Congruent Melt Research Articles

Determination of the Li/Nb ratio in LiNbO 3 crystals grown by Czochralski method with K 2O added to the melt

Off-congruent lithium niobate single crystals were grown by the Czochralski method with K 2O added to the congruent melts in a concentration varying between 2 and 10.5 wt%. The Li/Nb ratio was determined by means of indirect methods such as the fundamental absorption edge positon, the IR vibrational spectrum, lattice parameters and the variation of the refractive indices. The data presented in this work show that the Li/Nb ratio can be controlled by the amount of K 2O added to the congruent melt. Moreover, the techniques employed for characterization are good tools for the determination of the Li/Nb ratio.

Growth and characterization of MgO doped near stoichiometric LiNbO 3 crystals as a new nonlinear optical material

Nondoped and MgO-doped LiNbO 3 single crystals of 30 mm in diameter and 25–30 mm in length were grown by the top-seeded solution growth method. The solution with [Li]/[Nb] ratio of 1 contained K 2O component of 10.6 mol% (7 wt%). MgO of different concentrations was used to dope the solution. As-grown crystals were transparent, void-free and almost free from potassium contamination. The crystal grown without MgO-doping exhibited a Curie temperature of 1193°C, indicating that the crystal composition was considerably close to stoichiometry. The MgO-doped crystals grown from the above solution showed larger [Li]/[Nb] ratios (that is, closer to stoichiometry) than the crystals grown from the congruent composition melt doped with MgO of the same doping levels. The MgO-doped crystals with near stoichiometric composition showed a shift in absorption edge towards shorter wavelength, an increase in Curie temperature, and four orders of magnitude increase in photorefractive damage resistance compared to nondoped crystals grown from the congruent melt by the conventional Czochralski method.

Incorporation of hexavalent impurities into LiNbO3

The influence of the valence state on the lattice location of impurities in lithium niobate LiNbO3 crystals was investigated using hexavalent dopants (W6+ and U6+) introduced by doping of congruent melt. The distribution coefficients were determined by Rutherford Backscattering Spectrometry to be k = 0.2 for W and k = 0.07 for U. Channeling studies showed that the hexavalent impurities are located on the Nb site in both cases. The preferential occupation of Nb sites suggests that the formation of Li-vacancies is favored. This result is a strong evidence for the Li-vacancy defect model for congruent LiNbO3.

Bulk periodically poled lithium niobate doped with Yb3+ ions: Growth and characterization

We have obtained Yb3+-doped bulk periodically poled LiNbO3 crystals grown by adding Yb2O3 to the congruent melt. The structures obtained have been characterized by scanning electron microscopy, x-ray fluorescence, optical absorption, and second-harmonic generation. About 100 nm of fundamental tunable quasi-phase-matched second-harmonic generation at the pm/V level is obtained in some of the samples.

Stoichiometric LiTaO3 for Dynamic Holography in Near UV Wavelength Range

LiTaO3 single crystals with near-stoichiometric composition are grown using a double crucible Czochralski method equipped with an automatic powder supply system. Their photorefractive properties are compared with those of LiTaO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that nearly stoichiometric LiTaO3 crystals exhibit considerably higher photorefractive gain, speed and sensitivity than conventional congruent crystals. Due to the relatively large photoconductivity observed at near UV wavelengths, stoichiometric crystals are further characterized by small photogalvanic fields, making them very attractive for dynamic holography in this wavelength range.

On the opposite domain nature of Er-doped lithium niobate crystals

It has been shown that Erbium doping affects the domain structure of LiNbO 3 crystals grown from congruent melt. Doping up to 5 mol% produces opposite domains for congruent crystals. Additionally, the monodomain configuration of pure stoichiometric crystal becomes polydomain with the addition of Er in the melt.

