Growth of oriented C11b MoSi2 bicrystals using a modified Czochralski technique

Oriented bicrystals of pure C11b MoSi2 have been grown in a tri-arc furnace using the Czochralski technique. Two single crystal seeds were used to initiate the growth. Each seed had the orientation intended for one of the grains of the bicrystals, which resulted in a 60° twist boundary on the (110) plane. Seeds were attached to a water-cooled seed rod, which was pulled at 120 mm/h with the seed rod rotating at 45 rpm. The water- cooled copper hearth was counter-rotated at 160 rpm. Asymmetric growth ridges associated with each seed crystal were observed during growth and confirmed the existence of a bicrystal. It was also found that careful alignment of the seeds was needed to keep the grain boundary from growing out of the boule. The resulting boundary was characterized by imaging and crystallographic techniques in a scanning electron microscope. The boundary was found to be fairly sharp and the misorientation between the grains remained within 2° from the disorientation between the seeds

Mo{sub 5}Si{sub 3} single crystals: Physical properties and mechanical behavior

The materials processing, physical properties and mechanical behavior of an ultra-high temperature structural silicide, Mo{sub 5}Si{sub 3}, have been studied. High purity single crystals of Mo{sub 5}Si{sub 3} have been synthesized by both optical floating zone and Czochralski methods. The thermal and elastic properties of the MO{sub 5}Si{sub 3} single crystals were experimentally measured. Results show that Mo{sub 5}Si{sub 3} has significant thermal expansion anisotropy along the a and c directions with {alpha}{sub c}/{alpha}{sub a} = 2.2. Single crystal elastic moduli of Mo{sub 5}Si{sub 3} indicate that it has less elastic anisotropy and lower shear moduli than transition metal disilicides. Tensile stresses of up to 1.8 GPa can develop at grain boundaries after cooling from the melting point due to the thermal expansion mismatch in Mo{sub 5}Si{sub 3}, causing grain boundary cracking during processing of polycrystals. Room temperature Vickers indentation tests on (100) and (001) planes have been performed with different indenter diagonal orientations. The orientation dependence of hardness and fracture toughness of Mo{sub 5}Si{sub 3} single crystals have been obtained. The corresponding deformation and fracture modes have been revealed by microscopy studies. A comparison of Mo{sub 5}Si{sub 3} with other high temperature structural silicides, e.g., C11{sub b} and C40 transition metal disilicides, is discussed

Local lattice distortions and thermal transport in perovskite manganites

Measurements of thermal conductivity versus temperature and magnetic field are reported for perovskite manganites that exhibit ferromagnetic (FM), charge-ordering (CO), antiferromagnetic, and/or structural phase transitions. The data reveal a dominant lattice contribution to the heat conductivity with {kappa}{approximately}1{minus}2 W/mK near room temperature. The rather low values, implying a phonon mean free path on the order of a lattice spacing, are shown to correlate with static local distortions of the MnO{sub 6} octahedra. Modifications of the local structure are responsible for abrupt anomalies in the zero-field {kappa} at the FM, CO, and structural transitions, and for colossal magnetothermal resistance near the FM transition. {copyright} {ital 1997} {ital The American Physical Society

The effect of intrinsic defects on RE3Al5O12 Garnet Scintillator Performance

In order to enhance capabilities for the reliable detection of nuclear material, improved detector materials are required. There is considerable room for improvement within the scintillator family of materials. However, native defects are present in all materials and impurities are similarly common. In scintillators, these defects serve as trap sites for electrons or holes, and therefore may contribute to decreased and delayed light yield. In fact, Lempicki and Bartram [J. Lumin. 81 (1999) 13] have proposed that understanding defect related phenomena is vital to the improvement of scintillators. It follows that if the most egregious electron/hole trapping defects are removed from the system, light output should increase. However, defect removal is difficult to achieve since often the defect-type to be removed is not known. In this paper, we assist the optimization of scintillators by employing atomic scale simulation techniques to predict the intrinsic defect structure of RE3Al5O12 garnets (where RE ranges from Lu to Gd and Y). Specifically, we predict cation antisite defects to be the lowest energy intrinsic defect. Furthermore, we describe how our results can be used to interpret experimental observations

