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'Phase transition' in keywords Facet   Publication Year 1998  [X]
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1998[X]
1Author    R. Tchukvinskyi, R. Cach, Z. CzaplaRequires cookie*
 Title    Dielectric Study of the Ferroelectric Phase Transition in DMAGaS Crystal  
 Abstract    Electric permittivity measurments as functions of temperature at constant dc electric field and as func-tions of the dc electric field at constant temperatures in the paraelectric phase have been performed for DMAGaS crystal. The changes of the permittivity maximum with the electric field intensity evidence the first-order character of the ferroelectric phase transition at T cX . The field dependence of the permit-tivity e(E) in the paraelectric phase is discussed, using the classical electric equation of state. A meth-od for the determination of the corresponding coefficients is proposed. The obtained B and C coeffi-cients, together with double hysteresis loop observed above T cl prove the first-order character of the ferroelectric phase transition in DMAGaS crystal. 
  Reference    Z. Naturforsch. 53a, 105—111 (1998); received October 15 1998 
  Published    1998 
  Keywords    Ferroelectricity, Phase Transition, Dielectric Properties 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0105.pdf 
 Identifier    ZNA-1998-53a-0105 
 Volume    53 
2Author    M. Windhaus, B. D. Mosel, W. Müller-WarmuthRequires cookie*
 Title    Mössbauer Studies of Fe 2+ in Iron Langbeinites and other Crystals with Langbeinite Structure  
 Abstract    57 Fe Mössbauer spectra have been measured at various temperatures between 4.2 K and 300 K for iron langbeinites A 2 Fe 2^04)3 with A = K, NH 4 , Rb, T1 and magnesium, manganese and cadmium lang-beinites doped with Fe" + . The spectra revealed several contributions whose isomer shifts and quadru-pole splittings have been obtained by fitting program routines. For the high-temperature cubic phases two crystallographically non-equivalent iron sites have been identified, characteristic of Fe 2+ in the high-spin state. Abrupt changes of the quadrupole couplings indicated phase transitions; in some cases, the spectra have also revealed several sites for Fe 2+ in low temperature phases. From the temperature de-pendences, phase transition temperatures, crystal field splittings and Debye temperatures have been de-rived. 
  Reference    Z. Naturforsch. 53a, 27—37 (1998); received January 21 1998 
  Published    1998 
  Keywords    57 Fe Mössbauer Spectroscopy, Inorganic Crystals, Structure, Phase Transitions 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0027.pdf 
 Identifier    ZNA-1998-53a-0027 
 Volume    53 
3Author    M. Sandmann, A. WürflingerRequires cookie*
 Title    PVT Measurements on 4'-w-Hexyl-Biphenyl-4-Carbonitrile (6CB) and 4 -w-Heptyl-Biphenyl-4-Carbonitrile (7CB) up to 300 MPa  
 Abstract    P, V m , 7" data were established for 4'-n-hexyl-biphenyl-4-carbonitrile (6CB) and 4'-n-heptyl-biphen-yl-4-carbonitrile (7CB) between 300 and 370 K up to 300 MPa, and specific volumes were determined for the liquid crystalline, isotropic, and also partly for the crystal phases. Volume and enthalpy chang-es at the phase transitions are also presented. In the case of 6CB, a new crystal phase has been detect-ed, corresponding to a triple point at 338 K and 196 MPa. The p, V m , T data enabled us to separate the entropy change into a volume-dependent part and configurational part. From the molar volumes along the nematic-isotropic phase transition T Nl (p), the molecular field parameter y= 3lnT NI /3ln V NI was deter-mined. 
