Unusual paramagnetic centers in PbTe undoped crystals

We present the results of the first experimental observation of unusual paramagnetism in solid when magnetic susceptibility of paramagnetic centers doesn't depend on temperature but drastically decreases when the applied magnetic field increases. This unusual combination of the field and temperature dependences of magnetic susceptibility was observed in the studies of magnetization and magnetic susceptibility performed in the wide range of temperatures (1.7–300 K) and magnetic fields (0–5.0 T) on the bulk and surface PbTe powder samples manufactured from crystal ingots grown by Bridgman method out of high-purity Pb and Te. We believe that presence of these features indicate that we are dealing with unknown untypical paramagnetism of paramagnetic centers in solid. We observed that the concentration of such unusual paramagnetic centers in PbTe crystal ingots increases towards their surface. Increase of the concentration of the centers can be so strong that it causes a transition of PbTe from the diamagnetic state to the paramagnetic one in quite wide range of low magnetic fields. Possible nature of the observed unusual paramagnetic centers is discussed.     

On the possibility of aligning paramagnetic molecules or ions in a magnetic field

Strong magnetic fields can hybridize low rotational states of paramagnetic molecules or molecular ions whose electronic angular momentum is coupled to the molecular axis. The hybridization creates pendular states in which the molecular axis is confined to librate over a limited angular range about the field direction. In this way substantial spatial alignment associated with large Zeeman shifts can be attained for many ground-state radicals or ions and electronically excited states of diatomic or linear molecules. The magnetic hybridization is analogous to that recently demonstrated for polar molecules in electric fields. The magnetic version can only provide ensemble alignment rather than orientation, but offers complementary chemical scope by virtue of its applicability to nonpolar molecules and ions.     

Time-dependent alignment of molecules trapped in octahedral crystal fields

The hindered rotational states of molecules confined in crystal fields of octahedral symmetry, and their time-dependent alignment obtained by pulsed nonresonant laser fields, are studied computationally. The control over the molecular axis direction is discussed based on the evolution of the rotational wave packet generated in the cubic crystal-field potential. The alignment degree obtained in a cooperative case, where the alignment field is applied in a favorable crystal-field direction, or in a competitive direction, where the crystal field has a saddle point, is presented. The investigation is divided into two time regimes where the pulse duration is either ultrashort, leading to nonadiabatic dynamics, or long with respect to period of molecular libration, which leads to synchronous alignment due to nearly adiabatic following. The results are contrasted to existing gas phase studies. In particular, the irregularity of the crystal-field energies leads to persistent interference patterns in the alignment signals. The use of nonadiabatic alignment for interrogation of crystal-field energetics and the use of adiabatic alignment for directional control of molecular dynamics in solids are proposed as practical applications.     

Tb3+/Eu3+镧系金属有机框架材料在荧光检测的研究进展

镧系金属有机骨框架（Lanthanide metal organic frameworks,Ln-MOFs）是以镧系离子为中心,与配体有机物组合的多孔材料,具有良好的光学性质,在许多领域有广泛的应用。不同的镧系离子与相应的有机配体结合,可组成不同性能的Ln-MOFs。在众多镧系元素中,Tb3+和Eu3+是良好的发光中心,常被用于组建发光Ln-MOFs。由Tb3+/Eu3+与有机配体形成的Ln-MOFs具有独特的荧光特性、较大stokes位移以及长发光寿命等优点在荧光检测领域具有重要应用。本文综述了基于Tb3+/Eu3+的Ln-MOFs在离子检测、生物标志物检测以及小分子检测领域的研究进展,对Ln-MOFs的发展前景进行了展望。     

Theoretical study of magnetic properties of ammonia molecule in nonuniform magnetic field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electromagnetic field has been used to define magnetic multipole moment operators and operators for the magnetic field of electrons acting on the nuclei of a molecule in the presence of nonhomogeneous external magnetic field. Perturbation theory has been applied to evaluate the induced electronic moments and magnetic field at the nuclei. Multipole magnetic susceptibility and nuclear magnetic shielding tensors have been introduced to describe the contributions arising in nonuniform fields, and their origin dependence has been analyzed. Extended numerical tests on the ammonia molecule in a static, nonuniform magnetic field have been carried out, using the random-phase approximation within the framework of accurate Hartree-Fock zero-order wavefunctions, and allowing for both angular momentum and torque formalisms in the calculation of paramagnetic contributions.     

