Optical and electrical properties of some sulphates doped with cobalt

Doubly ionized cobalt ion which has a ⁴F ground state exhibits several optical bands in orthorhombic sulphates. In view of the low symmetry, many degenerate states split and at low temperature (77°K) well resolved bands have been observed which enable the detailed calculation of the crystal field parameters in orthorhombic symmetry.Electrical conductivity measurements in pure potassium and ammonium sulphates show only the extrinsic unassociated region while in cobalt doped crystals, extrinsic as well as association regions are observed. Three distinct regions with slopes 0·86, 1·2 and 0·5 eV are obtained in cobalt doped K₂SO₄.X-irradiated pure crystals give two prominent bands at 2200 and 3300 Å which are attributed to SO₃^? and SO₂^?. Divalent cobalt doped crystals give additional bands at 2100 and 3100 Å. These bands are attributed to Co⁺ in different surroundings. Three ESR lines with ‘g’ values 2·042, 2·02 and 2·004 are obtained in support of the assignments.     

Cobalt-doped magnesium oxide pressure-transmitting medium for high pressure and high-temperature apparatus

ABSTRACT

Magnesium oxide (MgO) doped with cobalt (Co) semi-sintered pressure-transmitting medium (MCOPM) was prepared at 1200°C in air, starting with a pre-compressed mixture of MgO and cobalt oxide (CoO). The samples were characterized using powder X-ray diffraction, scanning electron microscopy, and thermal conductivity measurements. The results demonstrate that during the sintering process, the exchange of metal ions between the mixture powders introduced lattice defects into the MgO lattice structure, and an MgO–CoO solid solution was formed. Compared to commercial semi-sintered MgO pressure-transmitting medium (SSMPM), the temperature-generation efficiency of MCOPM-17 (MgO?+?17?mol% CoO) improved by about 220% at the high pressure of 12?GPa with a cell assembly in a two-stage multi-anvil large volume press. Moreover, the thermal conductivity of CoO-doped MgO decreased with increasing CoO doping.     

Effects of doping and preliminary processing on the magnetically stimulated mobility of dislocations in InSb single crystals

The effect of the type of conductivity and the doping level of InSb single crystals on the mobility of fast 60° dislocations in a magnetic field is discovered. It is found that doping of a pure InSb crystal with tellurium (n-type impurity) to 10¹⁸ cm^?3 reduces the mobility of dislocations to the background level. At the same time, in p-type InSb crystals doped with Ge with the same carrier concentration (10¹⁸ cm^?3), the magnetoplastic effect is manifested clearly. It is shown that preliminary mechanical loading and, hence, internal stresses in the crystal affect not only the mean path length of dislocations in a magnetic field but also the magnitude of the threshold magnetic field below which the magnetoplastic effect is not observed. Possible reasons for these phenomena are discussed.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

Hydrogen doping of MgO thin films prepared by pulsed laser deposition

Hydrogen doped MgO films were grown by pulsed laser deposition method. Gaseous hydrogen stored in cavities of milky MgO single crystal targets provided doping in film deposition process. Clear MgO targets without hydrogen were used in the preparation of reference films. The influence of hydrogen doping on firing voltage (FV) of gas discharge and its AC frequency dependence was investigated. According to thermoluminescence experiments, the films grown from milky targets contained two kinds of electron traps with the activation energies of 0.051 and 0.31 eV, while latter traps were absent in reference samples. The 0.31 eV trap was assigned to the hydride ion H⁻ occupying an oxygen vacancy site in MgO crystal structure. Using standard gas mixture (Ne-10% Xe), FVs of hydrogen doped sample showed considerable frequency dependence and were up to 55 V lower in comparison to the reference sample. The FVs of reference sample were shifted 14-28 V to higher values when N₂ gas was added to the mixture. The N₂ addition lowered the FVs of hydrogen doped sample up to 38 V and almost eliminated the FV frequency dependence.     

