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The structures of the three dimeric cyclotrisilicate anions [Si6O17]10−, [Si6O16]8− and [Si6O15]6− forming part of the compounds Rb10[Si6O17] or Rb14[Si4][Si6O17], Cs8[Si6O16] and Na3Y[Si6O15], respectively, have been described as different ways of condensing the more elemental cyclotrisilicate anions [Si3O9]6− [Hlukhyy & Fässler (2013). Z. Anorg. Allg. Chem. 639, 231-233]. Such a description prevents a rational explanation of the connectivity between the Si atoms. So, in this article the Si skeletons of the above silicate anions are put on a common basis in light of the extended Zintl-Klemm concept. Their connectivity is directly related to the ratio Ψ-S/Ψ-P resulting from the number of electrons transferred from the electropositive Na, Y, Rb and Cs atoms, offering a new example of the fact that there exists a univocal link between chemical composition (in terms of pseudo-atoms) and structure: the generalized Zintl-Klemm concept. The three skeletons fit the connectivity of hypothetical Zintl polyanions of composition [Ψ-S4P2]10-, [Ψ-S2P4]8- and [Ψ-P6]6− respectively. [ABSTRACT FROM AUTHOR]

In this paper, we present a new study of the electromagnetic wave propagation inside a rectangular waveguide partially loaded with an isotropic chiral medium, by means of the coupled mode method. Along this analysis some new features related to the variation of the propagation constant versus frequency and the chirality parameter, which have not been previously reported in the literature, are introduced. Among them, we highlight the existence of a frequency point wherein all the dispersion curves cross each other, and the increasing of the cutoff frequency of the fundamental mode with the chirality of the medium. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1206-1212, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27534 [ABSTRACT FROM AUTHOR]

A topological analysis of the electron localization function (ELF) of a molecule of hexamethyldisiloxane, (H3C)3-Si-O-Si-(CH3)3, has been carried out, drawing a consistent picture of Si-O-Si bonding both in the linear and angular geometries. The ELF analysis confirms the idea that the O atom, in the linear geometry of (H3C)3-Si-O-Si-(CH3)3, is isolobal with the isoelectronic -CH- and -BH3- groups, the bonding in the Si-O-Si group being described as a two-electron, three-center (2 e, 3 c) bond. At the same time, the three oxygen lone pairs mirror the three C-H and B-H bonds, respectively. On the contrary, in the angular geometry the same O atoms form two Si-O bonds and its lone pairs mimic the geometry of the -CH2- group. In this model the O atoms would play the same role as the formally present O2− anions in the `so-called' ionic solids, such as in the skeletons of aluminate and silicate polyanions, thereby connecting molecular and solid-state chemistry as formulated by the `fragment formalism' or the `molecular unit-cell approach'. This unifying concept as well as the calculations we have carried out fully agree and also give support to earlier ideas developed by Bragg and Bent, among other authors. Bonding in the series of compounds P4, P4O6, P4O10, N4(CH2)6 (hexamethylenetetramine) and (CH)4(CH2)6 (adamantane) is discussed in the context of the isolobal model. [ABSTRACT FROM AUTHOR]

The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M2S subnets of M2SO x ( x = 3, 4) compounds ( M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M2S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397-398; Vegas (2000), Crystallogr. Rev. 7, 189-286; Vegas et al. (2002), Solid State Sci. 4, 1077-1081]. These structural changes suggest that if M2SO2 would exist, its cation array might well have an anti-CaF2 structure. On the other hand, in an analysis of the existing thermodynamic data for M2S, M2SO3 and M2SO4 we have identified, and report, a series of unique linear relationships between the known Δf Ho and Δf Go values of the alkali metal ( M) sulfide ( x = 0) and their oxyanion salts M2SO x ( x = 3 and 4), and the similarly between M2S2 disulfide ( x = 0) and disulfur oxyanion salts M2S2O x ( x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δf Ho) for as yet unprepared compounds, M2SO ( x = 1) and M2SO2 ( x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609-3620], the values of the absolute entropies were estimated and from them, the standard Δf So values, and then the Δf Go values of the salts. A tentative proposal is made for the synthesis of Na2SO2 which involves bubbling SO2 through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate Δ Go to be approximately −500 kJ mol−1. However, examination of the stability of Na2SO2 raises doubts and Na2SeO2 emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na2S and Na2SeO4. Like Na2SO2, this compound is predicted to have an anti-CaF2 Na2Se subnet. [ABSTRACT FROM AUTHOR]

In this paper, the characteristics of layered structures (photonic or electromagnetic bandgaps), including chiral media, are studied by means of two different numerical methods, one in the time domain (finite differences in the time domain, FDTD) and the other in the frequency domain (coupled-mode method, CMM). The results (reflection and transmission coefficients for a plane wave normally incident over a layered structure) obtained by means of both well different techniques are practically identical. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures ( fcc: face-centred cubic and sc: simple cubic, respectively) exhibit charge concentration zones which coincide with the positions occupied by the O atoms in their oxides. Similar features, also observed for the pairs Ca/CaF2 and BaSn/BaSnO3, are supported by recent high-pressure experiments as well as electron-localization function (ELF) calculations, carried out on elemental K. At very high pressures, the elemental K adopts the hP4 structure, topologically identical to that of the K atoms in high-pressure K2S and high-temperature α-K2SO4. Moreover, the ELF for the hP4 structure shows charge concentration (∼ 2 electrons) at the sites occupied by the S atoms in the high-pressure K2S phase. All these features confirm the oxidation/high-pressure equivalence as well as the prediction of how cation arrays should be metastable phases of the parent metals. For the first time to our knowledge, the structure type, dimension and topology of several oxides and fluorides (CaO, CaF2 and BaSnO3) are explained in univocal physical terms. [ABSTRACT FROM AUTHOR]

