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1	Author	T. Gruhn, S. Hess	Requires cookie*
	Title	Monte Carlo Simulation of the Director Field of a Nematic Liquid Crystal with Three Elastic Coefficients
	Abstract	In this article, a Monte Carlo simulation is presented, which generates the equilibrium director field of a nematic liquid crystal under the influence of an external field and fixed boundary conditions. The liquid crystal is characterized by a set of directors on a spatially fixed lattice. The simulation is based on an expression for the Frank free energy with three elastic coefficients. The chosen discretisation conserves the nematic symmetry, which means n and — n are equivalent. The results for several Frederiks geometries with homogeneous and spatially modulated external fields are shown, as well as an investigation of a capillary with homeotropic boundary conditions. Further we compare our method with the Lebwohl-Lasher model and introduce an extension of the latter which distinguishes between splay-, twist-and bend-configurations.
	Reference	Z. Naturforsch. 51a, 1 (1996); received December 15 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0001
	Volume	51

2	Author	R. K. Singla, R. K. Chhabra, S. K. Trehan	Requires cookie*
	Title	Effect of a Tangential Electric Field on the Second Harmonic Resonance in Kelvin-Helmholtz Flow
	Abstract	The stability of Kelvin-Helmholtz flow at the second harmonic resonance between two marginally unstable modes is investigated in the presence of a tangential electric field. We obtain coupled amplitude equations which are examined for the stability characteristics. Numerical integration of the equations reveals that at a certain determinate time, the amplitudes of both the fundamental mode and the second harmonic mode start increasing rapidly, resulting in what is termed as "Explosive Instability". It is demonstrated that for a fluid of given density and dielectric constant, the electric field plays an important role, and the explosive instability sets in much earlier as the electric field increases.
	Reference	Z. Naturforsch. 51a, 10—16 (1996); received November 24 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0010
	Volume	51

3	Author	Pardeep Kumar	Requires cookie*
	Title	Rayleigh-Taylor Instability of Viscous-Viscoelastic Fluids in Presence of Suspended Particles Through Porous Medium
	Abstract	The Rayleigh-Taylor instability of a Newtonian viscous fluid overlying an Oldroydian viscoelastic fluid containing suspended particles in a porous medium is considered. As in both Newtonian viscous-viscous fluids the system is stable in the potentially stable case and unstable in the poten-tially unstable case, this holds for the present problem also. The effects of a variable horizontal magnetic field and a uniform rotation are also considered. The presence of magnetic field stabilizes a certain wave-number band, whereas the system is unstable for all wave-numbers in the absence of the magnetic field for the potentially unstable configuration. However, the system is stable in the potentially stable case and unstable in the potentially unstable case for highly viscous fluids in the presence of a uniform rotation.
	Reference	Z. Naturforsch. 51a, 17—22 (1996); received September 19 1995
	Published	1996
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0017.pdf
	Identifier	ZNA-1996-51a-0017
	Volume	51

4	Author	F. Ucun, F. Köksal	Requires cookie*
	Title	An EPR Study on V0 2+ and Mn 2+ Ions in Some Zeolites
	Abstract	The electron paramagnetic resonance (EPR) of hydrated V0S0 4 -3H 2 0 and MnCl 2 -2H 2 0, adsorbed on the synthetic zeolites 3A, 4A, 5A and 13X of pore diameters 0.3, 0.4, 0.5, and 1.0 nm, respectively, and the natural zeolites heulandite and clinoptilolite was investigated. The spectra indicated that the coordination structures are V0(H 2 0)5 + and Mn(H 2 0)g + , and that their EPR line widths vary with the pore diameters and the surface areas of the zeolites. The spectra of V0(H 2 0)5 + in natural zeolites at room temperature display an isotropic behavior and therefore indicate that the water ligands are mobile. The EPR spin Hamiltonian parameters, the molecule orbital constant ß* 2 , and the Fermi contact term K e[[ were determined and are discussed. The spectra of Mn(H 2 0)^ for narrow-pore zeolites indicate the existence of mobile and immobile water, whereas for wide-pore and natural zeolites they indicated the existence of only mobile water. The obtained Mn(H 2 Ö)g + spectra are discussed.
	Reference	Z. Naturforsch. 51a, 23—26 (1996); received October 17 1995
	Published	1996
	Keywords	EPR, V0 2 +, Mn 2 +, Zeolites, Coordination
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0023.pdf
	Identifier	ZNA-1996-51a-0023
	Volume	51

