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'Phase transitions' in keywords Facet   Publication Year 1992  [X]
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1992[X]
1Author    S. Lewicki, B. Szafrańska, Z. PająkRequires cookie*
 Title    Molecular Motion in Solid Tetrapropylammonium Bromide and Iodide  
 Abstract    The proton NMR second moment and spin-lattice relaxation time for tetrapropylammonium bromide and iodide have been measured over a wide temperature range. A solid-solid phase transition related to the onset of cation tumbling was found for both salts and confirmed by DTA. In the low temperature phases methyl group reorientation was evidenced. For iodide a dynamic nonequivalence of the methyl groups and the onset of ethyl groups motion was also discovered. 
  Reference    Z. Naturforsch. 47a, 1115—1118 (1992); received July 14 1992 
  Published    1992 
  Keywords    NMR, Molecular motions, Phase transitions 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-1115.pdf 
 Identifier    ZNA-1992-47a-1115 
 Volume    47 
2Author    S. Torre, P. FerloniRequires cookie*
 Title    Molecular Motion in Solid Tetramethylammonium Tetrafluoroborate  
 Abstract    Measurements of n B and NMR relaxation times, differential scanning calorimetry and thermomechanical analysis, carried out on polycrystalline (CH 3) 4 NBF 4 from room temperature to about 100 K, have permitted to discuss the crystal dynamics of this salt and to analyze the nature of the phase transition occurring at «150K. Moreover, by means of an evaluation of the n B quadrupole coupling constant, information about the position of the F atoms has been obtained. An explanation of the NMR data in terms of an order-disorder phase transition involving the F~ ions is given. 
  Reference    Z. Naturforsch. 47a, 721—727 (1992); received March 24 1992 
  Published    1992 
  Keywords    (CH 3 ) 4 NBF 4, Molecular motion, Phase transition 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-0721.pdf 
 Identifier    ZNA-1992-47a-0721 
 Volume    47 
3Author    V. P. Tarasov, G. A. Kirakosyan, K. E. German, Acad Sei, UssrRequires cookie*
 Title    Anomalous Temperature Dependence of the "Tc Quadrupole Coupling and Disordering of Cesium Positions in CsTc0 4  
 Abstract    A "Tc and 133 Cs solid-state NMR study of polycrystalline CsTc04 at a field of 7.04 T and temperatures from 90 to 430 K revealed first-order quadrupole effects. "Tc quadrupole coupling constant, Cq, shows anomalous positive temperature coefficients and decreases from 2.0 MHz at 430 K to 0.45 MHz at 90 K. The asymmetry parameter, rj ("Tc), is zero at 430 K and at 210 K. The high-temperature minimum of rj is associated with a phase transition from orthorhombic to tetrag-onal and the second minimum reflects an orientational change of the principal axes q xx and q yy of the EFG-tensor. The 133 Cs NMR spectra are characterised by a superposition of the two powder multiplets from the first-order couplings which arise from magnetically and crystallographically nonequivalent cesium sites, Cs (1) and Cs (2); the population ratio of Cs (2) and Cs (1), p2/p1, varies from 2.0 at 120 K to 6.5 at 373 K. The nonequivalence and population changes are attributed to the temperature change of the crystal field potential in the vicinity of the cations. 
  Reference    Z. Naturforsch. 47a, 325—329 (1992); received July 26 1991 
  Published    1992 
  Keywords    Technetium, Cesium, Solid State NMR, Quadrupole Effects, Phase Transitions 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-0325.pdf 
 Identifier    ZNA-1992-47a-0325 
 Volume    47 
4Author    H. G. Kreul, R. Waldinger, A. WürflingerRequires cookie*
 Title    Differential Thermal Analysis and Dielectric Studies on Neopentanol under Pressure  
 Abstract    Differential thermal analysis (DTA) and dielectric measurements have been performed on 2,2-di-methyl-l-propanol (neopentanol) up to 200 MPa. Neopentanol exhibits at least one orientationally disordered (ODIC) phase (solid I) that transforms at lower temperatures to a non-plastic phase (solid II). There is evidence of a further ODIC phase denoted as solid I'. The pressure dependence of the phase transitions and the dielectric behaviour up to frequencies of 13 MHz are described. Activation enthalpies and volumes are derived from the dielectric relaxation time and compared with results for other alcohols. 
  Reference    Z. Naturforsch. 47a, 1127—1134 (1992); received July 27 1992 
  Published    1992 
  Keywords    High pressure, DTA, Phase transitions, Dielectric constant, and relaxation 
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 TEI-XML for    default:Reihe_A/47/ZNA-1992-47a-1127.pdf 
 Identifier    ZNA-1992-47a-1127 
 Volume    47