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

The development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Review: A gentle introduction to magnetism: units,fields, theory,and experiment

We present an introduction to the workings, units of measure, and general properties of magnetic materials. This is intended as a “primer to interpretation of magnetic data” for those who are entering the field, or those who are encountering magnetic measurements in the literature. We expect this work will serve as an initial guide to the reader to familiarize them with the basics in the hope that those working in the field of magnetochemistry will wish to explore additional, more detailed literature as their specific investigations demand. Topics covered include: magnetic fields and units (SI and cgs), paramagnetism (magnetization and magnetic susceptibility), Curie and Curie–Weiss behavior, magnetic exchange interactions, magnetic anisotropy, dimeric systems and exchange-coupled networks (including chains, ladders, and layers), and long-range order.     

Spectroscopic analysis of electrically polarized polyacrylonitrile

Electrically polarized and unpolarized films of polyacrylonitrile (PAN) have been investigated by both infrared attenuated total reflection (ATR) and thermally stimulated discharge (TSD). ATR analysis of PAN films has been aimed at explaining the molecular origin of thermally stimulated currents, and consequently that of phenomena contributing to electrical polarization in this material. Preferred orientation of nitrile dipoles along the thickness direction (applied electric field direction) has been detected by ATR in both polarized and unpolarized films. It is suggested that dipolar alignment in unpolarized solvent-cast films could result from internal electric fields associated with space charges. The observed orientational anisotropy is found to disappear gradually as films (both polarized and unpolarized) are heated from ambient temperature to 100°C and rotational motion in the backbone is thermally activated. TSD currents observed in this thermal range are thus associated with randomization of molecular dipoles. TSD currents observed above 100°C are suggested to originate from drift of space charges, since both an isotropic orientation of dipoles and onset of considerable diffusion are detected by ATR in this temperature range. PAN films polarized by high-intensity electric fields (5 × 10⁵Vcm^?1, as opposed to 5 × 10⁴Vcm^?1) are found to retain orientational anisotropy above 100°C, and this is believed to be associated with a structural rearrangement induced by electrical polarization.     

Neutron diffraction studies of Pr2C3, Nd2C3, and Dy2C3 at 300 to 1.6°K

Neutron diffraction measurements have shown that the body-centered cubic Pr₂C₃, Nd₂C₃, and Dy₂C₃ become antiferromagnetic below 8, 24, and 22°K, respectively, all exhibiting the Tb₂C₃-type magnetic structure. In the uniaxial moment model having two antiferromagnetic and two paramagnetic body diagonals, the saturation order moments per metal atom are 1.3, 3.0, and 9.5 Bohr magnetons, respectively, being 41, 92, and 95% of the respective free ion values. Pr₂C₃ shows an exceptionally large crystal field effect. The antiferromagnetic alignment is uninfluenced by the applied field of up to 21 kOe. The crystal structure data at 300 to 1.6°K are also given. A brief review is presented on the physical properties of the rare earth sesquicarbides.     

Magnetic memory based on magnetic alignment of a paramagnetic ionic liquid near room temperature

A paramagnetic ferrocenium-based ionic liquid that exhibits a magnetic memory effect coupled with a liquid-solid phase transformation has been developed. Based on field alignment of the magnetically anisotropic ferrocenium cation, the magnetic susceptibility in the solid state can be tuned by the weak magnetic fields (<1 T) of permanent magnets.     

Anisotropic Change in the Magnetic Susceptibility of a Dynamic Single Crystal of a Cobalt(II) Complex

Atypically anisotropic and large changes in magnetic susceptibility, along with a change in crystalline shape, were observed in a Co^II complex at near room temperature. This was achieved by combining oxalate molecules, acting as rotor, and a Co^II ion with unquenched orbital angular momentum. A thermally controlled 90° rotation of the oxalate counter anion triggered a symmetry‐breaking ferroelastic phase transition, accompanied by contraction–expansion behavior (ca. 4.5 %) along the long axis of a rod‐like single crystal. The molecular rotation induced a minute variation in the coordination geometry around the Co^II ion, resulting in an abrupt decrease and a remarkable increase in magnetic susceptibility along the direction perpendicular and parallel to the long axis of the crystal, respectively. Theoretical calculations suggested that such an unusual anisotropic change in magnetic susceptibility was due to a substantial reorientation of magnetic anisotropy induced by slight disruption in the ideal D ₃ coordination environment of the complex cation.     