5	Author	K.M T Yamada, A. Moravec, M. Niedenhoff, H. Bürger, G. Winnewisser	Requires cookie*
	Title	The 2V 5 Overtone Band of Cyanoacetylene by High Resolution FTIR Spectroscopy
	Abstract	The 2v 5 overtone band of cyanoacetylene, HCCCN, has been measured with a high resolution FTIR spectrometer. The analysis of the accidental resonances results in very accurate parameters for the observed v 5 = 2 state as well as for the hidden perturber state (r 4 = l, v 1 = 2)\ the vibrational energy and the rotational constant have been determined to be 1312.991921(23) cm" 1 and 4552.38440(23) MHz for the u ? = 2 state, and 1310.0627(6) cm" 1 and 4572.463(4) MHz for the (f 4 = l, v-j — 2) state. The associated hot bands from the v 7 = l, i> 6 = l, and u 5 = l state have also been analyzed.
	Reference	Z. Naturforsch. 51a, 27—35 (1996); received November 25 1995
	Published	1996
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0027.pdf
	Identifier	ZNA-1996-51a-0027
	Volume	51

6	Author	K.M T Yamada, A. Moravec, M. Niedenhoff, G. Winnewisser	Requires cookie*
	Title	Intensities and Widths of the Rovibrational Lines Observed in the 2v 5 Overtone Band of Cyanoacetylene
	Abstract	The relative intensities and Lorentz half-widths of rovibrational transitions observed for the 2v 5 overtone band of cyanoacetylene, HCCCN, have been determined. The line widths and the inten-sities were obtained by fitting the observed line profiles with Voigt functions. The Lorentz half-width shows a clear J dependence and vanishes at 7~ 100. In the intensity calculation, we took into account the effects of the accidental resonances between the v 5 = 2 state and the (r 4 = 1, v 7 = 2) state as well as those of the generic /-type resonance. Peak intensities have been found not to be the appropriate presentation of the line intensity because of the strong J dependence of the half-width.
	Reference	Z. Naturforsch. 51a, 36—40 (1996); received December 7 1995
	Published	1996
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0036.pdf
	Identifier	ZNA-1996-51a-0036
	Volume	51

7	Author	H. U. Suter, D. M. Marie, P. F. Meier	Requires cookie*
	Title	An ab initio Study of the Molecular Electric-field Gradients of the Chlorsilanes
	Abstract	The electric field gradient (EFG) of chlorine in the chlorine containing silanes (SiCl 4 , SiCl 3 H, SiCl 2 H 2 , and SiClH 3) was determined by means of ab initio methods and compared to recent experiments from which nuclear quadrupole moments are extracted. A careful estimation of the AO basis sets and the effect of the electron correlation is undertaken. The results showed the importance of the use of extended basis sets in the calculation of EFGs in second row atoms. Good agreement with deviations less than 5% from the experiments was found. The effect of the electron correlation was found to be small.
	Reference	Z. Naturforsch. 51a, 41—45 (1996); received November 22 1995
	Published	1996
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0041.pdf
	Identifier	ZNA-1996-51a-0041
	Volume	51