Effect of Co2+ on the growth,optical properties and laser damage threshold of potential nonlinear optical l-alanine single crystal

Good transparent bulk single crystals of pure l-alanine (LA) and cobalt doped LA crystals have been synthesized and successfully grown by slow-cooling method from their aqueous solutions. The concentration of metal dopants in the mother solution with 0.5 mol% for cobalt was carried out individually and crystals were obtained with well defined morphology. The as grown metal doped and pure single crystals were characterized by single crystal XRD studies which confirm that the incorporation of metallic dopants has not changed the basic structure of the parent crystal. The absorption of these crystals was analyzed and the result confirms that they possess low absorption in the range 230–1100 nm. Fourier transform infrared (FTIR) spectroscopy was carried out to investigate the molecular vibrations of these crystals and to confirm the incorporation of the dopants. The thermal properties have been studied by TGA/DTA curves. The EDAX measurement and surface morphology were studied for pure and metal doped LA crystals. The second harmonic generation (SHG) signals were observed using Nd: YAG laser with fundamental wavelength of 1064 nm in pure and metal doped crystals. The laser damage threshold was measured for pure and metal doped LA crystals and also tested by using a Q-switched Nd: YAG laser showed enhanced LDT value for metal (Co²⁺) doped LA crystal compared to pure LA crystal due to the metallic substitutions thus proving their useful candidature for nonlinear optical applications.     

Crystal shape effect on nuclear orientation thermometry at ultra-low temperatures

Absolute temperature measurement (T < 100 mK region) from anisotropy of gamma radiations emitted from oriented⁶⁰Co nuclei in a single crystal of hcp cobalt is found to depend on the shape of the crystal. This dependence is attributed to some closure magnetic domains not oriented along thec-axis of the disc shaped crystal studied. A long rectangular strip of cobalt crystal is found to give correct angular distribution ofγ-radiations and, therefore, suitable for thermometry.     

Growth and comparison of physicochemical properties of pure,Ca and Sr doped NH4Sb3F10 single crystals for electro optic applications

Single crystals of pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ are grown by slow evaporation technique. The effect of dopants on the growth and physicochemical properties also have been investigated and reported for the first time. The grown crystals are characterized with the aid of single crystal X-ray diffractometry to confirm the crystal structure. EDAX studies are done to confirm the presence of dopants in the crystal lattice. The vibrational frequencies of various group ligands in the crystals have been derived from the Fourier transform infrared (FT-IR) spectrum. From the optical absorption spectrum the band gap energy was calculated and it was found to be 5.76, 6.29 and 6.35 eV for pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ crystals respectively. Thermal stability of the sample has been analysed using TG-DTA analysis. The activation energy of pure, Ca²⁺ and Sr²⁺ doped NH₄Sb₃F₁₀ crystals were calculated from the dc conductivity measurements and it is found to be 0.2728, 0.2816 and 0.3622 eV Experimental results shows improved physicochemical properties when the dopant is added to the pure material.     

High-temperature electrical properties of yttrium iron garnet under varying oxygen pressures

The d.c. conductivity and Seebeck coefficient of Ca and Mg doped YIG have been measured at 1200–1700 K. A change of p-type to n-type conductivity was accomplished by varying the oxygen pressure between 1 and 10^-8 atm. From the position of the conductivity minimum as a function of temperature the band gap energy is estimated to be 2.85 eV.     

Dopant dependent stability of Co n TM+ (TM = Ti,V, Cr,and Mn) clusters

In this paper, we present a photofragmentation study of mass-selected transition metal-doped cobalt cluster cations Co _n TM⁺  (n = 8–18, TM = Ti, V, Cr, and Mn). Time-of-flight spectra recorded after laser excitation of mass-selected clusters in the gas phase show that the evaporation of a cobalt atom is the most facile dissociation channel for clusters with TM = Ti and V, suggesting an enhanced stability of the doped clusters compared to the bare ones. In contrast, for Co _n TM⁺ with TM = Cr and Mn, the loss of the dopant atom is found to be the preferred dissociation channel. Co₁₃Cr⁺ is a notable exception and favors dissociation by loss of a neutral Co atom. It is implied that substituting Mn and Cr generally destabilizes the cobalt clusters with the exception of Co₁₂Cr⁺, which is relatively more stable than Co ₁₃ ⁺ . Additional measurements of V _n Co⁺ (n = 9–16) show that the loss of a Co atom is still the most facile dissociation channel, which is in agreement with the predicted stronger V?V bond compared to the V?Co one.     

