Electronic Transport Properties of Pentacene Single Crystals upon Exposure to Air

We report the effect of air exposure on the electronic properties of pentacene single crystals. Air can diffuse reversibly in and out of the crystals and controls the physical properties. We discern two competing mechanisms that modulate the electronic transport. The presence of oxygen increases the hole conduction, as in dark four O2 molecules introduce one charge carrier. This effect is enhanced by the presence of visible light. Contrarily, water, present in ambient air, is incorporated in the crystal lattice and forms trapping sites for injected charges.Comment: 16 pages, 3 figure

A 2:1 cocrystal of 6,13-dihydropentacene and pentacene

6,13-Dihydropentacene and pentacene cocrystallize in a ratio of 2:1, i.e. C22H16·0.5C22H14, during vapour transport of commercial pentacene in a gas flow. The crystal structure is monoclinic, space group P21/n, and contains one dihydropentacene molecule and half a pentacene molecule in the asymmetric unit.

Polar Structure and Two-Dimensional Heisenberg Antiferromagnetic Properties of Arylamine-Based Manganese Chloride Layered Organic-Inorganic Perovskites

The breaking of inversion symmetry can enhance the multifunctional properties of layered hybrid organic-inorganic perovskites. However, the mechanisms by which inversion symmetry can be broken are not well-understood. Here, we study a series of MnCl4-based 2D perovskites with arylamine cations, namely, (C6H5CxH2xNH3)2MnCl4 (x = 0, 1, 2, 3), for which the x = 0, 1, and 3 members are reported for the first time. The compounds with x = 1, 2, and 3 adopt polar crystal structures to well above room temperature. We argue that the inversion symmetry breaking in these compounds is related to the rotational degree of freedom of the organic cations, which determine the hydrogen bonding pattern that links the organic and inorganic layers. We show that the tilting of MnCl6 octahedra is not the primary mechanism involved in inversion symmetry breaking in these materials. All four compounds show 2D Heisenberg antiferromagnetic behavior. A ferromagnetic component develops in each case below the long-range magnetic ordering temperature of μ42-46 K due to spin canting

Ratio effect of salt fluxes on structure, dielectric and magnetic properties of La,Mn-doped PbBi2Nb2O9 Aurivillius phase

The double-layer Aurivillius phase Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 was synthesized by a molten salt method using a K2SO4/Na2SO4 flux. The effect on the crystal structure, morphology, dielectric and magnetic properties of varying the molar ratio of the oxide precursors to salt flux was investigated. Single-phase products with an orthorhombic structure were obtained for oxide to salt ratios of between 1:5 and 1:9, whereas for lower concentrations of salt a pyrochlore impurity phase is found in the products. SEM showed anisotropic plate-like grains, the size of which increases for larger salt ratios. An investigation of the magnetic properties showed the presence of mixed Mn3+ and Mn4+; the unit cell volume of the single-phase products decreases as the proportion of salt increases, which implies a higher proportion of smaller Mn4+ cations. This can be explained by the oxide ion donating properties (oxobasicity) of the molten salt mixture, which produces an oxidizing environment during synthesis. The best dielectric properties are obtained for an oxide to salt ratio of 1:7, exhibiting relaxor ferroelectric behavior. This is also the ratio at which the most pronounced ferromagnetic properties are observed, resulting from double-exchange interactions between Mn3+ and Mn4+ the proportions of which are approximately equal. Pb(0.4)Bi(2.1)La(0.5)Nb(1.7)Mn(0.3)O(9 )synthesized under these conditions thus exhibits optimal multiferroic properties

Structure-property relationships in the lanthanide-substituted PbBi₂Nb₂O₉ Aurivillius phase synthesized by the molten salt method

Samples of PbBi2Nb2O9, PbBi1.5La0.5Nb2O9, and PbBi1.5Nd0.5Nb2O9 have been prepared by the molten salt method. The structure, morphology, and electrical properties were investigated. All samples are single-phase and crystallize in an orthorhombic structure with A21am symmetry. Neutron diffraction data indicate that the Ln3+ cations prefer to occupy the perovskite A-site, whereas Pb/Bi occupy the perovskite A-site and the Bi2O2 layer. Changes in unit cell volume are observed on substitution and are attributed to the ionic radii of the Ln3+ cations and also correlated to changes in the B-O bond distances in the BO6 octahedra, which are also observed in IR spectra. SEM images reveal anisotropic plate-like grains, which increase in size with the presence of Ln3+ ions. The ferroelectric transition temperature (Tc) decreases with decreasing degree of BO6 distortion as the influence of the 6s2 lone pair of Bi3+ is diminished. Relaxor ferroelectric behavior is observed with Ln3+ substitution, driven by the disorder of the A-site cations. The room temperature ferroelectric polarization increases with Ln3+ substitution, ascribed to the decreased dielectric loss

