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2000[X]
1Author    Tetsuo Asaji, Tomohiro IshizakaRequires cookie*
 Title    Effect of Deuteration on Ammonium Motion and Structural Phase Transition Studied by Nuclear Quadrupole Resonance  
 Abstract    35 C1 NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH4ICI4. By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH4ICI4, by the deuteration a remarkable change of the temperature dependence of the 35 C1 NQR frequencies was observed. The NQR spin-lattice relaxation-time minimum, ascribable to the modulation effect of the electric field gradient (EFG) due to a motion of the ammonium ions, was observed at 7 K and 11 K in NH4ICI4 and ND4ICI4, respectively. The activation energy of the ammonium motion and the fluctuation fraction q /q of the EFG were determined to be 0.32 kJ moP 1 and 1.3xlO~ 3 for NH4ICI4, and 0.30 kJ mol -1 and 0.88xl0~ 3 for ND4ICI4. The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl^ complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND4ICI4 than in NH4ICI4. This suggests that the lattice distortion in NH4ICI4 is suppressed to a great deal by the tunneling rotation of NH4 ions. The effective inter-ionic interaction responsible for the lattice distortion, the transition temperature and the enhancement of tunneling of NH4 as compared with ND4 is discussed for NH4ICI4 as well as for previously reported NH4AUCI4 and ct-N^HgCb. 
  Reference    Z. Naturforsch. 55a, 83—89 (2000); received August 26 1999 
  Published    2000 
  Keywords    Isotope Effect, Tunneling, Phase Transition, Ammonium, NQR 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0083.pdf 
 Identifier    ZNA-2000-55a-0083 
 Volume    55 
2Author    Y. M. Seo, S. K. Song, J. PelzpRequires cookie*
 Title    Impurity Effects on the Local Structure in the Mixed Hexachlorometallate (A 2 BC1 6 : A = K, Rb, B = Sn, Re, and Pb) Studied by the Chlorine NQR  
 Abstract    Chlorine NQR was studied for the isostructural hexachlorometallate mixed system. The study shows that Isomorphic hexachlorometallate solid solutions exhibit often impurity induced local structural order because of their relatively clear local site symmetry. This is manifested in the formation of a few satellite lines near the original resonance line and results from the random distribution of impurities on the lattice sites of the corresponding counterpart ions. Using the point charge model and a simple binomial function for the occupation probability of the guest ions on the host lattice sites, the position and the line intensity could be determined, the results of which are in good agreement with the NQR-observation. The temperature region of lattice dynamics in the crystal seems to shift in proportion to the impurity content. This fact explains the gradual change of the transition temperature in the mixed crystal between two starting materials. 
  Reference    Z. Naturforsch. 55a, 207—211 (2000); received August 23 1999 
  Published    2000 
  Keywords    NQR, Hexahalmetallates, Phase Transition, Mixed Crystals 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0207.pdf 
 Identifier    ZNA-2000-55a-0207 
 Volume    55 
3Author    Hideta Ishihara, Keizo Horiuchi, ThorstenM. Gesing, Shi-Qi Dou, J.-Christian Buhl, Hiromitsu TeraoRequires cookie*
 Title    I NQR and Crystal Structure Studies of [N(CH 3 ) 4 ] 2 CdI 4  
 Abstract    The temperature dependence of 127 1 NQR and DSC as well as the crystal structure at room temperature of the title compound were determined. This compound shows a first-order phase transition of an order-disorder type at 245 K. Eight 127 I(i^i: m = ±5 <-+ ±|) NQR lines of 79. corresponding eight 127 I(^ 2 : m = ±| <-» • ±|) NQR lines were observed at liquid nitrogen temperature. Three 127 I(^i) NQR lines wfth an intensity ratio of 1:1:2 in the order of decreasing frequency were observed just above the transition point and two NQR lines except for the middle-frequency line disappeared around room temperature. This temperature behavior of NQR lines is very similar to that observed in [N(CH 3)4] 2 Hgl4. Another first-order phase transition takes place at 527 K. The structure of the room-temperature phase was redetermined: orthorhombic, Pnma, Z = 4, a = 1342.8(3), b = 975.7(2), c = 1696.5(3) pm. The NQR result of three lines with an intensity ratio of 1:1:2 is in agreement with this structure. The thermal displacement parameters of atoms in both cations and anions are large. 
