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Now showing items 97 - 112 of 6688

  • Structures of the chain metaphosphates NaM(PO3)(3) (M = Ca or Sr)

    Abrahams, I.   Hawkes, G. E.   Ahmed, A.   Di Cristina, T.   Demetriou, D. Z.   Ivanova, G. I.  

    Solid-state Na-23 and P-31 magic-angle spinning nuclear magnetic resonance spectroscopy and X-ray crystallography have been used to study the structures of the chain metaphosphates NaCa(PO3)(3) and NaSr(PO3)(3). The compounds are isostructural and crystallise in space group P-1 with the following parameters: NaCa(PO3)(3), a = 6.711 angstrom, b = 6.934 angstrom, c = 7.619 angstrom, alpha = 83.44 degrees, beta = 81.41 degrees, gamma = 82.80 degrees; NaSr(PO3)(3) a = 6.805 angstrom, b = 7.133 angstrom, c = 7.720 angstrom and alpha = 83.71 degrees, beta = 80.48 degrees ,gamma = 82.87 degrees. Both structures contain anionic metaphosphate chains Of (PO3)(n)(n) with ionic contacts to Na+ ions in distorted octahedral sites and Ca2+ (or Sr2+) in distorted dodecahedral sites. P-31 and Na-23 NMR are entirely consistent with the crystallographic data and an empirical method for assigning P-31 resonances to particular crystal lographically unique P atoms is described. Copyright (C) 2008 John Wiley & Sons, Ltd.
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  • Soft-Mode Behavior in Ternary Silicides MAlSi (M=Ca, Sr, Ba)

    R. Heid   K. -P. Bohnen   B. Renker   P. Adelmann   T. Wolf   D. Ernst   H. Schober  

    We present a combined experimental and theoretical investigation of the lattice dynamics and electron–phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron–phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T c , among the investigated compounds can be explained by the presence of the soft mode.
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  • Soft-Mode Behavior in Ternary Silicides MAlSi (M=Ca, Sr, Ba)

    R. Heid   K. -P. Bohnen   B. Renker   P. Adelmann   T. Wolf   D. Ernst and H. Schober  

    We present a combined experimental and theoretical investigation of the lattice dynamics and electron–phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron–phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T c , among the investigated compounds can be explained by the presence of the soft mode.
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  • The non-centrosymmetric borate oxides, MBi2B2O7 (M = Ca, Sr)

    Barbier, J.   Cranswick, L. M. D.  

    Two novel noncentrosymmetric berates oxides, MBi2B2O7 or MBi2O(BO3)(2) (M=Ca, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600-700 degrees C range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi2B2O7 crystallizes in the orthorhombic Pna2(1) space group with a = 8.9371(5)angstrom, b = 5.4771(3)angstrom, c = 12.5912(7) angstrom, Z = 4, R-wp = 0.118, chi(2) = 2.30, SrBi2B2O7 crystallizes in the hexagonal P6(3) space group with a = 9.1404(4) angstrom, c = 13.0808(6) angstrom, Z = 6, R-wp = 0.115, chi(2) = 4.15. Large displacement parameters suggest the presence of disorder in SrBi2B2O7 as also revealed by diffuse 2 x a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO3 triangles and MO6 trigonal prisms forming six-membered rings in which Bi2O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi-O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi2B2O7 (d(eff) = 2d(eff)(KDP)). (c) 2006 Elsevier Inc. All rights reserved.
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  • The non-centrosymmetric borate oxides, MBi2B2O7 (M=Ca, Sr)

    J. Barbier   L.M.D. Cranswick  

    Two novel noncentrosymmetric borates oxides, MBi 2B 2O 7 or MBi 2 O(BO 3) 2 ( MCa, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600-700degC range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi 2 B 2O 7 crystallizes in the orthorhombic Pna 2 1 space group with a=8.9371(5)aring, b=5.4771(3)aring, c=12.5912(7)aring, Z=4, R wp=0.118, chi2=2.30. SrBi 2B 2O 7 crystallizes in the hexagonal P6 3 space group with a=9.1404(4)aring, c =13.0808(6)aring, Z=6, Rwp=0.115, chi2=4.15. Large displacement parameters suggest the presence of disorder in SrBi 2B 2O 7 as also revealed by diffuse 2times a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO 3 triangles and MO 6 trigonal prisms forming six-membered rings in which Bi 2O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi-O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi 2B 2O 7 ( deff=2 deff(KDP)). [All rights reserved Elsevier]
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  • Structures of the chain metaphosphates NaM(PO3)3 (M = Ca or Sr)