Controlling phase separation in thermoelectric Pb1-xGexTe to minimize thermal conductivity

Intensive studies have been carried out over the past decade to identify nanostructured thermoelectric materials that allow the efficient conversion of waste heat to electrical power. However, less attention has been paid to the stability of such materials under operating temperatures, typically 400 degrees C or higher. Conventionally nanostructured ceramics tend to undergo grain growth at high temperature, lowering the density of interfaces and raising the thermal conductivity, which is detrimental to device performance. Therefore it is preferable to identify materials with stable nanostructures, for example systems that undergo spontaneous phase separation. Here we investigate PbTe-GeTe alloys, in which spinodal decomposition occurs on initial cooling from above 580 degrees C, forming complex nanostructures consisting of Ge-rich and Pb-rich domains on different size scales. The resulting dense arrangement of interfaces, combined with mass fluctuation associated with Pb-Ge mixing, enhances phonon scattering and strongly reduces the thermal conductivity. Here we focus on the nominal composition Pb0.49Ge0.51Te and show that by tuning the synthesis procedure, we are able to control the pattern of compositional domains and the density of interfaces between them. This allows low lattice thermal conductivities to be maintained even after thermal cycling over the operating temperature range

Structural and multiferroic properties in double-layer Aurivillius phase Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 prepared by molten salt method

A single-phase sample of the Aurivillius compound Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 was prepared by a molten salt method using K2SO4/Na2SO4 as the flux. The crystal structure, morphology, ferroelectric, and magnetic properties were investigated. Neutron powder diffraction data confirmed a non-centrosymmetric orthorhombic crystal structure with space group A21am and Pb/Bi disorder in the bismuth oxide blocks, Bi/Pb/La disorder on the perovskite A-site, and Nb/Mn disorder on the perovskite B-site. The morphology of the sample showed anisotropic plate-like grains as probed by scanning electron microscopy. The dielectric constant exhibits a transition peak between 600 K and 640 K that depends on frequency, indicating relaxor ferroelectric behavior. Electrical polarization versus applied field loops are unsaturated, with a remnant polarization of 0.43 μC/cm2 at 40 Hz under the maximum electrical field applied of 160 kV/cm. The ferroelectricity originates from the displacement of oxygen atoms in the BO6 octahedra, resulting in a polar structural distortion. Magnetic susceptibility measurements showed the presence of mixed Mn3+ and Mn4+, resulting in short-range ferromagnetic order via double exchange interactions below 33 K. The remnant magnetization (Mr) is 0.01 emu/g at 5 K. This mixed valence of Mn cations is mainly responsible for the high electrical conductivity. Thus, Pb0.4Bi2.1La0.5Nb1.7Mn0.3O9 exhibits coexisting ferroelectric and ferromagnetic properties

Growth and Helicity of Noncentrosymmetric Cu₂OSeO₃ Crystals

We have grown Cu$_2$OSeO$_3$ single crystals with an optimized chemical vapor transport technique by using SeCl$_4$ as a transport agent. Our optimized growth method allows to selectively produce large high quality single crystals. The method is shown to consistently produce Cu$_2$OSeO$_3$ crystals of maximum size 8 mm x 7 mm x 4 mm with a transport duration of around three weeks. We found this method, with SeCl$_4$ as transport agent, more efficient and simple compared to the commonly used growth techniques reported in literature with HCl gas as transport agent. The Cu$_2$OSeO$_3$ crystals have very high quality and the absolute structure are fully determined by simple single crystal x-ray diffraction. We observed both type of crystals with left- and right-handed chiralities. Our magnetization and ferromagnetic resonance data show the same magnetic phase diagram as reported earlier

Gradual emergence of superconductivity in underdoped LSCO

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.Comment: 5 pages, 4 figures, and supplementary materia