  Reference    Z. Naturforsch. 55a, 225—229 (2000); received October 13 1999 
  Published    2000 
  Keywords    NQR, DSC, Crystal Structure, Phase Transition 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0225.pdf 
 Identifier    ZNA-2000-55a-0225 
 Volume    55 
4Author    Hideta Ishihara, Keizo Horiuchi, V. G. Krishnan, Ingrid Svoboda, Hartmut FuessRequires cookie*
 Title    Isolated versus Condensed Anion Structure VI: X-ray Structure Analysis and 81 Br NQR of Guanidinium Pentabromodicadmate(II), [C(NH 2 ) 3 ]Cd 2 Br 5 , fns-Hydrazinium Pentabromocadmate(II), [H 2 NNH 3 ] 3 CdBr 5 , and fo's-Hydrazinium Tetrabromocadmate(II)-Tetra Hydrate, [H 2 NNH 3 ] 2 CdBr 4 -4H 2 0  
 Abstract    The structure of the condensed bromocadmate anions in [C(NH 2) 3 ]Cd 2 Br 5 (1) and [H 2 NNH 3 ] 3 CdBr 5 (2) were studied at room temperature by X-ray diffraction. (1) crystallizes with double-chains bridged by Br atoms (orthorhombic, Pmmn, Z= 2, a = 1394.0(5), b = 394.5(1), c = 1086.9(5) pm). This chain structure was not described previously. (1) shows three 8 'Br NQR lines at temperatures between 77 and 323 K. Structural phase transitions take place at 283 K and at 535 K. (2) crystallizes with Br bridged zigzag-chains (monoclinic, P2[, Z=2,a = 943.1(1), b = 778.8(2), c = 942.0(2) pm, ß = 102.10(2)°) and shows a first-order phase transition around 304 K with a large thermal hysteresis. Below the transition point five 81 Br NQR lines are observed at temperatures between 122 and 304 K, and above the transition point four 81 Br NQR lines at tem-peratures between 288 and 353 K. Two 81 Br NQR lines are observed in [H 2 NNH 3 ] 2 CdBr 4 -4H 2 0 (3) at temperatures between 77 and around 320 K with positive temperature coefficients. 
  Reference    Z. Naturforsch. 55a, 390—396 (2000); received November 19 1999 
  Published    2000 
  Keywords    NQR, DSC, Crystal Structure, Phase Transition, Bromocadmate 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0390.pdf 
 Identifier    ZNA-2000-55a-0390 
 Volume    55 
5Author    Maciej Bujak, Jacek ZaleskiRequires cookie*
 Title    Structure and Phase Transition in (C 2 H 5 NH 3 ) 3 Sb 2 CI 9 (C 2 H 5 NH 3 )SbCl 4 ; X-ray, DSC and Dielectric Studies  
 Abstract    The structure of (C 2 H 5 NH 3) 3 Sb 2 Cl 9 • (C 2 H 5 NH 3)SbCl 4 at 295 K has been determined. The crystals are orthorhombic, space group Pna2! (a -16.925(3), b = 24.703(5), c = 7.956(2) A, V = 3326.4(12) A 3 , Z = 4, d c = 2.018, d m = 2.01(1) Mg m -3). They consist of an anionic sublattice composed of two different polymeric zig-zag chains. One is built of Sb 2 Cl 9 3-units (corner sharing octahedra) and the other one is made of corner sharing SbCl^ 2-square pyramids. In the cavites between the polyanionic chains four non-equivalent ethylammonium cations are located. Three of them are disordered. The cations are connected to the anions by weak N-H...C1 hydrogen bonds. A first order phase transition of the order-disorder type was found at 274 K. It was studied by DSC, dielectric and X-ray diffraction methods. The mechanism of the phase transition is attributed to the ordering of at least one of the ethylammonium cations. 
