Dielectric compounds, magnetic propertie

Barium titanate has many important commercial apphcations. It has both ferroelectric and piezoelectric properties. Also, it has a very high dielectric constant (about 1,000 times that of water). The compound has five crystalline modifications, each of which is stable over a particular temperature range. Ceramic bodies of barium titanate find wide applications in dielectric amplifiers, magnetic amplifiers, and capacitors. These storage devices are used in digital calculators, radio and television sets, ultrasonic apparatus, crystal microphone and telephone, sonar equipment, and many other electronic devices. 

In spite of the great number of measurements of the properties of this compound, including conductivity, dielectric constant, magnetic susceptibility, electron and nuclear spin resonance, specific heat, thermoelectric power, etc., many over wide temperature and frequency ranges, there is still no consensus as to how all the various pieces of the puzzle fit together. Even such a basic question as to whether most of the high temperature conduction is along the TTF or the TCNQ stacks or in hybridized orbitals of both remains open. 

Interpretation and systemization of the magnetic properties of lanthanide compounds are based on crystal field theory which has been rqjeatedly discussed in literature, in particular by Morrison and Leavitt (1982) in volume 5 of this Handbook. So we begin our chapter with a short account of crystal field theory in a comparatively simple form with a minimal number of initial parameters with a clear physical meaning. This immediately provides the interaction hamiltonians of 4f electrons with deformations and vibrations of the crystal lattice. Within the framework of this theory one can easily calculate the distortions of the crystal field near impurity ions. A clear idea of the nature of magnetic phenomena in simple dielectric lanthanide compounds is certainly useful for consideration of systems with a more complicated electron structirre. 

Just as the magnetic properties of rare earth perovskites exhibit very diverse behavior, the electrical conductivities also show wide variations. Some compounds have been utilized for their dielectric properties, others show... 

Figure 3.2 (a-d) The structure, magnetism and dielectric properties of compound 2. (Adapted from Ref. [39]. Reproduced by permission of The Royal Society of Chemistry.)... 

The physical properties of fluorides are determined mainly by the high ionicity of the metal-fluorine bond and the low polarizability of the fluoride ion (see Dielectric Polarizabilities of Oxides Fluorides). Therefore, magnetism based on superexchange interactions and high optical transparency in a wide spectral range are the most interesting properties. They define, together with the anionic conductivity in flnorite-like compounds, the main area of possible applications of flnorides in material science or industry. 

From the point of view of physics, LCs are partially oriented fluids that exhibit anisotropic optical, dielectric, magnetic, and mechanical properties. The most important property of LCs is the reorganization of their supramolecular structures on external stimuli such as electric and magnetic fields, temperatnre, and mechanical stress, which lead to changes in their optical properties. In particular, electric tiled-induced control of optical properties of LCs (electro-optical effects based on the Freedericksz transition ) is at the heart of the multi-billion dollar liquid crystal display (LCD) industry. Most current LCD technologies rely on nematic " and to a lesser extent on ferroelectric LCs, while the recently discovered bent-core and orthoconic LCs still require significant investment into fundamental research and development. These and other applications and technologies continne to drive the search for new liquid crystal materials, and provide impetus to continue fundamental studies on new, often exotic, classes of compounds. 

Tautomerism, an equilibrium involving two or more isomeric structures accomplished via migration of an atom or a small group within a molecule [1—4], has been attracting scientific interest from a fundamental as well as a practical point of view for more than a century [5, 6]. The tautomeric isomerization is a phenomenon traditionally related to solutions of compounds, where the tautomeric compounds exist in different tautomeric forms that usually interconvert rapidly. In the case of slower interconversion, the specific tautomers can be identified by spectroscopic methods, such as nuclear magnetic resonance (NMR) spectroscopy (Scheme 13.1) [8]. The abundance of a particular tautomer can be controlled by tuning the reaction conditions (proticity, dielectric constants, temperature, and pH of the solution) [9-11]. Numerous studies have dealt with the control of tautomeric systems and their corresponding properties, such as fluorescence [12], photo- [13], or thermochromism [14] and the bioavailability of proper tautomeric forms [15]. 

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