Effect of trace metals on reactive crystallization of gypsum

The effect of Fe²⁺, Fe³⁺, and Cr³⁺ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe²⁺, Fe³⁺, and Cr³⁺ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe³⁺ or Cr³⁺ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe³⁺ and Cr³⁺ ion concentrations. The change of morphology is related to the complex formation between Fe³⁺ or Cr³⁺ ions and citric acid at high ion concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental and theoretical study of recovery mechanism of impurity effect by the addition of EDTA

The impurity effect by trivalent metal ion such as Al³⁺, Fe³⁺ and Cr³⁺ during crystal growth of KDP is reasonably well documented. If a metal ion is adsorbed onto the crystal surface, it prevents the step propagation relevant to the crystal growth rate. However, this impurity adsorption mechanism is still not well understood. Recently, in our work on the addition of chelate agents, a recovery effect of the metal ion adsorption was discovered. However, its recovery mechanism is not clearly understood both theoretically and phenomenally. In this research, ethylene‐diamine‐tetra‐acetic acid, EDTA, which is the most common chelate agent, was used as a recovery agent. The recovery mechanism was considered from the correlation of experimental data and the interfacial distribution model that we proposed in our former study. Furthermore, quantum calculation of EDTA metal complex can explain the relaxation of impurity adsorption by the addition of EDTA. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparative characterization of defects formed by di-and trivalent metal dopants in KDP crystals and the structural mechanism of influence of impurities on face morphology

The structure of KDP crystals doped with trivalent (Al³⁺, Fe³⁺, Mn³⁺, V³⁺, and La³⁺) and divalent (Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) cations was simulated by minimizing the energy of atomic interactions. Three types of defects were revealed: isolated defect centers formed by M ³⁺ and Ni²⁺ ions, cluster chain centers formed by M ²⁺ ions with ionic radii exceeding 0.9 Å, and complex centers formed via the replacement of potassium ions by large Ba²⁺ dopants with the simultaneous replacement of some of the phosphorus atoms by silicon ones. The corresponding energies of defect formation are calculated. The surface morphology of the crystal faces is studied. The changes in morphology in the presence of M ³⁺ dopants are caused by their adsorption, whereas for M ²⁺ dopants, these changes are caused mainly by their incorporation into the crystal structure.     

Influence of La3+ ions on growth and NLO properties of KDP single crystals

Single crystals of pure and Lanthanum added KDP crystals were grown from aqueous solution. The influence of La³⁺ ions in KDP crystals is studied. Isolated centers are formed by La³⁺ ions in KDP structure along (100) plane. La³⁺ ions incorporated into superficial crystal growth layer and slightly affect the growth process as they generate weak lattice stresses. The creation of these weak lattice stress is confirmed by Vickers's micro hardness test. The HRXRD analysis showed reduced structural defects in the La added KDP in the (100) plane than pure KDP. The incorporation of La in the crystal was confirmed by EDAX analysis. The Kurtz's powder SHG efficiency was found to be 1.5 times that of pure KDP. The UV‐Vis transmission spectra of La added KDP showed excellent transmittance from 1100 nm to 340 nm and did not show any change in lower cutoff wavelength with respect to pure KDP. Laser damage threshold value has been determined using Q‐switched Nd:YAG laser operating at 1064 nm and with 65 ns pulses in single shot mode. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The combined effects of supersaturation and Ba2+ on the batch cooling crystallization of potassium dihydrogen phosphate

The combined effects of supersaturation and Ba²⁺ on potassium dihydrogen phosphate (KDP) were investigated in batch cooling suspension crystallization. Growth size, morphology, and impurity Ba²⁺ adsorbed in the KDP crystals were measured with changing Ba²⁺ concentration and supersaturation. Significant changes in shapes and volume of the grown crystals have been observed. The results further confirmed that the size and shape of crystals were greatly determined by supersaturation. Ba²⁺ ions significantly modified the growth habit of KDP crystals. The concentration of Ba²⁺ ions adsorbed in the crystals increases with the increasing Ba²⁺ ions in the solutions and supersaturation. The foggy phenomena caused by the addition of Ba to the KDP solution were also described. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and properties of Ba‐doped KDP crystals

