by two-beam coupling experiments and transmitted facula distortion method. The results showed that the photorefractive response speed of Mg:Fe:LiTaO₃ was about three times faster than that of Fe:LiTaO₃, whereas the photo-damage resistance was two orders of magnitude higher than that of Fe:LiTaO₃. In this paper, site occupation mechanism of impurities was also discussed to explain the high photo-damage resistance and fast response speed in Mg:Fe:LiTaO₃ crystal.     

Luminescent properties of La2LiTaO6:Mn4+ and its application as red emission LEDs phosphor

A novel phosphor, Mn⁴⁺ doped La₂LiTaO₆, was developed by solid-state reaction method. The luminescent spectra and emission efficiencies of La₂LiTaO₆:Mn _x ⁴⁺ (x = 0.001, 0.003, 0.005 and 0.01) were discussed. The effects of co-doped charge compensation ions, M = Mg²⁺, Ca²⁺, Na⁺, were investigated, respectively. The excitation spectra indicated that La₂LiTaO₆:Mn⁴⁺ could be effectively excited by both NUV light and blue light. The emission spectra of the phosphor exhibit a broadband ranging from 670 to 720 nm with the maximum at about 709 nm in deep red region. The co-doping of Mg²⁺ could significantly improve the luminescent properties of La₂LiTaO₆:Mn⁴⁺. Thus, phosphor La₂LiTaO₆:Mn⁴⁺, Mg²⁺ can serve as a key component to improve color rendering of blue-chip white-LEDs.     

Proton-exchange influence on the polar phonons of LiTaO3 optical waveguides

Refractive index change and Raman scattering of Z-cut proton-exchanged LiTaO₃ optical waveguides with different composition have been studied. Probing of the waveguide depth was carried out by using micro-Raman spectroscopy in order to distinguish layers with different phase states. Varying the proton-exchange and post-exchange annealing regimes, two phases –α and β– in addition to the pure LiTaO₃, were clearly observed. The behaviour of the A ₁ vibrations of the TaO₆ groups was found to be very sensitive to the phase changes. Using data from waveguide Raman spectra, normal and oblique E(TO, LO) phonons were sorted and their properties were followed in the α and β phases. The OH-stretchings are considered to correspond to dipoles turned out of the oxygen planes. Received: 9 July 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Optical spectrum, local lattice structure and EPR g factors for Cr impurity ions in MgTiO3 and LiTaO3

On the basis of the 120×120 complete energy matrices for a d³ configuration ion in a trigonal ligand field, for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃, the local structures and EPR g factors of the octahedral (CrO₆)⁹⁻ clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg²⁺, Ta⁵⁺) ions are obviously different, after Cr³⁺ replacing the M, the local lattice structures around the Cr³⁺ ions are quite similar and close to those of the Cr₂O₃. This may be ascribed to the fact that the octahedral Cr³⁺ center in MgTiO₃:Cr³⁺ and LiTaO₃:Cr³⁺ systems and that in Cr₂O₃ exhibit similar octahedral (CrO₆)⁹⁻ clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃.     

Non-equilibrum population of the Fe3+ electronic states after the electron capture in57Co: LiTaO3

Mössbauer emission spectra of⁵⁷Co: LiTaO₃ show as a consequence of the nuclear decay anomalous emission line intensities in the subspectra of Fe²⁺ and Fe³⁺. Magnetic field and angle dependence of Fe³⁺ intensities can be explained by taking into account polarization-and crystal-field effects in the excited states. The experimental results for LiTaO₃ are in good agreement with theory and previous measurements on LiNbO₃.     

Raman Spectra Study on LiTa0.9Nb0.1O3 Single Crystal

In this letter, Raman spectra of LiTa_0.9Nb_0.1O₃ single crystal in both of its ferroelectric phase and paraelectric phase are presented for the first time. Comparing with LiTaO₃ crystal, we find that the distribution and Raman shifts of normal vibrational mades are similar, but the line-shape and relative intensities of these modes have changed.     