Scintillation characteristics of nonstoichiometric phases formed in MF 2–GdF 3–CeF 3 systems Part III. Dense Gd 1− x− yM xCe yF 3− x tysonite-related crystals (M=Ca, Sr)

We evaluated the scintillation characteristics and the radiation hardness of Gd 1− x Ca x F 3− x and Gd 1− x Sr x F 3− x tysonite-type solid solutions (close to 6.6 g/cm 3 in density) doped with Ce which were grown from the congruent melt. As the Ce concentration increased, the Ce 3+ emission at 345 nm increased to a level comparable with the Gd 3+ emission at 310 nm. Both emissions, however, were slow with the decay constant in the order of ms. The radiation hardness, which was roughly 10 4 rad, tends to degrade as the wavelength decreases and/or the Ce concentration increases. Both spontaneous recovery and UV annealing of the radiation damages were significant. The intensity of the γ-ray excited luminescence, integrated over time, was as large as 50% of that in BaF 2 (12% of that in CdWO 4). When the Ce concentration is nil or only small, the yellow–green luminescence for UV excitation was as intense as in CdWO 4. Although the present crystals are not good for the applications in high-energy physics experiments, they may find applications in display devices, thermal neutron monitors, etc.

Site-selective spectroscopy of Nd 3+ in lanthanum magnesium hexaaluminate LaMgAl 11O 19 single crystals

Spectral and dynamical properties of the 4F 3/2 fluorescence of Nd 3+ in LaMgAl 11O 19 (Nd : LMA) crystals have been analysed by site-selective laser spectroscopy at low temperature. Three or four different Nd 3+ sites, with distinct relative contributions, have been characterized for two LMA crystals, grown from stoichiometric and congruent melts, respectively.

Imaging conoscope for investigation of optical inhomogeneity in large boules of uniaxial crystals

The name “conoscope” in Greek suggests that this tool should be used for observing interference patterns of birefringent crystals in the convergent beam of light. As such, the conoscope has been frequently used in laboratories for quick and usually qualitative estimation of optical inhomogeneity of crystals. In this paper we have described details of the computer-controlled imaging conoscope used for quantitative investigation of optical inhomogeneity in uniaxial crystals. To the best of our knowledge, this is the first conoscope used in an automated arrangement. Its working is based on equations derived for a plane–parallel uniaxial crystal plate cut out obliquely to the optical axis, which is next applied for two specific plate orientations practically investigated, i.e. for plates cut out perpendicularly and parallelly to this axis. It was found that these equations are more accurate than those published by other investigators. A practical investigation of a LiNbO3 crystal pulled by the Czochralski method from a congruent melt has been presented. Two birefringence inhomogeneity maps acquired for the above-mentioned two specific orientations in this crystal were used for eliminating its inhomogeneous areas from further use in optics. A theoretical error analysis carried out also in this work has shown that the optical inhomogeneity could be detected with a relative error usually not exceeding a small fraction of a percent.

Progress in single crystal growth of LiNbO3 using double crucible Czochralski method

In order to grow single crystals with high quality and high homogeneity from off-congruent melts, a double crucible Czochralski method has been developed. In this method, the melt was divided into two parts by a double structure (two chamber system) crucible. The crystal was grown from the inner melt, and powder with the same composition as the growing crystal was supplied to the outer melt. The powder was supplied smoothly at the rate of weight increase of the growing crystal. This rate was automatically controlled to match the weight increase of the growing crystal as monitored by a load cell. In combination with an automatic diameter control system, a fully automatic double crucible Czochralski method was established. Using this method, LiNbO3 single crystals with a composition close to stoichiometric were grown, and their photorefrative properties were compared with those of Nb-rich congruent melt LiNbO3 crystals. Two beam coupling experiments show that both non-doped and Fe-doped stoichiometric crystals exhibited larger exponential gain coefficients than the exponential gains obtained in Fe-doped congruent crystals. This fact suggested that nonstoichiometry control of LiNbO3 is a key for improvement of these properties.

