S.C. Sabharwal

Study of growth imperfections, optical absorption, thermoluminescence and radiation hardness of CdWO4 crystals

The effect of starting charge composition used for growth on the crystalline perfection, optical, thermoluminescent and radiation hardness of CdWO 4 (CWO) single crystals has been investigated. One of the major problems faced with the growth of defect free crystals is the presence of a core-like defect. The loss of CdO by evaporation, which makes the melt progressively rich in WO 3 , is found to be the main cause of core formation. This problem has been overcome by employing a starting charge rich in CdO, the pull rates in the range of 2-3 mm/h and maintaining a convex solid-melt interface during the growth. The crystals grown from starting charges containing ∼53 mol% CdO exhibit excellent optical transmission, high radiation hardness towards gamma rays and extremely weak thermoluminescence (TL) emission, the characteristics which are the most desirable for any scintillator crystal. In the TL glow curves recorded for stoichiometric crystals, only one glow peak around 148°C is observed. Minute stoichiometric deviations are found to shift the glow peak temperature towards higher temperature side and enhance the TL output considerably. The stoichiometric crystals are found not to develop coloration for gamma exposures of 10 Mrad.

Effect of sodium doping on thermoluminescence and optical properties of barium borate (BaB2O4) single crystals

Abstract Thermoluminescence (TL) emission from undoped and sodium doped α- and β-BaB 2 O 4 (BBO) crystals has been investigated. A single TL glow peak is observed for the undoped crystals of both phases. The glow peak temperatures found for α- and β-BBO crystals over the temperature range 35–300°C are 104 and 80°C, respectively. The presence of minute amount of sodium ions in the lattice gives rise to a glow peak around 200°C. For impurity concentrations higher than 0.02% (by wt), additional glow peaks around 123 and 240°C are also observed. The ratio of the TL intensities corresponding to the first and 200°C peaks is observed to increase with impurity concentration. This fact provides an easy and sensitive method of detecting the presence of sodium ions in BBO lattice. The results also show that sodium ions go substitutionally into the lattice and act as charge traps. The optical absorption in the ultra violet region and the crystal hardness towards ionizing radiations are found to be adversely affected by the presence of this impurity.

Crystal stoichiometry and thermoluminescence of Bi4Ge3O12 and Y3Al5O12

Abstract Single crystal and polycrystalline samples of Bi4Ge3O12 and Y3Al5O12 of known compositions have been prepared. Thermoluminesce nce (TL), X-ray diffraction and optical absorption measurements were made. The results show that TL can be very useful in the detection of stoichiometric deviations as small as 0.1%. This tech-nique has been found particularly useful in the synthesis and control of stoichiometry of a starting charge for crystal growth.

Growth of single crystals in Bi2(Sr, Ca)n + 1CunOx system

Abstract Flux growth of crystals in the general system Bi 2 (Sr, Ca) n + 1 Cu n O x with n = 1 and n = 2 is described. The solute concentration in solution has been found to govern the phase, morphology and quality of the grown crystals. A controlled excess of CuO and CaO when added to the solute of stoichiometry 2212 yielded crystals of superconducting phase free from secondary growth of CuO needles.

Effect of highest temperature invoked on the crystallization of LiB3O5 from boron-rich solution

Abstract The crystallization behavior of LiB3O5 (LBO) has been investigated by performing differential thermal analysis (DTA). When the measurements are carried out with the highest temperature limited to a few degrees above the melting point, a clear indication of the crystallization is revealed by the presence of exotherms at 787°C and 772°C in the DTA plot. On the other hand, the crystallization of LBO phase gets inhibited if the melt is subjected to a much higher temperature, prior to cooling. Powder X-ray diffraction patterns recorded in the two cases are also in agreement with these observations. The results obtained have been explained in terms of the microstructure of the melt, which besides other factors, is shown to be dependent upon the highest temperature to which the melt is raised prior to crystallization. The inference drawn has been vetted by growing LBO single crystals by top seeded solution growth technique. The results obtained also help to understand the difficulties encountered in nucleating LBO from a boron-rich solution with a Pt wire, as reported in literature [1] .

