Congruently Melting Composition Research Articles

Influence of Ultrahigh Dilution Treatment of the Charge on the Growth and Spectroscopic Properties of Nd:MgMoO4 Potential Laser Crystal

The influence of the charge treatment by ultrahigh dilution (UHD) technology on oxide single crystals grown by the Czochralski technique was studied for monoclinic MgMoO4 crystals doped by 1 at. % of Nd3+ ions. The series of 10 Nd:MgMoO4 crystals was grown from the charges that were subjected to UHD treatment, as well as from the charges treated with two types of control or with no special treatment at all. The grown crystals were studied by X-ray powder diffraction analysis, inductively coupled plasma atomic emission spectroscopy, mass-spectrometry, optical absorption, emission spectroscopy and luminescence kinetic analysis. We found that: (i) wetting of MgO + MoO3 mixture by a water-ethanol solution before calcining leads to some enrichment of the mixture with MoO3, whereas the wetting of the charge after the calcining leads to some enrichment of it with MgO; (ii) congruent melting composition of MgMoO4 crystal is in the field of some MoO3 excess; (iii) the solid-phase solubility of the excess MoO3 in MgMoO4 probably does not depend on temperature, whereas the solid-phase solubility of the excess MgO in MgMoO4 crystal depends on temperature. We suggest that the corresponding solidus line passes through the range of retrograde solubility; (iv) the crystals grown within this range are characterized by the enhanced Nd3+ segregation coefficient between the crystal and the melt (approximately 0.006 versus 0.004); (v) unit cell parameters of MgMoO4 crystal with the excess of MoO3 are larger than those of the crystal of the stoichiometric composition and of the crystal with the excess of MgO; (vi) the shapes of the optical absorption and luminescence spectra of Nd:MgMoO4 crystal do not depend on the charge treatment; (vii) luminescence decay kinetics are single-exponential for all the studied crystals, the luminescence decay time being different for the crystals grown from the charges that underwent different types of treatment; (viii) the luminescence intensity of Nd:MgMoO4 crystal grown from the charge that underwent UHD treatment before calcining (solid-phase synthesis) is reduced by an order of magnitude in comparison with the other studied crystals.

Refinement of the Congruently Melting Composition of Nonstoichiometric Fluorite Crystals Ca1-xYxF2x (x = 0.01–0.14)

The concentration series of nonstoichiometric crystals Ca1–xYxF2+x (x = 0.01–0.14) was obtained from a melt by directional crystallization to refine the composition of the temperature maximum on the melting curves. A precision (±9 × 10−5 Å) determination of lattice parameters of the Ca1–xYxF2+x crystals with the structure of fluorite (sp. gr. Fm-3m) was performed, and a linear equation of their concentration dependence was calculated: a(x) = 5.46385(5) + 0.1999(4) x. The distribution of yttrium along the crystals Ca1–xYxF2+x, the content of which is determined by the precision lattice parameters, is studied. The congruently melting composition x = 0.105(5) of the Ca1–xYxF2+x phase is refined by the method of directional crystallization.

Growth and the Actual Compositions of Cation‐Deficient Sodium‐Gadolinium Molybdate Single Crystals

AbstractThe concentration series of sodium‐gadolinium molybdate (NGM) crystals have been grown by Czochralski technique from stoichiometric Na0,5Gd0,5MoO4 melt, and from seven different nonstoichiometric melts in various atmospheres. The actual cation compositions of the crystals are measured by microprobe analysis. The location of the congruently melting composition is found for this compound, and the homogeneity field is evaluated. The unit cell parameters of the crystals have been calculated from X‐ray diffraction (XRD) analysis results. Hydrostatic densities of the crystals are measured. Based on these results, the actual molar weights of the samples are calculated that allows more precise determination of the crystals compositions. The studied crystals are shown to contain substantial sodium deficiency and excess of gadolinium compared to stoichiometry. The amount of cation vacancies occurs in both sublattices along with increase of Gd2O3/Na2O molar ratio in the crystals.

