On the Crystallization of Terbium Aluminium Garnet

Attempts to grow terbium aluminium garnet (Tb3Al5O12, TAG) by the Czochralski method lead to crystals of millimeter scale. Larger crystals could not be obtained. DTA measurements within the binary system showed that TAG melts incongruently at 1840 deg. C. The perovskite (TbAlO3, TAP) with a congruent melting point of 1930 deg. C is the most stable phase in this system. The region for primary crystallization of TAP covers the chemical composition of TAG and suppresses the primary crystallization of the terbium aluminium garnet.Comment: 6 pages, 2 figure

Growth Techniques for Bulk ZnO and Related Compounds

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3 and In2O3.Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Bosto

Growth of Oxide Compounds under Dynamic Atmosphere Composition

Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10^{-6} bar, independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga2O3, or Fe{1-x}O can be grown from the melt.Comment: 4 pages, 3 figures, talk on CGCT-4 Sendai, May 21-24, 200

Numerical Modeling of Heat Transfer and Thermal Stress at the Czochralski Growth of Neodymium Scandate Single Crystals

The Czochralski growth of NdScO3 single crystals along the [110]-direction is numerically analyzed with the focus on the influence of the optical thickness on the shape of the crystal–melt interface and on the generation of thermal stresses. Due to lack of data, the optical thickness (i.e., the absorption coefficient) is varied over the entire interval between optically thin and thick. While the thermal calculation in the entire furnace is treated as axisymmetric, the stress calculation of the crystal is done three-dimensionally in order to meet the spatial anisotropy of thermal expansion and elastic coefficients. The numerically obtained values of the deflection of the crystal/melt interface meet the experimental ones for absorption coefficients in the range between 40 and 200 m−1. The maximum values of the von Mises stress appear for the case of absorption coefficient between 20 and 40 m−1. Applying absorption coefficients in the range between 3 and 100 m−1 leads to local peaks of high temperature in the shoulder region and the tail region near the end of the cylindrical part

The Bold and the Beautiful: How Aspects of Personality Affect Foreign Language Pronunciation

In the main study, a group of Polish learners of English completed a number of mimicry tasks in three languages: Italian, Dutch and Chinese, as well as a narration task in English. Mimicry performance and English pronunciation were then assessed by native speakers and compared. Participants also completed a questionnaire concerning their feelings about the languages they were to mimic and a second questionnaire designed to detect affective factors such as language learning anxiety, as well as attitudes towards the pronunciation of Polish and English. The pilot study suggested that the perceived attractiveness of the foreign language to be mimicked did not affect the performance of most participants, and that mimicry skill was fairly constant across languages. However, those who were particularly concerned about their personal appearance showed greater fluctuation in their ability to mimic and their performance appeared to be influenced by their attitude towards the language. This is referred to by the author as the Cecily effect. That study also confirmed the results of my previous experimental work showing that mimicry skill is correlated to some degree with English language pronunciation and that both pronunciation and mimicry are negatively affected by high levels of anxiety. The main study sets out to investigate whether or not these conclusions hold true for a larger sample population and also seeks to determine the effect of confidence and willingness to take risks on scores for both foreign language pronunciation and mimicry exercises

The inverse perovskite BaLiF3: single-crystal neutron diffraction and analyses of potential ion pathways

Doped barium lithium trifluoride has attracted attention as component for scintillators, luminescent materials and electrodes. With lithium and fluoride, it contains two possibly mobile species, which may account for its ionic conductivity. In this study, neutron diffraction on oxide-containing BaLiF3 single-crystals is performed at up to 636.2°C. Unfortunately, ion-migration pathways could not be mapped by modelling anharmonic ion displacement or by inspecting the scattering-length density that was reconstructed via maximum-entropy methods. However, analyses of the topology and bond-valence site energies derived from the high-temperature structure reveal that the anions can migrate roughly along the edges of the LiF6 coordination octahedra with an estimated migration barrier of ∼0.64 eV (if a vacancy permits), whereas the lithium ions are confined to their crystallographic positions. This finding is not only valid for the title compound but for ion migration in all perovskites with Goldschmidt tolerance factors near unity