8	Author	Ulrich Wolschendorf, Uwe Kretschmer, DieterH. Sutter	Requires cookie*
	Title	Distortion of the Benzene Ring in 1,2,3-Trifluorobenzene; A High-Resolution Molecular Beam Fourier Transform Microwave Study
	Abstract	The results of a molecular beam microwave Fourier transform study of 1,2,3-trifluorobenzene and its monosubstituted 1 'C-isotopomers, all observed in natural abundance, are presented. Typically 30 transitions were observed for each isotopomer. Centrifugal perturbation theory up to fourth order was used to analyse the observed rotational spectra. Due to the high sensitivity and resolution of the spectrometer, very accurate vibronic ground state rotational constants can be reported: 13 C-> l? c, I3 C 4 l3 c, I2 C They are used to derive a partial ro-structure. which shows the distortions of the benzene ring caused by fluorine substitution.
	Reference	Z. Naturforsch. 51a, 46 (1996); received November 10 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0046
	Volume	51

9	Author	H. Zerbe-Foese, A. Guarnieri, O. L. Stiefvater	Requires cookie*
	Title	Microwave and Millimeterwave Spectrum of 1,1-Difluoroethylene and the Vibrational Mode v 10 from FT-IR Spectroscopy
	Abstract	Pure rotational spectra of 1,1-Difluoroethylene (1,1-DFE) in the vibrational ground state (GS) and in excited vibrational states (EVSs) have been investigated by double resonance modulation (DRM) microwave spectroscopy in the range from 12 GHz to 40 GHz, and by millimeterwave spectroscopy in the range from 90 to 260 GHz. In addition to the GS spectrum, rotational transitions have been observed for the EVSs v 10 = l, and 2, v 12 = l, v 6 = l, v n = l, v 4 = l, and v 9 = l. Along with other IR bands up to 1400cm" 1 , the fundamental vibrational band v 10 = l <-0 has been observed by FT-IR spectroscopy and has been analysed to yield v° 0 = 436.8851 cm -1 . Rotational constants and quartic distortion constants were deduced for all observed states. In addition, sextic distortion constants were obtained for the GS and for the EVSs v 10 = l, 2, v 6 = l, and v 12 = l by computer fits of combined microwave and millimeterwave data to Watson's A-re-duced Hamiltonian in I r representation. The S-reduced Hamiltonian in III r representation may be considered more appropriate for the calculation or the fitting of the transitions because 1,1-DFE is a near oblate asymmetric top. Thus, for comparison, results are given for both the A-and the S-reduction.
	Reference	Z. Naturforsch. 51a, 53—62 (1996); received November 11 1995
	Published	1996
	Keywords	1, 1-Difluoroethylene, Molecular Spectroscopy, Microwave Spectroscopy, FT-IR Spec-troscopy, Excited Vibration States
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0053.pdf
	Identifier	ZNA-1996-51a-0053
	Volume	51

10	Author	M. Maeda, T. Ito, M. Hori, G. Johansson	Requires cookie*
	Title	The Structure of Zinc Chloride Complexes in Aqueous Solution
	Abstract	The structure of zinc chloride complexes with different ratios of chloride to zinc, formed in concentrated ZnCl 2 aqueous solutions, were determined from large-angle X-ray scattering using concentrations of the chloride complexes estimated by complementary Raman spectroscopic mea-surements. The highest chloro complex, [ZnCl 4 ] 2 ~, is tetrahedral with a Zn-Cl bond length of 2.294(4) Ä. The trichloro complex, [ZnCl 3 ]~, which coordinates one water molecule, is pyramidal with the Cl-Zn-Cl angle 111°. The Zn-Cl and the Zn-H 2 0 bonds are 2.282(4) and 1.9 A, respec-tively. The two lower complexes, [ZnCl 2 ] and [ZnCl] + , cannot be separated by Raman spectra. o The average Zn-Cl distance in these complexes is 2.24 Ä, and the average Zn-H 2 Ö distance is 1.9 A. In [Zn(H 2 0) 6 ] 2+ the Zn-H 2 0 distance is 2.15 Ä. of zinc (II) chloride complexes; structure of hydrated non-complexed zinc (II) ion.
	Reference	Z. Naturforsch. 51a, 63—70 (1996); received November 7 1995
	Published	1996
	Keywords	X-ray diffraction, Raman spectra, IR spectra, structures
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0063.pdf
	Identifier	ZNA-1996-51a-0063
	Volume	51