    I. Abrahams   G. E. Hawkes   A. Ahmed   T. Di Cristina   D. Z. Demetriou   G. I. Ivanova  

    Abstract Solid-state 23Na and 31P magic-angle spinning nuclear magnetic resonance spectroscopy and X-ray crystallography have been used to study the structures of the chain metaphosphates NaCa(PO3)3 and NaSr(PO3)3. The compounds are isostructural and crystallise in space group P−1 with the following parameters: NaCa(PO3)3, a = 6.711 Å, b = 6.934 Å, c = 7.619 Å, α = 83.44° , β = 81.41° , γ = 82.80° ; NaSr(PO3)3a = 6.805 Å, b = 7.133 Å, c = 7.720 Å and α = 83.71° , β = 80.48° , γ = 82.87° . Both structures contain anionic metaphosphate chains of (PO3)nn with ionic contacts to Na+ ions in distorted octahedral sites and Ca2+ (or Sr2+) in distorted dodecahedral sites. 31P and 23Na NMR are entirely consistent with the crystallographic data and an empirical method for assigning 31P resonances to particular crystallographically unique P atoms is described. Copyright © 2008 John Wiley & Sons, Ltd.
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  • Soft-mode behavior in ternary silicides MAlSi (M=Ca, Sr, Ba)

    Heid, R.   Bohnen, K. -P.   Renker, B.   Adelmann, P.   Wolf, T.   Ernst, D.   Schober, H.  

    We present a combined experimental and theoretical investigation of the lattice dynamics and electron-phonon coupling of the ternary silicides MAlSi (M = Ca, Sr, Ba). Temperature dependent inelastic neutron-scattering measurements of the generalized phonon density-of-states provide evidence for a soft-mode behavior for CaAlSi which is absent for the other two compounds. Density-functional-based calculations of phonon spectra and electron-phonon coupling support this interpretation and identify the soft mode as strong coupling out-of-plane Al vibrations, in agreement with previous theoretical work. Observed differences in superconducting properties, in particular in T (c) , among the investigated compounds can be explained by the presence of the soft mode.
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  • Structure Prediction of Binary Pernitride MN2 Compounds (M=Ca, Sr, Ba, La, and Ti)

    Kulkarni, A.   Schoen, J. C.   Doll, K.   Jansen, M.  

    Metal-pernitride compounds belong to a class of chemical systems in which both the complex ions and the non-bonding electrons may play roles in the formation of their modified crystalline structures. To investigate this issue, the energy landscapes of pernitrides of metals with different maximum valence (M=Ca, Sr, Ba, La, and Ti) were globally explored on the abinitio level at standard and high pressures, thereby yielding possible (meta)stable modifications in these systems together with information on how the landscape changed as function of the valence of the metal cation. For all of the systems in which no compounds had been synthesized so far, we predicted the existence of kinetically stable modifications that should, in principle, be experimentally accessible. In particular, TiN2 should crystallize in a new structure type, TiN2-I.
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  • Sulfur Tolerance of La0.3M0.7Fe0.7Cr0.3O3 -? (M= Sr, Ca) Solid Oxide Fuel Cell Anodes

    Addo, P. K.   Molero-Sanchez, B.   Buyukaksoy, A.   Paulson, S.   Birss, V.  

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  • Structure Prediction of Binary Pernitride MN2 Compounds (M=Ca, Sr, Ba, La, and Ti)

    Dr. A. Kulkarni   Prof. Dr. J. C. Schön   Dr. K. Doll and Prof. Dr. M. Jansen  

    Metal-pernitride compounds belong to a class of chemical systems in which both the complex ions and the non-bonding electrons may play roles in the formation of their modified crystalline structures. To investigate this issue, the energy landscapes of pernitrides of metals with different maximum valence (M=Ca, Sr, Ba, La, and Ti) were globally explored on the ab initio level at standard and high pressures, thereby yielding possible (meta)stable modifications in these systems together with information on how the landscape changed as function of the valence of the metal cation. For all of the systems in which no compounds had been synthesized so far, we predicted the existence of kinetically stable modifications that should, in principle, be experimentally accessible. In particular, TiN2 should crystallize in a new structure type, TiN2-I.
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  • Sulfur Tolerance of La0.3M0.7Fe0.7Cr0.3O3 -  (M= Sr, Ca) Solid Oxide Fuel Cell Anodes

    Addo, P. K.; Molero-Sanchez, B.; Buyukaksoy, A.; Paulson, S.; Birss, V.  

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  • Electrical conductivity and thermal expansion behavior of MMoO4 (M =3D Ca,Sr and Ba)

    Maji, Binoy Kumar   Jena, Hrudananda   Asuvathraman, R.   Kutty, K. V. Govindan  

    Alkaline earth (Ca, Sr, Ba) molybdates were synthesized by solid state reaction route. The compounds were characterized by powder-XRD, TG-DTA techniques. The electrical conductivities of these compounds were measured by AC-impedance technique at 673-1073 K. The activation energies of electrical conduction of CaMoO4, SrMoO4 and BaMoO4 were found to be 1.29 +/- 0.01 eV, 1.33 +/- 0.01 eV and 1.31 +/- 0.01 eV respectively. The linear thermal expansion of these molybdates was measured by dilatometry. The mean coefficients (alpha(m)) of thermal expansion for these compounds were found to be in the range of 9.38 +/- 0.18 x 10(-6)-12.96 +/- 0.25 x 10(-6) K-1 at 305-1005 K temperature range. The diffusion coefficient (D) values of oxide ion conduction for these molybdates were determined and found to be in the range of 9.48 +/- 0.02 x 10(-14)-3.32 +/- 0.01 x 10(-10) for CaMoO4, 5.86 +/- 0.02 x 10(-14)-2.50 +/- 0.01 x 10(-10) for SrMoO4 and 3.46 +/- 0.02 x 10(-14)-3.32 +/- 0.01 x 10(-10) cm(2) s(-1) for BaMoO4 at 673-1073 K range of temperature. (C) 2015 Elsevier B.V. All rights reserved.
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  • Study on Red Long-Time Luminescent Material Y2O2S:Eu, M (M = Mg, Ca, Sr, Ba)