Assessment of sigma-diatropicity of the cyclopropane molecule

Spatial models of the current density field induced in the cyclopropane molecule by stationary, homogeneous magnetic fields, parallel to either the C3 or the C2 symmetry axis, have been constructed. A compact, abridged representation of the models is given via stagnation graphs that convey essential information. Maps of streamlines and moduli are also reported to complete current models that have proven useful to rationalize magnetic tensor properties, that is, magnetizability, 1H and 13C nuclear shieldings, and magnetic shielding along the C3 symmetry axis. Plots of Biot-Savart magnetic shielding density combined with current density visualization yield an accurate, detailed account of the shielding mechanisms. The magnetropicity of the system described by the current density model is fully consistent with the magnitude of magnetic tensors calculated at near Hartree-Fock level. In a field perpendicular to the molecular plane, cyclopropane sustains a diatropic sigma-ring current with the following peculiar features: (i) it follows the molecular periphery rather than the CC framework; (ii) it bifurcates in the proximity of the methylene moieties flowing along the CH bonds, both above and below the sigma(h) plane; (iii) it has an effect on the values of response properties, although it is not as large as expected from naive arguments (e.g., the center-of-mass value of the magnetic shielding constant is dominated by in-plane components rather than the out-of-plane component, which is in contrast to pi-aromatic systems such as benzene); (iv) it has a negligible effect on the strong anisotropy of carbon magnetic shielding, which is shown to arise from local currents. No evidence for strong diatropism, and therefore sigma-aromaticity of the cyclopropane molecule, was found on the magnetic criterion.     

The paramagentic resonance principles and fundamental parameters in powder spectra

The EPR principles are reviewed. The interaction of the unpaired electron with various local fields was shown to give rise to perturbations which help to define the environment of the electron spin. The principal magnetic parameters are defined and related to the physical parameters of the paramagnetic substances. The use of the g̃ and à magnetic parameters in the case of a single unpaired electron is illustrated in various characteristic cases. The identification of the paramagnetic center and the determination of lattice crystal fields were shown to be possible through the use of g̃ principal components. Further information about the nature and the structure of the paramegnetic center and its close environment is shown to be obtainable from the variation of spin-spin and spin-nuclear interactions.The cases where more than a single unpaired electron exist at a particular center are considered and the relevant additional parameters defined and their significance with respect to the symmetry of the center is examined.     

Angular‐Resolved Magnetometry Beyond Triclinic Crystals Part II: Torque Magnetometry of Cp*ErCOT Single‐Molecule Magnets

The experimental investigation of the molecular magnetic anisotropy in crystals in which the magnetic centers are symmetry related, but do not have a parallel orientation has been approached by using torque magnetometry. A single crystal of the orthorhombic organometallic Cp*ErCOT [Cp*=pentamethylcyclopentadiene anion (C₅Me₅^?); COT=cyclooctatetraenedianion (C₈H₈^2?)] single‐molecule magnet, characterized by the presence of two nonparallel families of molecules in the crystal, has been investigated above its blocking temperature. The results confirm an Ising‐type anisotropy with the easy direction pointing along the pseudosymmetry axis of the complex, as previously suggested by out‐of‐equilibrium angular‐resolved magnetometry. The use of torque magnetometry, not requiring the presence of magnetic hysteresis, proves to be even more powerful for these purposes than standard single‐crystal magnetometry. Furthermore, exploiting the sensitivity and versatility of this technique, magnetic anisotropy has been investigated up to 150 K, providing additional information on the crystal‐field splitting of the ground J multiplet of the Er^III ion.     

Magnetic sensitivity of the 3330.8 Å state of s-triazine

We report more precise measurements of the magnetic field effect on the supposed first singlet state of s-triazine at 3330.8 Å, and find a g-factor of 0.11 ± 0.08 assuming the observed effects are caused by a splitting. We also report new results that are in agreement with previous work on the magnetic dipole nature of the transition, but a concern about the assumed cylindrical symmetry of the 4.2°K s-triazine crystal is raised.     