This paper studies the stability and numerical dispersion of an extension of the original finite-difference time-domain (FDTD) method to incorporate magnetized ferrites. This extension is based on discretizing the ferrite equations by means of central differences and averages. Both partially magnetized and saturated ferrites are considered. With the aim of studying the numerical features of the resulting FDTD algorithms, we have considered the propagation of plane-waves along longitudinally magnetized ferrite materials. This is a simple 1D problem that enables the numerical properties of the algorithms to be studied analytically. A von Neumann stability analysis shows that the stability condition of the original FDTD method is preserved and that the algorithms do not exhibit numerical dissipation. The accuracy of the numerical results is not deteriorated with respect to those obtained in the demagnetized case. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We introduce and discuss an interpretative model of the structure and bonding of inorganic crystals containing metallic elements. The central idea is the conception of the crystal structure of such an inorganic compound as a metallic matrix whose geometric and electronic structures govern the formation and localization of the anions in the lattice. This is the reason for labelling the model anions in metallic matrices (AMM). Taking the Al X3 crystal family ( X = F, Cl, OH) as a suitable test-bed class of compounds, we illustrate how this approach gives a direct interpretation of the crystalline structures and explains the variable coordination that Al exhibits in crystalline materials. An exhaustive analysis of the topology of the electron density allows us to provide a quantum-mechanical assessment of the main hypotheses of the AMM model and to uncover, using microscopic arguments, the behavior of anions as chemical pressure agents. [ABSTRACT FROM AUTHOR]

This Letter introduces a novel impedance boundary condition (IBC) for the matched termination of waveguide ports in finite-difference–time-domain (FDTD) simulators. It is based on approximating the wave impedance by a rational function over the frequency band of interest. To show the validity of the proposed IBC, the reflection coefficient of a wideband matched 90° H-plane corner in rectangular waveguide is computed. The results obtained, which are less than -45 dB over the whole waveguide band, are in good agreement with those obtained by a commercial finite-element simulator. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 151–155, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10400 [ABSTRACT FROM AUTHOR]

The main drawback of the coupled-mode method is the poor convergence and accuracy of the discontinuous field components. The greater the discontinuity, the worse these problems become. This Letter presents an improvement of the method, allowing this drawback to be minimized. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 408–414, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10337 [ABSTRACT FROM AUTHOR]

The accurate time-domain simulation of the nonlinear dynamics of circuits containing distributed elements is of great interest. Specially at high frequencies, it is essential to consider phenomena, such as crosstalk, packaging effects, and electromagnetic interaction between active and passive elements. These effects are mainly due to the distribute components of the circuit. Therefore, the accuracy in the instability or oscillation analysis of nonlinear circuits is often limited by the models used for such elements. Commercial simulators based on the equivalent-circuit approach hardly take into account the mentioned effects. In this work, the finite-difference time-domain (FDTD) method is applied to the nonlinear simulation of a diode-based microstrip circuit exhibiting a parametric frequency division by two. The FDTD method rigorously solves Maxwell's curl equations in the time domain providing a full-wave characterization of the distributed elements of the circuit. In the example considered, a relatively low operating frequency has been selected to allow a reliable comparison between the results obtained by the proposed technique and those provided by a circuit-based commercial simulator. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1300–1302, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23352 [ABSTRACT FROM AUTHOR]

At the pre- and post-processing stages of a finite-difference time-domain (FDTD) simulation, important tasks are carried out that require knowledge of the port data (propagation constants, fields, impedances, and so on) of the problem structure. This paper introduces a 2D finite-difference frequency-domain (FDFD) eigenvalue formulation specifically tailored for the computation of port data to be used in conjunction with the 3D-FDTD method. A key feature of the proposed FDFD scheme is that it leads to the same numerical dispersion equation as that of the 3D-FDTD method. This means that, for a given frequency, the numerical propagation constants and mode patterns calculated by the two methods are identical. This is desirable for preserving the accuracy of the FDTD simulation. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 1–3, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20704 [ABSTRACT FROM AUTHOR]

A topological analysis of the electron localization function (ELF) of a molecule of hexamethyldisiloxane, (H3C)3-Si-O-Si-(CH3)3, has been carried out, drawing a consistent picture of Si-O-Si bonding both in the linear and angular geometries. The ELF analysis confirms the idea that the O atom, in the linear geometry of (H3C)3-Si-O-Si-(CH3)3, is isolobal with the isoelectronic -CH3(+)- and -BH3- groups, the bonding in the Si-O-Si group being described as a two-electron, three-center (2e, 3c) bond. At the same time, the three oxygen lone pairs mirror the three C-H and B-H bonds, respectively. On the contrary, in the angular geometry the same O atoms form two Si-O bonds and its lone pairs mimic the geometry of the -CH2- group. In this model the O atoms would play the same role as the formally present O(2-) anions in the `so-called' ionic solids, such as in the skeletons of aluminate and silicate polyanions, thereby connecting molecular and solid-state chemistry as formulated by the `fragment formalism' or the `molecular unit-cell approach'. This unifying concept as well as the calculations we have carried out fully agree and also give support to earlier ideas developed by Bragg and Bent, among other authors. Bonding in the series of compounds P4, P4O6, P4O10, N4(CH2)6 (hexamethylenetetramine) and (CH)4(CH2)6 (adamantane) is discussed in the context of the isolobal model.