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A numerical dispersion analysis of the alternating-direction implicit finite-difference time-domain method for transverse-electric waves in lossy materials is presented. Two different finite-difference approximations for the conduction terms are considered: the double-average and the synchronized schemes. The numerical dispersion relation is derived in a closed form and validated through numerical simulations. This study shows that, despite its popularity, the accuracy of the double-average scheme is sensitive to how well the relaxation-time constant of the material is resolved by the time step. Poor resolutions lead to unacceptably large numerical errors. On the other hand, for good conductors, the synchronized scheme allows stability factors as large as 100 to be used without deteriorating the accuracy significantly. Index Terms--Alternating-direction implicit finite-difference time-domain (FDTD) method, lossy media, numerical dispersion, stability.

The lumped-network finite-difference time-domain (LN-FDTD) technique is an extension of the conventional FDTD method that enables the incorporation of linear one-port LNs in a single FDTD cell. This paper studies the stability and the numerical dispersion of this technique. To this end, an isotropic medium that is uniformly loaded with LNs in the x-direction is considered as a working model. The stability analysis, based on the von Neumann method, is performed for general Mth-order LNs and closed-form stability conditions are derived for some particular cases. The numerical dispersion relation is obtained for plane-wave propagation in the proposed LN-loaded medium. It is shown that LNs can be interpreted in terms of an effective frequency-dependent permittivity and, as a consequence, the LN-loaded medium can be viewed as a uniaxial medium. The numerical admittance of the LNs is also obtained showing that, as a side-effect of the time discretization, the LN parameters become frequency-dependent, e.g. for the resistor case, the resistance becomes a function of the frequency. Index Terms--Finite-difference time-domain (FDTD) methods, lumped elements, numerical dispersion, stability.

Numerical results obtained using a full-wave hybrid technique (comprising a new formulation of the coupled-mode method and the mode-matching method) for treating 3-D isotropic-chiral-material discontinuities in rectangular waveguides with perfect electrically conducting walls were compared with experimental data. For that purpose, samples of an isotropic chiral material were manufactured as composite materials. These samples were then fully characterized in the X-band. The measured constitutive parameters were used in the full-wave hybrid technique to predict the scattering parameters of three different structures--each a waveguide rifled partially with the isotropic chiral material. The scattering parameters were also measured. The predicted and measured scattering parameters were in reasonably good agreement. Index Terms--Chiral material, coupled-mode method, discontinuities, generalized scattering matrix, mode-matching method, rectangular waveguide, scattering parameters, vector network analyzer.

This paper introduces an extension of the original finite-difference time-domain (FDTD) method for modeling double-negative media characterized by high-order frequency-dependent permittivity and permeability. The approach basically consists of adding electric and magnetic current densities to Maxwell's curl equations and considering Ohm's law as a constitutive relationship. Current densities are discretized by using a weighted average in time and Ohm's law by applying the Mobius transformation technique. The extended FDTD formulation is validated and its numerical features are carefully examined. More specifically, analytical stability conditions are derived for several types of double-negative media and the numerical dissipation issue is discussed. In addition, the numerical dispersion equation for general high-order double-negative media is given and the order of accuracy of the scheme is studied. Finally, the definition of numerical refractive index is addressed and it is shown that, when the discretization parameters of the problem ate not properly chosen, a negative refractive index may become a positive one in the discrete world, thus changing the physics of the problem. Index Terms--Double-negative media, finite-difference time-domain (FDTD) methods, Mobius transformation, numerical dispersion, stability.

The lumped-network finite-difference time-domain (LN-FDTD) technique is an extension of the conventional finite-difference time-domain (FDTD) method that allows the systematic incorporation of linear one-port lumped networks (LNs) into a single FDTD cell. This paper presents an extension of the LN-FDTD technique, which allows linear two-port (TP)-LNs to be incorporated into the FDTD framework. The method basically consists of describing a TP-LN by means of its admittance matrix in the Laplace domain. By applying the Mobius transformation technique, we then obtain the admittance matrix of the TP-LN in the Z-transform domain. Finally, appropriate digital signal-processing methodologies are used to derive a set of difference equations that models the TP-LN behavior in the discrete-time domain. These equations are solved in combination with the Maxwell-Ampere's equation. To show the validity of the TP-LN-FDTD technique introduced here, we have considered the equivalent circuit of a chip capacitor and a linear circuit model of a generic metal--semiconductor field-effect transistor. These LNs have been placed on a microstrip gap and the scattering parameters of the resulting hybrid circuit have been computed. The results are compared with those obtained by using the electromagnetic simulator Agilent HFSS in combination with the circuital simulator ADS, and with those calculated by ADS alone. For the chip capacitor, experimental measurements have also been carried out. The agreement among all the simulated results is good. Generally speaking, the measured results agree with the simulated ones. The differences observed are mainly due to the influence of the subminiature A connectors and some mismatching at the ports. Index Terms--Finite-difference time-domain (FDTD) methods, global modeling, lumped networks (LNs).

