Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski growth and constitutional studies on single crystals of potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2−xNb_5+xO₁₅ (KLN) is a very promising material for the conversion of infrared light to light in the visible region. However, growing of single crystals is known to be complicated due to the considerable anisotropy of the growth rate and the thermal expansion behaviour. The single crystals of KLN, Mg²⁺‐doped KLN, as well as the mixed crystals of potassium lithium tantalate niobate K₃Li₂(Nb_1−xTa_x)₅O₁₅ (KLTN) were grown by the Czochralski technique. The chemical analyses of the samples were performed by atomic absorption spectroscopy (AAS) and X‐ray fluorescence analysis (XRF). The element concentrations along the single crystals were measured by the electron microprobe analysis (EMPA) to clarify the segregation phenomena in the grown crystals. The elements distribution coefficients were also calculated. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of nanocrystalline lithium niobate powders at low temperature

A facile route to prepare lithium niobate (LiNbO₃) powders was proposed by an alternative solid‐state method. Stoichiometric Li₂C₂O₄ and ammonium niobium oxalate were mixed with small amounts of water and then dried at room temperature. It was demonstrated that Li[NbO(C₂O₄)₂]·n H₂O intermediate was produced by an ion‐exchange reaction. Pure LiNbO₃ powders were successfully synthesized by heating the intermediate at 500, 600 and 700 °C for 3 h. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Fourier‐transform infrared (FTIR) spectroscopy, UV‐Vis diffuse reflectance (UV‐Vis) spectroscopy and thermogravimetric (TG) analysis were used to characterize the precursor compound and as‐prepared samples. XRD results reveal that all the products are identified as hexagonal structure with high relative crystallinity (>87%). The particle size is found to be about 40 nm for the mixture calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The as‐prepared LiNbO₃ powders by this method are high quality according to FTIR spectra. (Li_0.996Nb_0.005)Nb_0.999O₃ phase was formed when the calcination temperature was raised to 800 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation on photorefractive properties of doped lithium niobate

Cu:LiNbO₃ crystal and Fe:Cu:LiNbO₃ crystals were grown by the Czochralski method from congruent melt. The OH^‐ absorption spectrum of doped lithium niobate crystals was measured. The photorefractive properties of doped crystals were studied by the two‐wave coupling method. The results of the two‐wave coupling experiments showed that as the concentration of doping ions increased, the diffraction efficiency and the dynamic range enhanced, the holographic response time shortened. The recording time of Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal is only a tenth of that of Cu(0.05wt%): LiNbO₃ crystal. Among all samples, the dynamic range of the Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal was the most largest (up to 40.78). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bridgman growth and properties of potassium lithium niobate single crystals

Transparent KLN crystals 10mm in diameter and 25 to 45mm in length have been grown by the modified vertical Bridgman technique from different melts in the range of 3035mol% K₂O, 1723mol% Li₂O and 4350mol% Nb₂O₅. The growth conditions are a growth rate of less than 0.25 mm/hr, temperature gradient in solid-liquid interface of 23 °C/mm and growth direction of <110>. As-grown KLN crystals have tetragonal tungsten bronze structure. Most of the as-grown crystals do not crack when cooling through the paraelectric/ferroelectric phase transition. 180° domain structures are observed after the KLN crystal was etched in boiling 2HNO₃:Hf. Dielectric properties and transmission spectrum of the as-grown KLN crystals are measured.     

Optimisation of chromium doping in LiNbO3 single crystals

Growth of undoped and Cr doped (0.1, 0.25 and 0.5 mol % Cr₂O₃) congruently‐ melting‐composition LiNbO₃ single crystals by Czochralski technique is reported. Chromium doping was optimised to get crystals with potential for an integrated, broadband, tunable laser in the 700‐1100 nm spectral range. Typical sizes of the grown crystals are 25‐30 mm in diameter and 30‐40 mm in length. Symmetrical and sharp conoscopy pictures confirm the optical homogeneity of the crystals. Optical transmission was recorded for both undoped and doped crystals. 70% transmittance was observed. The grown crystals have reasonably good laser damage threshold. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lattice vibration and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate

Raman scattering and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate were examined. The A₁(TO) modes appear in E symmetry spectrum of the doped lithium niobate. Their intensities vary with different dopings. In the spectrum of z(yy)x geometry, the properties of the lowest-frequency E(TO) mode of the Cr-doped lithium niobate are different from those of pure, Co- and Fe-doped lithium niobate. The intensity of the A₁(TO) mode at 637cm^-1 I is decreased in doped lithium niobate compared with the pure crystal. We attribute these properties to both the photorefractive effect which is enhanced by dopants and to the different occupation of the doping ions. A light climbing effect was observed in Co- and Cr-doped lithium niobate for the first time. A higher photodamage threshold and quicker light climbing speed were found in Co- and Cr-doped lithium niobate in comparison with the light climbing effect in the Fe-doped lithium niobate. The results from the photoinduced light scattering experiments were compared with those from a Raman spectroscopic study.     