KDP crystals were rapidly grown from solution doped with different Ba²⁺ concentrations. The effects of Ba²⁺ on the growth rate, morphology and quality of KDP crystals were discussed. Significant changes in shapes and volume of the grown crystals have been observed. During the growth process, defect region expands gradually with the increasing Ba²⁺ concentration. Samples were cut from different parts of the as‐grown crystals for investigating the optical quality, including transmission spectrum, scattering centers. Through comparison, it is found that the nonuniform distribution of Ba²⁺ ions causes remarkable difference in optical quality between prismatic and pyramidal sectors. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effect of metallic ions on tapering phenomenon of DKDP crystals and its elimination

The anomalous wedge‐like shape of KH₂PO₄‐type single crystal, also called tapering, is discussed in this paper. DKDP single crystals grew on seeds from the solutions containing high concentrations of Al³⁺, Fe³⁺ and Cr³⁺ impurities. The tapering phenomenon occurred on the prismatic {010} faces, and the tapering angle θ was nearly measured as 10°. We adjusted the pD value of the solution from 3.0 to 4.2 by adding K₂CO₃, the tapering angle of DKDP crystal grown from the same solution at same supercooling was reduced to 0°. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Nd3+ on the growth and quality of KDP crystals

KDP crystals were rapidly grown from solutions doped with different Nd³⁺ concentrations. During the growth process, “foggy” inclusions were selectively captured in the pyramidal sector of KDP crystal and hourglass shaped crystals were obtained. It is found that the nonuniform distribution of Nd³⁺ ions causes remarkable differences in optical quality between prismatic and pyramidal sectors. With increasing Nd³⁺ concentration, the optical quality is greatly decreased for pyramidal sectors, while the change is not so obvious for prismatic sectors. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Coarse crystal layer growth and liquid entrapment study with gradient freeze technology

The coarse crystal layer growth and liquid entrapment processes were investigated with gradient freeze technology in this paper. The research system was hemihydrate phosphoric acid (H₃PO₄·0.5H₂O) crystal‐phosphoric acid aqueous solution. The distribution coefficients of ions (Ca²⁺, Fe³⁺ and Na⁺) in this system were measured. The effect of supercooling degree gradient on layer growth and the effect layer growth rate on ions redistribution were studied. The result indicated that the layer growth rate increased with supercooling degree gradient as an exponential curve. The distribution coefficient tended to increase as an approximate ‘S’ curve when coarse crystal layer growth rate increased. The ‘three region’ theory was applied to explain this phenomenon. Each ion's diffusion parameter was obtained, which contributed to explain the separation differences between different ions. The work in this paper also indicated that layer crystallization was an effective separation technology for electronic grade phosphoric acid preparation.     

Characterization of defects generated by di-and trivalent cations in the potassium-dihydrophosphate structure and their influence on growth kinetics and face morphology

Different types of defect sites generated by the impurities of divalent (M²⁺) and trivalent (M³⁺) metals in the structure of potassium dihydrophosphate KH₂PO₄ (KDP) were revealed by crystal-chemical analysis and computer simulation. These sites cause different deformations of the crystal matrix by generating different local strains, which enhance the inhibiting effect of impurity atoms adsorbed on the surface. This fact accounts for the different influence of di-and trivalent cations on the growth kinetics and face morphology of KDP crystals. The effect of the M³⁺ ions is associated primarily with their adsorption on the face surfaces, whereas the influence of the M²⁺ ions results from their insertion into the surface layer of the crystal.     

Synergy of organic dyes for KDP crystal growth

Single potassium dihydrogen phosphate (KDP) crystals were grown in a supersaturated solution containing an organic dye (sunset yellow FCF, brilliant blue FCF, and sky blue). The growth rate, morphology, and impurity dye distribution of faces, (100) and (101) in a KDP crystal were measured as dye concentration and the supersatutation of KDP were changed. Complete expressions for the effect of dye on all aspects of the growth of KDP crystals were discussed. The growth rates of (100) and (101) faces were well correlated by the empirical equation, and resulted in good estimation of the morphology. The distribution of dye in a KDP crystal was represented by the distribution model containing the minimum growth rate for coloring. The growth rate equation and distribution equation were expressed by functions of the supersaturation and dye concentration, and they could effectively provide the operational conditions with coloring the KDP crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cr3+‐doped LiNbO3 crystals grown by the Bridgman method