  Reference    Z. Naturforsch. 55a, 526—532 (2000); received January 10 2000 
  Published    2000 
  Keywords    Ethylamine, Chloroantimonate(III), Structure, Phase Transition 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0526.pdf 
 Identifier    ZNA-2000-55a-0526 
 Volume    55 
6Author    A. Wiirflinger, M. SandmannRequires cookie*
 Title    Thermodynamic Measurements on w-Hexadecane (C 16 H 34 ) and /z-Heptadecane (C 17 H 36 ) at Elevated Pressures  
 Abstract    Specific volumes of/7-hexadecane (H-Ci 6 H 34) and n-heptadecane (fl-C 17 H 36) are presented for tem-peratures between 298 and 348 K and pressures up to 280 MPa. The melting curve for /7-C j6 H 34 was determined with differential thermal analysis. ThepVTdata are established for the liquid, solid and (in the case of J?-C| 7 H 36) rotator phases. Forn-C 16 H 34 the density immediately below the freezing temper-ature is larger than for n-Ci 7 H 36 immediately below the rotational transition temperature. Changes of volume, enthalpy, and entropy along the phase transitions are reported. The volume change of melting decreases distinctly with increasing pressure, whereas the volume change at the rotational transition is much less pressure-dependent. Enthalpy and entropy changes have been calculated with the aid of the Clausius-Clapeyron equation. 
  Reference    Z. Naturforsch. 55a, 533—538 (2000); received February 16 2000 
  Published    2000 
  Keywords    Hexadecane, Heptadecane, DTA, /?V7, High Pressure, Phase Transitions 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0533.pdf 
 Identifier    ZNA-2000-55a-0533 
 Volume    55 
7Author    M. F. Mostafa, S. S. ArafatRequires cookie*
 Title    Dielectric Dispersion of New Ferroelectric Cobalt Halide Dimers: Bis-ethanolammonium-hexahalocobaltate, (C 2 H 8 NO) 2 Co 2 X 6 , X = CI/Br  
 Abstract    The AC conductivity in the frequency range 5.0 Hz -10.0 kHz, the magnetic susceptibility in a field of 14.7 and 17.810 4 A/m, and differential thermal analysis at 78 K up to room temperature for bis-(ethanolammonium)Co 2 X 6 ,X = CI and Br are reported. The bromide dimer undergoes an order-disorder transition at 302 K and a displacive type ferroelectric transition at T ~ 220 K. The chloride dimer shows two transitions, the first being in a displacive ferroelectric one at T ~ 210 K showing critical slowing down. The second phase transition, occuring at 282 K, is found to be inactive in the electric measurements. PACS: #76. 74 
  Reference    Z. Naturforsch. 55a, 595—604 (2000); received February 29 2000 
  Published    2000 
  Keywords    Dielectric Permittivity, Phase Transition, Ferroelctric Transition 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0595.pdf 
 Identifier    ZNA-2000-55a-0595 
 Volume    55 
8Author    Z. Czapla, S. Dacko, B. KosturekRequires cookie*
 Title    Ferroelectricity and Phase Transitions in Pyridinium Periodate Single Crystal  
 Abstract    Single crystals of pyridinium periodate were grown, and their physical properties were studied. Anomalies of the electric permittivity and birefringence were observed at T CL = 321 K and T C2 = 211 K. These observations gave evidences for the existence of three phases denoted as I, II, and III. Hysteresis loops were observed both in phase II and III. Pyroelectric measurements showed two anomalies at T CL and T C2 . The anomaly at T CL is related to the transition between a para-and ferroelectric phase, and the anomaly at T cl to the transition between two ferroelectric phases. 