11	Author	M. Bionducci, F. Buffa, G. Licheri, G. Navarra, B. Bouchet-Fabre, J. M. Tonnerre	Requires cookie*
	Title	Determination of the Partial Structure Factors of Amorphous CuZr by Anomalous X-Ray Scattering and Reverse Monte Carlo
	Abstract	Anomalous X-ray Scattering experiments have been performed to study the local order in an amorphous CuZr sample prepared by mechanical alloying. The three partial structure factors were extracted both using a regularization algorithm to solve an ill-conditioned system of linear equations and applying the reverse Monte Carlo technique to the experimental data. Then the short range atomic distributions of similar samples prepared by different methods and investigated by different techniques are compared.
	Reference	Z. Naturforsch. 51a, 71—82 (1996); received November 15 1995
	Published	1996
	Keywords	Amorphous Alloys, Anomalous X-ray Scattering, Regularization Algorithm, Reverse Monte Carlo, Partial Structure Factors
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0071.pdf
	Identifier	ZNA-1996-51a-0071
	Volume	51

12	Author	Hiroyuki Ishida, Yoshihiro Furukawa	Requires cookie*
	Title	Self-Diffusion in the Ionic Plastic Phase of (CH 3 ) 3 NHC10 4 Studied by *H NMR and Electrical Conductivity
	Abstract	Spin-lattice relaxation times (7^) and spin-spin relaxation times (T 2) of 'H NMR and the electrical conductivity (er) of trimethylammonium Perchlorate were measured in the ionic plastic phase obtainable above 480 K. In this phase, both the cation and anion were revealed to perform self-diffusion. The activation energy (£ a) of the cationic diffusion was evaluated to be 55 ±4 and 50 + 4kJmol _1 from 7j and 'H 7 2 respectively, while E a of the anionic diffusion was 64 ±3 kJ mol" 1 from the electrical conductivity.
	Reference	Z. Naturforsch. 51a, 83—86 (1996); received December 4 1995
	Published	1996
	Keywords	Ionic plastic phase Self-diffusion, Nuclear magnetic resonance, Electrical conductivity
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0083.pdf
	Identifier	ZNA-1996-51a-0083
	Volume	51

13	Author	J. Blétry, D.E M	Requires cookie*
	Title	Description of the Glass Transition by a Percolation Blocking of Local Chemical Order
	Abstract	A model for the liquid-glass transition, based on a percolation blocking of local chemical order, is proposed. The case of metallic liquids and glasses, whose structure is dominated by first neighbour chemical arrangement, is first treated. The chemical ordering "reaction" of the liquid phase is studied at thermodynamical equilibrium and the increase of the chemical order parameter with decreasing temperature is calculated. Within a given composition interval, however, a geometrical percolation process is shown to block this reaction below a "percolation temperature" (correspond-ing to null cooling rate) where the liquid is irreversibly frozen into a glass. The liquid-glass "phase diagram" is established and kinetic arguments, involving "frustrated" finite clusters which are formed close to the percolation threshold, provide an evaluation of the experimentally measured "glass transition temperature" as a function of cooling rate. The validity of this one order parameter model is then discussed with the help of the irreversible thermodynamics theory of Prigogine. The formation of tetracoordinated glasses is explained by the formation of tetrahedral bonds, when the liquid temperature decreases, and represented by a "hole ordering" reaction. A general description of the structure of tetracoordinated glasses is thus achieved, which applies to amorphous silicon and germanium, 111 -V compounds, silica, amorphous water etc. Furthermore, an estimation of the temperature interval for the glass transformation of silica is obtained, which agrees well with experiment. The existence of frustrated clusters gives to glasses a composite structure in the "medium distance order", which could explain the "fractal nature" of glass fracture surfaces down to the nanometer scale.
	Reference	Z. Naturforsch. 51a, 87 (1996); received September 6 1995
	Published	1996
	Keywords	Glass transition Percolation, Short range chemical order, Metallic glasses, Tetracoordi-nated glasses, Fractography
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0087.pdf
	Identifier	ZNA-1996-51a-0087
	Volume	51