Structure and real composition of undoped and Cr- and Ni-doped Sr0.61Ba0.39Nb2O6 single crystals

Strontium barium niobate crystals with congruent melting composition Sr0.61Ba0.39Nb2O6 (SBN-61), both nominally pure and doped with Cr3+ и Ni3+ ions, have been investigated by neutron diffraction. Different strontium and barium contents as well as their different distribution over the Sr1, of Sr2 and Ba2 crystallographic sites of SBN-61 structure, caused by introduction of dopants, have been revealed. Coordination polyhedra of cations have been established based on the analysis of cation–anion internuclear distances together with the calculation of bond-valence sums for cations, which are equal to their formal charge. It was found that the Nb1 and Nb2 atoms are located in distorted octahedra with quadfurcated (the Nb1O6 polyhedron) or bifurcated (the Nb2O6 polyhedron) vertices, and the Sr1 atoms are located in a cuboctahedron with bifurcated vertices in the base plane. Different polyhedra have been revealed for the Sr2 and Ba2 atoms: Sr2 atoms are coordinated by 15 oxygen atoms to form a highly distorted five-capped pentagonal prism, whereas Ba2 atoms are located in a highly distorted three-capped trigonal prism with a coordination number 9. Comparison of interatomic and internuclear distances, determined by X-ray and neutron diffraction analyses, respectively, allowed to reveal a highly pronounced shift of electron density in Nb1 and Sr2 polyhedra, responsible for the covalent bond and properties of crystals. Location of Cr3+ и Ni3+ dopant ions in the SBN-61 structure as well as their formal charges has been discussed.

Optical nonlinearities of small polarons in lithium niobate

An overview of optical nonlinearities of small bound polarons is given, which can occur in the congruently melting composition of LiNbO3. Such polarons decisively influence the linear and nonlinear optical performance of this material that is important for the field of optics and photonics. On the basis of an elementary phenomenological approach, the localization of carriers in a periodic lattice with intrinsic defects is introduced. It is applied to describe the binding energies of four electron and hole small polarons in LiNbO3: small free NbNb4+ polarons, small bound NbLi4+ polarons, small bound NbLi4+:NbNb4+ bipolarons, and small bound O− hole polarons. For the understanding of their linear interaction with light, an optically induced transfer between nearest-neighboring polaronic sites is assumed. It reveals spectrally well separated optical absorption features in the visible and near-infrared spectral range, their small polaron peak energies and lineshapes. Nonlinear interaction of light is assigned to the optical formation of short-lived small polarons as a result of carrier excitation by means of band-to-band transitions. It is accompanied by the appearance of a transient absorption being spectrally constituted by the individual fingerprints of the small polarons involved. The relaxation dynamics of the transients is thermally activated and characterized phenomenologically by a stretched exponential behavior, according to incoherent 3D small polaron hopping between regular and defect sites of the crystal lattice. It is shown that the analysis of the dynamics is a useful tool for revealing the recombination processes between small polarons of different charge. Nonlinear interaction of small polarons with light furthermore results in changes of the index of refraction. Besides its causal relation to the transients via Kramers-Kronig relation, pronounced index changes may occur due to optically generated electric fields modulating the index of refraction via the linear electro-optic effect, also. Based on a microscopic picture and by considering the local structural environment of bound polarons, the appearance of photovoltaic currents is explained straightforwardly as a result of the optically induced carrier transfer. Both transient absorption and index changes are spatially confined to the intensity profile of the interacting light allowing for the recording of efficient mixed absorption and phase volume holograms. By means of holographic spectroscopy, these small-polaron based optical nonlinearities are verified either without or with the action of the linear electro-optic effect; their prominent features are highlighted by appropriate experimental studies wherin the ultrafast response on the picosecond time scale is the most recognized one. Based on these findings, the consequences for applications of LiNbO3 in the field of nonlinear optics and photonics are presented. Besides visionary examples like real-time, 3D holographic displays, the impact of optical nonlinearities of small polarons for present applications are discussed with frequency conversion and respective limiting effects, such as green-induced infrared absorption and optical damage, as important example.

Growth, Structural Considerations, and Characterization of Ce-Doped (La,Gd)2Si2O7 Scintillating Crystals

Ce-doped lantanium-gadolinium pyrosilicate (La,Ce,Gd)2Si2O7 (Ce:La-GPS), crystals with various content of rare-earth elements were produced from the melt, and their optimal La/Gd ratio was examined. It was found that Ce:La-GPS single crystals of acceptable optical quality can be produced from the melts ranging from La0.5Gd1.5Si2O7 to La1.0Gd1.0Si2O7 and containing about 1 atom % Ce with respect to the host rare-earths of La and Gd. The crystal growth was performed by the micro-pulling-down and Czochralski methods. The crystals were chemically uniform along the growth axis, and their composition was equal to that of the melt, thus corresponding to vicinity of congruent melting composition. Spatial distribution of Ce in La-GPS was also inspected, and no variation of Ce content was detected as a result of its similarity to one of the host cations (La) regarding the size. Basic optical and scintillation properties of the Ce:La-GPS crystals are also reported, and it is demonstrated that partial substitution of...