The origin of transverse anisotropy in axially symmetric single molecule magnets

Single-crystal high-frequency electron paramagnetic resonance spectroscopy has been employed on a truly axial single molecule magnet of formula [Mn(12)O(12)(tBu-CH(2)CO(2))16(CH(3)OH)4].CH(3)OH to investigate the origin of the transverse magnetic anisotropy, a crucial parameter that rules the quantum tunneling of the magnetization. The crystal structure, including the absolute structure of the crystal used for EPR experiments, has been fully determined and found to belong to I4 tetragonal space group. The angular dependence of the resonance fields in the crystallographic ab plane shows the presence of high-order tetragonal anisotropy and strong dependence on the MS sublevels with the second-highest-field transition being angular independent. This was rationalized including competing fourth- and sixth-order transverse parameters in a giant spin Hamiltonian which describes the magnetic anisotropy in the ground S = 10 spin state of the cluster. To establish the origin of these anisotropy terms, the experimental results have been further analyzed using a simplified multispin Hamiltonian which takes into account the exchange interactions and the single ion magnetic anisotropy of the Mn(III) centers. It has been possible to establish magnetostructural correlations with spin Hamiltonian parameters up to the sixth order. Transverse anisotropy in axial single molecule magnets was found to originate from the multispin nature of the system and from the breakdown of the strong exchange approximation. The tilting of the single-ion easy axes of magnetization with respect to the 4-fold molecular axis of the cluster plays the major role in determining the transverse anisotropy. Counterintuitively, the projections of the single ion easy axes on the ab plane correspond to hard axes of magnetization.     

Correlation between magnetic properties and molecular structure of some metallo-mesogens

The behaviour of a large number of paramagnetic metallo-mesogenic molecules with Cu and VO in different mesophases in a magnetic field was investigated by EPR techniques and magnetic susceptibility measurements. The investigation of the angular dependence of the EPR spectra enabled conclusions to be reached concerning the molecular orientation in the external magnetic field. Temperature dependence magnetic susceptibility measurements were carried out in order to obtain information about the overall susceptibility anisotropy. The good agreement between experimental results and calculated data based on the known increment scheme is obvious. It is shown that the direction of orientation of the molecules in a magnetic field is predetermined by the sum of the anisotropy of the phenyl ring and the chelate core in the molecular structure.     

Luminescent properties and recent progress in applications of lanthanide metal-organic frameworks

Lanthanide metal-organic frameworks(Ln-MOFs), which is composed of organic bridging ligands and Ln3+ions/clusters, is an important component of luminescent MOFs. Compared with transition metal ions,lanthanide ions have a higher coordination number and abundant coordination geometry. Moreover, LnMOFs have special characteristics such as good porosity, topological diversity, high surface area and highly adjustable structure. The energy transfer(ET) process in Ln-MOFs could be easily affected by th...     

Lanthanides in molecular magnetism: old tools in a new field

In this tutorial review we discuss some basic aspects concerning the magnetic properties of rare-earth ions, which are currently the subject of a renovated interest in the field of molecular magnetism, after the discovery that slow relaxation of the magnetization at liquid nitrogen temperature can occur in mononuclear complexes of these ions. Focusing on Dy(III) derivatives a tutorial discussion is given of the relation of the crystal field parameters, which determine the anisotropy of these systems and consequently their interesting magnetic properties, with the geometry of the coordination sphere around the lanthanide centre and with the pattern of f orbitals. The problem of systems of low point symmetry is also addressed by showing how detailed single crystal investigation, coupled to more sophisticated calculation procedures, is an absolute necessity to obtain meaningful structure-property relationships in these systems.     

Detailed magnetic study on the formato-bridged MOFs with anion-tunable magnetic properties

Detailed studies of the structures, magnetic properties and photodimerization of a series of formato-bridged MOFs with the general formula M2(HCOO)3(4,4′-bpe)3(H2O)3(X) (4,4′-bpe = 4,4′-bipyridylethylene, M = Mn (1-X-), X- = ClO4-, NO3-, BF4-, I-, Br-; M = Co (2-X-), X- = ClO4-, NO-3; M = Zn (3-X-), X- = NO3-) were reported. Careful magnetic measurements on an ori- ented single crystal of 1-ClO4- determined the spin-flop magnetic phase diagram and some intrinsic parameters, such as the intralayer coupling J, the anisotropy field HA and the exchange field HE. Different anions can remarkably tune the magnetic properties of 1-X-, especially the critical fields of the spin-flop transition. Compound 2-ClO4- remained paramagnetic down to 2 K.