  Reference    Z. Naturforsch. 53a, 233—238 (1998); received March 11 1998 
  Published    1998 
  Keywords    6CB, 7CB, High Pressure, pVT, Phase transitions, Thermodynamics 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0233.pdf 
 Identifier    ZNA-1998-53a-0233 
 Volume    53 
4Author    Tetsuo Asaji, Keizo Horiuchi, Takehiko Chiba, Takashige Shimizu, Ryuichi IkedaRequires cookie*
 Title    NMR * NQR and DTA * DSC Studies of Phase Transitions in Pyridinium Tetrachloropalladate(II) and Pyridinium Tetrachloroplatinate(II)  
 Abstract    From the measurements of DTA • DSC and the temperature dependences of 35 C1 NQR frequen-cies, phase transitions were detected at 150 K, 168 K, and 172 K for (pyH) 2 [PtCl 4 ], and at 241 K for (P yH) 2 [PdCl 4 ], In order to elucidate the motional state of the constituent ions in the crystals in connec-tion with the structural phase transitions, the 35 C1 NQR and 'H NMR spin-lattice relaxation times and the second moment of the 'H NMR line were measured as functions of temperature. For both com-pounds, the potential wells for the cationic reorientation are suggested to be highly nonequivalent at low temperatures. Above 168 K, the pyridinium ions in (pyH) 2 [PtCl 4 ] are expected to reorient between almost equivalent potential wells. As for (pyH) 2 [PdCl 4 j, it is expected that the orientational order of the cation still remains even above the second order transition at 241 K. A change of the potential curve from two-unequal to three-unequal wells is proposed as a possible mechanism of the second order tran-sition. The activation energies for the cationic motion in the respective model potential are derived for both compounds at high and low temperatures. 
  Reference    Z. Naturforsch. 53a, 419—426 (1998); received December 30 1997 
  Published    1998 
  Keywords    Phase transition, NQR, NMR, Spin-lattice relaxation, DSC 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0419.pdf 
 Identifier    ZNA-1998-53a-0419 
 Volume    53 
5Author    Masao Hashimoto, Yuko Monobe, Hiromitsu Terao, Haruo Niki, Koichi ManoRequires cookie*
 Title    Phase Transition and Crystal Dynamics of 4-Bromobenzyl Alcohol  
 Abstract    For the title compound a phase transition from Phase II to Phase I (low and room temperature phases, respectively) was found at ca. 217 K. The temperature dependence of the 8 'Br NQR frequency and that of the dielectric constant showed anomalies at ca. 195 K that were tentatively attributed to a higher order phase transition. A similar anomaly was found at ca. 218 K for 4-chlorobenzyl alcohol which showed a II-I transition at 236 K. The dielectric dispersion observed for both compounds at low temperatures indicates an excitation of a molecular motion with the dielectric relaxation rate of ca. 1 kHz. The temperature dependence of the sl Br NQR frequencies of 2-and 3-bromobenzyl alcohol, measured at T > 77 K, gave no evidence of phase transition in their crystals. 
  Reference    Z. Naturforsch. 53a, 436—441 (1998); received March 24 1998 
  Published    1998 
  Keywords    NQR, Phase Transition, Molecular Motion, Dielectric Dispersion 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0436.pdf 
 Identifier    ZNA-1998-53a-0436 
 Volume    53 
6Author    M. Sandmann, A. WürflingerRequires cookie*
 Title    PVT Measurements on 4'-/i-Octyl-Biphenyl-4-Carbonitrile (8CB) up to 300 MPa  
 Abstract    P, V m , Tdata have been measured for the smectic, nematic and isotropic phases of 4'-n-octyl-biphen-yl-4-carbonitrile (8CB) in the temperature range 300-370 K and pressures up to 300 MPa. At atmos-pheric pressure all phase transitions appear to be of first order due to a discontinuity in the density. The volume change at the smectic A -nematic transition is only a tenth of the volume change at the clear-ing temperature. At moderate pressures below 80 MPa the S A -N transition could be detected as a dis-continuity in the period of oscillation in measurements with a high-pressure vibrating tube densimeter. At higher pressures the discontinuity seems to die away, possibly indicating a change from first order to second order transition. From the volume changes and the slopes of the transition lines we calculate the enthalpy changes at the phase transition. The p, V m , T data enable us to calculate the volume part of the entropy and the molecular field parameter y=9ln 7 N i/3ln V NI . 