The growth of LiNbO₃ crystals doped with Cr³⁺ ions in 0.1, 0.2, and 0.5 mol % concentrations by Bridgman method were reported. The Cr³⁺ ion concentrations in crystals were measured by inductively coupled plasma spectrometry. Electron paramagnetic resonance had been used to investigate the sites occupied by the Cr³⁺ ions. Two Cr³⁺ ion centers located at Li⁺ and Nb⁵⁺ sites (Cr_Li³⁺ and Cr_Nb³⁺ centers, respectively) were observed. Optical absorption and temperature‐dependence emission spectra of the Cr³⁺ ions were reported. The crystal‐field parameters and Racah parameters of the Cr³⁺ ion defect sites were reported and compared with those grown by Czochralski technique. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Intensities of circular dichroism and absorption bands and the states of <Emphasis Type="Italic">d</Emphasis> ions in absorbing media: II. Tetrahedrally coordinated positions of <Emphasis Type="Italic">d</Emphasis> ions

The factors affecting the band intensity in the circular dichroism (CD) and absorption spectra of tetrahedrally coordinated d ions in an absorbing medium (symmetry selection rules, structural position, and bond covalence) are analyzed. It is shown by the examples of the Cr⁴⁺ ion in Ca₃Ga₂Ge₄O₁₄ crystal and the Fe²⁺ and Fe³⁺ ions in SiO₄ crystal that the symmetry forbiddenness of the transitions from orbitally degenerate states and the covalence of the d-ion-ligand bond lead to changes in the intensity of the corresponding CD bands in a wide range, beginning from zero. It is shown by the example of Ca₃Ga₂Ge₄O₁₄, LiAlGeO₄, LiGaGeO₄, and LiGaSiO₄ crystals activated with Cr⁴⁺ ions that the preferred ion localization position corresponds to a higher effective symmetry.     

Single crystals growth and absorption spectra of Cr-doped Al2−xInx(WO4)3 solid solutions

The growth conditions of pure and Cr³⁺-doped Al_2−xIn_x(WO₄)₃ single crystals, using top-seeded solution growth (TSSG) technique, have been studied. A series of experiments have been performed at different In concentrations, x=0.0, 0.3, 0.6 and 1.0, as well as at different concentrations of Cr³⁺ (0.0, 0.1, 0.2, 0.5 and 1.0) in at% with respect to the initial total concentration of Al and In in the starting solutions. The basic parameters of the crystal growth are varied over a wide range: seed orientation, speed of rotation, axial and radial temperature differences in the solution and the solution cooling rate. The investigated relations between the basic defects in the crystals and these parameters result in determination of the optimal conditions for growth of defect-free crystals. Distribution coefficients of Al, In and Cr have been determined, so the growth of crystals with given compositions is possible. Values of Dq/B (crystal field strength) for the various crystal compositions are calculated from the optical absorption spectra. The calculated values show that the discussed solid solutions have weak crystal field and are suitable for media with broadband emission spectra.     

Simulation of a defect region in KDP crystals doped with divalent iron ions. Comparison of defects induced by di-and trivalent metals

The crystal structure of Fe^II-doped KDP crystals was simulated with the use of specially developed partly covalent potentials. Different variants of introduction of impurity into the structure were analyzed. The M1 position was found to be more favorable for both isolated divalent and trivalent metal ions. Upon optimization, the coordinates of the Fe^II ion are (0.25, 0.44, 0.125). The Fe^II ions can aggregate to form “clusters” energetically more favorable than isolated defects. It seems that Fe^III ions cannot form aggregates.     

State of Fe3+ ion and Fe3+ −F− interaction in calcium fluorosilicate glasses

The state of Fe³⁺ ions and Fe³⁺ ?F^? interaction in calcium fluorosilicate glasses xCaF₂·(1- x)CaO·SiO₂) (0 ≤ x ≤ 0.3) containing a small amount of iron were investigated by ESR spectroscopy. Two resonances observed near g = 2.0 and g = 4.3 were assigned to dipole-dipole interacted Fe³⁺ ions and Fe³⁺ ions in a rhombic crystal field, respectively. The fraction of Fe³⁺ ions in a rhombic crystal field decreased and that of dipole-dipole interacted Fe³⁺ ions increased with increasing Fe₂O₃ content. It was found that the quantity of dipole-dipole interacted Fe³⁺ ions depends on the negative partial charge of fluorine ions and shows a maximum at 10 mol% CaF₂ (x = 0.2). The maximum is attributed to the largest difference between absolute values of the ionic potentials of ferric and fluorine ions which is caused by the smallest negative partial charge of flourine at 10 mol% CaF₂.     

