Podlojenov, Serguei (2003). Contributions to crystal growth and characterization of the ferroelectric tetragonal tungsten bronze potassium-lithium-niobate (K3Li2Nb5O15 KLN). PhD thesis, University of Cologne.
Single crystals with the structure of the tetragonal tungsten bronzes (TTB) represent a potential material for nonlinear optical applications. In this substance family, potassium-lithium-niobate (KLN) is the only known ferroelectric substance with the structure of the closed TTB, which in comparison to others TTB favorable properties in relation to nonlinear optical applications result, such as e.g. B. the increased damage threshold for irradiation of laser light. Until the beginning of this work, however, it had not yet been possible to reproducibly grow optically homogeneous and crack-free single crystals of KLN. The main reasons for this are the incongruent melting character and the considerable range of existence of the ferroelectric KLN phase. The investigations on the phase diagram of the three-component system K2O-Li2O-Nb2O5 have based on the information from Scott et al. (1970) confirmed, but in most cases they showed a deviation of approx. 20-50 K downwards compared to Scott's results. The liquidus area belonging to the KLN extends over the range of approx. 30-38 mol% K2O. Crack-free or crack-free single crystals of undoped and Mg-doped KLN were grown using the Czochralski technique. To reduce the temperature gradients in the growing single crystal, a combination of an active post-heater (induction double coil) and a passive post-heater in the form of a platinum cylinder was used. This reduced the axial temperature gradient to only approx. 10-20 K / cm. The optimal growth conditions for the Czochralski single crystal growth are 0.7 mm / h for the translation rate and 10 / min for the rotation rate. However, the reproducibility of good growth results remained low, which is mainly due to the behavior of the crystals during cooling. A series of Czochralski experiments to grow KLTN mixed crystals was also carried out. Optically clear single crystal samples up to a size of 5 × 5 × 5 mm were obtained. Because of the large distribution coefficient of Ta (> 2), the translation rate had to be reduced to up to 0.5 mm / h. The suitability for doubling the frequency of the KLN crystals was examined with the aid of the powder SHG test. For the undoped and Mg-doped KLN single crystals, a clear correlation was observed between the composition (Nb content) and the intensity of the SHG radiation. An Nb content of 49 mol% (based on the melt composition) can be assumed as the limit value between the paraelectric and ferroelectric KLN phase. All of the grown KLTN mixed crystals were paraelectric. The composition of the grown single crystals was determined with the aid of XRF and AAS. Axial and radial concentration profiles were determined using ESMA. This has shown that segregation in the KLN single crystals grown here is negligibly small. However, Ta segregation on the order of 1 mol% / 1 cm crystal length was found in KLTN single crystals. The ferroelectric phase transition of KLN was investigated using several comparative methods. The ferroelectric phase transition (PU) was clearly demonstrated in the range of 470-480 ° C. The phase transition can be classified as 2nd order PU according to the measurements of the dielectric constant and the temperature-dependent birefringence. DTA measurements, however, showed a slight exotherm in the area of the phase transition, which indicates a first-order component. The refractive index of the paraelectric and the ferroelectric KLN phase was measured with an accuracy of 0.00004. The refractive index of the ferroelectric KLN single crystals allow the generation of the second harmonic at a fundamental wavelength of 910 nm with non-critical phase matching at room temperature (type I).
|Item Type:|| Thesis (PhD thesis) |
|Contributions to crystal growth and characterization of the ferroelectric tetragonal tungsten bronze potassium lithium niobate K3Li2Nb5O15 KLN||English|
|Single crystals with the structure of tetragonal tungsten bronzes (TTB) are a potentially useful material for non-linear optical applications. In this materials group potassium lithium niobate (KLN) is the only known ferroelectric substance with the structure of the so-called stuffed TTB, i.e. with completely occupied cation sites. This type should provide better optical properties, e.g. the increased (optical damage) threshold under laser radiation compared to other TTB. The growth of KLN single crystals of optical quality is a difficult task, and the production of homogeneous and defect-free single crystals of KLN in a reproducible manner had not yet succeeded up to the beginning of this work. The main reasons are the incongruent melting behavior and the considerable existence range of the ferroelectric KLN phase. The investigations of the phase diagram of the ternary system K2O-Li2O-Nb2O5 have confirmed the results of Scott et al. (1970). However, the correction of the thermal effects of 20-50 K downward of Scotts results was detected. The liquidus field of KLN extends at about 30-38 mole% of K2O. Undoped and Mg-doped KLN single crystals of optical quality were grown by the Czochralski technique. For decreasing temperature gradients in a growing single crystal an inductive afterheater was applied. Thereby the axial temperature gradient was reduced to 10-20 K / cm. The optimal growth conditions were found to be 0.7 mm / h for the translation rate and 10 / min for the rotation rate. The reproducibility of the crystal quality was poor because of the crystal cracking during the cooling. Several potassium lithium tantalate niobate (KLTN) single crystals were also grown by the Czochralski technique. Specimens suitable for crystal examinations were obtained. Due to the large distribution coefficient of Ta (> 2) the translation rate must be reduced to 0.5 mm / h. The applicability for the frequency doubling of the KLN crystals was examined by using of powder SHG tests. There was a clear correlation between the crystal composition (Nb-content) and the intensity of the second harmonic radiation. The niobium concentration of 49 mole% in the liquid phase can be appropriated as the boundary between the crystallization of paraelectric and ferroelectric KLN phase. All the grown KLTN crystals were paraelectric. The composition of the grown crystals was determined by X-ray fluorescence analysis and the atomic absorption analysis. The axial and radial concentration profiles of K and Nb over single crystals were determined by means of electron microprobe analysis. The segregation of elements is negligibly small in the investigated KLN single crystals. But in the KLTN single crystals the segregation of Ta was estimated to be in the order of 1 mole% / 1 cm crystal length. The ferroelectric phase transition in KLN was investigated with several techniques. The ferroelectric phase transition was estimated to lie within the range of 470-480 ° C. The second-order phase transition was detected by temperature-dependent measurements of the dielectric constant and the birefringence. But the DTA measurements indicated small thermal effect at the same temperature region, what refers to a first-order phase transition The refractive indices of the paraelectric and the ferroelectric KLN phase were measured with an accuracy of 0.00004 over the wavelength region of 404- 1083 nm. The data indicate that second harmonic generation in ferroelectric KLN crystals is possible at the primary wavelength 910 nm with non-critical phase matching (type I).||English|
|Creators||E-mail||ORCID||ORCID put code|
|Podlojenov, Serguei||[email protected]||UNSPECIFIED||UNSPECIFIED|
|URN:||urn: nbn: de: hbz: 38-9511|
|Faculty:||Faculty of Mathematics and Natural Sciences|
|Divisions:||Faculty of Mathematics and Natural Sciences> Department of Geosciences> Institute for Crystallography|
|tetragonal tungsten bronze, nonlinear optical materials, crystal growth, potassium lithium niobate||German|
|tetragonal tungsten bronzes, non-linear optic materials, crystal growth, potassium lithium niobate||English|
|Date of oral exam:||14 May 2003|
|Mühlberg, Manfred||Prof. Dr.|
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