Inhomogeneity of composition in near‐stoichiometric LiNbO3 single crystal grown from Li rich melt

A near‐stoichiometric LiNbO₃ single crystal has been grown by the Czochralski technique from a melt of 58.5 mol% Li₂O. Its composition homogeneity was assessed by measuring the UV absorption edge. It was found that the maximum composition difference is about 0.03 mol% in the radial direction and 0.05 mol% in the axial direction. Differential scanning calorimetry (DSC) analysis was performed on the powder from the synthesized raw material and the frozen melt after crystal growth. The analytical results indicate that, during crystal growth, the magnitude of lithium volatilization from the melt surface is more than the degree of segregation from the crystal. The volatilized lithium diffuses into the crystal to compensate for the lithium segregation in the LiNbO₃ crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Domain structures and etching morphologies of lithium niobate crystals with different Li contents grown by TSSG and double crucible Czochralski method

Stoichiometric lithium niobate single crystals with different Li contents have been grown both by the top‐seeded solution growth (TSSG) method from potassium containing flux and by the double crucible Czochralski (DC Cz) method. Spectroscopic properties (e.g. the UV absorption edge, Raman linewidth) and the Curie temperature measurement have been used for the characterization of the crystal composition. The double crucible Czochralski method is found to be suitable for mass production of stoichiometric LiNbO₃ with Li content larger than 49.7 mol% and homogeneity of 0.03 mol%. The domain structures and etching morphologies on negative and positive c‐surface were also investigated by chemical etching method. A new domain structure of three‐fold symmetric sectors were observed in near‐stoichiometric LiNbO₃ grown by TSSG method. The straight line arrangement hillocks on negative c‐surface and the net‐like arrangement etch lines were observed and explained by stress etching mechanism. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Contribution to the tin‐rich region of the Sn‐Zn‐Ti system

The tin‐rich region of the system Sn‐Zn‐Ti system has been studied by diffusion couples, differential scanning calorimetry and electron microprobe analyses. Ternary eutectic reaction occurs at 193.7°C near to the binary tin‐zinc eutectic point and titanium content less than 0.9 at.% Ti. Three ternary compounds with approximate formulae: Ti₈Sn₅Zn₂ to Ti₅Sn₃Zn, TiSn₄Zn₅ and Ti₂Sn₄Zn₃ have been observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Second Harmonic Generation in Zn‐doped Li‐rich LiNbO3 Crystals

6.0 mol. % ZnO doped LiNbO₃ crystals were grown by Czochralski technique. Various Li/Nb mole ratios of 0.942, 0.970, 1.000, and 1.020 were used to prepare the starting materials. Second harmonic generation (SHG) experimental results show that the phase matching temperature increases near linearly with the increasing of Li/Nb ratio, and the SHG efficiency is enhanced by the Zn doping and the increasing of Li/Nb ratio. The intrinsic and extrinsic defects are discussed in this paper to explain the SHG behavior and photo‐damage resistance in the Zn doped Li‐rich LiNbO₃ crystals.     

Influence of Non‐Stoichiometric Defects on Optical Properties in LiNbO3

We present a band structure approach with a molecular dynamics cluster optimization which accounts for the various structural modifications related to the non‐stoichiometry of LiNbO₃ crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Particular role of the electron‐phonon contributions to the electrooptics coefficient is shown. Model calculations yield a large dependence of the electrooptis coefficient r₂₂ on the crystal composition, in agreement with the experimental data. The observed minimum of the r₂₂ coefficient versus the non‐stoichiometry is interpreted as originated from the non‐centrosymmetry in the electrostatic potential distribution around Nb‐O₆ clusters.     

Synthesis and characterization of fine lithium niobate powders by sol‐ gel method

Lithium niobate (LN) nanocrystal powders were prepared by low‐temperature sol‐gel method. Dihydrate lithium acetate as lithium source, and niobium chloride as niobium source were used as starting materials. The gel and powders were characterized by thermogravimetry and differential scanning calorimetry (TG/DSC), X‐ray diffraction (XRD), transmission electronmicroscopy (TEM) and Fourier transform infrared (FTIR) spectra. The results show that when the gel was heat‐treated at 600°C, the fine LN nanocrystals with the size of 40‐60 nm were obtained, and the size of the powders become larger with the heat‐treated temperature increasing. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate

The effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate was studied. The absorption spectra examination of Ru‐doped LiNbO₃ crystals with different post treatments showed that the absorption coefficient at 530 nm increased after the reduced treatment was employed and the absorption edge of the reduced crystal shifted towards the infrared band. On the contrary, the trend reversed after the oxidized treatment was employed. In addition, the photorefractive properties were investigated with the two‐beam coupling method conducted via a 532 nm solid state laser. It was found that the oxidized Ru:LiNbO₃ had smaller exponential gain coefficient and diffraction efficiency because the charges in the shallow level were exchanged to the deep level. On the other hand, the reduced Ru:LiNbO₃ crystals had larger exponential gain coefficient and diffraction efficiency due to the increase of the Ru³⁺ which existed in the shallow level. The response times of both oxidized and reduced Ru:LiNbO₃ were longer than those of the as‐grown ones. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On‐line monitoring of lithium carbonate dissolution

