Mixed Single Crystals Research Articles

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.

Pseudomorphic transformations of euhedral crystals of γ-AlOOH into aluminas

The thermal transformation of synthetic well-shaped hexagonal laths of boehmite-AlOOH was studied between 200°C and 1500°C by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD), to characterize the transition aluminas that appear before the formation of α-Al 2O 3. It was observed in the single crystals that dehydroxylation produces, between 400°C and 500°C, pores regularly organized and parallel to the larger diameter cf the hexagons which have the same direction (100) as the original boehmite crystal; this gives a striated appearance to the pseudomorphs, which is maintained until after heating at 900°C, when the transformation from the γ phase coexist; the pores have lost their orderly disposition and larger pores or openings are now irregularly distributed in the mixed single crystals, whose crystalline structure can be observed in the lattice image by the use of high-resolution electron microscopy. The α-alumina pseudomorphs exists as platy elongated single crystals at 1100°C and coalesce at 1200°C; from 1300°C to 1500°C the pseudomorphs coalesce into round polygonal particles of α-alumina, whose crystalline structure could be demonstrated by SAED, XRD or lattice images.

Electrical and Dielectric Properties of Polymerizing Mixed Single Crystals of Diacetylenes: pTS-pFBS System

Abstract The paper presents results of measurements of evolution of the photoconductivity and the low-frequency electric permittivity of the mixed pTS-pFBS diacetylene single crystals during the thermally driven solid-state polymerization. The kinetics of the polymerization was found to be a function of the composition of the crystals, the rate constants in the induction and autocatalytic periods following different composition dependences. Changes in the kinetics of the photocurrent build-up essentially follow the changes in the polymerization kinetics. Temperatures of the phase transitions in the mixed crystals decrease with increasing concentration of pFBS, probably due to weaker dipole-dipole interactions between fluorobenzenesulfonyl side groups.

Spontaneous polarization in the deuterated and undeuterated proton glass Rb1-x(NH4)xH2AsO4

Spontaneous polarization of mixed single crystals of Rubidium Ammonium Dihydrogen Arsenate (RADA) and its deuterated counterpart (DRADA) are presented together with the pure ferroelectric Rubidium Dihydrogen Arsenate (RDA) and its deuterated counterpart (DRDA). There is a sharp rise in the spontaneous polarization at the ferroelectric transition temperature Tc for the pure crystals as this transition is first order. The change in the spontaneous polarization in the mixed (x = 0.08) crystals below Tc is gradual however, indicating the presence of acid hydrogens and ammonium cations that are still mobile and in the paraelectric phase. These mixed crystals show coexistence of paraclectric/proton glass and ferroelectric order below the glass “transition” temperature Tg. The maximum values of the spontaneous polarization in the deuterated samples are greater than in the undeuterated samples. Also, the maximum values of the spontaneous polarization in mixed crystals below 1c. at the lowest temperatures reported...

Growth and characterization of rare-earth mixed single crystals of samarium barium molybdate

Molybdate crystals doped with rare-earth ions are finding immense use as laser materials. Rare-earth mixed single crystals of samarium barium molybdate grown in silica gel are important for use in optical equipment and electronic and acoustic studies. These octahedral bipyramidal crystals of samarium barium molybdates are characterized by IR, EDAX and thermal analyses.

Formation of mixed crystals in microbial conversion of sterols and steroids

The phenomenon of formation of mixed crystals during microbial conversions involving water-insoluble substrate and product was examined through the investigation of two biotransformations: (a) dehydrogenation of hydrocortisone by Arthrobacter simplex and (b) oxidation of cholesterol by Rhodococcus erythropolis. Evidence for the intrinsic ability of hydrocortisone and prednisolone molecules to form mixed crystals from both artificial solutions and microbial conversion systems was provided by x-ray powder diffractometry and differential scanning calorimetry. Mechanical agitation was shown to further promote the bioconversion of mixed crystals from an initial value of 83% prednisolone to an ultimate value of 95%. The solids in the bioconversion system of cholesterol were shown by their x-ray diffractions to be a mixture of single substrate and product crystals. A scheme of the various crystal structures likely to be formed in bioconversions—mixed single crystals, mixed crystals, and coated crystals—was presented. A solubility-based classification of the sterols and steroids with regard to their likelihood to form mixed crystals in bioconversion systems was suggested.

Raman spectra in non-linear A1ndash;xBxTiOAsO4mixed single crystal (A,B=K,Rb,Cs)

Abstract The vibrational modes of A1–xBxTiOAsO4 mixed single crystals with A, B = K, Rb, Cs, are investigated by Raman spectroscopy in the frequency range from 10 cm−1 to 1200 cm−1 between 10 and 900 K and for various geometrical configurations. The recorded spectra are interpreted in terms of internal and external vibrational modes by comparing the observed phonon optical modes with those of compounds possessing equivalent molecular groups such as KDP and ABO3 families. Furthermore, the peak assignment is confirmed coherently with the Raman structures measured in pure KTiOPO4 and RbTiOPO4 single crystals. The dependence of the various spectra on A and B ions substitution and on temperature is analyzed in connection with the typical physical properties like ionic conduction, dielectric behaviour and occurence of strong optical non-linearities which is of particular interest in this kind of materials

Dynamic behavior of quadrupolar orientational glasses: KxNa1-xCN mixed crystals.