A study of the stability and numerical dissipation of the finite-difference time-domain technique including passive and active lumped elements has been reported recently by Thiel and Katehi. In particular, three different formulations for lumped inductors were analyzed: the explicit, semi-implicit, and implicit schemes. The implicit scheme was identified to be the same as the formulation previously introduced by Piket-May et al., and its dissipative nature was modeled by a series resistor. In this paper, we perform a numerical dispersion analysis of the above-mentioned schemes, which has not been reported thus far. Other numerical properties of these schemes such as the stability are also discussed. The dispersion analysis allows the numerical impedance of the lumped inductor to be defined or, alternatively, to show that a region of lumped inductors can be interpreted as a dielectric material with a frequency-dependent permittivity. Special attention is given to the implicit scheme. For this scheme, three main conclusions are drawn, i.e., 1) the formulation introduced by Piket-May et al. does not correspond to the implicit scheme, but to the explicit one, 2) the implicit scheme is unconditionally stable for a range of inductance values, and 3) by means of the dispersion analysis, it is shown that the dissipative nature of the implicit scheme is more naturally modeled by a parallel resistor. Index Terms--Finite-difference time-domain (FDTD) methods, lumped inductors, numerical dispersion, stability.

The borate Ag16B4O10 was synthesized at high temperature and at elevated oxygen pressures [Kovalevskiy et al. (2020). Chem. Sci.11, 962–969]. Its structure consists of [B4O10]8− polyanions (isostructural to P4O10) embedded in an Ag matrix. According to the standard valences Ag+, B3+ and O2−, the formula has an excess of eight e− which the above authors proposed were located, pairwise, in four Ag4 tetrahedra within the silver substructure. That conclusion was based on the semiconducting and diamagnetic properties, as well as the very small 'attractors' of the Electron Localization Function (ELF) found at the centre of these Ag4 tetrahedra. However, a large overlap of the projected density of states (DOS) of silver and oxygen indicated possible dispersion interactions between both atomic species. In this article, an alternative description is proposed based on the extended Zintl–Klemm concept. The anion [B4O10]8− can be formulated as Ψ‐[N4O10] P4O10, if it is assumed that the eight e− are transferred to the four B atoms, so converting them into Ψ‐N, this then makes sense of its similarity with P4O10, [N4(CH2)6], adamantane and tetraisopropyladamantane. When the Ag atoms connect to the O atoms, they arrange as the H atoms do in hexamethylenetetramine (HMTA). If the two lone pairs of each of the bridging O atoms in Ψ‐[N4O10] are equated to the C—H bonds in HMTA, then, this same equivalence exists between the C—H bonds and the O—Ag bonds in the compound Ag16B4O10. The 24 Ag atoms surrounding each [B4O10]8− group prolong the sphalerite structure of the borate anion by means of Ag—O bonds which also fit the sphalerite structure formed of AgO. The eight excess electrons might then be distributed between the Ag and the O atoms, so making sense of the mixing of the Ag and O states. The Ag atoms bonded to the O atoms of the [B4O10]8− groups form a coat that interconnects the borate anions through Ag—O bonds. To establish the validity of this new proposal, the study needs to be extended to the compound Ag3B5O9. [ABSTRACT FROM AUTHOR]

The structure of eudidymite is described in light of the extended Zintl–Klemm concept which considers that Na and Be atoms transfer their six valence electrons to the six Si atoms, converting them into Ψ‐P which forms a skeleton characteristic of pentels (Group 15 elements) and is similar to that described in the compound (NH4)2Ge[6][Ge[4]6O15] when analysed in the same manner. The Si[4] skeleton is formed of bilayers that are connected through Be2O6 groups which are in fact fragments of the β‐BeO structure which bridge the two contiguous Si‐bilayers by sharing O atoms. In this context, the Be atoms play a dual role, i.e. on the one hand converting the Si atoms into Ψ‐P, on the other hand replicating fragments of its own β‐BeO structure. The Be atoms partially reproduce their own structure despite it being enclosed in a more complex network such as in Na2Be2[Si[4]6O15]·H2O. Calculations of the ionic strength I considering Si as Ψ‐P is energetically more favourable than when I is calculated on the basis of tetravalent Si in the silicate, justifying this new approach of developing the theory of pseudo‐structure generation. This approach offers a major new development in the study of crystal structures. [ABSTRACT FROM AUTHOR]

