Antiparallel Ordering Research Articles

The Elastic Constants and Electric Permittivities of Mixtures Containing Terminally Cyano Substituted Nematogens

Abstract Measurements of electric permittivities and elastic constants for various terminally cyano substituted nematics and their mixtures with terminally dialkyl substituted nematogens are reported. The mixtures showed a considerably reduced ratio of the bend to splay elastic constants (k 33/k 11) compared with the pure p-cyano substituted nematics. Also the large diversity in k 33/k 11 observed for the p-cyano substituted nematics was found to converge upon dilution. It is suggested that these results can be attributed to the reduced anti-parallel local ordering observed in the mixtures.

A new pair of M13 vectors for selecting either DNA strand of double-digest restriction fragments

The strategy of shotgun cloning with M13 is based on obtaining random fragments used for the rapid accumulation of sequence data. A strategy, however, is sometimes needed for obtaining subcloned sequences preferentially out of a mixture of fragments. Shotgun sequencing experiments have shown that not all DNA fragments are obtained with the same frequency and that the redundant information increases during the last third of a sequencing project. In addition, experiments have shown that particular fragments are obtained more frequently in one orientation, allowing the use of only one of the two DNA strands as a template for M13 shotgun sequencing. Two new M13 vectors, M13mp8 and M13mp9, have been constructed that permit the cloning of the same restriction fragment in both possible orientations. Consequently, each of the two strands becomes a (+) strand in a pair of vectors. The fragments to be cloned are cleaved with two restriction enzymes to produce a fragment with two different ends. The insertion of such a fragment into the vector can occur only in one orientation. Since M13mp8 and M13mp9 have their array of cloning sites in an antiparallel order, either orientation for inserting a double-digest fragment can be selected by the choice of the vector.

Neutron powder diffraction on α-Tl 4CrI 6 and β-Tl 4CrI 6

α-Tl 4CrI 6 ( a = 9.132(1), c = 9.667(1) Å, Z = 2, P4 mnc at 293 K) adopts a distorted Tl 4HgBr 6 structure. In α-Tl 4CrI 6 there occurs a random distribution of Jahn-Teller distorted octahedra which are elongated perpendicular to the c axis. Between 77 and 4.2 K a phase transition occurs. In β-Tl 4CrI 6 ( a = 12.941(3), b = 12.596(3), c = 9.602(2) Å, Z = 4, Cccm at 4.2 K) the directions of elongation of the octahedra are ordered. The structure is very much related to that of α-Tl 4CrI 6. A three-dimensional magnetic ordering takes place at 2.7(2) K. The magnetic space group at 1.2 K is C I 22′2′. The magnetic moments (3.48(6) μ B) are parallel to (0 0 1) and have an angle of 41(9)° with the a axis. Four magnetic sublattices are present, forming two independent magnetic lattices which have no interaction due to the antiparallel ordering.

Pre-Transitional Effects in the Electric Permittivity of Cyano Nematics

Abstract Measurements we have made of electric permittivities have revealed new pre-transitional effects in nematics with cyano end groups. These effects have been found to be related to the discontinuity in the mean permittivity at TNI observed for these nematics and they have been attributed to pre-transitional changes in the anti-parallel ordering.

ChemInform Abstract: HYDROGEN BONDING AND ORDER‐DISORDER PHENOMENA IN AMMONIUM HALIDE CRYSTALS

AbstractEs wird gezeigt, daß sich eine große Menge von Informationen zur Ordnung in NHZ‐ halogenid‐Kristallen vom CsCl‐Typ (NH4X (X: Cl, Br, I), ND4X (X: Cl, Br), NH4Brx‐ Cl,‐x, (NH4)yM,‐yCl und (NH4)yM,‐yBr (M: Alkalimetallionem), z.B. alle z. Zt.

Hydrogen bonding and order–disorder phenomena in ammonium halide crystals

The ordering of NH4+ ions in CsCl-type ammonium halide crystals is analyzed in terms of a model involving Ising pseudospin–phonon coupling. Direct interactions between NH4+ ions stablize parallel (’’ferro’’) ordering, and indirect interactions due to spin–phonon coupling stabilize antiparallel (’’antiferro’’) ordering. The strength of this coupling depends on the mass of the halide ion X, the frequency of an M-point transverse acoustic lattice mode, and the gradient ΔV′ in the potential for the hydrogen bond between NH4+ and X−. All available data for ordering in NH4Cl, NH4Br, NH4I, ND4Cl, ND4Br, NH4BrxCl1–x, (NH4)yM1–yCl, and (NH4)yM1–yBr are used to obtain empirical values of the potential parameter ΔV′. The magnitude of this quantity and the variations in its value with changes in halide ion, pressure, deuteration, and substitution of NH4+ by alkali ions are completely consistent with expectations for systems with weak N–H⋅⋅⋅X hydrogen bonding.

Nachweis einer antiparallelen Spinkopplung in paramagnetischem Manganoxid / Proof of Antiparallel Spin-Coupling in Paramagnetic Manganese Oxid

Abstract For temperatures above the Néel-point it is shown that there are severe deviations from pure paramagnetism in the ESR-behaviour of stoechiometric MnO. The experimental results in the temperature range between 130 K and 480 K are interpreted by a model of local order. The excitation energy necessary to decouple the antiferromagnetic bonding is computed to be about ΔE= 250 ± 15 cm-1 . For a temperature of T=TN +10 we derive that according to this theory 73% of all spins in MnO still remain in antiparallel order. Further calculations show that the bonding is compatible with an isotropic exchange interaction of next nearest neighbours having a coupling constant of J2 = 3.42 K. Finally, an interesting aspect is discussed connecting the formation of antiferromagnetic clusters and the absorption behaviour of MnO in the far infrared.