AgBiS2 single crystal grown using slow cooling method and its characterization

Silver bismuth sulfide (AgBiS2) single crystal was successfully grown using the slow cooling method. Characterization of a grown crystal using power X-ray diffraction (XRD) revealed a single AgBiS2 phase and a cubic structure with lattice parameter a=5.641Å. The crystal was found to have a high-temperature phase with a cubic structure. XRD measurement and composition analysis showed that the stoichiometric composition of the AgBiS2 was equivalent to the congruent melt composition. The grown AgBiS2 crystal was confirmed to be single crystal by observing the Laue X-ray backscattering pattern. Differential thermal analysis showed that the melting point of the AgBiS2 was 805°C. The grown crystal had n-type conductivity, a carrier concentration of 2.6×1018cm−3, electrical resistivity of 2.1Ωcm, and hole mobility of 1.1cm2/Vs at room temperature.

SnS crystal grown using horizontal gradient freeze method and its electrical properties

Tin sulfide (SnS) single crystal was successfully grown using the horizontal gradient freeze method. Characterization of the grown crystal revealed a single SnS phase and an orthorhombic structure with lattice parameters a=4.327, b=11.18, and c=3.981Å. Composition analysis revealed a stoichiometric composition equivalent to a congruent melt composition. The grown SnS crystal was confirmed to be single crystal by the X-ray Laue pattern. The melting point of the SnS was measured by differential thermal analysis and found to be 878°C. The grown crystal had p-type conductivity, a carrier concentration of 8×1017cm−3, electrical resistivity of 0.08Ωcm, and hole mobility perpendicular to the (010) direction of 98cm2/Vs at room temperature.

Ellipsometric study of near band gap optical properties of SrxBa1−xNb2O6 crystals

Complex refractive indices N=n+ik for a-cut SrxBa1−xNb2O6 single crystals with four nominal values of composition x=0.40; 0.50; 0.61; 0.75 were studied by spectroscopic ellipsometry in the photon energy range 2–5eV of crystal transparency and fundamental electronic absorption. The ellipsometric data were evaluated taking into account an uniaxial anisotropy and an unintentional surface roughness. We use a global composition dependent Sellmeier fit as a reference for monitoring the crystals composition. The optical model yields indirect and direct optical band gap energies Eg in the spectral range 3.77–4.03eV and 4.28–4.55eV, respectively, depending on the Sr/(Ba+Sr) content ratio x and light polarization. The congruently melting composition Sr0.61Ba0.39Nb2O6 shows distinct dispersion features in the imaginary part of the UV complex refractive index with respect to other compositions.

Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements

This work revisits the melting behaviour of neptunium dioxide, an actinide compound which can be produced in the nuclear fuel during operation, and which has an important impact on the nuclear fuel and waste radioactivity especially on the very long term. The present experimental approach employs remote laser heating under controlled atmosphere and fast pyrometry. This technique circumvents problems encountered by more traditional heating techniques, in particular, the reaction between sample and containment at temperatures beyond 2500 K. In addition, only a small amount of sample material is required, which is an advantage with respect to the radioactivity and limited availability of neptunium. The NpO2 melting/freezing temperature has been measured to be 3070 K ± 62 K, much higher than previous values (around 2830 K) obtained by more traditional thermal analysis methods. The large amount of experimental data collected allowed a consistent statistical analysis. It seems likely, although not fully evident from the present results, that the high oxygen potential at temperatures around melting leads to a slightly hypo-stoichiometric congruent melting composition, as already observed in other actinide (ThO2, PuO2) and lanthanide oxides (e.g., CeO2). Finally, a recently developed phase-field model was used for the simulation of the observed thermograms, allowing a deeper insight in material properties that are difficult to directly measure. For example, a polaron contribution to the high-temperature thermal conductivity, well accepted for the commonly studied actinide oxide UO2, is shown here to likely be present in NpO2.

Electronic and Optical Properties of Strontium Barium Niobate Single Crystals

Electronic structure of Sr x Ba1- x Nb2O6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr0.61Ba0.39Nb2O6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function ϵ(E) compared to the non congruently melting compositions of Sr x Ba1- x Nb2O6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr0.60Ba0.40Nb2O6 and LiNbO3 crystals in the energy range of 6–8 eV and the corresponding dielectric functions ϵ(E) in the photon energy range of 3–10 eV have been found to be similar, what is caused by similar octahedral NbO6 complexes in the crystals.