Negative refraction has many potential applications in the information and communications technology field. However, the high losses of most negative refraction materials prevent the use of this novel property in the implementation of practical devices in the HF range. In an attempt to overcome this inconvenience, we present two gammadion–fishnet chiral metamaterial structures, constituted by the combination of a lossy chiral metamaterial and different fishnet structures. The electromagnetic characterization of the proposed metamaterials provides a larger bandwidth with negative refraction as well as lower losses when compared with the initial structures. The results were obtained by using both numerical and experimental data, which support our designs. [ABSTRACT FROM AUTHOR]

This paper reports on the experimental measurement of power threshold levels for the multipactor effect between samples of ferrite material typically used in the practical implementation of L- and S-band circulators and isolators. For this purposes, a new family of wideband, nonreciprocal rectangular waveguide structures loaded with ferrites has been designed with a full-wave electromagnetic simulation tool. The design also includes the required magnetostatic field biasing circuits. The multipactor breakdown power levels have also been predicted with an accurate electron tracking code using measured values for the secondary electron yield (SEY) coefficient. The measured results agree well with simulations, thereby fully validating the experimental campaign. [ABSTRACT FROM AUTHOR]

This article examines the comparison between the classical formulations used to describe silicates and that derived from the application of the extended Zintl-Klemm concept (EZKC). The ionic strength, I, for 25 silicate lattices is calculated taking into account both formulations, and the results show that, in every single one of the examples, the ionic strength of the Zintl polyanion is higher than that of the classical model which assigns a formal charge of 4+ for silicon. Our earlier study, firstly applied to the germanate (NH4)2Ge[6][Ge[4]6O15] [Vegas & Jenkins (2017). Acta Cryst. B 73, 94-100] and to the polyanion [Ge[4]6O15]6− equivalent to the pseudo-As2O5 derived from it, explained satisfactorily the charge transfer that takes place in the Zintl compounds. The value of I = ½∑ nizi2 for the Zintl polyanion was greater than for the compound as formulated in the classical way. In that article, a meaningful relationship was found between the electron transfers as defined by the EZKC and the ionic strength I of the anion [Ge[4]6O15]6− ≡ Ψ-As2O5. Because the ionic strength, I, of a lattice is directly proportional to the lattice potential energy, UPOT, the higher the I the greater the UPOT; thus it is harder to break up the lattice into its constituent ions and hence the lattice itself is more stable, giving support to the idea that the application of the EZKC and the resulting electron shifts yields structures which are inherently thermodynamically more stable than the starting configuration. [ABSTRACT FROM AUTHOR]

Taking into account new experimental data [Barkalov et al. (2016). Solid State Sci. 61, 220-224] on the pressure-induced Ni2In phase of Li2S, at 30 GPa, three concepts related to high-pressure phase transitions are reviewed here. This paper firstly reviews evidence that chemical oxidation (by inclusion of oxygen atoms) can produce a similar effect to the application of physical high pressure and temperature, in an effect labelled as the oxidation-pressure concept. Secondly, the pressure-induced Ni2In phase of Li2S is the final phase in the double transition antifluorite → anticotunnite → Ni2In, as is observed in other alkali metal sulfides. This new phase for Li2S could be expected after knowledge of the high-pressure Cmcm phase of Li2SO4, which is a distortion of the hexagonal I-Na2SO4 phase, both having M2S subarrays of the Ni2In-type. Thirdly, in order to clarify these links, a simple methodology is proposed for gauging the level of increased stability (by defining a stability enhancement ratio, S) when the extended Zintl-Klemm concept (EZKC) has been applied. The method uses relative values of the lattice potential energies estimated for Li2S and for the pseudo-lattice Ψ-BeS derived by applying the EZKC to Li2S, after which, Li2S can be reformulated as Li+[LiS]− ≡ Li+[Ψ-BeS]. [ABSTRACT FROM AUTHOR]