    Chen Yongjie; Qiu Guanming; Geng Xiujuan; Yang Ying; Shi Shuang; Wang Hui; Zhang Zhongqiu  

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  • High thermoelectric figure of merit in nanostructured p-type PbTe-MTe (M = Ca, Ba)

    Biswas, Kanishka   He, Jiaqing   Wang, Guoyu   Lo, Shih-Han   Uher, Ctirad   Dravid, Vinayak P.   Kanatzidis, Mercouri G.  

    We demonstrate that by endotaxially embedding nanocrystals of the alkaline earth tellurides CaTe or BaTe in PbTe doped with Na(2)Te heat carrying phonons can be strongly scattered with little effect on the power factor. An intriguing observation is that the insertion of CaTe in PbTe increases the solubility of Na(2)Te, thereby allowing high levels of carrier concentration. Powder X-ray diffraction patterns reveal that all samples crystallize in the NaCl-type structure without noticeable secondary phase, while scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) demonstrate the presence of endotaxially coherent/semi-coherent nanostructures in the PbTe-CaTe/BaTe samples. We show that this is the principal factor for the very low thermal conductivity. Electron energy loss spectroscopy (EELS) confirms the presence of Ca in the nanoprecipitates. We present a comparison in the lattice thermal conductivity in going from nanocrystals of CaTe to SrTe to BaTe in PbTe. Temperature dependent Hall coefficient measurements of PbTe-CaTe materials suggest that the carrier population in the so-called heavy hole valence band in PbTe becomes important above 450 K and is responsible for the high power factors. The highest thermoelectric figure of merit, ZT, value achieved is similar to 1.5 at 765 K for 6% CaTe containing sample, while 3% BaTe containing sample shows the ZT of similar to 1.3 at 760 K.
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  • Electrical Properties of (CeO2)(0.9)(MO)(0.1) (M=Ca or Mg) Solid Electrolyte System

    Montalvo-Lozano, R. A.   Padmasree, K. P.   Fuentes, A. F.   Montemayor, S. M.  

    Electrical conductivity studies of alkaline earth oxides doped ceria Ce0.9M0.1O2-delta, (where M=Ca and Mg) is reported in this work. The samples were prepared at room temperature by mechanical milling. X-ray diffraction patterns confirm the cubic fluorite structure. Scanning electron micrographs of MgO doped sample shows the presence of the segregation of Mg2+ which results a negative effect on the electric properties of the MgO doped sample. Frequency dependence of ac conductivity (sigma(ac)) of these oxide ion conductors showed a remarkably different behavior from that of the cation ones. As temperature increases above 350 degrees C, ac conductivity sigma(ac) in the low frequency range was strongly influenced by space charge polarization at grain boundary and the high frequency plateau agreed with the bulk conductivity of the system. Bulk and grain boundary conductivity studies show that CaO doped ceria exhibit better electrical properties compared to MgO doped sample.
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  • M3V2B10O23 (M =3D Ca,Sr):two new vanadoborates with [V2B10O23](infinity) double layers

    Chen, Zhaohui   Liu, Yaqin   Shi, Yunjing   Zhang, Wenyao   Jing, Qun   Fang, Yuan   Dong, Xiaoyu  

    Two M3V2B10O23 (M =3D Ca, Sr) vanadoborates were synthesized by using a high-temperature solid-state technique for the first time. The single-crystal X-ray diffraction results showed that they both crystallized in the triclinic space group P (1) over bar, with unit cell parameters a =3D 6.525(3) angstrom, b =3D 11.462(4) angstrom, c =3D 12.714(5) angstrom, alpha =3D 114.126(10)degrees 1, beta =3D 103.656(10) 1, gamma =3D 90.594(10)degrees for Ca3V2B10O23 and a =3D 6.639(2) angstrom, b =3D 11.571(4) angstrom c =3D 12.998(4) angstrom, alpha =3D 114.333(3) 1, beta =3D 103.334(3) 1, gamma =3D 90.432(3)degrees for Sr3V2B10O23. Their main structural frameworks were observed to be novel "infinite'' 2D [V2B10O23](infinity) double layers composed of the [B5O9](infinity) infinite layers and VO4 tetrahedra. Functional groups present in the M3V2B10O23 (M =3D Ca, Sr) samples were identified in their infrared spectra. Their thermal properties, UV-Vis-NIR diffuse-reflectance spectra and elements analyses are also presented.
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