Effect of non-stoichiometry on some properties of lead tungstate single crystals

The effect of stoichiometric deviations on the optical absorption, gamma rays induced absorption and thermoluminescence (TL) from PbWO4 (PWO) single crystals has been investigated. The stoichiometric deviations are found to develop yellowish coloration in this crystal. This problem has been overcome by purifying the starting powder for minute stoichiometric deviations through repeated crystallisations. In the case of non-stoichiometric crystals a broad absorption over the wavelength range 400–500 nm is observed. This absorption gets further enhanced when the crystal is subjected to γ-ray irradiation. Whereas, the stoichiometric crystals, which are colourless, are found not to develop coloration for gamma exposures as high as 10 Mrad. The presence of weak TL in both types of crystals is observed for the radiation exposures of 10 Mrad. For colourless crystals, only one TL glow peak at 170°C is recorded over the temperature range 30–400°C. A shift in the glow peak temperature towards the lower temperature side is observed in the case of non-stoichiometric crystals. This result can be used to detect minute stoichiometric deviations in PWO.

Growth of yttria and dysprosium thin films by molecular beam epitaxy and their characterization

Abstract The thin film deposition of yttria (Y 2 O 3 ) and dysprosium (Dy) on single crystal substrates of YSZ and SrTiO 3 has been studied under MBE conditions. Yttria is found to be very stable under electron beam evaporation. Depending upon the growth temperatures, two growth regimes which give rise to epitaxial growth of Y 2 O 3 on 〈100〉 YSZ substrates and predominantly oriented films along 〈222〉 are identified. In the case of Dy films on 〈100〉SrTiO 3 substrates, for growth temperatures above 600°C the growth orientation is found to be 〈101〉, whereas below this temperature the films are polycrystalline in nature. ESCA measurements made on Dy films reveal their special affinity to oxygen.

Characterization of YBa2Cu3O7- delta single crystals grown from charges containing silver nitrate

Single-crystal growth of YBa2Cu3O7- delta has been studied using starting charges containing known amounts of Ag in the form of AgNO3. The as-grown crystals are found to undergo a sharp transition to the superconducting state at 90 K, showing that the addition of AgNO3 facilitates their oxygenation. The measured room-temperature resistivities, both in plane ( rho ab) and out of plane ( rho c), for the grown crystals are found to be the lowest among the values reported so far. The anisotropy in resistivity ( rho c/ rho ab) at 300 K is found to be roughly one-half of the value measured for the crystals grown from charges free from AgNO3. The angular dependence of transport critical current for the doped crystals has been measured at 82 K and for an applied magnetic field of 0.9 T. The results have been analysed using the existing theoretical models.

Thermally stimulated luminescence from single crystals of LiYF4 and its constituent fluorides

Abstract Thermally stimulated luminescence (TSL) from YF 3 , LiYF 4 and LiYF 4 (U 4+ ) crystals has been investigated for the first time. While an undoped LiYF 4 crystal exhibits very weak TSL, the emission from YF 3 is found to be intense. On doping a LiYF 4 crystal with uranium, its TSL output is observed to get considerably enhanced. The emission spectra recorded for all the crystals are found to be identical and characteristic of the constituent Y 3+ ions. This is an important result which shows that in the process of TSL, U 4+ ion behaves as a donor of excitation energy to Y 3+ ions.

Thin film deposition of BaO by molecular beam epitaxy

Thin film deposition of BaO using BaO2 as the source material is studied by molecular beam epitaxy. The crystalline nature of the films grown at different substrate temperatures is studied by RHEED. Chemical composition of the films has been ascertained from ESCA measurements carried out without breaking the vacuum. The substrate temperature has been found to govern the composition of the films. Whilst at low growth temperature metallic Ba films are obtained, those grown at high temperatures (∼ 600°C) are of BaO.