Growth of Oriented C11(b) MoSi(2) Bicrystals Using a Modified Czochralski Technique

Oriented bicrystals of pure C11b MoSi2 have been grown in a tri-arc furnace using the Czochralski technique. Two single crystal seeds were used to initiate the growth. Each seed had the orientation intended for one of the grains of the bicrystals, which resulted in a 60° twist boundary on the (110) plane. Seeds were attached to a water-cooled seed rod, which was pulled at 120 mm/h with the seed rod rotating at 45 rpm. The water- cooled copper hearth was counter-rotated at 160 rpm. Asymmetric growth ridges associated with each seed crystal were observed during growth and confirmed the existence of a bicrystal. It was also found that careful alignment of the seeds was needed to keep the grain boundary from growing out of the boule. The resulting boundary was characterized by imaging and crystallographic techniques in a scanning electron microscope. The boundary was found to be fairly sharp and the misorientation between the grains remained within 2° from the disorientation between the seeds

Inelastic magnetic neutron scattering in CePd3

We have performed time-of-flight neutron scattering measurements on a single crystal of the intermediate valence compound CePd3. At 10 K, a Kondo-esque inelastic magnetic scattering peak occurs near with maximum intensity for momentum transfer Q near the zone boundary. Spectral weight is transferred to lower energy as Q varies until at zone center the intensity at 60 meV is considerably weaker. These results are in qualitative accord with predictions of the Anderson lattice. The Q-dependence may resolve an older controversy concerning the low-temperature scattering. We discuss the relationship of these results to our recent results in YbAl3.<br/

Phonon Raman Scattering In R1-xaxmno3+δ (r=la,pr; A=ca,sr)