Dissolution of lithium carbonate (Li₂CO₃) in aqueous solution was investigated using three on‐line apparatuses: the concentration of Li₂CO₃ was measured by electrical conductivity equipment; CLD (Chord Length Distribution) was monitored by FBRM (Focused Beam Reflectance Measurement); crystal image was observed by PVM (Particle Video Microscope). Results show dissolution rate goes up with a decrease of particle size, and with an increase in temperature; stirring speed causes little impact on dissolution; ultrasound facilitates dissolution obviously. The CLD evolution and crystal images of Li₂CO₃ powders in stirred fluid were observed detailedly by FBRM and PVM during dissolution. Experimental data were fitted to Avrami model, through which the activation energy was found to be 34.35 kJ/mol. PBE (Population Balance Equation) and moment transform were introduced to calculate dissolution kinetics, obtaining correlation equations of particle size decreasing rate as a function of temperature and undersaturation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Leaf‐like copper oxide nanocrystals synthesized by a gas‐liquid diffusion method at room temperature

The leaf‐like copper oxide (CuO) nanocrystals have been synthesized by a gas‐liquid diffusion method in pure aqueous solution at room temperature. The structure, morphologies and related properties of the as‐prepared crystals were characterized with X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED) and thermogravimetric analysis (TG). The influence of copper concentration was investigated, which plays an important role in the formation of nanostructured CuO crystals. Only when the copper concentration was low enough (0.005 M) that the leaf‐like CuO could be obtained directly. Additionally, a growth mechanism of CuO was also proposed based on the observed results.     

Synthesis,characterization, and tribological properties of two‐dimensional Ti3C2

We report on the preparation of two‐dimensional (2D) Ti₃C₂ and its friction and wear properties. Laminated Ti₃AlC₂ was synthesized by pressureless sintering using Ti, Al, and graphite, followed by HF exfoliation and sonication treatment to form 2D‐layered Ti₃C₂, which exhibited individual layer or stack of several layers. Analysis of microstructure and composition was used to confirm the successful exfoliation of laminated Ti₃AlC₂. The tribological behaviors of the as‐prepared 2D Ti₃C₂ as a lubrication additive in base oil were investigated. Results indicate that 2D‐layered Ti₃C₂ can greatly enhance the friction‐reducing and anti‐friction properties of base oil, especially with 1.0 wt% Ti₃C₂. This novel 2D‐layered Ti₃C₂ would be a good candidate for lubricating materials to meet emerging needs in practical applications.     

Polymeric metal‐organic octupolar NLO materials: lithium(I) and sodium(I) 3,5‐dinitrobenzoates

Lithium 3,5‐dinitrobenzoate (Li(dnb)) exhibits a 1D propeller chain structure of D ₃ point symmetry and the chains are trigonally assembled in the crystal under the chiral space group P 3₁21. Sodium 3,5‐dinitrobenzoate (Na(dnb)) also crystallizes in space group P 3₁21 and exhibits a 3D structure. The structure of Na(dnb) could be regarded to be similar to that of Li(dnb) if the weaker Na‐O(nitro) bonds were to be ignored. These two compounds represent rare examples of octupoles arranged in an octupolar environment and show modest powder SHG effects. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High photorefractive sensitivity and fast response time of near‐stoichiometric and MgO doped LiNbO3:Fe crystals

Congruent LiNbO₃:Fe and LiNbO₃:Mg,Fe crystals were grown by Czochralski method, and vapor transport equilibration technique was employed to improve the [Li]/[Nb] ratios of these crystals. The influence of stoichiometry and MgO dopant on the photorefractive sensitivity and response time of LiNbO₃:Fe crystals was investigated. Both stoichiometry and MgO dopant can effectively reduce the amount of intrinsic defects, but MgO can also decrease the concentration of Fe²⁺ ions in Li‐sites. Near‐stoichiometric and MgO doped LiNbO₃:Fe crystal has high photorefractive sensitivity and fast response time. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Angular stability of electric field‐induced effects in crystalline materials

We analyze angular bandwidths of extrema of the indicative surfaces describing spatial anisotropy of piezoelectric (PE) and electrooptic (EO) properties of doped lithium niobate (LiNbO₃:MgO) and langasite (La₃Ga₅SiO₁₄) crystals. A number of highly efficient experimental geometries are suggested, which are promising for PE and EO devices. Our data obtained with both analytical and numerical techniques characterize angular stability of those devices and, in particular, their angular aperture. We show that, besides of a maximal size of the electric field‐induced effects, ‘nondirect crystal cuts’ offer considerably higher angular stability of their characteristics, when compared with that typical for ‘direct crystal cuts’ usually employed in PE and EO devices.