Mixed Single Crystals Research Articles

F‐center effects in the luminescent properties of KCl 1–x Br x with divalent lead impurity

F-centers, generated in mixed single crystals KCl1–xBrx:Pb2+ by gamma radiation exposure in the range of 0.5–30 kGy, were studied in order to elucidate the effect of these defects in the thermally stimulated luminescence emission of the doped crystal. The F-centers were identified by its characteristic F absorption band which was located between 520 and 640 nm, depending on the ion composition in the mixed crystal. The thermoluminescence emission was measured before and after room temperature optical bleaching with F-light and significant effects in the TL glow curve were observed. The results suggested that the defects like F-centers play a very important role in the recombination mechanism responsible for the thermoluminescence emission process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GROWTH OF 4-(4-DIMETHYLAMINOSTYRYL)-1-METHYLPYRIDINIUM TOSYLATE (DAST) MIXED SINGLE CRYSTAL BY BRIDGMAN–STOCKBARGER METHOD

4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST) is a very promising organic nonlinear optical (NLO) material for future optoelectronic and nonlinear optical applications, though it cannot be grown from molten state. Hence, a derivative of DAST was synthesized and mixed with DAST. The melting point of the mixed crystal decreased drastically, making it possible to grow a single crystal from molten state. In the present investigation, a mixed single crystal of DAST has been grown from molten state by vertical Bridgman–Stockbarger method.

Kinetics of decomposition in ionic solids: III. Time evolution of phonons during spinodaldecomposition in AgCl–NaCl

Real-time inelastic neutron scattering has been used to study the kinetics of the demixingprocess in AgCl–NaCl mixed single crystals. The variation of transverse acoustic phononspectra provides the most direct information about the microscopic non-equilibriumbehaviour. It is shown that a well defined splitting of phonon peaks corresponding to theformation of the product phases takes place on a timescale of seconds. Along with theobservation of satellite reflections these findings clearly demonstrate that thephase separation is governed by the mechanism of spinodal decomposition inthis ionic system. During the entire process the lattice parameter keeps almostunchanged, thus leading to a metastable state characterized by large coherency strains.

The deformation tensor ∊ of the spin transition in the mixed crystal [Fe0.46Zn0.54(ptz)6](BF4)2

The conversion of the spin state of complexes exhibiting thermal spin crossover from the1A1low-spin (LS) state to the5T2high-spin (HS) state is accompanied by a deformation of the lattice due to the larger bond lengths in the HS state as compared with the LS state. In a previous work [Kuszet al.(2000).J. Appl. Cryst.33, 201–205], it has been shown that the deformation of the lattice, corrected for its temperature dependence, can be described by an almost temperature-independent tensor ∊ multiplied by the fraction of molecules in the HS state, γHS. Here the dependence of ∊ in a mixed single crystal of [Fe0.46Zn0.54(ptz)6](BF4)2(ptz = propyltetrazole) with a transition temperature near 110 K is reported. In order to study the dependence of the metal–ligand bond length on the concentration of Zn, five structures were determined at 10 K: the neat iron and the mixed-crystal compound in both the excited HS state (LIESST) and the LS state, and the neat Zn compound. The comparison of the structures shows that neither the bond lengths nor their differences in the HS and LS states depend linearly on the Zn concentration. These deviations can be considered as consequences of elastic interactions directly observed by X-ray diffraction analysis. The lattice constantsaandcalso do not depend linearly on the Zn concentration, contrary to the unit-cell volume and the differences ofaandcbetween the HS and LS states, a fact which is in agreement with the observed linear dependence of these parameters on the temperature-dependent HS fraction.

Luminescence and lifetime characteristics of microphases in KCl:KI:Pb mixed crystals

Absorption and luminiscence measurements were performed on lead-doped KCl:KI mixed single crystals. Strong differences were found between as-grown and quenched samples. New bands were observed in the absorption spectra of unquenched samples in the spectral region between the limit positions of the A bands, corresponding to chloride or iodide lattices. The strong band at 449nm dominates the luminescence of these samples at low temperatures. Its kinetics have a complicated form and the decay time is in the interval from ns to ms. Quenched samples have a well-known luminescence spectrum caused by octahedral lead impurity centres in KCl single crystals. The kinetics, however, also indicate mixed chlorine and iodine coordination of lead in the investigated crystals.

Defects generated by irradiation with gamma rays in lead doped KCl–KBr mixed single crystals

In this work we report the behavior of solid solutions of lead-doped KCl–KBr mixed single crystals subjected to gamma irradiation. Various composition of KCl1−xBrx (x=0.02, 0.35, 0.50, 0.65, 0.85) doped with 20–40ppm of Pb2+ were used and exposed to ionizing radiation. The defect production seems to be correlated with the observed increasing integrated thermoluminescence glow curve as a function of irradiation at high 10kGy dose as well as in the low 2.2–130 dose Gy range. The analysis shows the potential use of these mixed crystals as radiation detector. The defects generated by irradiation were monitored by the optical absorption spectrum and the thermally stimulated luminescence.