The ultrasonic velocity and attenuation of TA[100] acoustic waves have been measured as a function of temperature and frequency in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{Na}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$CN mixed single crystals. Relaxation behavior is revealed by comparing our ultrasonic ${\mathit{c}}_{44}$ elastic stiffness data with hypersonic ${\mathit{c}}_{44}$ data obtained from Brillouin scattering. Combining ultrasonic velocity and attenuation data we are able to determine the Kramers-Kronig ratio R in the disordered paraelastic phase and to estimate it in the orientational glass phase. For the regime \ensuremath{\omega}${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$\ensuremath{\ll}1, R is a direct measure of the cooperative quadrupolar clamped relaxation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$, which can be represented by the empirical form ln${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$=${\mathit{A}}_{\mathit{s}}$-${\mathit{B}}_{\mathit{s}}$T in the paraelastic phase. Quadrupolar relaxation processes in the orientational glass state are also discussed.

Optical absorption edge and Urbach's rule in mixed single crystals of (Ga xIn 1− x) 2Se 3 in the indium rich region

The absorption processes at the absorption edge of the ternary system (Ga x In 1− x ) 2Se 3 in the In-rich region were investigated. For all concentrations the absorption coefficient exhibited exponential dependence on photon energy following Urbach's rule. The characteristic Urbach parameters were determined and their temperature and compositional dependence investigated. Analyzing the results along the lines of the general model which takes into account the possible role of several different types of disorder we concluded that the absorption processes in the absorption edge for our mixed solution of binary compounds are the results of the superposition of at least two different mechanisms, one related to the crystal phonons and the other resulting from structural and compositional disorder.

ESR Study of Mo3+ Containing Mixed Yttrium–Scandium–Aluminium Garnet Single Crystals

AbstractESR of Mo3+ ions in mixed single crystals of yttrium–scandium–aluminium garnets Y3Al2−xScx. Al3O12, where 0 ≦ x ≦ 2 is investigated. Several new centers of Mo3+ differing from those for Y3Al5O12 single crystals are observed due to changes of symmetry and crystal field parameters associated with isomorphous substitution of octahedrally coordinated Al3+ (a) by those of Sc3+ in the third coordination sphere. Symmetry, nearest coordination, and spin–Hamiltonian parameters are estimated concerning with all Mo3+ centers in Y3Al2−xScxAl3O12 crystals for x < 0.15. It is found that in Y3Sc2Al3O12 (x = 2) the Sc3+ ions substitute also Y3+ at dodecahedral sites.

Dielectric relaxation of mixed crystals of Rb1-x(NH4)xH2PO4 at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb1-x(NH4)xH2PO4,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to ≊ 100 MHz, with a freezing temperature ofT o≅8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.

Niobium-induced central peak in AgTa1-xNbxO3solid solutions

Abstract Temperature dependent Raman measurements have been performed on mixed AgTa1-xNbO3 single crystals with x = 0.11, 0.21, 0.35, 0.65 and 0.82. The strong central scattering which is observed is analysed with a response function involving a coupling between a phonon and a relaxational motion. The quasi-harmonic frequency ω and the damping γ∘ of the phonon mode as well as the coupling parameter δ and the inverse relaxation time γ are deduced and discussed.

Fundamental absorption edge in mixed single crystals of II-VI compounds.

The energy and temperature dependence of the absorption coefficient near the fundamental absorption edge has been measured in mixed single crystals of II-VI compounds. In all the crystals the Urbach rule is valid. Prominent differences have been found in the temperature dependence of the \ensuremath{\sigma} parameter of these crystals, according to which class the crystal belongs: amalgamated or persistent type. Most of the results are explained by the Dow-Redfield theory of the internal Franz-Keldysh effect. In order to explain the differences in the optical properties of the crystals of the two classes (Raman, infrared reflection, and the optical absorption near the fundamental absorption edge), a model of coupled oscillators was developed. The application of that model, together with Toyozawa's approach of the self-trapped excitons, helps to explain the above differences. The latter theory was found to be applicable especially in the case where localization of phonons occurs.

Temperature shift of the absorption edge in mixed single crystals of ZnxCd1-xSe.

The thermal dependence of the absorption-edge shift has been measured in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se, in the temperature range 13--300 K. The experimental results support the argument that the energy gap shift is caused by an internal Franz-Keldysh effect which results from electric fields induced by phonons. However, the exact dependences of the energy gap shift and the increase in the absorption coefficient on the electric fields differ from those calculated by Franz and Keldysh, because of final-state interaction correction. It is shown that in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se, as well as in other single crystals of pure II-VI compounds, the electric fields which determine the thermal shift of the absorption edge are dominated by the LO-phonon-induced electric fields.