  Reference    Z. Naturforsch. 55a, 891—894 (2000); received August 8 2000 
  Published    2000 
  Keywords    Phase Transitions, Ferroelecticity, Dielectric Properties, Birefringence 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0891.pdf 
 Identifier    ZNA-2000-55a-0891 
 Volume    55 
9Author    Michael Uhrmacher, Klaus-Peter LiebRequires cookie*
 Title    Phase Transitions in Oxides Studied by Perturbed Angular Correlation Spectroscopy  
 Abstract    Radioactive atoms located on cation sites in oxide matrices can be used to monitor phase transitions by measuring the electric or magnetic hyperfine interactions by means of Perturbed Angular Correlations spectroscopy. The article illustrates three types of phase transitions studied with 11 'in tracers and their daughter nuclei m Cd, namely magnetic, structural and REDOX phase transitions in binary and ternary polycrystalline or single-crystalline oxides. In this context, we also discuss the question of identifying the probes' lattice site(s), the scaling of the Electric Field Gradients in oxides, the influence of the (impurity) probes themselves on the phase transitions, and the occurrence and mechanisms of dynamic interactions. Recent results on " 1 In in pure and Li-doped In 2 S 3 will also be presented. 
  Reference    Z. Naturforsch. 55a, 90—104 (2000); received August 26 1999 
  Published    2000 
  Keywords    Phase Transitions, Oxides, Perturbed Angular Correlations, 111 In, Point Charge Model 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0090.pdf 
 Identifier    ZNA-2000-55a-0090 
 Volume    55 
10Author    Masao Hashimoto, Shinichi Hashimoto, Hiromitsu Terao, Masayuki Kuma, Haruo Niki, Hiroyuki InoRequires cookie*
 Title    Crystal Structure and Phase Transition of 4-Aminopyridinium Tetrabromoantimonate(III) as Studied by Bromine and Antimony NQR, Proton NMR, and Single Crystal X-Ray Diffraction  
 Abstract    The crystal structure of the room temperature phase (RTP) of the title compound was determined at 297 K (monoclinic, space group C2/c, a = 1384.2(2), b = 1377.8(3), c = 755.5(2) pm, ß = 121.58(1)°). A complicated disorder was found for the cation. A phase transition from the low-temperature phase (LTP) to the RTP was found at (224 ±1) K (T c). The 'H NMR spectra showed a sharp motional narrowing at ca. T=T C , indicating the occurrence of a reorientational motion of the cation in the RTP in accord with the disorder. It was found that another reorientational motion is excited in the LTP. Four 81 Br NQR lines (132.71, 115.38, 61.54 and 59.31 MHz at 77 K) and two Sb NQR lines (53.78 and 33.76 MHz at 77 K) were found in the LTP, while a single 81 Br NQR line was observed at T> 276 K (ca. 121.80 MHz at 300 K). Crystal dynamics are discussed on the basis of the temperature dependence of the NQR, *H NMR line width, and 'H NMR Tj. 
  Reference    Z. Naturforsch. 55a, 167—172 (2000); received August 24 1999 
  Published    2000 
  Keywords    4-NH 2 PyHSbBr 4, Crystal Structure, Phase Transition, NQR, 'H NMR 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0167.pdf 
 Identifier    ZNA-2000-55a-0167 
 Volume    55 
11Author    J. Pirnat, Z. Trontelj, J. Lužnik, D. KirinRequires cookie*
 Title    Halogen NQR and the Phase Transition in CH 3 Hg-Halide Family  
 Abstract    The temperature dependence of the 35 C1 NQR line and its relaxation has been measured from 77 K to room temperature. The known displacive phase transition near 162 K is reflected in the change of the slope of the temperature dependence of the quadrupole line, but not so in the chlorine or proton relaxation. The results are discussed in view of the proposed structural changes and thermal vibrations. 