14	Author	Gerhard Raabe, Klaus Breuer, Dieter Enders, J.Henrique Teles	Requires cookie*
	Title
	Abstract	The role of conjugative interaction in stable carbenes of the l,2,4-triazol-5-ylidene type was studied at different levels of ab initio theory employing model compounds. In the case of parent 4,5-dihydro-l//-l,2,4-triazol-5-ylidene this type of interaction was found to be of similar importance as in 1,2,4-triazole and pyridine. Five stationary points were located on the hypersurface of the dimer (C 2 N 3 H 3) 2 . AE for all considered dimerization reactions was found to be negative, and the change of energy associated with the formation of the most stable isomer is about — 9.5 kcal / mo i. In spite of a high value for the singlet-triplet gap of about 90 kcal / mol for the carbene, the heavy atom skeleton of the most stable dimer is close to planarity. This finding is in perfect accordance with the predictions of other authors regarding the geometry of dimers from singlet carbenes stabilized by ^-conjugation.
	Reference	Z. Naturforsch. 51a, 95—101 (1996); received November 15 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0095
	Volume	51

15	Author	C. Kerst, P. Potzinger, H. Gg, Wagner	Requires cookie*
	Title	Reaction of Trimethyl-and Dimethyl-Silyl Radicals with Pentamethyldisilane
	Abstract	The rate constant for the H atom abstraction of trimethylsilyl radicals from pentamethyldisilane k (4) (&(4)) was measured relative to the trimethylsilyl combination reaction k(3). A value for • ]/k (3) = (8.53±0.08) • 10" 11 cm 3 ' 2 s~ 1/2 was obtained. For the dimethylsilyl radical, a smaller value for the corresponding rate constant ratio (5.9 + 0.2) • 10" 11 cm 3 ' 2 s" 1/2 was measured, and this was attributed to a disproportionation reaction between the dimethylsilyl and the pentamethyldisilyl radical leading to dimethylsilylene.
	Reference	Z. Naturforsch. 51a, 102—104 (1996); received October 28 1995
	Published	1996
	Keywords	Hydrogen abstraction, silyl radicals, pentamethyldisilane, disproportionation reaction
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0102.pdf
	Identifier	ZNA-1996-51a-0102
	Volume	51

16	Author	C. Kerst, P. Potzinger, H. Gg, Wagner	Requires cookie*
	Title	The Mercury-Sensitized Photolysis of Pentamethyldisilane
	Abstract	Two primary processes were observed in the Hg-sensitized photolysis of Me 5 Si 2 H: (I) hydrogen abstraction from the Si-H bond with a quantum yield of 0(1) = 0.85, (V) Si-Si bond breaking with 0(V) = 0.04. The hydrogen atoms formed in (/) undergo an H atom abstraction reaction (k(3)), as well as substitution reactions at the Si centers resulting in the formation of dimethylsilane and trimethylsilyl radical (k(4)) or trimethylsilane and dimethylsilyl radical (k(5)). The following branching ratios have been determined: M = 0.87. M 0.096, M = 0.034. k(3) + k(4) + k(5) k(3) + k(4) + k(5) k(3) + k(4) + k(5) The ratio of disproportionation (A: (2)) to combination (£(1)) for the pentamethyldisilyl radical has been determined with MeOH as the scavenger for 1-methyl-l-trimethylsilylsilene, 0.046 < k(2)/ A: C1) < 0.071. A mechanism with pertinent rate constants has been proposed which accounts for the results.
	Reference	Z. Naturforsch. 51a, 105—115 (1996); received October 28 1995
	Published	1996
	Keywords	Hg-sensitized photolysis, pentamethyldisilane, mechanism, substitution reaction, silyl rad-ical disproportionation
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	TEI-XML for	default:Reihe_A/51/ZNA-1996-51a-0105.pdf
	Identifier	ZNA-1996-51a-0105
	Volume	51