The influence of demixing on the dynamics of ionic solids: inelastic neutron scattering fromAgxNa1−xCl single crystals

The dynamics of mixed single crystals ofAgxNa1−xCl has been investigated by inelastic neutron scattering before and after chemical demixing. Inthe homogeneous phase, the concentration dependence of acoustic phonons reveals thatdoping of NaCl with silver chloride leads to a considerable softening of the lattice, while theelastic properties of AgCl are almost independent of sodium chloride additives. Afterquenching into the miscibility gap, the phase separation is associated with a well-definedsplitting of acoustic phonons which provide the most direct information aboutthe underlying mechanism. In contrast to the local dynamical properties, thelattice structure is essentially determined by coherency strains which hinder therelaxation of lattice parameters. Thus diffraction and inelastic scattering yieldindependent and complementary information about demixing processes in ionic solids.

Growth and transport properties of tin monosulphoselenide single crystals

The binary and ternary mixed single crystals of the orthorhombic IV–VI family in the form of SnS x Se 1− x (where x=0, 0.25, 0.50, 0.75 and 1) were grown by direct vapor transport technique. The lattice parameters of each grown crystal were determined from the X-ray diffraction analysis. The grown crystals were examined under the optical zoom microscope for their surface microstructure study. The low-temperature resistivity measurements were carried out in the temperature range 123–303 K using four-probe method. The Hall coefficient, carrier concentration and Hall mobility of each sample was determined from Hall effect measurements at room temperature. From the optical absorption spectra obtained in the range 1000–1500 nm, optical band gap of each sample was determined at room temperature. The results obtained are reported systematically.

Time-evolution of phonon spectra during decomposition in Ag x Na 1-x Br ionic crystals

Real-time inelastic neutron scattering has been used to characterise the mechanism of decomposition in AgxNa1-xBr mixed single crystals well below the critical temperature on a microscopic scale. The chemical demixing process is associated with a splitting of acoustic phonon branches, which is observed to take place within some 100 s. Interestingly, the Bragg reflections exhibit a completely different time behaviour: the adaptation of the equilibrium lattice parameters occurs on a much longer time scale. Hence, decomposition is governed by large coherency strains which persist over a long period of time. Only in a second stage does the crystal lattice relax into the final equilibrium state.

Progressive Parkinsonism in a Young Experimental Physicist Following Long-Term Exposure to Methanol

A case is described of an experimental physicist who developed parkinsonism, apparently as delayed toxic effect of long exposure to vapors of methanol in the laboratory. Clinical and magnetic resonance imaging (MRI) supported the diagnosis, after exclusion of hereditary diseases and primary degenerative diseases. Screening for heavy metals in urine and plasma ceruloplasmin was negative. This case illustrates the neurotoxic delayed effect of long-term exposure to methanol with no episodes of acute intoxication. The setting of a research laboratory with prolonged exposure to mixed single crystals and inhalation of methanol vapors may exist in other academic and hi-tech environments, and pose the risk of similar delayed toxic influences.

Supramolecular fullerene chemistry: synthesis, crystal structure and potassium intercalation of [(C60)][(C6H5)2

Cocrystallization of C60 and biphenyl affords neutral mixed single crystals, which present close compact arrays of C60 molecules and vacant sites. Intercalation with potassium at low temperature (80 °C) affords a new, ternary, fulleride salt. Additionally, a simple model has been developed to describe the kinetics of intercalation.

Dielectric properties of lithium niobate–tantalate crystals

Dielectric properties, including linear (refractive index) and second-order nonlinear optical responses, of LiNb 1− y Ta y O 3-type mixed single crystals with y=0.00, 0.81, 0.92, 0.97, and 1.00 have been quantitatively studied. Based on the corresponding crystal compositions, the chemical bond method was applied. The results show that the dielectric susceptibilities at 1.064 μm decrease remarkably with increasing tantalum content in the mixed-crystal system lithium niobate–tantalate.

Photoluminescence spectra of Zn1−xCdxAl2S4 (0.0⩽x⩽0.2;0.8⩽x⩽1.0) single crystals

The photoluminescence spectra as well as the lattice constants and band gaps for the mixed single crystals Zn1−xCdxAl2S4 with 0.0⩽x⩽0.2 and 0.8⩽x⩽1.0 grown by the chemical transport reaction method were investigated. The Zn1−xCdxAl2S4 crystals were a cubic spinel phase α in the range of 0.0⩽x⩽0.2 and a defect chalcopyrite in the range of 0.8⩽x⩽1.0, and showed a miscibility range from x=0.2 to x=0.8 in the composition dependence of the lattice constants and band gaps. We observed two emission bands consisting of a strong blue emission band and a weak broad emission band due to donor-acceptor pair recombinations in the crystals with a cubic spinel and a defect chalcopyrite structure. These emission bands showed a different behavior in their temperature and composition dependence. An energy band scheme for the radiative mechanism of the Zn1−xCdxAl2S4 was proposed on the basis of our experimental results along with the measurements of photoinduced current transient spectroscopy and thermoluminescence.

