Growth and luminescence of CsBr:Cu crystals

Special features of the incorporation of a copper impurity into CsBr crystals are studied for different types of impurities (metallic copper, CuO, and CuBr₂) and crystal-growth techniques (the Bridgman method or the method of isothermal evaporation from solution). The optical characteristics of these crystals (absorption, photoluminescence, and photostimulated-luminescence spectra) are investigated. The copper impurity is shown to enter CsBr:Cu_met crystals in the monovalent state Cu⁺ and CsBr:CuO and CsBr:CuBr₂ crystals in the divalent state Cu²⁺. It is found that the CsBr crystals doped with Cu⁺ and Cu²⁺ ions exhibit intense photostimulated luminescence and can be used as storage phosphors for visualization of X-ray images.     

UV‐visible emission of (O2‐ ‐ F+) centres in KBr

Single crystals of KBr containing the (O^2‐ ‐ F⁺) color centres and elaborated in atmospheric air by the Czochralski method are studied. The measure of the UV‐visible emission spectra at 78 K present several emission bands situated between 340 and 800 nm. The photoluminescence excitation shows that these emissions are attributed to O^‐₂ and (O^2‐ ‐ F⁺) centres. This study show the new emission bands of (O^2‐ ‐ F⁺) centres which are situated in the UV‐visible spectral domain. The comparison between the emission spectra before and after the thermal treatment of samples shows clearly the transformation of O^‐₂ centers at high temperature to (O^2‐ ‐ F⁺) centres. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Complex study of the structural and optical homogeneity of lithium niobate crystals

Methods of Raman spectroscopy, laser conoscopy, optical microscopy, and electron spin resonance have been used to study the photorefractive properties and structural and optical homogeneity of the following lithium niobate (LiNbO₃) crystals: nominally pure crystals of congruent composition (LiNbO_3con); LiNbO₃:Cu[0.015 wt %] crystals grown from a melt of congruent composition and nominally pure crystals of stoichiometric composition grown from a melt with 58.6 mol % Li₂O (LiNbO_3st). A small deformation of optical indicatrix and regular microdomain structures of fractal type are revealed for the LiNbO₃:Cu[0.015 wt %]; the microdomain structures may be due to the nonuniform impurity incorporation into the structure. It is shown that oxygen octahedra in the LiNbO₃:Cu[0.015 wt %] crystal are deformed in comparison with the octahedra in LiNbO_3st and LiNbO_3con crystals and that the main and impurity cations are clusterized along the polar axis. It is established that the LiNbO₃:Cu[0.015 wt %] crystal exhibits photorefractive properties not only due to the presence of intrinsic defects with localized electrons, as in the case of LiNbO_3st, but also due to the charge exchange in copper cations (Cu²⁺ → Cu⁺) under illumination.     

Potassium and ammonium dihydrogen phosphates activated with thallium: Growth and luminescence and scintillation properties

The conditions for the growth of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals with concentrations of the thallium activator in the initial solution of 0.01, 0.1, and 1.0 wt % are investigated. It is shown that the character of incorporation and distribution of thallium in the KDP and ADP lattices is limited, apparently, by the difference in the ionic radii of K⁺, NH ₄ ⁺ , and Tl⁺ cations and the charge state of prismatic {100} and pyramidal {101} growth planes. Doping of KDP and ADP with thallium (to 0.1 and 1.0 wt %, respectively) does not deteriorate the structural quality of these crystals. The dependence of the lattice parameters a and c on the thallium impurity concentration is investigated. The absorption bands of thallium in the KDP:Tl⁺ and ADP:Tl⁺ crystals peak at 218 and 215 nm, respectively, while the photoluminescence band peaks at 280 nm for both types of crystals. The relative light yield upon excitation of scintillations by α particles (Pu²³⁹) and β particles (Bi²⁰⁷) is measured.     

Color centers and charge state change in Ce:YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F⁺-type color center. Charge state change processes of Fe³⁺ impurity and Ce³⁺ ions take place in the irradiation process. The variations of Ce³⁺ ions concentration clearly indicate that Ce⁴⁺ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce³⁺ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals.     

An EPR Study of Scapolite

In single crystals of scapolite from two different localities, three paramagnetic centres are detected by electron paramagnetic resonance (EPR): 1. One isotropic singlet with g_iso = 2.005; 2. One triclinic singlet with g‖ = 2.005 ± 0.001 and g⟂= 2.009; 3. One triclinic sextet with g‖= 2.005 ± 0.001, g⟂ = 2.011; A‖ = 85.4 × 10⁻⁴ cm⁻⁴, A⟂ = 85.3 × 10⁻⁴ cm⁻¹. Centres 1 and 2 can be attributed to colour centres as they are bleached after annealing. Centre 3 can be due to Mn²⁺ (only the central M_s = ± 1/2 transition is observable) most likely substituting for Ca²⁺ The site symmetry must be triclinic but due to Al, Si disorder and mixed Na, Ca composition the different components from magnetically non-equivalent sites are averaged out for many orientations.     

