Melt growth of highly pure CdBr2 crystals and study of their polytypism

Very high purity single crystals have been successfully grown by the method of vertical zone refining. The problem of excessive evaporation of the material, confronted by the earlier workers, has been successfully overcome. The purity of the crystals has been tested by D. C. conductivity measurements and further confirmed by U. V. spectroscopy. X-ray diffraction has been employed for the characterization of the crystals. The crystals have been found to crystallize exclusively in 6R polytypic modifications, without any arcing and streaking on their X-ray photographs. The results have been discussed in relation to the effect of impurities on polytype formation.     

Phase transformations in PbI2 crystals with temperature

A systematic study of phase transformation with temperature has been undertaken in single crystals of lead iodide grown in gel. The crystals of the polytype 2H, which is known to be the common modification of PbI₂ at room temperature, have been finally found to transform into polytype 12R after heating at 150°C. During the intermediate period of heating the crystals show features of disorder, viz. streaking or arcing or both, on their X-ray diffraction photographs. The higher polytypes 12H and 16H do not show any change even after prolonged heating at 150°C. If silver iodide is added in a minute quantity during crystallization, the process of phase transformation is appreciably altered. The results have been discussed.     

Electrical conductivity in undoped and Mn2+-doped NaNO2 single crystals

Electrical conductivity studies in NaNO₂ single crystals with inherent impurities and also in crystals with added Mn²⁺ impurities have been reported. The heating conductivity runs of undoped and doped NaNO₂ crystals have been compared. The decrease in conductivity in cooling following a heating run has been attributed to the oxidation during heating leading to the bulk precipitation of impurities in the host. Above 170°C however the intrinsic defects are responsible for conduction. An anomaly is noticed in both the heating and cooling conductivity runs of the sample at about the Curie temperatures and has been found to show thermal hysteresis.     

Optical properties of the surface transition-metal structures in fluoride ionic crystals and peculiarities of their formation during thermal diffusion

The possibility of forming surface films with an elevated concentration of an impurity metal during high-temperature diffusion has been analyzed for a wide series of ionic crystals: LiF with Co, Ni, Mg, Ca, Ba, and Sr impurities; NaF with Co, Mn, Mg, Ca, and Sr; MgF₂ with Co and Ni; and CaF₂ with Co. It is established that films are formed only on alkali halide crystals with impurities of transition metals and are not formed on alkaline earth fluorides with transition metals, as well as on alkali halide crystals activated with other divalent cationic impurities. The dynamics of the increase and decrease in the intensity of centers related to impurity-vacancy dipoles during thermal diffusion is shown. The mechanisms of film formation are explained in terms of the features of growth and structure of ionic crystals with cationic impurities and on the basis of isomorphism rules.     

Growth and scintillation properties of doped ZnWO4 crystals

A series of ZnWO₄ crystals with different purities of WO₃ and ZnO were grown by conventional Czochralski method and compared with Fe:ZnWO₄, Nb:ZnWO₄, Ag:ZnWO₄ and Ce:ZnWO₄ crystals grown by the same technology. Scintillation experiments revealed that the light yield of undoped ZnWO₄ crystals varied with the purities of raw materials WO₃ and ZnO. Raising the concentration of iron ions in ZnWO₄ crystals resulted in reduction of the light yield from the lighter appearance. On the contrary, the light yield was enhanced by doping Ag₂O, Nb₂O₅ and CeO₂, equivalent to that observed in ZnWO₄ crystals after oxygen-rich annealing at high temperature. The mechanisms of coloration and decolorization in doped ZnWO₄ crystals were analyzed. And the relationship between enhanced scintillation properties and the color center complex of iron impurity and oxygen vacancy in the crystals was discussed.     