Urbach rule in mixed single crystals of ZnxCd

The dependence of the absorption coefficient on the photon energy and on temperature near the fundamental absorption edge was measured in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Se, for x=0, 0.04, 0.3, 0.4, and 1. Exponential dependence of that coefficient according to the Urbach rule was observed. The thermal dependence of the slope parameter indicates that the absorption is influenced by the phonon-induced electric microfields and not by compositional or structural potential fluctuations. Up to about 90 K the magnitude of \ensuremath{\Elzxh}${\ensuremath{\omega}}_{0}$ fits the energy of the piezoelectric LA phonon, and at higher temperatures \ensuremath{\Elzxh}${\ensuremath{\omega}}_{0}$ fits the energy of the LO phonon. The energy \ensuremath{\Elzxh}${\ensuremath{\omega}}_{0}$ of the LO phonons varies linearly within the experimental error with x, from 27 meV for CdSe to 31 meV for ZnSe, and is in good agreement with the Raman spectra measured for these amalgamated behavior-type crystals. The energy gap, measured at room temperature, and the value of ${E}_{0}$ change linearly with composition for the hexagonal phase crystals. The absorption process can be considered as an internal Franz-Keldysh effect, caused by the phonon-generated electric fields. It is described in the framework of the Dow-Redfield model, which ascribes the Urbach rule to the ionization of the exciton, as an extension of the Stark shift.

Lattice vibrations in ternary II-VI compounds.

The ratio of the difference between the LO-phonon frequencies to the sum of these frequencies of the pure constituent crystals is proposed as an a priori indication for the classification of the ternary compounds as amalgamated- or persistent-type crystals. This criterion has been found to be valid for all the investigated II-VI compound mixed single crystals, and is explained in the framework of a diatomic linear-chain model and the localization of phonons.

EFFECTS ON PRESSURE ON THE SPECTRA, LIFETIMES AND ENERGY TRANSFER OF Eu P5O14, Tb P5O14 AND Eu1-xTbxP5O14

The effect of pressure (up to 210 kbars) on the fluorescence of Tb3+ and Eu3+ ions in the mixed single crystals Tbx Eu1-x P5O14 is investigated. A strong Tb3+ → Eu3+ energy transfer is observed and the pressure dependence of this transfer is shown to be governed by diffusion-limited relaxation. Using time-resolved spectroscopy under high pressure this energy transfer is found to be faster with pressure.

Triplet state of the double molecule 2,2′-biquinoline: A study of the phosphorescence, Raman spectra, and ODMR transitions

An investigation of the low temperature phosphorescence and the triplet state zero-field ODMR spectra of 2,2′-biquinoline is presented. In order to characterize the normal modes of the molecule the Raman spectrum of the neat crystal at room temperature has been studied and the observed molecular vibrations assigned. The phosphorescence at 1.9 K in mixed single crystal (benzoic acid) and Shpol’skii hosts show a highly structured vibronic spectrum. The spectra reveal that to a good approximation the molecule retains its trans-planar geometry in the triplet state in these matrices at low temperatures. The zero-field transitions of the molecule in its lowest triplet state were investigated by ODMR spectroscopy on the various optically resolvable sites observed in the phosphorescence. Each of the ODMR transitions exhibit two resonances both of which have well-resolved 14N hyperfine and quadrupolar satellites. On the basis of a hyperfine analysis and the frequencies of the resonances the existence of two close lying triplet states (unresolved in the phosphorescence) was determined. One of these is a localized quinolinelike 3ππ* state while the other possesses a delocalized character.

Effect of Solid Dilution and Temperature on the Crystal Field Spectra of Chromium Ions in Single Crystals of Chrome Alum

AbstractThe optical absorption spectra of mixed single crystals of chrome alum and potash alum are studied for different concentrations at room temperature and for one concentration at different temperatures between 100 and 310 K. The cubic crystal field splitting parameter Dq, and the interelectronic repulsion parameter B are obtained using the observed values of the peak positions. It is found that Dq, B, and half width at half maxima Γ of the spectral bands associated with the paramagnetic chromium ion in chrome alum do not change on replacing varying amounts of chromium ions with the diamagnetic aluminium ions. The temperature dependence of Γ is explained on the basis of the vibronic transition and that of Dq on the basis of the results of crystal field theory. A simple model is proposed to explain the temperature dependence of B.

Ordering Effects in the Random-Anisotropy System Dy(P1−xVx)O4

Measurements are reported of the magnetization, magnetic ordering, and optical absorption of mixed single crystals of Dy$({\mathrm{P}}_{1\ensuremath{-}x}{\mathrm{V}}_{x})$${\mathrm{O}}^{4}$. This compound is shown to be a good example of a system with random spin anisotropy. In contrast to the results on ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Cl}}_{2}$, no tetracritical point is found, and long-range magnetic order is destroyed by rather small concentrations of the minority component. These results agree with recent theories on random-anisotropy systems.