Photorefractive effect in LiNbO3 crystals enhanced by stoichiometry control

LiNbO 3 single crystals with a composition close to stoichiometry are grown by using a double crucible Czochralski method equipped with an automatic powder supply system. The photorefractive properties are compared with those of LiNbO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that both nondoped and Fe-doped stoichiometric crystals exhibit stronger photorefractive effect than the Fe-doped congruent crystals.

The effect of native defects on the domain structures of :Fe - a case study by means of the addition of MgO and to the congruent melt

The effect of native defects on the domain structure of has been investigated. To reduce the number of native defects, two methodologies have been adopted: firstly, co-doping with Mg; and secondly, the addition of to the congruent melt. When grown from congruent melts, has a multi-domain nature. Co-doping with Mg changes the domain structure, although the crystals are still multi-domain. On addition of , single-domain crystals are obtained. The change in domain structure brought about by doping with Mg is due to compensation of the stoichiometric defects. On the other hand, reduction in the number of intrinsic defects due to change in the Li/Nb ratio in the crystal as a result of the addition of to the melt is responsible for the mono-domain structure.

Photoluminescence of LiTaO3:Pr

Single crystals of LiTaO3 doped with praseodymium were grown by the Czochralski method from congruent melt and their optical properties were examined. It has been found that the Pr3+ ions in LiTaO3 occupy several nonequivalent sites differing in the crystal field strength. In addition, the structural disorder associated with Li deficiency gives rise to strong inhomogeneous broadening of both the optical absorption and emission lines. In contrast to isostructural LiNbO3:Pr, the relaxation of the excited P03 level of praseodymium is governed by radiative transitions, which terminate on excited multiplets and make the LiTaO3:Pr3+ crystals promising for four-level lasing in the visible range.

On the single domain nature of stoichiometric LiNbO3 grown from melts containing K2O

Stoichiometric crystals of LiNbO3 were grown by the Czochralski technique from a congruent melt with 6 wt% of K2O flux dopant. The crystals were always single domain irrespective of the seed history. Analysis of the domain structure at the seed-crystal boundary reveals a domain inversion in certain cases. During crystal growth, self-poling of initially unpoled congruent LiNbO3 seeds was observed. It is thought that the monodomain nature of the crystals is attributed to the reduction of native defects due to their stoichiometric Li-Nb composition.

Congruent composition and solid solution range of Li 2B 4O 7 crystal

Congruent melt composition for Li 2B 4O 7 single crystal growth was investigated by surveying the residual melt obtained after the quick growth of Li 2B 4O 7 polycrystals under strong stirring in the melt. Residual products were examined by X-ray diffraction (XRD) and differential thermal analysis (DTA) to determine if there were phases formed other than Li 2B 4O 7, which represents the crystallization from the off-congruent melt. This “residual melt” method confined the congruent composition within the range of 33.232–33.537 mol% Li 2O in which the stoichiometric composition, 33.334 mol% Li 2O, lies. Thus, the congruent melt composition is very close to the stoichiometric composition for Li 2B 4O 7 within the error range of 0.2 mol% Li 2O. The solid solution range of Li 2B 4O 7 in the Li 2O-rich side is constrained to be less than 0.35 mol% Li 2O by investigating the solubility limit of Li 2O into the Li 2B 4O 7 phase.

Critical free energy for nucleation from the congruent melt of MoSi 2

As a potential candidate for high-temperature applications, the intermetallic compound molybdenum disilicide, MoSi 2, has been the focus of many recent investigations. Plasma spraying has been demonstrated as a viable technique for producing dense monolithic and composite forms of MoSi 2. It has been reported that in the plasma spraying process, in which the melting of powders and the rapid solidification are involved, the metastable hexagonal structure of β-MoSi 2 is the major phase observed in the as-sprayed molybdenum disilicide deposit and/or coating. This study examines the formation of β-MoSi 2 in terms of the classical nucleation theory. It is concluded that the critical free energy for nucleation of β-MoSi 2 is lower than that of the stable phase of α-MoSi 2 on solidification and thereby β-MoSi 2 will be the prevailing phase formed by the homogeneous nucleation mechanism from liquid droplets of a certain undercooling, resulting from the rapid solidification rate.