The structure of ((NH4)2Ge7O15 recently described as being a microporous material containing rings, in which GeO6 octahedra coexist with GeO4 tetrahedra, is re-examined in the light of the Extended Zintl-Klemm Concept as applied to cations in oxides. The Ge[6] atoms together with the NH4+ groups act as true cations, transferring their 6 valence electrons to the acceptor Ge2O5 moiety, so converting it into the [Ge6O15]6-≡3(Ψ-As2O5) ion (where Ψ refers to a pseudo-lattice) and yielding threefold connectivity. The tetrahedral Ge network shows similarities with the Sb2O3 analogue. At the same time, the Ge[6] atoms are connected to other Ge[4] atoms forming blocks that are part of a rutile-type GeO2 structure. Such an analysis shows that both substructures (the Zintl polyanion and the rutile fragments) must be satisfied simultaneously as has already been illustrated in previous articles which considered stuffed-bixbyites [Vegas et al. (2009). Acta Cryst. B65, 11-21] as well as the compound FeLiPO4 [Vegas (2011). Struct. Bond. 138, 67-91]. This new insight conforms well to previous (differential thermal analysis) DTA-TGA (thermogravimetric analysis) experiments [Cascales et al. (1998). Angew. Chem. Int. Ed. 37, 129-131], which show endothermic loss of NH3 and H2O to give rise to the metastable structure Ge7O14, which further collapses to the rutile-type GeO2 structure. We analyze the stability change in terms of ionic strength, I, and so provide a means of rationalizing the driving force behind this concept capable of explaining the atomic arrangements found in these types of crystal structures. Although the concept was formulated in 2003, later than the publication of the germanate structure, it was not used or else ignored by colleagues who solved this crystal structure. [ABSTRACT FROM AUTHOR]

In this paper, the multipactor RF breakdown in a parallel-plate waveguide partially filled with a ferrite slab magnetized normal to the metallic plates is studied. An external magnetic field is applied along the vertical direction between the plates in order to magnetize the ferrite. Numerical simulations using an in-house 3-D code are carried out to obtain the multipactor RF voltage threshold in this kind of structures. The presented results show that the multipactor RF voltage threshold at certain frequencies becomes considerably lower than for the corresponding classical metallic parallel-plate waveguide with the same vacuum gap. [ABSTRACT FROM PUBLISHER]

Neodymium aluminate coatings have been prepared in-situ by the laser zone melting (LZM) method, using a CO2 SLAB-type laser emitting at 10.6 µm. Polycrystalline Al2O3 commercial plates have been used as substrates, and coatings were prepared from the corresponding mixtures of powdered neodymium and aluminium oxides as starting materials. Microstructure, studied by SEM and phase composition, studied by XRD, proved the in-situ formation of a NdAlO3/NdAl11O18 eutectic. As a result, a well integrated composite coating was formed. Nanoindentation tests are consistent with excellent integration between coating and substrate. Structural similarities between the eutectic components within the coating, as well as between these and the substrate, are consistent with the crystallographic concepts proposed by Vegas (Ramos-Gallardo & Vegas, 1997), where cation sub-arrays play an important role governing metal oxide structures. These structure sublattices are suggested as the driving force behind eutectic oxide formation. [ABSTRACT FROM AUTHOR]

The aim of this paper is the analysis of the multipactor effect in a parallel-plate waveguide when a ferrite slab, transversally magnetized by a static magnetic field parallel to the waveguide walls, is present. Employing an in-house developed code, numerical simulations are performed to predict the multipactor radio frequency voltage threshold in such a ferrite-loaded waveguide. Variations of the ferrite magnetization field strength and the ferrite slab height are analyzed. Effective electron trajectories are also shown for a better understanding of the breakdown phenomenon, finding different multipactor regimes. [ABSTRACT FROM AUTHOR]

This work presents an experimental study in W band about the behavior of a plane Fresnel reflector when the feeder changes its position on the surface of a sphere whose centre is the same of the Fresnel plate zones. For this purpose, an experimental system based on seven Fresnel plate zones and two di®erent levels has been developed. The center frequency of the reflector is 96 GHz, the focal length is 100mm and height between levels is 0.78 mm. Based on this Fresnel reflector, an experimental set up has been developed. The horn antenna feeder is fixed and situated in far field and the receiver is also a horn antenna located at the Fresnel focal distance. Both the reflector and the receiving antenna have some rotation capability to enable measurements from di®erent angles. The experimental results show a good, stable behavior in gain versus the angular position of the feeder. This special property of Fresnel reflectors is impossible in parabolic reflectors and consequently, Fresnel reflectors could be used in new applications as radar imaging, increasing the radar field of view or improving the resolution by means of several squint feeders working simultaneously on the same lens or reflector. Therefore, the main objective of this paper is to analyze the behavior of this experimental set up for developing new Fresnel reflector-based applications. [ABSTRACT FROM AUTHOR]