Conductivity and dielectric relaxation phenomena in (NH4)2SO4 single crystal

AC impedance measurements have been carried out on (NH₄)₂SO₄ single crystals for the temperatures from 300 to 473 K and frequency range between 100 Hz and 4 MHz. The results reveal two distinct relaxation processes in the sample crystal. One is the dipolar relaxation with a peak at frequency slightly higher than 4 × 10⁶ Hz. The other is the charge carrier relaxation at lower frequencies. The frequency dependence of conductivity is described by the relation σ(ω) = Bωⁿ, and n = 1.32 is obtained at temperatures below 413 K. This value drops to 0.2 and then decreases slightly with increasing temperature. The dipolar response of the (NH₄)₂SO₄ single crystal under an ac field is attributed to the reorientation of dipoles. The contribution of charge carriers is increasing substantially with increasing temperature at temperatures above 413 K. The temperature variation of conductivity relaxation peaks follows the Arrhenius relation. The obtained activation energy for migration of the mobile ions for (NH₄)₂SO₄ single crystal was 1.24 eV in the temperature range between 433 and 468 K in this intrinsic region. It is proposed that the NH₄⁺ in the sample crystal has the contribution to the electrical conduction.     

Electron and lattice stages in the valence transition in SmTe under a high hydrostatic pressure

The results of precision measurements of the resistivity, thermopower, volume, and thermal conductivity of the compound SmTe under truly hydrostatic pressure conditions at room temperature are reported. High quality stoichiometric and doped (n-type, n ≈ 8 × 10¹⁸ cm^?3) single crystals are studied. It is found that the valence transition occurs as consecutive stages of rearrangement of the electron subsystem and the crystal lattice, which take place under different pressures. At the initial stage of the transition, metallization is observed, which is accompanied by anomalies in kinetic coefficients; the curve describing the pressure dependence of the volume deviates from the curve corresponding to the initial semiconductor phase only slightly. The next stage is accompanied by a substantial change in the sample volume (lattice collapse); in this pressure range, however, the resistivity and thermopower become independent of pressure. At the final stage of the transition, the sample compressibility decreases; the resistivity and thermopower become again functions of pressure; and a state emerging in the sample in this case corresponds to the “golden” phase of SmS in all the properties being measured.     

Diffusion of 51Cr in Cr-doped MgO

An arc fusion technique was used to grow single crystals of MgO and Cr-doped MgO. Diffusion coefficients for ⁵¹Cr in Cr-doped single crystals were measured at three temperatures 1383, 1444 and 1495°C using a high specific activity isotope ⁵¹Cr. An approximately linear relationship between the concentration of Cr-ions in MgO and diffusion coefficients of ⁵¹Cr was obtained.It is shown that the activation energy of 19.6 $\text{kcal}\text{mole}$ obtained for the doped crystals is the difference between the energy for motion and the energy for association of the Cr-vacancy complexes. Using a previously determined value of 39.9 kcal/mol for the energy of motion, the energy of association for the Cr-vacancy complex is calculated to 20.3 ± 3 kcal/mol or 0.88 ± 0.13 eV.     

Electrical conductivity studies of cobalt-precipitation in RbCl crystals

The results of electrical conductivity measurements in heavily doped RbCl:CoCl₂ crystals with 2500 and 6000 ppm of cobalt are being reported in this paper. The different regions of the conductivity plots for the crystals with the two concentrations of cobalt have been explained and relevant energy parameters determined. The crystals with 6000 ppm of cobalt have been found to contain two types of precipitates one of these being of the same type as that existing in RbCl crystals with 2500 ppm of cobalt while the other being of a different composition. Further, it has been found that the crystals appear blackened after heating during measurements. The blackening has been attributed to the expulsion of cobalt from the bulk of the crystal which forms an oxide at the surface.     