The temperature dependence of the band edge energy in calcium barium niobate

AbstractThe temperature dependent band edge energy of the novel tungsten bronze type calcium barium niobate (CBN) is measured over a temperature range from 80 K to 573 K. CBN has been under investigation for serveral years and is believed to be a promising material for future applications. Czochralski‐grown, colorless single crystals from the congruently melting composition with a calcium content of 0.281 were used. A numerical fit has been applied, comparing the results of CBN to those of the well known relaxor ferroelectric strontium barium niobate (SBN). The band edge energy of CBN was found to be higher than in SBN over the whole temperature range. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Axial stoichiometry variations in lithium niobate crystals grown from near-congruent melts

Eleven lithium niobate crystals were grown using the Czochralski method from melts deviating from the congruent melting composition and the resulting axial variation of the crystal composition was analyzed using Curie temperature measurements. The weight of the melts was 7 kg, and grown crystals had solidifying mass ratios in excess of 90%. A mathematical model to predict axial compositional variations in the crystals was developed and integrated numerically to estimate the value of the effective distribution coefficient. A linear relationship between the effective distribution coefficient and the melt composition accurately describes the solid compositions of crystals grown from melts ranging from 45.1% to 51.7% Li 2O. It is shown that for growth conditions suitable for volume production and large crystal diameters, the difference between the effective and equilibrium distribution coefficient is well below 0.2%.

Spectroscopy ofEu3+ions in congruent strontium barium niobate crystals

The optical properties of single crystals of strontium barium niobate, grown from the congruently melting composition and doped with different amounts of europium, are investigated. Absorption and emission spectra are measured in the visible and infrared spectral regions at different temperatures. From the spectra, conclusions on the structural sites can be drawn, revealing the major occupations of the B1 and B2 sites, and a minor one of the A2 site. The data are used to calculate crystal-field parameters for the mainly occupied site. A Judd--Ofelt analysis shows that the radiative quantum efficiency is approximately 70%. Furthermore, the influence of europium doping on the phase-transition temperature is determined.

Electric current induced compositional variation in LiNbO 3 fiber crystal grown by a micro-pulling down method

The electric-field modified partitioning of Li ion species was studied for the growth of LiNbO 3 fiber crystals via a micro-pulling down (μ-PD) method. DC power was applied between the platinum seed and the outlet of a capillary pipe attached to the bottom of the crucible so as to generate a constant electric current. The electric current, passing through the molten zone, the solid–liquid interface and the crystal, induced an electric field near the interface which influences the transport and segregation of ionic species in the melt. The compositional change in terms of mol% Li 2O was examined by measuring the Curie temperature of crystals grown from melts with various compositions. The melt with Li 2O concentration larger than the congruent-melting composition ( k 0 Li 2 O < 1 ) decreased its Li 2O content near the interface by an electric current flowing from the crystal to the melt, while it did not change by the inverse current. In contrast, the melt with Li 2O concentration smaller than the congruent composition ( k 0 Li 2 O > 1 ) increased its Li 2O content by the electric current from the melt to the crystal, while it did not change by the inverse current. The above variation was peculiar to growth via a μ-PD method under an electric field induced by an imposed electric current, which confirmed that the major ionic species driven by the electric field was the Li ion species resulting in a change in the bulk Li 2O concentration both in the melt near the interface and in the grown crystal.

Crystal growth and characterization of [formula omitted] (CBN) in the composition range of [formula omitted

Transparent and nearly colorless Ca x Ba 1 - x Nb 2 O 6 (CBN) single crystals with five different compositions in the range 0.22 ⩽ x Ca ⩽ 0.35 have been grown by the Czochralski method. An improved crystalline quality could be achieved by adjusting optimized thermal conditions and by using an accurately oriented [0 0 1] seed. The congruent melting composition was calculated to be located at about 28.1 Ca mol fraction by correlating the initial melt compositions and the crystal compositions. The analysis showed that the distribution coefficients on both sides of the congruent melting composition were close to unity. The density of CBN, measured by the buoyancy method, is a linear function of the composition. On the other side, only lattice parameter c 0 is markedly influenced by the variation of the Ca content, whereas a 0 shows only minor changes.