Refractive indices of Zn-doped lithium niobate

We measured the index of refraction n( λ) of Zn-doped LiNbO 3 grown from a congruent melt as a function of the Zn concentration in the crystal. The Zn concentration c Zn in a sample was determined by electron microprobe technique. The refractive index n( λ, c Zn) is described by a generalized Sellmeier equation for 400 < λ < 1200 nm and c Zn up to 8 mol% ZnO. Calculated phase-matching conditions of nonlinear effects like second-harmonic generation are in excellent agreement with experimental results obtained with the same set of crystals.

The influence of Mg2 , Gd3 and Ta5 admixtures on cation structural ordering in lithium niobate single crystals

The influence of magnesium, gadolinium and tantalum admixtures in the processes of ordering and disordering of lithium niobate single crystals structure has been investigated. We selected the above admixtures based on the fact that the locations they occupy when introduced into the crystal lattice are non-identical in terms of crystallochemistry. Admixture concentrations in the single crystals varied in the limits [Mg2+]—0.01-0.8% mass, [Gd3+]—0.001-0.5% mass, [Ta5+]—1-5% mass. Single crystals were grown using the Czochralski method from a congruent melt. Properties of crystals obtained were studied by Raman spectroscopy, by microprobe electron analysis, by selective etching and optical microscopy. Threshold concentration of magnesium (∼0.3% mass) and gadolinium (∼0.5% mass) were shown to exist, beneath which level the doping element influence is of an ordering character while above it their action is a disordering one. For tantalum, the disordering influence is more characteristic. Crystals doped with tantalum exhibit a noticeable heterogeneity of composition. This results in the formation of a developed microdomain structure.

Increased Optical Damage Resistance and Transparency in MgO-Doped LiTaO3 Single Crystals

Clear and transparent 2-inch diameter undoped and MgO-doped LiTaO3 single crystals with few Fe impurities have been grown by the conventional Czochralski method from a congruent melt. The optical damage resistance of undoped LiTaO3 has been characterized by the measurement of photoinduced birefringence change and is lower than that of commercially available LiNbO3, which is not consistent with the previously reported data in the literature. In addition, doping of a few mol% MgO further improved the optical damage resistance, transmission in the visible light region, and crystal decolorization, and produced a favorable shift of absorption edge toward shorter wavelengths for lithium tantalate. These are important advantages when considering shorter- wavelength accessibility and high conversion efficiency in second-harmonic- generation devices.

Analysis of optically detected compositional inhomogeneities in Czochralski-grown LiNbO 3

During the analysis of birefringence inhomogeneities due to axially compositional variations of Li 2O in Czochralski-grown LiNbO 3 crystals, compositional inhomogeneities were revealed which could not be explained by an axial distribution from a well-mixed melt following Pfann's segregation. These inhomogeneities in the first-to-freeze region, crystallized fraction g<0.1, locally resolved observable by orthoscopy, were also found in LiNbO 3 crystals grown from a melt with a well-established congruent melt composition (±0.005 mol% Li 2O). The observed δ n e inhomogeneities showed two typical features: the existence of a Li 2O-rich skin and the formation of a local maximum of extraordinary refractive index n e at 0.045⩽ g⩽0.065. The first is due to Li 2O indiffusion of preferentially evaporated Li 2O from the melt and the second is due to build-up of an axial Li 2O concentration profile in the melt. From calculations based on a simple model, it can be shown that this concentration profile is due to the incongruent evaporation of Li 2O from the melt and the separation and interaction of different convective stream modes in the melt: a surface near convection mode due to the axially thermal gradient near the melt surface and the Marangoni convection — and the thermal convection in the bulk of the melt. The compositional inhomogeneity is axially limited by the change of the convective stream pattern due to dominating forced convection in the melt.