  Reference    Z. Naturforsch. 55a, 215—218 (2000); received October 13 1999 
  Published    2000 
  Keywords    Methylmercury Chloride, NQR, Crystal Structure, Phase Transition, Thermal Vibrations 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0215.pdf 
 Identifier    ZNA-2000-55a-0215 
 Volume    55 
12Author    J. Roth, M. Uhrmacher, R. De, La Presa, L. Ziegeler, K. P. LiebRequires cookie*
 Title    Structural Phase Transitions in BaTi0 3 Studied via Perturbed Angular Correlations  
 Abstract    Phase transitions in the ferroelectric perovskite BaTi0 3 were studied for 111 In-implanted poly-crystalline samples by measuring the electric field gradients by means of Perturbed Angular Correla-tion spectroscopy. The phase transitions between the orthorhombic rhombohedral tetragonal cubic lattices were investigated in 2 -10 K steps, for increasing and decreasing temperatures, in order to determine their hysteresis. The transition parameters are compared with results from measurements of the spontaneous polarization, electric susceptibility and neutron scattering. 
  Reference    Z. Naturforsch. 55a, 242—246 (2000); received August 24 1999 
  Published    2000 
  Keywords    Perturbed Angular Correlations, BaTi0 3, Phase Transitions, Hysteresis, 1 " In 
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 Identifier    ZNA-2000-55a-0242 
 Volume    55 
13Author    Ichi Suzuki, Shin'. Ichi, Ishimaru, Ryuichi IkedaRequires cookie*
 Title    Koh  
 Abstract    133 Cs NMR spin-lattice relaxation times(Ti) in crystalline Cs 2 CdI 4 were measured at 225 -373 K. The critical exponent (of T\ observed near the normal-incommensurate transition in the normal phase was determined to be 0.62 ± 0.03, in good agreement with the predicted value for three-dimensional XK-model. The frequency dependent T\ in the incommensurate phase could be explained by the fluctuation of amplitudon and small gap phason. 
  Reference    Z. Naturforsch. 55a, 339—342 (2000); received August 27 1999 
  Published    2000 
  Keywords    l33 Cs NMR, T\ \ Incommensurate, Phase Transition, Critical Exponent 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0339.pdf 
 Identifier    ZNA-2000-55a-0339 
 Volume    55 
14Author    E. Mikuli, A. Migdał-Mikuli, I. Natkaniec, J. MayerRequires cookie*
 Title    Phase Transitions and Water Dynamics of [Mn(H 2 0) 6 ](C10 4 ) 2 Studied by Differential Scanning Calorimetry and Neutron Scattering Methods  
 Abstract    DSC measurements performed at 95 -290 K have shown that [Mn(H 2 0) 6 ](C10 4) 2 possesses, besides a high-temperature phase, existing above 323 K, four low-temperature solid phases. The inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) pat-terns registered at 20 -290 K have supported the DSC results and provided evidence that the investigated substance possesses even more than five solid phases. The IINS spectra have shown that in the room-temperature phase, water molecules perform fast stochastic reorientation at the picosecond scale. The orientational disorder characteristic for the room-temperature phase can be easily overcooled and frozen. Even by relatively slow cooling at ca. 40 K/hour a metastable, orientational (protonic) glass phase is formed below ca. 160 K. Below ca. 100 K, a structural phase transition was observed by the NPD, however the IINS spectra indicate existence of the pure ordered low-temperature phase only after annealing the sample for a few hours at 100 K. On heating, a structural phase transition takes place at ca. 120 K, and at ca. 225 K water molecules begin fast reorientation. 
  Reference    Z. Naturforsch. 55a, 759—764 (2000); received July 7 2000 
  Published    2000 
  Keywords    Hexaaquamanganese(II) chlorate(VII), Phase Transitions, DSC, Neutron Scattering 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0759.pdf 
 Identifier    ZNA-2000-55a-0759 
 Volume    55 
15Author    Takahiro Iijima, Motohiro Mizuno, Masahiko SuharaRequires cookie*
 Title    H NMR Study of Molecular and Electron Spin Dynamics in Paramagnetic [Co(H 2 0) 6 ][SiF 6 ]  
 Abstract    The temperature dependences of 2 H NMR spectra and the spin-lattice relaxation time T\ were measured for [Co(H20)f,][SiF6]. The variation of the spectrum above room temperature can be explained by the reorientation of [Co(H20)fi] 2+ about the C3 axis. The activation energy E a and the jumping rate at infinite temperature fco for the three site jump of [Co(H20)6] 2+ were obtained as 82 kJmol -1 and 2x 10 17 s~ 1 from the spectral simulation. Below room temperature, the spectral line shape was dominated by the 180° flip of the water molecule. The minimum of T\ caused by the 180° flip of the water molecule was observed at ca. 260 K. The jumping rate of the 180° flip of the water molecule was estimated from the 2 H NMR T\ and the spectral simulation. E* = 38 kJmol -1 and fco = 6x 10 l5 s -1 for the 180° flip of the water molecule were obtained from T\. 