Investigation of the localization of H+ ions in stoichiometric LiNbO3

The possible positions of H⁺ ions in stoichiometric crystals of lithium niobate are analyzed using computer simulation. It is shown that all hypothetical positions of H⁺ ions at the midpoints of the O-O bonds are unstable and must be excluded from further consideration. An analysis of the trajectories of displacement of H⁺ ions in the structure of LiNbO₃ shows that an H⁺ ion is associated with one of the O^2? ions of the upper oxygen face of the NbO₆ octahedron and that the direction of the dipole moment of the OH^? group differs only slightly from the direction of the short O-O bond.     

Dye centres in magnesium doped corundum monocrystal

Results of absorption spectra studies in Mg doped Al₂O₃ monocrystals are described. It is shown that due to Mg doping dye centres appear together with the associated additional absorption bands with maxima at 585, 475 and 260 nm. Kinetics of the formation (destruction) of these centres) of these centres in the process of thermal treatment is considered. An assumption on the nature of dye centres and the related absorption bands is proposed together with an evaluation of oscillator strength. Dependences of diffusion coefficients of the subject centres on thermal treatment temperatures are derived.     

Internal stress in doped NH<Subscript>4</Subscript>Cl:Mn<Superscript>2+</Superscript> crystals

The relationship of the effect of impurity on crystal growth and morphology, along with the internal stress and anomalous birefringence arising upon impurity trapping by a growing crystal, is considered. The NH₄Cl-MnCl₂-H₂O-CONH₃ model system and the heterostructural NH₄Cl:Mn²⁺ crystals formed in it are experimentally studied. It is found that up to 6.63 wt % Mn²⁺ impurity is effectively captured by growing NH₄Cl crystals at an impurity trapping coefficient only slightly below unity. The effect of Mn impurity stabilizes the full-face growth of NH₄Cl crystals with a rhombododecahedral habit in aqueous solutions and a cubic habit in water-formamide solutions. The trapping of manganese impurity by ammonium chloride crystals causes high internal stress (up to 4 GPa) in them, which manifests itself in the form of anomalous birefringence and leads to splitting, twinning, and cracking in NH₄Cl crystals.     

Stoichiometry and point defects in lithium niobate crystals

Investigations of the OH⁻ ion incorporation into LiNbO₃ crystals and the comparison of calculated and measured densities show that undoped congruent lithium niobate crystals contain Li⁺ and O^{− −} ion vavancies O and Li⁺. If really present, stacking faults are of inferior importance. The good agreement of the absorption edges of stoichiometric and of 2.7 mol% MgO containing congruent lithium niobate crystals is explained by the occupance of all oxygen sites within these crystals.     

Growth and optical characteristics of Ce-doped and Ce : Na-codoped BaLiF3 single crystals

Ce-doped and Ce : Na-codoped BaLiF₃ single crystals were grown by the Czochralski technique under reactive atmosphere. Na⁺-ions, used as a charge-compensating impurity, compete with Ce³⁺-ions for Ba²⁺ sites in the host, reducing the Ce³⁺ incorporation and degrading the mechanical integrity of crystals. Ce-doped BaLiF₃ showed potential for UV tunable lasers.     

Low‐Temperature Raman Scattering Spectra of GaSexS1‐x Layered Mixed Crystals

Raman scattering has been used to study the vibrational spectra of GaSe_xS_1‐x layered mixed crystals at 10 K. We report the frequency dependencies of different modes on composition x, with particular emphasis on A′₁⁽²⁾ (A_1g¹) and A′₁⁽⁴⁾ (A_1g²) intralayer compressional modes having low dispersion in the Brillouin zone. The appearance of additional bands is attributed to multimode behavior typically exhibited by mixed crystals of anisotropic compounds.     

Growth, structure, and properties of KTiOPO4 crystals doped with iron

A series of iron-doped KTiOPO₄ (KTP: Fe) single crystals in which iron substitutes for 0.1–0.3% titanium was grown. The structure of the KTP: Fe crystals was determined, and their dielectric and conducting properties were studied. An X-ray diffraction analysis failed to reveal such asmall amount of Fe⁺³ ions in titanium octahedral positions of the structure. It was found that an increase in the iron concentration results in a lowering of the symmetry of Ti(1)O₆ and Ti(2)O₆ octahedra. The splitting of the dielectric anomaly due to the ferroelectric phase transition was explained by the mechanism of incorporation of an impurity into different growth pyramids of the crystals. It was established that the aging of the KTP: Fe crystals leads to changes in the permittivity and electrical conductivity during long storage.     