A study on the influence of ytterbium and impurities on lattice parameters and the phase transition temperature of Czochralski-grown LiNbO3

LiNbO₃ crystals were grown from a congruent melt by the Czochralski method. Special care was taken with respect to purity and an analysis of impurity levels. Present crystals are as pure as 3N-4N. In particular, doping by Yb³⁺ was investigated. An upper limit for the inclusion of Yb³⁺ in LiNbO₃ may be set at 2.5 mol% for a congruent melt. Present results demonstrate the complex interaction between intrinsic and extrinsic defects in LiNbO₃. The variation of lattice parameters supports evidence for the Li⁺ vacancy model and an occupation of Li sites for Yb³⁺. Data on purities, laser properties and analytical methods provide a short review on a field, where analytical results gained by different techniques often may disagree.     

硅中替位式和间隙式IB族杂质的电子态

我们对硅中IB族杂质Cu,Ag和Au的单粒子电子结构用自洽的Xα-SW法作了计算。用集团XSi₄Si′₁₂和XSi₁₀Si′₁₆分别模拟替位式和四面体间隙式杂质X所局域微扰的硅晶体。这两类集团已广泛地得到采用,我们已经成功地用它研究硅中4d过渡金属杂质的性质。计算得到;替位杂质在带隙中导致一个t₂能级,它几乎不具有d性质而是类悬挂键的。间隙杂质在带隙中导致一个a₁能级,对于Si:Au,此能级位置接近导带底,轨道是非常离域的。此外,还得到一个位于价带底之下类s的超深能级。对硅中1B族杂质的电子性质的趋势作了详细讨论。 关键词：     

Structural and radiation color centers and the dielectric properties of doped yttrium aluminum garnet crystals

The results of experimental and theoretical studies of the spectral and dielectric properties of garnet Y₃Al₅O₁₂ crystals are analyzed. The crystals studied were grown under different conditions and were either pure or contained iron-group ion impurities (Ti, Cr, Fe). The crystals were studied before and after irradiation by γ rays and high-energy electrons. A model is proposed for the formation of color centers and their influence on luminescent and other optical and dielectric properties.     

Space charge regions at silver halide surfaces: Effects of divalent impurities and halogen pressure

The model first proposed by Poeppel and Blakely to account for equilibrium space charge regions in ionic crystals is extended to include effects of mobile divalent impurities. Equations are developed that account for impurities segregating to free surfaces of crystals of the AgCl type. Numerical computations have been performed and the results indicate the variation with temperature of the potential difference across the surface space charge region. The importance of accounting for the density and energy of surface sites is demonstrated. The calculation includes effects of varying the doping level of the impurities as well as the binding energy of the impurity-vacancy complex. The effect of a chlorine atmosphere on intrinsic crystals is discussed and the variation of the chemical potential of the silver ions with Cl₂ pressure is presented.     

Polytypism in alloys

This paper summarizes polytypes in alloys having layer structures such as Laves phases and Sm-Ni intermetallics which were studied by X-ray diffraction and electron microscopy. In the Mg-base pseudo-binary Laves phases, many stacking variants were found as a function of electron concentration. They are 4H, 6H, 3C, 4H, 21R, 10H, 16H, 9R, 6H', 14H, 8H and 2H in order of increasing e/a from 0.67 to 2.0. Many polytypes were also observed in the specimens of Sm₂Ni₇ and Sm₅Ni₁₉ compounds. Polytypes found in Sm₂Ni₇ are 2H, 3R, 4H, 5T, 9R, 12R, and for Sm₅Ni₁₉ 2H, 3R, 4H, 5T, 6H, 7T, 9R, 12R, 15R, 18R were recognized. Stacking faults, intergrowth and many defect structures were observed under the electron microscope.     

Effect of quenching on the electrical conductivity of doped KCl crystals

Electrical conductivity and optical absorption of divalent cation and anion doped KCl crystals have been measured before and after quenching from 750 °C. The conductivity after quenching is found to decrease in undoped and cation doped crystals while it increases in the anion doped crystals. This is due to higher coagulation rate of cation impurities as compared to anion impurities. Since the divalent impurities diffuse in the form of impurity-vacancy complexs, the higher mobility of cation vacancies (which form dipoles with divalent cation impurities) accounts for the higher aggregation rate of the cation impurities. The aggregation rate, during annealing after quenching, has also been found to depend on concentration.The author is indebted to Prof. H. N.Bose for helpful discussions. Thanks are also due to Dr. M. L.Mukherjee for providing the crystals.     