The structures of ternary oxides and chalcogenides of alkali metals are dissected in light of the extended Zintl-Klemm concept. This model, which has been successfully extended to other compounds different to the Zintl phases, assumes that crystal structures can be better understood if the cation substructures are contemplated as Zintl polyanions. This implies the occurrence of charge transfer between cations, even if they are of the same kind. In this article, the charge transfer between cations is even more illustrative because the two alkali atoms have different electronegativity, so that the less electropositive alkali metal and the O/S atom always form skeletons characteristic of the group 14 elements. Thus, partial structures of the zincblende-, wurtzite-, PbO- and SrAl2-type are found in the oxides/sulfides. In this work, such an interpretation of the structures remains at a topological level. The analysis also shows that this interpretation is complementary to the model developed by Andersson and Hyde which contemplates the structures as the intergrowth of structural slabs of more simple compounds. [ABSTRACT FROM AUTHOR]

The article presents an experimental study which examines the high-pressure behaviour of dirubidium sulfide (Rb2) with antifluorite-type system at room conditions. The study synthesized Rb2 from stoichometric quantities of the elements by heating them inside a quarz ampoule. Results from the angle-dispersive X-ray powder diffraction analysis confirm two phase transitions such as an anticotunnite-type and a Ni2In-type structures.

The structures of the dimorphic FeLi[PO4] phosphate are analysed on the basis of their cation arrays. At ambient conditions, the compound, known as the mineral triphylite, is olivine type (Pnma) (CaMg[SiO4]) and transforms into the olivine-related NaCaVO4-type (Cmcm) structure, at high temperature and high pressure. This phase transition takes place with an exchange in the positions of the Li and Fe atoms, both filling octahedral voids. This feature, impossible to explain with the traditional model of cation-centred anion polyhedra, can be understood in the light of the extended Zintl-Klemm concept (EZKC) which contemplates as possible the electron transfer even between "cations". A detailed dissection of both structures leads to the conclusion that the observed phase transition triphylite → 3-LiFe[PO4] is controlled by the [FeP] cation subarrays which undergo a PbO-type → NiAs-type transition at high pressure. In addition to this relevant feature, it is shown that the [FeP] subarray, PbO type in triphylite, exits in the delithiated phase FePO4. The EZKC allows that LiFe[PO4] can be written with the pseudo- formula Li+[Ψ-FeSO4], so that the [FeP] subarray is converted into Ψ-[FeS], whose structure coincides with that of real FeS. The significant result is that the structure of FeS persists in the pseudoarrays Ψ-[FeS] of both Fe[PO4] and LiFe[PO4]. The "autopsy" also reveals that several substructures, formed by different pairs of atoms, co-exist with those of the PbO type and NiAs type, in triphylite and β-LiFe[PO4], respectively. Fragments of metallic lithium, layers of metallic iron and the rocksalt LiCl are some of the structures that can be revealed through the dissection process. The conclusion is that the formation of any structure implies that several substructures need to be satisfied simultaneously. These multiple substructures act as if they were resonance structures which cooperate to the stabilization of the whole network. [ABSTRACT FROM AUTHOR]

A novel full-wave hybrid technique was formulated to treat three-dimensional discontinuities in isotropic chiral materials present in rectangular waveguides with perfect electrically conducting walls. Our formulation comprises (i) the coupled-mode method for obtaining the electromagnetic field in each section that is homogeneous in the propagation direction and (ii) the mode-matching method for obtaining the generalised scattering matrix of each discontinuity (formed by two adjacent homogeneous sections). In particular, we reformulated the coupled-mode method based on appropriate representations of E and B. This formulation ensures the satisfaction of the boundary conditions of the magnetic field on perfect electrically conducting walls, which is not ensured by the conventional formulation of the coupled-mode method as it is based on representations of E and H. A new expansion for H emerges indirectly from our formulation. Knowledge of this expansion is required by the mode-matching method for incorporating the effects of discontinuities between adjacent homogeneous sections. Calculations confirmed that our new formulation delivers correct results, whereas the conventional formulation may not. [ABSTRACT FROM AUTHOR]