  Reference    Z. Naturforsch. 53a, 787—792 (1998); received July 10 1998 
  Published    1998 
  Keywords    8CB, High Pressure, pVT, Phase Transitions, Thermodynamics 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0787.pdf 
 Identifier    ZNA-1998-53a-0787 
 Volume    53 
7Author    Hiroyuki IshidaRequires cookie*
 Title    H and 19 F NMR Studies on Molecular Motions in Two Solid Phases of t-Butylammonium Tetrafluoroborate  
 Abstract    Differential thermal analysis (DTA), differential scanning calorimetry (DSC), and the temperature dependence of the spin-lattice relaxation time (T {) and the second moment (M 2) of 1 H and F NMR were studied in (CH3) 3 CNH 3 BF4 and (CH 3) 3 CND 3 BF 4 . DTA and DSC revealed a solid-solid phase transition at 219 K for (CH 3) 3 CNH3BF4 and at 221 K for (CH 3) 3 CND 3 BF 4 . The motions of cations and anions in the two solid phases were studied by T x and M 2 experiments. The motional modes of the ions and their motional parameters were determined. 
  Reference    Z. Naturforsch. 53a, 796—800 (1998); received May 30 1998 
  Published    1998 
  Keywords    Molecular motion, Phase transition, Nuclear magnetic resonance 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0796.pdf 
 Identifier    ZNA-1998-53a-0796 
 Volume    53 
8Author    Yoshio Kume, Tetsuo AsajiRequires cookie*
 Title    Chlorine-35 NQR Study of a Structural Phase Transition in (ND 4 ) 2 PdCV  
 Abstract    Temperature dependences of 35 C1 NQR frequencies and spin-lattice relaxation times were measured at 4.2 to 400 K for natural and deuterated ammonium hexachloropalladate. It was confirmed that only the deuterated salt undergoes a first order phase transition at 30 K. The crystal structure of the low-tem-perature phase is predicted to be the same as that of the deuterated ammonium hexachloroplatinate and hexachloroplumbate. The mechanism of the deuteration-induced phase transition is discussed. 
  Reference    Z. Naturforsch. 53a, 514—517 (1998); received March 24 1998 
  Published    1998 
  Keywords    Isotope Effect, Phase Transition, Chlorine NQR, Spin-lattice Relaxation, Deuterated Ammonium Hexachloropalladate 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0514.pdf 
 Identifier    ZNA-1998-53a-0514 
 Volume    53 
9Author    Hiromitsu Terao, Masao Hashimoto\, Tsutomu Okuda, Alarich WeissRequires cookie*
 Title    Phase Transitions and Crystal Structure of Dimethylammonium Tribromomercurate(II), (CH 3 ) 2 NH 2 HgBr 3 , as Studied by 81 Br NQR and Single Crystal X-ray Diffraction  
 Abstract    The temperature dependence of the 81 Br NQR frequencies of the title compound has revealed the presence of three phases (I, II and III, in the order of decreasing temperature): T cl (II -1) = (318 ± 10) K and T c2 (III -II) = (202 ± 1) K. The transitions were confirmed by thermal analysis (DTA). Both of the II-I and I-II transitions exhibited strong hysteresis (/. e., superheating and supercooling, respectively). The crystal data, determined by single crystal X-ray diffraction, are: monoclinic, space group P2/a, a = 1384.8(3), b = 934.7(4), c = 1452.0(2) pm, ß = 104.10(1)°, Z= 8, R = 0.076. The crystal has two crystallographically independent cations (CH 3) 2 NH 2 + and an infinite chain of anions almost in the c-direction. The anion chain is considered to consist of a HgBr 2 molecule and two different HgBr 4 2 ~ anions which are interconnected via weak intermolecular Hg-Br bonds. The splitting patterns of the 8 'Br NQR spectra indicate that rearrangements of the Hg-Br bonds are slight at the III-II transition but serious at the II-I transition. Some of the 81 Br NQR lines show anomalous temperature coefficients, attributable probably to thermal motions of cations. 
  Reference    Z. Naturforsch. 53a, 559—567 (1998); received January 26 1998 
  Published    1998 
  Keywords    (CH 3 ) 2 NH 2 HgBr 3, X-Ray Analysis, NQR, Phase Transition, Crystal Structure 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0559.pdf 
 Identifier    ZNA-1998-53a-0559 
 Volume    53