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In the present paper, we have investigated the electronic structure of some sillenites - Bi12MO20 (M = Ti, Ge, and Si) compounds based on the density functional theory. The mechanical and optical properties of Bi12MO20 have also been computed. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The band gap trend in Bi12MO20 can be understood from the nature of their electronic structures. The obtained electronic band structure for all Bi12MO20 compounds is semiconductor in nature. Similar to other oxides, there is a pronounced hybridization of electronic states between M-site cations and anions in Bi12MO20. Based on the obtained electronic structures, we further calculate the frequency-dependent dielectric function and other optical functions.

In this work, we present an investigation of the optical properties and band structures for the photonic crystal structures (PCs) based on Sn2P2X6: X = S, Se) with Fibonacci superlattices. The optical properties of PCs can be tuned by varying structure parameters such as the lengths of poled domains, filling factor, and dispersion relation. In our simulation, we employed the finite-difference time domain technique and the plane wave expansion method, which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives.

In the present paper we have investigated the electronic structure of some orthorhombic A(3)X(2)S(7) (X = Ti, Zr, and Hf) compounds based on the density functional theory. Then we extend the RuddlesdenPopper (RP) A(3)X(2)S(7) sulfides and examine how ferroelectricity is induced by coupled octahedral rotation modes. The mechanical and optical properties have also been computed. The second-order elastic constants have been calculated, and the other related quantities have also been estimated in the present work. The band gap trend in A(3)X(2)S(7) can be understood from the nature of their electronic structures. The obtained electronic band structure for Ba3Hf2S7 and Ba3Hf2S7 compounds are semiconductor in nature, and the Ba3Hf2S7 compound is also semi-metal. Similar to ferroelectric oxides, there is a pronounced hybridization of electronic states between X-site cations and anions in A(3)X(2)S(7). Based on the obtained electronic structures, we further calculate the frequency-dependent dielectric function and other optical functions in different phases.

In this work, we study the band gap properties of a two-dimensional phononic crystal composed of a hexagonal cavity inside a circular enclosure embedded in a homogeneous matrix. A simple two-dimensional structure with a cavity that is modeled with an opening in one side that allows air to enter was studied theoretically using the finite element method (FEM). The properties of acoustic wave propagation in a phononic crystal consisting of periodic inclusions with a cavity placed in an air host material are analyzed. The phononic crystal dispersion relations, transmission loss spectrum, and displacement fields of the eigenmodes of this phononic crystal are investigated. The pressure field distribution inside the domain is solved parametrically through a two-dimensional acoustic wave equation in the horizontal plane as a function of frequency. We have found that several complete band gaps with a variable bandwidth exist for the acoustic waves of longitudinal polarization.

In the present work the acoustic band structure of a two-dimensional phononic crystal (PC) containing an organic ferroelectric (PVDFpolyvinylidene fluoride) and topological insulator (Bi2Te3) were investigated by the plane-wave-expansion (PWE) method. Two dimensional PC with square lattices composed of Bi2Te3 cylindrical rods embedded in the PVDF matrix are studied to find the existence of stop bands for the waves of certain energy. Phononic band diagram w= w(k) for a 2D PC along the G-X-M-G path in the square Brillouin zone show four stop bands in the frequency range 0.01–8.0 kHz. This work is supported by the projects DPT-HAMIT, DPT-FOTON, and NATO-SET-193 as well as TUBITAK under the project nos., 113E331, 109A015, and 109E301. One of the authors (Ekmel Ozbay) also acknowledges partial support from the Turkish Academy of Sciences.

Based on density functional theory, we have studied the electronic, and optical properties of narrow-band ferroelectric compounds – (Ge,Sn) Te. Generalized gradient approximation has been used for modeling exchange-correlation effects. The lattice parameters of the considered compounds have been calculated. The calculated electronic band structure shows that GeTe and SnTe compounds have a direct forbidden band gap of 0.742 eV and 0.359 eV. The real and imaginary parts of dielectric functions and therefore, the optical functions such as energy-loss function, as well as the effective number of valance electrons and the effective optical dielectric constant are all calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data. This work is supported by the projects DPT-HAMIT, DPT-FOTON, NATO-SET-193 and TUBITAK under Project Nos. 113E331, 109A015, 109E301. One of the authors (Ekmel Ozbay) also acknowledges partial support from the Turkish Academy of Sciences.

The authors are grateful to the ABINIT group for the ABINIT project that we used in our computations. The nonlinear optical properties and electro-optic effects of some oxygen-octahedra ferroelectrics are studied by the density functional theory (DFT) in the local density approximation (LDA) expressions based on first principle calculations without the scissor approximation. We present calculations of the frequency- dependent complex dielectric function (?) and the second harmonic generation response coefficient ?(2)(-2?, ?, ?) over a large frequency range in tetragonal and rhombohedral phases. The electronic linear electro-optic susceptibility ?(2)(-?, ?, 0) is also evaluated below the band gap. These results are based on a series of the LDA calculation using DFT. The results for ?(2)(-?, ?, 0) are in agreement with the experiment below the band gap and those for ?(2)(-2?, ?, ?) are compared with the experimental data where available. This work is supported by the projects DPT-HAMIT, DPT-FOTON, and NATO-SET-193 and TUBITAK under the project nos., 113E331, 109A015, and 109E301. One of the authors (Ekmel Ozbay) also acknowledges partial support fromthe Turkish Academy of Sciences.

We have performed a first principles study of structural, electronic, and optical properties of rhombohedral Sb2Te3 and Bi 2Te3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated and presented in the study. © 2013 Copyright Taylor and Francis Group, LLC.