  Reference    Z. Naturforsch. 55a, 173—177 (2000); received August 23 1999 
  Published    2000 
  Keywords    Phase Transition, 2 H NMR, Nuclear Quadrupole Interaction, Paramagnetic Shift, Molecular Dynamics 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0173.pdf 
 Identifier    ZNA-2000-55a-0173 
 Volume    55 
16Author    Motohiro Mizuno, Takahiro Iijima, Kengo Orii, Masahiko SuharaRequires cookie*
 Title    Water Molecular Motion and Hydrogen Bond in Paramagnetic [Cu(H 2 0) 6 ][PtCl 6 ] as Studied by Single Crystal 2 H NMR  
 Abstract    The temperature and angular dependences of the 2 H NMR spectrum were measured for single crystal [Cu(H 2 0) 6 ] [PtCl 6 ]. (e 2 Qq/h, r?) = (132 kHz, 0.72) were obtained for the D nucleus averaged by the fast 180° flip of the water molecule at 297 K. D,(236 kHz, 0.10) and D 2 (246 kHz, 0.09) were obtained for two unequal D nuclei of the water molecule at 133 K. Below T c , at least three nonequivalent waters were found to exist. The jumping rate for the 180° flip of the water molecule (k) was obtained from the simulation of 2 H NMR spectra at //o||[l 11] direction. The activation energy E d and the jumping rate at infinite temperature were estimated from the temperature dependence of k to be 24kJmol _1 and 1 x 10 13 s -1 . 
  Reference    Z. Naturforsch. 55a, 178—182 (2000); received August 23 1999 
  Published    2000 
  Keywords    Phase Transition, 2 H NMR, Nuclear Quadrupole Interaction, Paramagnetic Shift, Molecular Dynamics 
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 Identifier    ZNA-2000-55a-0178 
 Volume    55 
17Author    Hiromitsu Terao, Masao Hashimoto, Shinichi Hashimoto, Yoshihiro FurukawaRequires cookie*
 Title    Studies of Structure and Phase Transition in [C(NH 2 ) 3 ]HgBr 3 and [C(NH 2 ) 3 ]HgI 3 by Means of Halogen NQR, NMR, and Single Crystal X-Ray Diffraction  
 Abstract    The crystal structure of [C(NH 2) 3 ]HgBr 3 was determined at room temperature: monoclinic, space group C2/c, Z = 4, a = 775.0(2), b = 1564.6(2), c = 772.7(2) pm, ß = 109.12(2)°. In the crystal, almost planar HgBr 3 ~ ions are connected via Hg -Br bonds, resulting in single chains of trigonal bipyramidal HgBr 5 units which run along the c direction. [C(NH 2) 3 ]HgI 3 was found to be isomorphous with the bromide at room temperature. The temperature dependence of the halogen NQR frequencies (77 < 77K < ca. 380) and the DTA measurements evidenced no phase transition for the bromide, but a second-order phase transition at (251 ± 1) K (7 tl) and a first-order one at (210 ± 1) K (T C2) for the iodide. The transitions at T C2 are accompanied with strong supercooling and significant superheating. The room temperature phase (RTP) and the intermediate temperature phase (ITP) of the iodide are characterized by two I(m =i/2~3/2) NQR lines which are assigned to the terminal and the bridging I atoms, respectively. There exist three lines in the lowest temperature phase (LTP), indicating that the resonance line of the bridging atom splits into two. The signal intensities of the 127 I (m =i/2—3/2) NQR lines in the LTP decrease with decreasing temperature resulting in no detection below ca. 100 K. The 127 I(m =i/ 2 ~3/2) NQR frequency vs. temperature curves are continuous at T CL , but they are unusual in the LTP. The T X vs. Tcurves of 'H NMR for the bromide and iodide are explainable by the reorientational motions of the cations about their pseudo three-fold axes. The estimated activation energies of the motions are 35.0 kJ/mol for the bromide, and 24.1, 30.1, and 23.0 kJ/mol for the RTP, FTP, and LTP of the iodide, respectively. 