Transport,thermal, and magnetic properties in heavy-fermion superconductor Ce2CoIn8

We have grown high purity single crystals of Ce₂CoIn₈ using the self-flux technique, and have investigated its transport, thermal, and magnetic properties, including the anisotropic features. Single crystals of Ce₂CoIn₈ were grown in the lower temperature region to avoid the formation of un-wanted phases such as CeCoIn₅ and CeIn₃ impurities. The results of the structural and physical measurements imply that the present single crystals have high purity. The electrical resistivity and specific heat measurements demonstrate that Ce₂CoIn₈ has a superconducting ground state with a distinct non-Fermi liquid character. This indicates that the superconductivity in Ce₂CoIn₈ arises out of the verge of the underlying quantum critical instability mediated by the antiferromagnetic correlations. Additionally, we investigate the crystalline electric field (CEF) energy scheme based on the temperature dependence of the specific heat and the anisotropic features in the susceptibility. We propose one of the CEF level schemes calculated on the basis of the CEF model that the first and second CEF states are located at Δ₁ = 82 K and Δ₂ = 102 K above the ground state doublet, respectively.     

Luminescent and photochemical processes in CdBr<Subscript>2</Subscript> : AgCl crystals

This paper reports on the results of the investigation into the optical luminescence properties of photochromic crystals CdBr₂ : AgCl grown by the Bridgman-Stockbarger method. It has been shown that, under X-ray, optical, and N₂-laser excitations of the grown crystals, there occurs emission due to Ag⁺ impurities in addition to emission from centers characteristic of CdBr₂. The photostimulated chemical reactions occurring in CdBr₂ : AgCl lead to a weakening of the luminescence and to a change in its spectral composition. Models of photosensitive centers and centers of photochemical coloring have been proposed. The mechanisms of the photochromic effect have been considered. The nature of luminescent and trapping centers has been discussed.     

The role of residual technological impurities in the formation of radiative recombination centres in irradiated germanium

A comparison study of the photoluminescent (PL) spectra in pure and ultra-pure irradiated and annealed germanium has been performed. Some radiative recombination centres including impurities have been found. The spectra of samples bombarded by neutrons were found to include, along with the lines from intrinsic defects (Z, Y, X, W, V), the centres due to hydrogen and oxygen impurities (the lines T₁, T₂, U). A considerable number of the lines (A₁-A₃, B₁, B₂, C, D₁, D₂) in the PL spectra of crystals exposed to gamma-quanta and electrons were identified with the centres including carbon atoms. Such a great set of lines, presumably, results from the transformation of the above mentioned complexes in the course of annealing. The observed differences in the PL spectra within 0.72 to 0.74 eV range probably indicate a contribution of several types of intrinsic defects and other impurities, along with carbon, to complexing.     

Thermo-and photostimulated depolarization in self-compensated CdF2:Ga and CdF2:In crystals

A study is reported of the conductivity of CdF₂ semiconductor crystals doped by indium and gallium donor impurities and residing in a semi-isolated state. The latter results from self-compensation of the impurities, in the course of which one half of them creates two-electron DX centers, and the second is ionized. Photo-and thermostimulated depolarization of these crystals has been studied. It was shown that the observed polarization/depolarization phenomena have a nonlocal nature and are due to the charges present in these crystals changing their positions. These changes may be formally considered as charge displacement to macroscopic distances considerably in excess of the interatomic ones. The mechanisms responsible for these phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1575–1581 (September 1999)     