Induced emission cross sections of near-stoichiometric LiNbO3:Mg, Nd

Optical absorption and emission spectra are measured on Nd3+ ion in near-stoichiometric LiNbO3 single crystals codoped with MgO. From the decomposition of the nonpolarized spectra, the Stark splitting scheme is established for 2wt% Nd3+ ion in the near-stoichiometric LiNbO3:Mg, Nd system. The induced emission cross sections are calculated to be 2.70×10−19 and 5.57×10−20cm2 for the π and σ polarizations, respectively, based on the decomposition of the polarized spectra. These values agree well with the reported values of 1.8×10−19 and 5.1×10−20cm2 for the π and σ polarizations of the conventional congruent-melt composition crystal through the branching ratio of emission spectra despite the different approach of calculations and difference in the near-stoichiometric and congruent-melt compositions. This fact shows that the slight difference in the host composition does not affect the cross section. The population inversion threshold is calculated to be 2.0×1016 for the Nd(2wt%) near-stoichiometric LiNbO3:Mg, Nd in the π polarization, the value of which is almost the same as that of 1.5×1016 for Nd:Y3Al5O12, one of the representative laser materials.

Growth and properties of Ru-doped lithium niobate crystal

Ruthenium (Ru)-doped lithium niobate (LiNbO 3) single crystals were grown by the Czochralski method from a congruent melt composition. The color of the Ru: LiNbO 3 was red and darkened with increasing Ru concentration. The Ru concentration in the grown crystal gradually decreased along the pulling direction because the effective segregation coefficient of Ru in lithium niobate is greater than one, and also because RuO 2 evaporated during the crystal growth period. However, a solubility limit does exist in Ru: LiNbO 3 crystals, so the maximum amount of Ru doped into LiNbO 3 single crystals is about 0.2 mol%. In addition, the lattice constants of Ru: LiNbO 3 on the A- and C-axis decreased with the concentration of Ru in the crystal. An absorption spectra examination of Ru: LiNbO 3 showed that there were two absorption peaks around 370 and 530 nm that is within the UV/VIS region. The absorption coefficients should increase, and the absorption edges shift toward longer wavelength as the Ru concentration increases. The OH − absorption spectra confirm that Ru ions may have substituted for both Li vacancies (V − Li) and antisite Nb ions (Nb 4+ Li) as the Ru doping concentration in the present case is well below the threshold limit.

Crystal growth of Sc-doped near-stoichiometric LiNbO 3 and its characteristics

Sc-doped near-stoichiometric LiNbO 3 (Sc:SLN) crystals were grown from Li-rich solutions with Sc concentrations of up to 1.3 mol% by a top-seeded solution-growth method. We investigated the Sc-doping effects on the effective distribution coefficient, photorefractive damage (optical damage), the OH-absorption peak position, transmittance spectra, the ordinary and extraordinary refractive indices ( n o and n e), and the electric field needed for domain inversion. The Sc:SLN crystals doped with a Sc-doping concentration of 0.4 mol% or more showed no photorefractive damage. This critical Sc-doping concentration in the SLN crystal was one-fifth that in a congruent-melt composition LN (CLN) crystal which requires at least 2 mol% Sc (1 mol% Sc 2O 3) to suppress photorefractive damage. The 0.4 mol% Sc doping needed to suppress photorefractive damage in the SLN crystal was also low compared to the 1 mol% required for Mg doping. Except for n o and n e, all of the measured properties were influenced by the Sc-doping effect.

Characterization of photorefractive BCT:Rh crystals at 1.06 µm by two-wave mixing

We present an experimental investigation of the photorefractive properties of rhodium-doped barium calcium titanate (BCT) crystals of the congruent melting composition Ba0.77Ca0.23TiO3. Considering the results previously obtained on these crystals in the visible region and at 850 nm wavelength, it should be a good alternative to BaTiO3:Rh. Besides, the rhodium doping has the effect of making this material sensitive up to the wavelength of 1.06 µm—a very technologically significant wavelength, because of the existence of high-power lasers. Therefore we present a study of rhodium-doped BCT crystals at 1.06 µm. These crystals are characterized by two-wave-mixing experiments. Two-beam coupling gain factors of up to 7.5 cm−1, with ordinary polarization and a response time of the order of 1 s at an incident power of 10 W cm−2, have been obtained. All properties are explained by a three-centre charge transport model, together with the existence of an iron impurity which masks the influence of rhodium at low doping concentrations.