The coupled mode method (CMM) is a seminumerical method for studying electromagnetic propagation, originally formulated for closed structures. We show how this method can be used for obtaining the propagation constants and electromagnetic field in unbounded isotropic chiral media. We also characterize single and periodic cascade discontinuities made of isotropic chiral slabs by means of the mode matching method combined with the CMM. The results are tested with a robust FDTD technique, modified to model bi-isotropic media and with analytical solutions. In all cases good agreement is found. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2771–2779, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22863 [ABSTRACT FROM AUTHOR]

The generalization of the Zintl–Klemm concept provides a universal formulation of a crystal structure in terms of universal building skeletons formed by Klemm's pseudoatoms: atoms that behave structurally according to their formal total electron charge. An important difference in this novel view is that charge is considered to be transferred, in the strict Zintl's sense, from the donor cations to the building skeleton as a whole, not specifically to a given atom or ion. Although application is restricted to (IV)–(IV) compounds (group 14 structures), the principle seems to be universal and can be applied to understand, to relate and to predict the structure of complex compounds on the basis of more simple structures, e.g. a given AB skeleton provides the building block for A2 B, AB2, ABX m etc. compounds of a very different nature. The application of such a principle only requires information on the constituent atoms and on the existing phases of the p-block elements (observed under ambient and high-pressure and/or high-temperature conditions). The ideas introduced here demonstrate, for the first time, that a generalization of the Zintl–Klemm concept is possible and that such a generalization helps to establish a univocal link between chemical composition (in terms of pseudoatoms) and the crystalline structures observed experimentally. [ABSTRACT FROM AUTHOR]

As in SiO2 which, at high pressures, undergoes the α-quartz → stishovite transition, GaAsO4 transforms into a dirutile structure at 9 GPa and 1173 K. In 2002, a new GaAsO4 polymorph was found by quenching the compound from 6 GPa and 1273 K to ambient conditions. The powder diagram was indexed on the basis of a hexagonal cell ( a = 8.2033, c = 4.3941 Å, V = 256.08 Å3), but the structure did not correspond to any known structure of other AXO4 compounds. We report here the ab initio crystal structure determination of this hexagonal polymorph from powder data. The new phase is isostructural to β-MnSb2O6 and it can be described as a lacunary derivative of NiAs with half the octahedral sites being vacant, but it also contains fragments of the rutile-like structure. [ABSTRACT FROM AUTHOR]

Electromagnetic band gap (EBG) structures of two different configurations implemented inside R120 rectangular waveguides (frequency range 10–15 GHz) are examined. The first configuration has a periodic, piecewise uniform variation of permittivity in the propagation direction. Two types of such one-dimensional (1D) EBG structures are analyzed, one with the unit cell comprising two dielectric layers, the other with three dielectric layers per unit cell. The second configuration is two-dimensional, with the unit cell along the propagation direction containing two sections, each of which is made of alternating pillars of two dielectric materials. The Bloch theorem is invoked for the ideal EBG structures, which are infinitely long in the propagation direction. The mode-matching method (MMM) is used for real 1D-EBG structures, which contain a finite number of unit cells, whereas a combination of the coupled-mode method (CMM) and the MMM is used for real 2D-EBG structures. Spectrums of the transmission coefficient of the fundamental mode for the 1D-EBG structures are computed and shown to compare favorably against experimental data. The effect of inserting a defect in 1D-EBG structures is demonstrated theoretically as well as experimentally. The band gaps computed for real and ideal 2D-EBG structures are compared as well. [ABSTRACT FROM AUTHOR]

