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'Phase Transitions' in keywords Facet   Publication Year 1997  [X]
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1997[X]
1Author    M. Grottel, M. Szafrański, Z. PająkRequires cookie*
 Title    NMR Study of Cation Motion in Guanidinium Iodoplumbates  
 Abstract    Proton NMR second moments and spin-lattice relaxation times of polycrystalline C(NH 2) 3 PbI 3 and [C(NH 2) 3 ]2PbI 4 were studied in a wide temperature range. The order-disorder character of the existing solid-solid phase transitions was confirmed and for tetraiodoplumbate a new phase transition at 170 K was discovered. For both compounds a dynamical inequivalence and unusual high mobility of the guanidinium cations were revealed. For the reorientations discovered (NH 2 group flipping, C 3 in-plane reorientation and a tumbling) the activation parameters were determined. 
  Reference    Z. Naturforsch. 52a, 783—788 (1997); received October 6 1997 
  Published    1997 
  Keywords    NMR, Molecular Motions, Phase Transitions 
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 TEI-XML for    default:Reihe_A/52/ZNA-1997-52a-0783.pdf 
 Identifier    ZNA-1997-52a-0783 
 Volume    52 
2Author    Roman Goc, JanW. WąsickiRequires cookie*
 Title    Computer Simulation of the Order-Disorder Transition in Ammonium Chloride  
 Abstract    The phase transition at T c = 243 K in ammonium chloride is well established to be of the order-disorder type. The ammonium chloride has a CsCl-type cubic structure with hydrogen atoms pointing toward four of the eight chloride ions at the corners of the unit cell. This means that there are two possible orientations of ammonium tetrahedron; a and b. In the ordered phase below the T c temperature all ammonium ions have the same, let it be a, orientation. Above the T c temperature ions are randomly distributed among the two possible orientations a and b. An attempt was made to correlate such structural transition with the second moment of the proton NMR absorption line. This approach was chosen because the second moment is the parameter of the NMR line for which the exact theoretical formula exist, and which is mainly a function of the structure and the reorien-tational state of the studied material. It was found that the NMR second moments calculated for the described above model of the structural transition properly reflect the changes of the experimental NMR second moments measured at temperatures below and above this transition. 
  Reference    Z. Naturforsch. 52a, 609—613 (1997); received June 18 1997 
  Published    1997 
  Keywords    NMR, Ammonium chloride, Phase transition, Simulation 
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 TEI-XML for    default:Reihe_A/52/ZNA-1997-52a-0609.pdf 
 Identifier    ZNA-1997-52a-0609 
 Volume    52 
3Author    Hiroyuki Ishida, Hiroshi Ono, Ryuichi IkedaRequires cookie*
 Title    Crystal Structure of the High-temperature Solid Phases of Choline Tetrafluoroborate and Iodide  
 Abstract    The crystal structure of the highest-and second highest-temperature solid phases of choline tetrafluoroborate and iodide was determined by X-ray powder diffraction. The structure in the highest-temperature phase of both salts is NaCl-type cubic (a = 10.16(2) Ä, Z = 4 for tetrafluorobo-rate; a = 10.08(2) A, Z = 4 for iodide). The second highest-temperature phase of tetrafluoroborate and iodide is CsCl-type cubic (a = 6.198(6) Ä and Z = 1) and tetragonal (a = 8.706(2) A, c = 6.144(6) Ä, and Z = 2), respectively. DSC was carried out for the iodide, where the presence of three solid-solid phase transitions was confirmed. Enthalpy and entropy changes of these transitions were evaluated. 
  Reference    Z. Naturforsch. 52a, 679—680 (1997); received June 17 1997 
  Published    1997 
  Keywords    Crystal structure, Powder X-ray diffraction, Phase transition 
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 TEI-XML for    default:Reihe_A/52/ZNA-1997-52a-0679_n.pdf 
 Identifier    ZNA-1997-52a-0679_n 
 Volume    52 
4Author    J. Furtak, Z. Czapla, A. V. KitykRequires cookie*
 Title    Ultrasonic Studies of Ferroelectric Phase Transition in Gly-H 3 P0 3 Crystals  
 Abstract    The temperature behaviour of the longitudinal and transverse ultrasound wave velocities and attenuation in the region of the ferroelectric phase transition (T c — 225 K.) of the glycine phosphite (Gly-H 3 P0 3) crystals is studied. The obtained results are considered within the framework of phenomenological theory. 
  Reference    Z. Naturforsch. 52a, 778—782 (1997); received October 4 1997 
  Published    1997 
  Keywords    Ferroelectricity, Phase Transition, Ultrasonic Velocity, Attenuation 
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 TEI-XML for    default:Reihe_A/52/ZNA-1997-52a-0778.pdf 
 Identifier    ZNA-1997-52a-0778 
 Volume    52 
5Author    M. Sandmann, F. Hamann, A. WürflingerRequires cookie*
 Title    PVT Measurements on 4-/i-Pentyl-4'-Cyano-Biphenyl (5CB) and trans-4-(4'-Octyl-Cyclohexyl)-Benzonitrile (8PCH) up to 300 MPa  
 Abstract    Specific volumes are presented for 4-n-pentyl-4'-cyanobiphenyl (5CB) and trans-4-(4'-octyl-cyclo-hexyl)-benzonitrile (8PCH) for temperatures between 300 and 370 K up to 300 MPa. The p,V m ,T data were determined for the liquid crystalline and isotropic phases, and partly also for the solid phase adjacent to the melting curve. Stable and metastable crystal phases can be distinguished. The density and melting temperature of the metastable form are lower than for the stable one. Volume and enthalpy changes accompanying the phase transitions are reported as well. The p,V m ,T data allow to calculate the entropy change for a hypothetical transition at constant volume. The molar volumes along the nematic-isotropic phase transition T NI (p) allow to determine the molecular field parameter y = S In T NI /0 In F N ,. 
  Reference    Z. Naturforsch. 52a, 739—747 (1997); received August 13 1997 
  Published    1997 
  Keywords    5CB, 8PCH, High pressure, pVT, Phase transitions, Thermodynamics, DTA 
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 TEI-XML for    default:Reihe_A/52/ZNA-1997-52a-0739.pdf 
 Identifier    ZNA-1997-52a-0739 
 Volume    52