Phase diagram of the solid solution system NH 4Cl 1-xBr x as determined by optical methods

The phase diagram of NH 4Cl 1- x Br x has been determined by means of optical anisotropy and transmission measurements using solution-grown single crystals. It is shown that the change from the parallel to the anti-parallel orientational ordering of the NH 4-tetrahedra already takes place at X tr ≈ 0.08. In NH 4Br, the γ - δ transition temperature decreases rapidly with slowly increasing substitution of chloride for bromide. From the hysteresis along the β-δ and β-γ transition lines the existence of a tricritical point at X tc ≈ 0.25 is concluded. The relation between the x- T phase diagram and the p- T diagram of the ammonium halides is discussed.

A Microscopic Theory on the Phase Transitions in NH4Br –An Ising Spin Phonon Coupled System–

CsCl type NH 4 Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with the orientational order of NH 4 + ions. The appearance of two ordered phases is explained as essentially due to the fact that the direct interaction between NH 4 + ions stabilizes parallel ordering while the indirect interaction through phonons stabilizes the antiparallel ordering. Based on a microscopic hamiltonian describing the spin-phonon coupled system, several experimental results have been analized such as (i) anomalous lattice expansions (ii) orientational order parameters (iii) spontaneous displacement of Br - ions (iv) P - T phase diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially the property of correlated fluctuations of displacement of Br - ions seems to give a support on the validity of the microscopic model used.

Proprietes Magnetiques Des Alliages De Formule Tco 3, Dans Laquelle T Designe L'yttrium Ou Une Terre Rare

We describe the crystallographic structure and the magnetostatic properties of alloys of the formula TCo 3, in which T designates yttrium or one of the following rare earths: Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm. These properties are interpreted by considering an antiparallel ordering of the spins of the rare earth atoms with those of the atoms of cobalt. The band structure of cobalt is modified by the exchange field acting upon the cobalt atom and the moment of this atom increases with the spin of the rare earth atom.

Upper Limits to Electric-Field-Induced Nuclear Magnetic Dipole—Dipole Couplings in Polar Liquids

The alignment of polar molecules in liquids by an external electric field is investigated by pulsed nuclear magnetic resonance methods. Sensitive rotating-frame techniques are described for the detection of the expected induced nuclear magnetic dipole—dipole coupling. In no case could an induced coupling be detected. The upper experimental limits for the induced couplings disagree with the predictions of the Lorentz localfield model. For nitrobenzene and p-nitrotoluene, disagreement is also obtained with the Onsager local-field model by a reduction of almost an order of magnitude in the expected size of the effect. The negative results for these two compounds are indicative of strong local antiparallel ordering of the molecular electric dipoles. The induced coupling in p-nitrotoluene, reported by Buckingham and McLauchlan using standard NMR methods, is found to be absent in this investigation, which employs a measuring technique more sensitive by at least one order of magnitude.

Measurement of the Spin Dependence of theHe3(n, p)TReaction and of the Nuclear Susceptibility of AdsorbedHe3

The spin dependence of the ${\mathrm{He}}^{3}(n, p)\mathrm{T}$ reaction was investigated by studying the transmission of polarized thermal neutrons through a polarized ${\mathrm{He}}^{3}$ target which consisted of ${\mathrm{He}}^{3}$ adsorbed on zeolite. It was found that the interaction cross section was essentially all associated with an $I\ensuremath{-}\frac{1}{2}=0$ channel, i.e., $\frac{{\ensuremath{\sigma}}_{I\ensuremath{-}\frac{1}{2}}}{{\ensuremath{\sigma}}_{0}}=1.010\ifmmode\pm\else\textpm\fi{}0.032$. Once the spin dependence of the cross section was established, polarized neutrons provided a convenient method of studying the nuclear spin susceptibility of ${\mathrm{He}}^{3}$. Susceptibility measurements were made on our adsorbed-phase target over the temperature range 0.06-1.0\ifmmode^\circ\else\textdegree\fi{}K. It was found that the nuclear susceptibility began to fall below Curie's law at about 0.2\ifmmode^\circ\else\textdegree\fi{}K, indicating the onset of some type of antiparallel ordering in the nuclear spin system.

Impurity dipole interactions in alkali halides at low temperature

Calculations of dipole correlations and internal-energy densities are reported for dilute dipole systems in the NaCl and CsCl lattices. The tensorial dipole interaction potential is used in thermal averaging in a two-dipole approximation. Actual lattice geometries are taken into account in the three-dimensional spatial averages. The dipoles are assumed to have equilibrium orientations along [100] in the NaCl lattice, and along either [100] or [111] in the CsCl lattice. It is found thatlocal antiparallel ordering is favored in both lattices at low temperatures. Comparison with recent, low-temperature dielectric data indicates that relaxation mechanisms are as important as local antiparallel ordering in limiting theT −1 increase of (ɛ−1)/(ɛ+2) with decreasing temperature for OH− doped KCl. A simplified, nonequilibrium mechanism is studied whereby parallel-correlated dipoles with interaction energies greater than2 kT in absolute value become nonalignable and do not contribute toɛ. This mechanism together with antiparallel ordering are found to describe the experimental data satisfactorily; but additional relaxation mechanisms are indicated. The contribution of the dipole interactions to the specific heat is also calculated, and is in qualitative agreement with measured specific heat data for OH− doped KCl.