核磁共振光谱法测定(S)-(-)-α-苯乙胺的对映体纯度

以 (2R ,3R) 二苯甲酰酒石酸为手性溶解剂 ,用1HNMR的方法测定了 (S) (- ) α 苯乙胺样品的对映体纯度。以氘代氯仿为溶剂 ,当样品浓度为 0 0 5 1mol·L-1,手性溶解剂与样品的摩尔比为 0 33时 ,α 苯乙胺甲基双峰的化学位移不等价为 0 0 8ppm(基线分离 )。由甲基吸收峰的面积可以计算样品的对映体纯度。测量结果的相对标准偏差与样品的对映体纯度有关 ,当样品的ee值高达 99 1%时 ,RSD为 0 3 % (n =4)。     

Localised vibrations due to substitutional impurities in calcium fluoride

The localized modes in CaF₂ due to the introduction of substitutional impurities have been investigated using the greens functions method. The weakening of the nearest neighbour interaction in all the alkaline earth fluorides, is estimated from the infrared absorption spectra on localized modes in these crystals. The splitting of the localized modes in CaF₂, due to the presence of additional Sr²⁺ or Ba²⁺ impurities is estimated and satisfactory agreement exists between theory and experiment, within the limits of the simplifications in our calculations.     

Interaction of impurity and intrinsic defects in forsterite and its role in the formation of spectral and luminescence properties

This paper reports on the results of investigations of the coefficients of chromium distribution between a crystal and a melt of forsterite, the absorption and luminescence spectra, and the electron paramagnetic resonance spectra of chromium centers in Mg₂SiO₄: Cr, Mg₂SiO₄: Cr: Sc, and Mg₂SiO₄: Cr: Li crystals. It has been established that the concentration dependences of these properties vary upon changing over from the range of trace concentrations of chromium, scandium, or lithium impurities in the melt to the range of higher concentrations of these impurities. The observed phenomenon is explained by the interaction of impurities with intrinsic defects of the crystal, which is called as the microimpurity trapping effect. According to the performed estimations, the concentration of predominant intrinsic defects (magnesium Frenkel defects) in the forsterite crystals grown from the melt is equal to (7.5 ± 0.3) × 10⁻⁶ atomic fractions. The energy of the formation of magnesium Frenkel defects can be estimated as 4.2 ± 0.2 eV.     

Electrolytic preparation of CaB<Subscript>6</Subscript> by molten salt technique

Calcium hexaboride (CaB₆) crystals with high melting point (2,235 °C) have been conveniently synthesized at low temperature (900 °C) from molten salt electrolysis. The synthesis was carried out using CaO–B₂O₃–LiF melt under argon atmosphere. Electrochemical experiments were carried out in an inconel reactor to having a high purity graphite crucible, which served as an electrolyte holding vessel as well as the anode. An electropolished molybdenum rod was employed as the cathode. The electrolysis was performed at 900 °C under argon atmosphere at current densities ranging from 0.5 to 1.5 A/cm² at 1:6 M ratios of calcium and boron content. After the electrolysis, the cathode product was removed and cleaned using dilute HCl solution followed by triple distilled water. Characterization of the crystalline product by TG/DTA, XRD, CHNS, EDAX, XPS, EPR, and SEM were reported. From the studies, it has been observed that CaB₆ can be synthesized at all current densities and the products have some impurities.     

Phase transitions in the (Cs1−x Rbx)2ZnI4 system

(Cs_1−x Rb_x)₂ZnI₄ crystals were grown by two different methods with Rb concentrations varying from x=0 to 2.5%. ¹²⁷I NQR and calorimetric measurements showed that crystals grown by the Bridgman technique contain residual impurities (∼0.5%) for all x. While x=0 crystals grown from solution do not contain detectable impurities, they allow incorporation only of a low Rb concentration, not above 0.5%. A transition-temperature-concentration (x) phase diagram has been constructed for Bridgman-grown crystals from NQR data. Rb doping shifts the normal-incommensurate and incommensurate-ferroelastic phase-transition points toward higher temperatures with different rates. The P2₁/m↔P1 first-order transition shifts toward lower temperatures. The region of low Rb concentrations lies closest to the critical point. Fiz. Tverd. Tela (St. Petersburg) 41, 143–147 (January 1999)