17	Author	Cornelia Fooken, Wolfram Baumann	Requires cookie*
	Title	Charge-Transfer Complexes Between Iodine and Substituted Thioureas: Determination of Thermodynamic and Spectroscopic Properties
	Abstract	15 iodine complexes with substituted thioureas were investigated in dichloromethane solution by UV-visible spectroscopy, and their equilibrium constants, reaction enthalpies and molar absorption coefficients were determined. The influence of the substituents on the properties of the charge-trans-fer complexes is examined. A critical discussion of previously published data is given, where comparable data are available.
	Reference	Z. Naturforsch. 51a, 116—122 (1996); received January 19 1996
	Published	1996
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	Identifier	ZNA-1996-51a-0116
	Volume	51

18	Author	H. Klein, S. P. Belov, G. Winnewisser	Requires cookie*
	Title	Terahertz Spectrum of Trioxane
	Abstract	The pure rotational spectrum of trioxane, (fTCO)? the trimer of formaldehyde, has been recorded with high resolution in the frequency range between 326 and 947 GHz for the main isotopomer, the l3 C, and the l8 0 isotopic species in the vibrational ground state. These new high J and K data reveal that the molecule is fairly rigid. For the constants determinable from the recorded high J and K spectra (J = 90 and K = 75) the rotational constant B = 5273.257 180(33) MHz, the two quartic centrifugal distortion constants DJ, and DJK, and the three sextic constants HJ, HJK, and HKJ are needed in the fit to reproduce the measured spectra within experimental accuracy. In addition, for the l3 C isotopomer the sextic constants HJK and HKJ are determined as well as the off-diagonal parameters d\ and di.
	Reference	Z. Naturforsch. 51a, 123—128 (1996); received January 26 1996
	Published	1996
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	Identifier	ZNA-1996-51a-0123
	Volume	51

19	Author	Susana Blanco, Alberto Lesarri, JuanC. López, JoséL. Alonso, Antonio Guarnieri	Requires cookie*
	Title	The Rotational Spectrum and Nuclear Quadrupole Coupling of CH 35 C1F 2
	Abstract	The rotational spectrum of CH 35 C1F 2 in the ground vibrational state has been measured in the frequency range 8-18 GHz using a waveguide Fourier transform microwave (FTMW) spectrometer in order to determine accurate nuclear quadrupole coupling constants. The spectra of the excited vibrational states v 5 = 1, v 6 = 1 and v Q = 1 have been also observed and analyzed in the frequency region 8-250 GHz using FTMW, Stark, and source modulation spectrometers. Quadrupole cou-pling constants are also reported for these states.
	Reference	Z. Naturforsch. 51a, 129—132 (1996); received December 28 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0129
	Volume	51

20	Author	P. Andonov, S. Kimura, T. Sawada, A. Kobayashi	Requires cookie*
	Title	Cluster Formation in Liquid Lithium Niobate A Small Angle X-ray Scattering Investigation
	Abstract	Structural analysis of the clustering phenomenon in the LiNb0 3 melt was carried out by means of small-angle X-ray scattering using synchrotron radiation in the range of momentum transfer 0.2 nm -1 <£< 14.0 nm -1 . Microclusters were present in the melt throughout the entire explored temperature-domain from 1673 K to 1513 K which includes undercooling (melting point T m = 1526 K). The radius of gyration, size, shape and distance of particles were determined. Chains of few Nb0 6 octahedra and small blocks constituted by two layers of four corner-shared octahedra, interlocked by a lithium atom, could be good models for the microclusters in a large temperature-domain. By lowering the temperature, the particle size increased. Below 1553 K, probable macroclusters would be correlated to the rapid increase of the viscosity. Using the viscosity data and previously published and the present SAXS results, it was possible to evaluate the volume of the clusters. Near the freezing point about a quarter of the volume would be occupied by clusters. Indexing codes: 61.60.Lx experimental diffraction and scattering techniques; 61.25.Ks molten salts; 36.40.+ d atomic and molecular clusters.
	Reference	Z. Naturforsch. 51a, 133—146 (1996); received December 27 1995
	Published	1996
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	Identifier	ZNA-1996-51a-0133
	Volume	51

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