E(transverse optical)-mode properties in the LiTaO3: Nd crystal at room temperature

E(TO)-mode properties in LiTaO₃:Nd crystal were examined by analyzing the Raman spectra measured.E(TO) modes appear in the transverseA ₁ spectrum. Their intensities obviously increase in theE andE+A ₁ mixed-symmetry spectra but decrease in theE spectrum which shows new vibrational modes. In particular, in the transverse-E spectrum of y(xz) geometry, the properties ofE(TO) modes are similar to those of pure LiTaO₃ of the same geometry, whereas in the transverse-E spectrum in x(yz) geometry these modes are turned intoA ₁ (TO) modes. We attribute these properties to both the surface strain produced by mechanical polishing of the sample and the microstructural change of the LiTaO₃ crystal resulting from Nd doping.     

Soft vibrational mode in LiNbO3 and LiTaO3

The temperature dependence of the far-infrared reflectivity as obtained with a scanning interferometer for the A₁- and E-type modes of both LiNbO₃ and LiTaO₃ is reported in the ferroelectric phase. Results of a Kramers-Kronig analysis are compared with Raman and neutron scattering data which are controversial about the existence of a soft vibrational mode. For LiTaO₃, spectra are obtained 300 K above the Curie temperature, in the paraelectric phase. The lowest-frequency A₁ (TO) mode is unambiguously found as soft and becomes rapidly overdamped.     

Nonvolatile holographic recording in Ti, Fe Co-doped LiTaO3 crystal

Ti, Fe Co-doped LiTaO₃ (LT) crystals have been grown by the Czochralski method from the congruent melts. The absorption spectra of crystal were measured before and after ultraviolet illumination. Holograms have been recorded in doubly doped crystals with continuous-wave laser light by use of two-color method. The maximum value of refractive-index changes 7×10⁻⁵ is achieved.     

LiTaO3 as holographic storage material

The optical storage properties of LiTaO₃:Fe are investigated and compared with those of the isomorphous compound LiNbO₃:Fe. Absorption, photocurrent, photoconductivity and holographic measurements are reported. In the case of photovoltaic writing similar results for LiTaO₃- and LiNbO₃-crystals are obtained. However, in the case of photoconductive writing using external electric fields LiTaO₃:Fe-crystals yield much better results due to large photoconductivity values. Considering the recording sensitivity and the extremely large storage time LiTaO₃:Fe turns out to be one of the most promising materials for photorefractive storage of volume phase holograms.     

Influence of X-rays on optical properties of lithium tantalite crystals

The influence of X-ray radiation on the optical absorptance, photorefractive sensitivity, and the birefringence of pure and doped (with Cu and Rh impurities) LiTaO₃ crystals has been investigated. The radiation-induced birefringence δΔn ^R of LiTaO₃:Cu exceeds the same quantity of other previously investigated oxygen-octahedral ferroelectrics by more than an order of magnitude. The increased photorefractive sensitivities of LiTaO₃ and LiTaO₃:Cu observed after irradiation are related to metastable radiation-induced photovoltaic centers, which is assumed to arise from charge exchange between existing oxygen vacancies.     

晶体电光和非线性光学效应的离子基团理论(Ⅲ)——利用畸变氧八面体的离子基团模型计算LiNbO3,LiTaO3,KNbO3,BNN晶体的电光和倍频系数

本文用文献[1]中所提出的(MO₆)离子基团模型,计算LiNbO₃,LiTaO₃,KNbO₃,BNN等晶体的电光和倍频系数。假设在O_h对称时,这些晶体中的氧八面体具有相同的能级和对称波函数,则通过群表示理论就可得到在C_3ν,C_2ν对称性时氧八面体的能级和对称波函数,并进而用ABDP理论计算它们的电光和倍频系数。计算结果和实验符合得相当好。本文并从理论上解释了这些晶体的倍频系数大小、符号和氧八面体畸变间的关系,由此可以得到以下两个结论:(1)畸变氧八面体的离子基团模型不但适用于钙钛矿型材料,同时也适用于钨青铜型、LiNbO₃型材料。(2)在钨青铜型、LiNbO₃型的材料中,仍是“离子键”对电光和倍频效应作出主要贡献,同时由于LiTaO₃的共价键成份比LiNbO₃大,因而LiTaO₃的非线性光学效应比LiNbO₃小。     