Conversion of broadband thermal radiation in lithium niobate crystals of various compositions

The conversion of the broadband thermal radiation in stoichiometric (R = 1) lithium niobate single crystals that are grown from melt with 58.6 mol % of LiO₂, congruent (R = Li/Nb = 0.946) melt with the K₂O flux admixture (4.5 and 6.0 wt %), and congruent melt and in congruent single crystals doped with the Zn²⁺, Gd³⁺, and Er³⁺ cations is studied. It is demonstrated that the conversion efficiency of the stoichiometric crystal that is grown from the melt with 58.6 mol % of LiO₂ is less than the conversion efficiency of congruent crystal. In addition, the stoichiometric and almost stoichiometric crystals and the doped congruent crystals exhibit the blue shift of the peak conversion intensity in comparison with a nominally pure congruent crystal. For the congruent crystals, the conversion intensities peak at 520 and 495 nm, respectively.     

The effect of MgO doping on optical properties of LiNbO3 single crystals

Undoped and MgO doped Lithium niobate single crystals were grown by the Czochralski technique. Comparative study of the optical properties of undoped and 7 mol% MgO doped LiNbO₃ crystal was undertaken. The effect of doping on refractive indices as well as second harmonic generation has been experimentally analyzed. The results of the polarization characteristics of second harmonic generation (SHG) support the major contribution of Li-O bonds to optical nonlinearity. MgO doping reduces the number of localized excitons and the grown LiNbO₃ crystal approaches the stoichiometric composition. This causes blue shift in the absorption edge of the crystals.     

Microstructure and discharge properties of Mg-Zr-O protective films in plasma display panel

Mg-Zr-O protective films for plasma display panels (PDPs) were deposited on soda-lime glass substrates by magnetron sputtering method. The effects of Zr doping on both the discharge properties (firing voltage, V_f and the minimum sustaining voltage, V_s) and the microstructure of the Mg-Zr-O films were investigated. The results show that the deposited Mg-Zr-O films retain the NaCl-type structure as the pure MgO crystal. The doped Zr exists in the form of Zr⁴⁺ substitution solution in MgO crystal and an appropriate amount of Zr can improve the surface characteristics of the Mg-Zr-O films effectively. When the Zr atomic concentration is about 2%, the Mg-Zr-O films have the strongest (2 0 0) preferred orientation and the minimum surface roughness. The firing voltage and the minimum sustaining voltage of Mg-Zr-O protective layer are reduced at most by about 25 V and 15 V, respectively, compared with those of the pure MgO film. Mg-Zr-O protective layers with an appropriate amount of Zr are promising to meet the demands of advanced high-vision PDPs.     

Electronic absorption spectrum of cobalt antipyrine complex

The electronic absorption spectrum of cobalt antipyrine complex in the form of a single crystal is studied both at laboratory and liquid nitrogen temperatures. The spectrum is characteristic of Co²⁺ ion in an octahedral crystal field with bands exhibiting spin-orbit splittings at liquid nitrogen temperature. A good fit is obtained with the following crystal field parameters. B = 0.85 kK, C = 37.00 kK, Dq = 0.84 kK, ζ = 0.50 kK.     

Absorption and emission spectra of yttria-stabilized zirconia and magnesium oxide

We have investigated the luminescence and absorption spectra of doped and undoped ZrO₂-Y₂O₃ and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni²⁺, Co²⁺ and Pr³⁺ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr³⁺/Pr⁴⁺ is investigated.     

Investigation of dielectric and electrical behavior of Mn doped YCrO3 nanoparticles synthesized by the sol gel method

In the present work Mn doped YCrO₃ nanoparticles are synthesized by the sol–gel method. Samples have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis absorption spectroscopy. The optical band gap of Mn doped YCrO₃ nanoparticles increases with increase of doping concentration. The dc resistivity of the prepared samples decreases with increasing temperature. The variation of ac conductivity with frequency has been explained by the Correlated Barrier Hoping (CBH) conduction mechanism. Dielectric permittivity of the samples was studied and it follows the power law ε^/(f)∝Tⁿ, where the temperature exponent n is found to be frequency dependent. The dielectric properties of the samples have been discussed in terms of electric modulus vector. Both activation energies due to dc resistance and dielectric response have been measured for the different samples and it is observed that it increases with the Mn content.