Abstract: We present two new approaches to describe the structures of the tin oxide fluorides Sn2OF2, Sn2OF5 and Sn4OF6, formerly described in terms of linked, cation-centered anion polyhedra. One approach considers the structures as cation arrays with inserted anions. The arrangement of the tin atoms in Sn2OF2 corresponds to that of the tin atoms in the rutile-like structure of SnO2 which, in turn, can be related to the structure of the high pressure phase γ-Sn. The anions in Sn2OF2 take positions like the O atoms in SnO2, but one fourth of them remain vacant. Sn2OF5 contains layers that match layers which can be cut out of the SnO2 structure parallel to (); compared to SnO2, the layers are mutually shifted, but within the layers the distribution of the tin atoms is alike. Sn4OF6 contains parallel columns that correspond to columnar fragments taken out of SnO2 parallel to b. These columns correspond to a composition [Sn3OF3]−; they are interconnected by [Sn2F6]2+ groups. The similarity of the tin atom arrangement in a [Sn3OF3]− column to that in SnO2 helps to recognize this. The other approach is to derive the structures of the oxide fluorides from the SnO2 structure by symmetry reduction, i.e., with the aid of group–subgroup relations between their space groups. It takes three steps of symmetry reduction from , the space group of SnO2, to the space group of Sn2OF2, . All atomic positions occupied in Sn2OF2 can be derived from positions of SnO2, but one fourth of the anion positions remain vacant. Group–subgroup relations reveal only a partial agreement for Sn2OF5 (within the layers), but the Sn atoms between the layers cannot be related to positions in SnO2. The structure of Sn4OF6 cannot be derived group theoretically from SnO2 because of the interfering [Sn2F6]2+ groups between the [Sn3OF3]− columns. [Copyright &y& Elsevier]

Guided wave propagation in a Ku -band rectangular waveguide filled with a series of cells of dielectric materials in the Kronig-Penney morphology is examined. Allowed and forbidden bands in these ideal electromagnetic band-gap (EBG) filters are obtained for the TE z and TM z propagation modes by invoking the Floquet theorem. Real EBG structures, with a finite number of unit cells, are also studied using the scattering matrix technique. As the number of unit cells in a real structure increases, its transmission characteristics converge to those of its ideal analogue. [ABSTRACT FROM AUTHOR]

Guided wave propagation in a parallel-plate waveguide with Kronig–Penney morphology is analyzed. Modes in the photonic band-gap (PBG) structure can be classified as either transverse electric or transverse magnetic with respect to the propagation direction. Above the modal cut-off frequencies, band gaps exist where propagation is forbidden. The locations and the widths of the band gaps depend on modal order, waveguide height, and the permittivity contrast and relative volumetric proportion of the two materials constituting the unit cell of the PBG structure. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 511–514, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11106 [ABSTRACT FROM AUTHOR]

Recently a finite-difference frequency-domain (FDFD) formulation has been reported for the dispersion analysis of uniform waveguides loaded with anisotropic dielectrics characterized by a diagonal tensor [4]. This formulation, which leads to an eigenvalue problem for the propagation constant γ, involves four field components. This paper shows that this formulation can be improved by taking advantage of the fact that the structures considered are strictly bidirectional waveguides and thus the dispersion analysis problem can be formulated as an eigenvalue problem for γ2, involving only two field components. This improved formulation is applied to several waveguiding structures and a comparison of the results with those obtained by the commercial simulator Agilent-HFSS 5.6 show good agreement. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 331–335, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11052 [ABSTRACT FROM AUTHOR]

The structure of 94 ternary aluminates are reinterpreted in the basis f the Zintl-Klemm concept and Pearsons generalized octet rule. In aluminates of highly electroositve metals such as alkali, alkaline-earth and rare-earth metals, the Al atoms from three-dimensional skeletal which can be interpreted as if the Al atoms were behaving as Intel polyanions, adopting the structure f either main-group elements or Zintl plyanions showing the either main-group elements or Zintl plyanions showing the same connectivity. The atoms are then located close to both the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. When more electronegative elements, such as W or Si, are present in the compound, the electron transfer towards the Al atoms does not take place. In this case, aluminum behaves as a base, transferring ties electrons to the more electronegative atoms and the coordination sphere of aluminum becomes octahedral. In some compounds the Al atoms clearly show amphoteric character s that some Al atoms act as donors (bases) and hence are octahedrally coordinated, whereas others behave as acceptors (acids), adopting a tetrahedral coordination. From this it is concluded that the coordination sphere of aluminum is not a function of the ionic radius of the Al[SUP3+] cations, but it depends on the nature f the other cations accompanying them in the structure. The networks formed by these aluminates are, in many instances, similar to those of the binary oxides of the main-group elements. For this reason, a systematic survey of these oxides is also reported. Compounds such as stuffed cristobalites and trydimits and also perovskites are examples of this new interpretation. Perovskites are then reinterpreted as a stuffed psudo-Te[SUB3] structure. there families of compounds such as silicates and phosphates are susceptible to a similar interpretation. This study provides additional examples of how cations recognize themselves in spite of... [ABSTRACT FROM AUTHOR]