Magnetically ordered phases in triangular-lattice antiferromagnets CsNi 1− xFe xCl 3 and Cs 1− yK yNiCl 3

Magnetization measurements have been made on triangular-lattice antiferromagnetic mixed single crystals CsNi 1− x Fe x Cl 3 and Cs 1− y K y NiCl 3. The mother crystal CsNiCl 3 shows a characteristic behavior of triangular-lattice Heisenberg antiferromagnet with easy-axis (Ising) anisotropy exhibiting two successive magnetic phase transitions due to spin frustration effect. This crystal is changed to be a triangular-lattice antiferromagnet with easy-plane (XY) anisotropy by replacing Ni with above only 0.7 at% Fe. The replacement of Cs with above 50 at% K also changes the anisotropy from Ising-like to XY. In addition, the presence of a novel phase appearing only under the magnetic field perpendicular to the c-axis are confirmed for both replaced systems with XY anisotropy.

Photoluminescence properties of MgxCd1−xSe mixed single crystals

The photoluminescence spectra of MgxCd1−xSe (0.00⩽x⩽0.46) mixed single crystals grown by the chemical transport reaction technique were investigated. We observed an emission band (I2) due to excitons bound to a neutral donor, an emission band (DA) due to donor-acceptor pair recombination with LO-phonon replicas and a self-activated emission band. These emission bands were shifted to higher energy with increasing x. For the crystals with x=0.46, the donor-acceptor emission band was observed at 2.661 eV in the blue region.

Preparation and Structural Characterization of Lithium Silylborates

Borane triethylamine reacts with lithium triphenylsilanide or lithium tert-butyldiphenylsilanide with formation of lithium (triphenylsilyl)trihydridoborate or lithium (tert-butyldiphenylsilyl)trihydridoborate. Complexation of the lithium cation with various ligands allows the isolation of compounds 1, 2a, and 2b. Trimethoxyborane reacts with lithium trimethylsilanide to form lithium tetrakis(trimethylsilyl)borate 3 and lithium tris(trimethylsilyl)methylborate 4. Mixed single crystals of 3 and 4 show an unexpected coordination of the lithium cation due to the lack of any supporting donor molecule. All silylborates exhibit short Si–B bond lengths compared to tricoordinated silylboranes.

Growth and characterization of amino acids mixed triglycine sulpho-phosphate single crystals

The growth of amino acids mixed triglycine sulpho-phosphate (TGSP) crystals has been investigated. The crystals were grown by a slow cooling technique. The amino acids mixed were 1-alanine (ATGSP), 1-valine (VTGSP), 1-asparagine (ASP-TGSP) and 1-arginine (ARG-TGSP), the structure of these amino acids are similar to glycine. Changes in the morphology of the crystals with substitution have been identified. The presence of amino acids in the crystal has been identified by FTIR analysis. The relative permittivity decreases in the case of asparagine mixed TGSP crystal (ASP-TGSP). Powder X-ray diffraction shows changes in the peak positions and relative intensity for amino acids mixed TGSP crystals.

The cooperative spin transition in [FexZn1 − x(ptz)6](BF4)2: II. Structural properties and calculation of the elastic interaction

The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor e) accompanying the spin transition is determined. For diluted mixed crystals with x 0.44 a first order crystallographic phase transition (R3i → P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm − 1 for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is ≃ 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.

Hardness Variation in Undoped and Doped Mixed NaCl-NaBr Single Crystal

Hardness Variation in Undoped and Doped Mixed NaCl-NaBr Single Crystal

A single crystal X-ray and electron spin resonance study of the 5-nitro-1,1,3,3-tetramethylisoindolin-2-yloxyl free radical

X-ray structural analyses have been carried out on the 5-nitro-1,1,3,3-tetramethylisoindolin-2-yloxyl free radical (NTMIO) and its precursor 5-nitro-1,1,3,3-tetramethylisoindoline (NTMI). A mixed single crystal of about 2% NTMIO in NTMI has been prepared and subjected to a detailed ESR examination. The X-ray analyses of NTMIO and NTMI showed that the two have different crystal habits but the dimensions of the two molecules are almost identical hence a small amount of NTMIO in NTMI would not be expected to perturb the lattice of the latter. The unit cell of NTMIO shows that the packing of the structure is normal and that there are no intermolecular distances below 3.41 A showing that hydrogen bonding is not present. The bond distances and angles of the heterocyclic ring of NTMIO are generally more closely related to saturated rather than to unsaturated five-membered ring nitroxyl radicals. ESR parameters are compared for TMIO in solution, in frozen solution and in a single crystal of NTMI.