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'Phase Transition' in keywords Facet   Publication Year 1996  [X]
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1996[X]
1Author    Tetsuo Asaji, Masaki Ohkawa, Takehiko Chiba, Makoto KagaRequires cookie*
 Title    Successive Phase Transitions in Potassium Hexachloroselenate(IV) Revealed by 35 C1 NQR  
 Abstract    K 2 SeCl 6 undergoes structural phase transitions of first order at 35 K, 65 K, and 79 K. The temperature dependence of the ? C1 NQR spin-lattice relaxation time suggests that the phase transitions at 65 K and 79 K are displacive ones associated with a resonant soft mode. The presence of a precursor cluster is suggested by the temperature dependence of the signal intensity above 79 K. 
  Reference    Z. Naturforsch. 51a, 705—709 (1996); received October 20 1995 
  Published    1996 
  Keywords    Phase transition, Soft mode, NQR, Spin-lattice relaxation 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0705.pdf 
 Identifier    ZNA-1996-51a-0705 
 Volume    51 
2Author    M. Grottel, A. Kozak, Z. PająkRequires cookie*
 Title    *H and 19 F NMR Study of Cation and Anion Motions in Guanidinium Hexafluorozirconate  
 Abstract    Proton and fluorine NMR second moments and spin-lattice relaxation times of polycrystalline guanidinium hexafluorozirconate and its deuterated analogue were studied in laboratory (60 MHz) and rotating (H, = 20 G) frames over a wide range of temperature. An analysis of the experimental results enabled us to reveal a dynamical inequivalence of two crystallographically independent cations and an unexpected high mobility of nonspherical anion dimers. A comparison of the ions dynamics in 2:1 complex studied with the guanidinium 1:1 and 3:1 complexes has shown a significant contribution of the hydrogen bonds to the potential barriers hindering the anion reorien-tations. At low temperatures a proton motion in the hydrogen bond and at 400 K a solid-solid phase transition have been discerned. 
  Reference    Z. Naturforsch. 51a, 991—996 (1996); received May 14 1996 
  Published    1996 
  Keywords    NMR, Ion Motions, Hydrogen Bonds, Phase Transitions 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0991.pdf 
 Identifier    ZNA-1996-51a-0991 
 Volume    51 
3Author    GaryL. Catchen, EdwardF. Hollinger, ToddM. RearickRequires cookie*
 Title    Characterizing Phase Transitions in the Perovskites PbTi0 3 and BaTi0 3 Using Perturbed-Angular-Correlation Spectroscopy  
 Abstract    Perturbed-angular-correlation (PAC) spectroscopy was used to measure in ceramic samples of PbTi0 3 and BaTi0 3 the temperature dependence of the Ti-site electric-field gradients (EFGs) at temperatures very close to the ferroelectric-to-paraelectric transition temperatures T c . The samples were doped with small amounts of Hf that carried the 181 Hf — l8l Ta probe radioactivity. A high-frequency nuclear quadrupole interaction that decreases very little as the temperature approaches T c , characterizes the PbTi0 3 transition. The tetragonal and cubic phases for PbTi0 3 appear to coexist over a temperature interval of 8 ± 1 K, and the transition shows a thermal hysteresis of about 4 K. In contrast, a lower-frequency interaction that decreases rapidly as temperature approaches T c , characterizes the BaTiO, transition. Both phases of BaTi0 3 appear to coexist over an interval of about 2 K, and the thermal hysteresis is about 1 K. At temperatures above T c , both PbTi0 3 and BaTi0 3 show weak, non-vanishing Ti-site EFGs. Although, for BaTi0 3 , this effect limits the accuracy with which critical effects can be measured, we estimate a power-law exponent ß = 0.21 ± 0.05, which most likely is somewhat lower in magnitude than the actual critical exponent. For the explanation of our observations we assume the existence of a distribution of T c -values. This distribution would arise because the crystals could have spatially non-uniform distributions of nucleation sites, which for PbTi0 3 and BaTi0 3 could be point defects. 
  Reference    Z. Naturforsch. 51a, 411—421 (1996); received November 20 1995 
  Published    1996 
  Keywords    Ferroelectric, perovskite, nuclear electric-quadrupole, perturbed-angular-correlations, phase transition 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0411.pdf 
 Identifier    ZNA-1996-51a-0411 
 Volume    51 
4Author    Tsutomu Okuda, Shusaku Gotou, Toshirou Takahashi, Hiromitsu Terao, Koji YamadaRequires cookie*
 Title    Structure and Successive Phase Transitions of RGeBr 3 (R = Alkylammonium) Studied by Means of NQR  
 Abstract    A series of tribromogermanate(II) complexes RGeBr 3 (R = CH 3 NH 3 , (CH 3) 2 NH 2 , (CH 3) 4 N, C 2 H 5 NH 3 , (C 2 H 5) 4 N) have been synthesized and characterized by 81 Br NQR, AC conductivity, DTA, and X-rax diffraction. These measurements revealed the presence of successive phase transi-tions in these complexes. The crystal structures of Phases I and II in (NH 3) 4 NGeBr 3 were derived from the Rietveld analysis of powder X-ray diffraction. The electric conductivity was considerably high in CH 3 NH 3 GeBr 3 and (NH 3) 4 NGeBr 3 . The conduction mechanism is discussed on the basis of 81 Br NQR spin-lattice relaxation times and crystal structures. 
  Reference    Z. Naturforsch. 51a, 686—692 (1996); received October 10 1995 
  Published    1996 
  Keywords    81 Br NQR, T x, Phase Transition, Ionic Motion, Ionic Conductivity 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0686.pdf 
 Identifier    ZNA-1996-51a-0686 
 Volume    51 
5Author    K. Yamada, T. Ohtani, S. Shirakawa, H. Ohki, T. Okuda, T. Kamiyama, K. OikawaRequires cookie*
 Title    Phase Transition of C 5 H 5 NHSbBr 4 Having a Hypervalent Bond: A 2 H NMR and X-Ray Diffraction Study* [Phase Transition of C 5 H 5 NHSbBr 4 l  
 Abstract    The second order phase transition at 253 K of pyridinium tetrabromoantimonate(III), C 5 H 5 NHSbBr 4 , has been characterized by means of X-ray diffraction and 2 H NMR. As was suggested from the 81 Br NQR spectra, the Rietveld refinements of the X-ray diffraction patterns at 297 K and 116 K confirm the structural change of the anion from SbBr 4 to SbBr 3 • Br -. The line shape analysis of the 2 H NMR using the deuterated analog revealed that the librational amplitude of the pyridinium ring about the axis normal to the plane increased with temperature and its rms amplitude reaching ca. 25° at T c . The librational motion may induce the phase transition breaking the hydrogen bonds of the type N-H -Br. However, nearest-neighbor 2n/6 jump or reorientation such as seen in benzene were not observed even at 350 K. 
  Reference    Z. Naturforsch. 51a, 739—744 (1996); received October 11 1995 
  Published    1996 
  Keywords    2 H NMR, 81 Br NQR, Rietveld analysis, Phase transition, Hypervalent bond 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0739.pdf 
 Identifier    ZNA-1996-51a-0739 
 Volume    51