Spectroscopic properties of Cr3+ ions in nanocrystalline glass–ceramic composites

Luminescence properties of Cr³⁺ ions in a silica-based precursor glass, and in fabricated optically transparent glass-based nanocrystalline composites, have been investigated. The luminescence spectra of the precursor glass revealed a wide range of crystal fields and showed the ⁴T₂–⁴A₂ broadband emission of Cr³⁺ ions in a weak crystal field, combined with ²E–⁴A₂ emission characteristic for Cr³⁺ ions in a strong crystal field. Glass–ceramic nanocomposites, with gallium oxide nanocrystals nucleated in a host glass matrix, indicated the prevailing contribution of the crystal-like ²E–⁴A₂ emissions (R-lines) of Cr³⁺ ions in a strong crystal field. The low-temperature studies demonstrated that the fluorescence of Cr³⁺ ions could be altered from sharp R-lines of the ²E–⁴A₂ transition, below 70 K, to a combination of R-lines and their sidebands, above 70 K. Our results indicate that, in the developed glass–ceramic nanocomposites, most of the Cr³⁺ ions have migrated from the host glass matrix to the nucleated gallium oxide nanocrystalline phase.     

Recharging processes of Cr ions in Mg2SiO4 and Y3Al5O12 crystals under influence of annealing and γ ‐ irradiation

Recharging processes of chromium ions were investigated for Mg₂SiO₄:Mg, Cr single crystals using annealing in O₂ and in air and γ‐irradiation, as compare to YAG :Ca, Cr single crystals. The formation of tetravalent Cr ions in the Mg₂SiO₄ :Mg, Cr is related not only to the initial Cr content in the melt, oxygen partial pressure and O₂‐ vacancy existing in the crystal, but also to the external field such as γ‐irradiation. The additional absorption after γ‐irradiation shows the decrease in intensity of the absorption of Cr³⁺ and Cr⁴⁺ ions in some part of the spectrum and increase in the other giving evidence on recharging effects between Cr³⁺ and Cr⁴⁺. There arises also color centers observed between 380 nm and 570 nm that may participate in energy transfer of any excitation to Cr⁴⁺ giving rise to Cr⁴⁺ emission. Opposite to forsterite crystal, absorption spectrum of YAG:Ca, Cr crystal after γ‐irradiation reveals only increase in the absorption of the Cr bands. The observed behavior of the absorption spectrum of YAG:Ca, Cr crystal under influence of γ‐irradiation suggests that γ‐irradiation ionizes only Cr ions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron and X-ray diffraction study of superstructure and localized magnetic moments in Cu0.5Fe0.5Cr2S4 and Cu0.5In0.5Cr2S4 compounds

The superstructure parameters for the Cu_0.5Fe_0.5Cr₂S₄ and Cu_0.5In_0.5Cr₂S₄ compounds have been determined by neutron and X-ray diffraction. The localized magnetic moments in different sublattices measured for Cu_0.5Fe_0.5Cr₂S₄ are equal to 3.06 ± 0.17 μB for Fe³⁺ ions in the A-site and 2.76 ± 0.22 μB for Cr³⁺ ions in the B-site (Cu⁺ possess no magnetic moment), which are much less than the magnetic moments for the ions in the purely ionic state.     

Growth and Properties of BeO · 3 Al2O3 (BHA) Single Crystals

Large single crystal of BeO · 3 Al₂O₃ (beryllium hexa-aluminate or BHA) doped with chromium up to 20 mm in maximum diameter and 60 mm long were grown by Czochralski method. Thechnological conditions of crystal growth have been introduced and key factor leading to a successful growth is a adequate composition of starting charge mixture other than stoichiometry. The crystal structure, absorption spectra and fluorescence spectra of BHA: Cr³⁺ have been measured. Vibronic side band of the fluorescence spectra is from 700 nm to 900 nm.