Crystal and molecular structures of potassium cobalt hydrogen phthalate K2[Co(H2O)6](C8H5O4)4 ⋅ 4H2O and mechanism of Co2+ impurity trapping in KAP crystals

The single-crystal X-ray diffraction analysis of K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O has been carried out. The K₂[Co(H₂O)₆](C₈H₅O₄)₄ ? 4H₂O single crystals are obtained in attempting to grow the KAP crystals with the maximum possible content of Co²⁺ impurity cations. The crystals are isostructural to the earlier-studied similar crystals with Ni(II). The structure is formed by double layers of biphthalate anions and the Co²⁺ and K⁺ cations in between. The Co²⁺ cations are coordinated only by water molecules, whereas the coordination of the K⁺ cations involves both the biphthalate anions and water molecules. A detailed crystal chemical analysis, together with the data on the growth kinetics of KAP crystals in the presence of Co²⁺ and the mass-spectrometric data obtained earlier for the KAP crystals, leads to the conclusion that the Co²⁺ impurity cations should be located in the form of the [Co(H₂O)₆]²⁺ cationic complexes in the interblock layers of the KAP crystals.     

Complex formation and background impurity of Oxygen in the PbTe:Eu doped crystals grown from melt by the Bridgman method

The magnetic field dependence of magnetization M(B) at the temperature 1.72 K in magnetic fields up to 5 T and the temperature dependence of magnetic susceptibility (M_S) χ(T) in the temperature range 1.7–400 K of six PbTe:Eu samples with the concentration of Eu impurity of the order of 1×10¹⁹–1×10²⁰ cm^?3, prepared from the doped crystals grown from the melt by the Bridgman method, have been investigated. It is shown that the dependence of M(B) and χ(T) can be quantitatively explained by the contribution of the single centers of Eu ions, their pairs, and the matrix of the doped crystals using the same set of parameters for each sample. This is true provided we use in our analysis the values of the exchange integrals between Eu ions in EuO normalized with the lattice constant of PbTe, i.e., J₁/k_B=0.056 K for the ferromagnetic interaction of the NN (nearest neighbor) pairs and J₂/k_B=?0.13 K for the antiferromagnetic interaction of the NNN (next nearest neighbor) pairs, as well as different values of the M_S of crystal χ_matrix. It is revealed that the probability of the formation of complexes based on the magnetic impurity pairs is higher in the incipient section of a doped ingot, and it decreases towards the ingot end where the single centers of Eu ions become the only centers of the impurity. We conclude that the pairs of Eu²⁺ ions, which are formed during the growth of the PbTe:Eu ingots from melt by the Bridgman method, are the constituents of the complexes of the magnetic impurities with the background Oxygen impurities in the crystal matrix of the doped lead telluride. It is shown that the formation of the complexes leads to an increase of the M_S of crystal matrix χ_matrix and can even cause the change of its sign from minus to plus, i.e., it can convert the crystal matrix from the diamagnetic to paramagnetic state. The possible causes of this effect are analyzed.     

Explanations of the optical absorption and ESR spectrum for tetragonal Ni+ centers in CuAlS2:Ni+ crystals

In this paper, the optical absorption and electron spin resonance (ESR) spectrum of Ni⁺‐doped CuAlS₂ crystals have been studied by using a double spin‐orbit (SO) coupling approximation model, where the effects due to the SO coupling of the central metal 3d¹ ion and those of ligands are included. From this model, the formulas of the ESR g factors g_‖, g_⊥ and hyperfine structure constants A_‖, A_⊥ for 3d¹ ions in the tetragonal MX₄ clusters are constructed. The optical absorption and ESR parameters for Cu⁺ sites of CuAlS₂ have been calculated. The results obtained show that Ni⁺ ions substitute for Cu⁺ ions sites. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

The role of hydration and complexing in the mechanism of impurity influence on crystal growth

The influence of impurities of chlorides and nitrates of divalent Ca²⁺ and Ba²⁺ cations on the kinetic growth of potassium dihydrophosphate KH₂PO₄ crystals at a saturation temperature of 323 K and relative supersaturation of 0.03 has been investigated experimentally. It is established that the impurity acts differently, depending on the face index, the impurity concentration, cation hydration, and the stability of the complexes formed in the solution by impurity salt ions. A model is proposed to explain the different influence of impurity ions on the growth of crystal faces. This influence is determined by the different hydration of cations and enhanced association of cations and anions of impurity salts in the surface layer with a lower dielectric constant.     

Micromorphology and defects of non-doped gallium arsenide layers

Electron microscopy was used to study non-doped GaAs layers obtained in a chloride system Ga-AsCl₃-H₂. Electron concentrations is 10¹³–10¹⁵ cm⁻³, mobility at liquid nitrogen temprature reaches 200000 cm²/v. sec. It is shown that a fraction of the growth surface taken with (110) faces is less for films with less electron concentration. The main type of defects in pure GaAs layers is precipitates at pinning centres of growth steps. Estimations based on microdiffraction patterns and irradiation effects show that the impurity concentration in them is 10¹⁶–10¹⁹ cm⁻³. The impurity in precipitates is assumed to be electrically non-active, mainly, in interstitial positions.     

Optical Properties of CuInSe2 Single Crystals Grown by the Vertical Bridgman Technique

The optical absorption near the fundamental edge of n-type CuInSe₂ single crystals was studied for samples having different impurity concentrations. It is found that with increasing impurity concentration the gap energy decreases whilst the tail absorption below the edge and its characteristic energy increase. It is concluded that band-gap narrowing due to high ionized impurity concentrations plays an important role in CuInSe₂ single crystals.