Temperature dependence of the anomalous emission line intensities in57Co:LiTaO3-initial population and relaxation

Mössbauer spectra of⁵⁷Co: LiTaO₃ single crystals were recorded in an external longitudinal magnetic field of 6 T at different temperatures between 4.2 K and 150 K. The spectra were taken at two different orientations of the crystallographicc-axis relative to the magnetic field. The line intensities of the Fe³⁺-subspectra show a temperature dependent anomalous population of the three Kramers doublets of the Fe³⁺ spinS=5/2 ground state. A relaxation broadening is observed at higher temperatures, which cannot successfully be reproduced within a relaxation model taking into account only the six lowest lying electronic states.     

Design and comparison of single-mode planar and ridge type electro-optic waveguide modulators

Electro-optic waveguide modulators utilizing phase retardation of two orthogonally polarized optical modes in LiNbO₃ and LiTaO₃ waveguides have been designed taking into consideration the optical field distribution in the waveguides and the electrical properties of the electrodes. The analysis has revealed that a driving-voltage to frequency-bandwidth ratio of 1 V/GHz is attainable at the wavelength of 1.05 m using presently available embedded and ridge waveguides. Improvement in waveguide fabrication techniques may reduce the ratio by at least a half. Thus, LiNbO₃ and LiTaO₃ waveguide modulators are considered to be promising candidates for practical application to single-mode optical-fibre transmission systems of higher than 1 Gbit/s.     

DFT - based study of electronic structures and mechanical properties of LiTaO3: ferroelectric and paraelectric phases

Abstract

By applying ab initio calculation within density functional theory (DFT), we study the structure parameters, electronic band structure, elastic coefficients, polycrystalline elastic properties, anisotropy factors and Debye temperature of ferroelectric and paraelectric phases of LiTaO₃ within the generalised gradient approximation at ambient pressure. The atomic structure in both phases is fully relaxed and the lattice constant, angle and atomic positions are well consistent with experimental values. The computed single-crystal elastic coefficients indicate that mechanical stability of LiTaO₃ in both phases is confirmed using the generalised Born criteria. The shear, bulk and Young’s modulus, Poisson’s ratio, and Vickers hardness were computed according to theoretical elastic constants by Voight–Reuss–Hill method. Several anisotropy factors and indexes are computed to illustrate mechanical anisotropy. Both phases are shown to be weakly anisotropic. The Debye temperature is estimated using the longitude and transverse elastic wave velocity of the ideal polycrystalline LiTaO₃ aggregates. We have found that LiTaO₃ in both phases has an indirect energy band gap. The differences in the electronic structure and density of states for both phases are quite small. Our results indicate that the mechanical and bonding properties of both phases are very similar. The obtained results were compared with the available experimental and theoretical values.     

Crystal growth and spectroscopic characterization of Yb:LiTaO3

Spectroscopic properties are presented for Yb³⁺ incorporated into single crystals of LiTaO₃ grown by the top-seeded solution growth method. From an analysis of the absorption and fluorescence spectra, we are able to determine the Stark-level components of the ²F_7/2 (the ground-state multiplet manifold) and the ²F_5/2 (the excited-state multiplet manifold of Yb³⁺ (4f¹³)). The room-temperature fluorescence lifetime of ²F_5/2 is 678 μs as measured on a thin sample to reduce possibilities for reabsorption. Spectral comparisons of Yb³⁺-doped LiTaO₃ and LiNbO₃ are drawn. The crystal-field splitting of Yb³⁺(4f¹³) in both crystal hosts is modeled using a set of crystal-field splitting parameters, B_nm, determined from a recent spectroscopic analysis of Er³⁺(4f¹¹) in LiNbO₃. Without adjustment of the B_nm parameters, the model predicts the Stark-level energy and the symmetry label for each level in reasonable agreement with the experimental values. Less photorefractive than its niobate cousin, LiTaO₃ has potential for use in numerous integrated electro-optical circuits and devices.