Polarized Raman spectra of single and polycrystalline R1-xAxMnO3 (R=La,Pr; A=Ca,Sr) ceramic samples were studied as a function of temperature. For the rhombohedral LaMnO3.1 and La0.7Sr0.3MnO3, the observed Raman peaks were associated with modes arising from the folding of the Brillouin zone under lattice deformation. For the orthorhombic LaMnO3.0, the Raman spectra are consistent with the Pnma structure and show an anomalous softening of the 494 and 604 cm-1 modes below the antiferromagnetic ordering temperature TN≃140 K. Polycrystalline samples of La0.5Ca0.5MnO3 show a dramatic change of the Raman spectra between 100 and 160 K, which was associated with the increase of the orthorhombic distortion observed by others for T≲240 K. Other R1-xAxMnO3 single crystals, with small orthorhombic distortions, show Raman spectra which are similar to those observed in the rhombohedral samples.58171143511440Jonker, G.M., Van Santen, J.H., (1950) Physica, 16, p. 337. , UtrechtKusters, K.M., Singleton, J., Keen, D.A., McGreevy, R., Hayes, W., (1989) Physica B, 155, p. 362Von Helmolt, R., Wecker, J., Holzapfel, B., Schultz, L., Samwer, K., (1993) Phys. Rev. Lett., 71, p. 2331Hwang, H.Y., Palstra, T.T.M., Cheong, S.-W., Batlogg, B., (1995) Phys. Rev. B, 52, p. 15046Millis, A.J., Littlewood, P.B., Shraiman, B.I., (1995) Phys. Rev. Lett., 74, p. 5144Millis, A.J., (1996) Phys. Rev. B, 53, p. 8434Millis, A.J., Shraiman, B.I., Mueller, R., (1996) Phys. Rev. Lett., 77, p. 175Billinge, S.J.L., DiFrancesco, R.G., Kwei, G.H., Neumeier, J.J., Thompson, J.D., (1996) Phys. Rev. Lett., 77, p. 715Dai, P., Zhang, J., Mook, H.A., Liou, S.-H., Dowben, P.A., Plummer, E.W., (1996) Phys. Rev. B, 54, pp. R3694Zhao, G., Conder, K., Keller, H., Muller, K.A., (1996) Nature (London), 381, p. 676Oseroff, S.B., Torikachvili, M., Singley, J., Ali, S., Cheong, S.-W., Schultz, S., (1996) Phys. Rev. B, 53, p. 6521Ritter, C., Ibarra, M.R., De Teresa, J.M., Algarabel, P.A., Marquina, C., Blasco, J., García, J., Cheong, S.-W., (1997) Phys. Rev. B, 56, p. 8902Urushibara, A., Moritomo, Y., Arima, T., Asamitsu, A., Kido, G., Tokura, Y., (1995) Phys. Rev. B, 51, p. 14103Wiles, D.B., Young, R.A., (1981) J. Appl. Crystallogr., 14, p. 149Last, J.T., (1957) Phys. Rev., 105, p. 1740Silverman, B.D., Koster, G.F., (1961) Z. Phys., 163, p. 158Kim, K.H., Gu, J.Y., Choi, H.S., Park, G.W., Noh, T.W., (1996) Phys. Rev. Lett., 77, p. 1877Verelst, M., Rangavittal, N., Rao, C.N.R., Rousset, A., (1993) J. Solid State Chem., 104, p. 74Cochran, W., Zia, A., (1968) Phys. Status Solidi, 25, p. 273Thomas, H., Muller, K.A., (1968) Phys. Rev. Lett., 21, p. 1256Scott, J.F., (1969) Phys. Rev., 183, p. 823Harley, R.T., Hayes, W., Perry, A.M., Smith, S.R.P., (1973) J. Phys. C, 6, p. 2382Goodenough, J.B., Longo, J.M., (1970) Magnetic and Other Properties of Oxides and Related Compounds, 4 (PART A), p. 126. , edited by K.-H. Hellwege and A. M. Hellwege, Landolt-Borstein, Tabellen, New Series, Group 3, Springer-Verlag, BerlinElemans, A.A., Van Laab, B., Van Der Veen, K.R., Loopstra, B.O., (1971) J. Solid State Chem., 3, p. 238Iliev, M.N., Abrashev, M.V., Lee, H.-G., Popov, V.N., Sun, Y.Y., Thomsen, C., Meng, R.L., Chu, C.W., (1998) Phys. Rev. B, 57, p. 2872Wold, A., Arnott, R., (1959) J. Phys. Chem. Solids, 9, p. 176Goodenough, J.B., Wold, A., Arnott, R.J., Menyuk, N., (1961) Phys. Rev., 124, p. 373Radaelli, P.G., Cox, D.E., Marezio, M., Cheong, S.-W., Schiffer, P.E., Ramirez, A.P., (1995) Phys. Rev. Lett., 75, p. 4488Knízek, K., Jirák, Z., Pollert, E., Zounová, F., Vratislav, S., (1992) J. Solid State Chem., 100, p. 292Asamitsu, A., Moritomo, Y., Tomioka, Y., Arima, T., Tokura, Y., (1995) Nature (London), 373, p. 407Mitchell, J., Argyriou, D.N., Potter, C.D., Hinks, D.G., Jorgensen, J.D., Bader, S.D., (1996) Phys. Rev., 54, p. 6172Wollan, E.O., Koehler, W.C., (1955) Phys. Rev., 100, p. 545Udagawa, M., Kohn, K., Kashizuka, N., Tsushima, T., (1975) Solid State Commun., 16, p. 779Radaelli, P.G., Cox, D.E., Marezio, M., Cheong, S.-W., (1997) Phys. Rev. B, 55, p. 301

Multiple system atrophy: current and future approaches to management

Multiple system atrophy (MSA) is a rare neurodegenerative disorder without any effective treatment in slowing or stopping disease progression. It is characterized by poor levodopa responsive Parkinsonism, cerebellar ataxia, pyramidal signs and autonomic failure in any combination. Current therapeutic strategies are primarily based on dopamine replacement and improvement of autonomic failure. However, symptomatic management remains disappointing and no curative treatment is yet available. Recent experimental evidence has confirmed the key role of alpha-synuclein aggregation in the pathogenesis of MSA. Referring to this hypothesis, transgenic and toxic animal models have been developed to assess candidate drugs for MSA. The standardization of diagnosis criteria and assessment procedures will allow large multicentre clinical trials to be conducted. In this article we review the available symptomatic treatment, recent results of studies investigating potential neuroprotective drugs, and future approaches for the management in MSA