  Reference    Z. Naturforsch. 55a, 230—236 (2000); received August 23 1999 
  Published    2000 
  Keywords    [C(NH 2 ) 3 ]HgX 3, Crystal Structure, Phase Transition, NQR, 'H NMR 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0230.pdf 
 Identifier    ZNA-2000-55a-0230 
 Volume    55 
18Author    N. Ulbrich, W. Tröger, T. Butz, P. Blaha, Nukleare FestkörperphysikRequires cookie*
 Title    Investigation of the Negative Thermal Expansion of ZrW 2 O g  
 Abstract    The negative thermal expansion in ZrW 2 0 8 was investigated on a microscopic scale by temperature dependent measurements of the electric field gradients at the nuclear probe 187 W (ß~) 187 Re using time differential perturbed angular correlation spectroscopy. Two distinct nuclear quadrupole interactions I VJ? cl l= 18.92(4) • 10 21 V/m 2 , q Rcl = 0.0 and I V5 e2 l = 4.55(2) • 10 21 V/m 2 , r? Re2 = 0.053(3) were observed at 295 K, which are assigned to the two crystallographically distinct W0 4 tetrahedra of the room temperature structure. Ab initio calculations of electron densities and electric field gradients with 1:7 Re-impurities using the full potential linearized augmented plane wave package WIEN97 yield the electric field gradients VJ? C| = 12.63 • 10 21 V/m 2 ,7y Rcl = 0.0 and vJ? e2 =4.90 • 10 21 V/m 2 , rj Re2 =0.0. The observed temperature dependence of the nuclear quadrupole interactions agrees well with the structu-ral phase transition at 428 K observed by neutron and x-ray diffraction. Our experiments corroborate the suggested mechanism of coupled librations of rigid Zr0 6 octahedra and W0 4 tetrahedra, which is an alternative description of transverse vibrations of oxygen atoms in Zr-O-W bonds, for the negative thermal expansion in ZrW 2 0 8 . 
  Reference    Z. Naturforsch. 55a, 301—310 (2000); received October 28 1999 
  Published    2000 
  Keywords    ZrW 2 0 8, Negative Thermal Expansion, Phase Transition, Time Differential Perturbed Angular Correlation (TDPAC), Hyperfine Spectroscopy 
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 Identifier    ZNA-2000-55a-0301 
 Volume    55 
19Author    L. Guibé, A. Péneau, M. Gourdji, A. PanichRequires cookie*
 Title    Thiourea and Thiourea Inclusion Compounds. A 14 N NQR Investigation  
 Abstract    A reinvestigation of the nitrogen-14 NQR in thiourea has permitted to record, apparently for the first time, the resonances above 169 K. The first order and second order of the transitions at 169 K and 202 K, respectively, appear clearly from the temperature dependence of the resonance frequencies; the mod-ulated structure, incommensurate or commensurate, is also seen on the 14 N NQR spectrum. This study is the first step of a new investigation of thiourea and its inclusion compounds. 
  Reference    Z. Naturforsch. 55a, 331—334 (2000); received October 13 1999 
  Published    2000 
  Keywords    Thiourea, Nitrogen-14 NQR, Phase Transitions, Commensurate, Incommensurate Modu-lations, Inclusion Compounds 
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 TEI-XML for    default:Reihe_A/55/ZNA-2000-55a-0331.pdf 
 Identifier    ZNA-2000-55a-0331 
 Volume    55