Cohesive and elastic properties of alkaline earth chalcogenide crystals

In the present study an analysis of the cohesive and elastic properties has been performed in fourteen alkali chalcogenide crystals using the composite potential forms based on the Drude oscillator model. The short-range overlap repulsion has been considered up to the second nearest neighbour interactions. The van der Waals coefficients have been estimated from the Kirkwood-Muller formulae. Cohesive energies, bulk-modulus and its pressure derivatives calculated for the entire family of crystals under study are found to present a good agreement with available experimental data, better than those obtained by previous workers using the traditional Born potential model.On leave from theDepartment of Physics, G. B. Pant University, Pantnagar-263 145, India.Authors are thankful to Mr. Saleh O. Harraga for the computational help.     

Dielectric properties of nickel-doped NaCl crystals

The influence of nickel impurities upon dielectric losses in natrium chloride crystals is studied. Experiments have shown that in non-heated samples, stored over prolonged periods of time, the majority of nickel is present in the form of aggregates and higher complexes. Their decomposition and the building up of dipoles depends in the first instance on sample heating and cooling methods. The aggregation of dipoles in time at room temperature is studied.It is shown that with nickel concentrations reaching approximately up to 2. 10^–3 mol% in heated and quickly cooled samples, all nickel builds itself into the NaCl lattice substitutionally in the form of dipoles Ni²⁺ + cation vacancy, while at higher concentrations some nickel precipitates or builds itself in in another non-substitutional way.The author wishes to express his thanks to Dr. Funk for making the polarographic analysis of the samples and to Dr. Bohun for valuable discussions of the results obtained.     

Study of static and thermophysical properties of alkaline earth fluoride crystals

An analysis for cohesive energy, bulk modulus and its pressure derivatives in BaF₂, SrF₂ and CaF₂ crystals has been carried out by means of interionic potential model. Interionic potential consist of long range coulomb and three-body interactions along with short-range attractive van der Waals and repulsive Hafemeister-Zarht type interactions. The three body force parameter $\text{?(r)}$ and vdWI coefficients C and D are determined using the Cochran's formula and Slater-Kirkwood variational (SKV) integrals. The calculated values of cohesive energy, bulk modulus and its pressure derivatives for solids under study are found generally in good agreement with available experimental data. The results achieved by other investigations [13, 14] have also been compared with results calculated in the present study.     

Electrolytic coloration and spectral properties of OH- and Cu-codoped NaCl crystals

OH⁻- and Cu⁺-codoped NaCl crystals are colored electrolytically by using a pointed anode and a flat cathode at various temperatures and under various voltages. Absorption and fluorescence spectral properties of colored crystals are investigated. Absorption, excitation and fluorescence spectra reveal existence of V₂, V₂^m, V₃, OH⁻, U, U_A, O²⁻–V_a⁺, O₂⁺, Cu⁺, Cu⁰, Cu⁻ and OH⁻-perturbed Cu⁺ color centers in colored crystals. Formations of color centers are explained.     

Vibrational properties of crystals with TlSe type structure

The lattice vibration spectra of TlS and TllnSe₂ crystals were investigated by infrared reflectivity in the wavenumber range from 20 to 5000 cm^?1. The frequencies of K = 0 phonons were determined by Kramers-Kronig analysis of the spectra. All five infrared active lattice vibrations predicted by the group theoretical analysis were identified. The normal coordinates of vibrations for five infrared active phonons were determined. In the approximation which does not take into consideration long range Coulomb interaction values of the force constant and effective changes are obtained for TlSe, TlS and TllnSe₂ crystals.     

Spontaneous excitation of discrete breathers in crystals with the NaCl structure at elevated temperatures

The density of phonon states at various temperatures has been calculated for crystals with the NaCl structure with equal and strongly differing weights of anions and cations. It has been shown that, in the crystal with a considerable difference in the weights of components, a wide band gap exists in the spectrum of phonon states, which leads to spontaneous excitation of nonlinear localized vibrational modes??gap discrete breathers having frequencies inside the band gap, if the temperature is sufficiently high. It has been found that the peak of the density of phonon states lying above the spectrum of linear vibrations appears at elevated temperatures, which can indicate the existence of discrete breathers with corresponding frequencies. It has been noted that, previously, the existence of gap discrete breathers in the NaI crystal at 555 K was shown experimentally. The presented results bring up the question of the theoretical justification and experimental observation of the breathers with frequencies above the phonon spectrum.     

Reversible and irreversible magnetic-field-induced changes in the plastic properties of NaCl crystals

It is established that a weak magnetic field with induction B∼1 T gives rise to irreversible changes in NaCl crystals without freshly introduced dislocations, while a strong magnetic field with induction 16<B<35 T gives rise predominantly to reversible changes. It is inferred that there exist two different channels whereby a magnetic field influences the state of point defects. Fiz. Tverd. Tela (St. Petersburg) 40, 2065–2068 (November 1998)     

Thermal expansion and elastic properties of the crystals with modulated structure

An experimental study of structure characteristics and elastic properties of the incommensurate crystals is reported.     

Dispersion properties of two-dimensional phonon crystals with a hexagonal structure

Acoustic and optical properties of two-dimensional phonon crystals with a hexagonal symmetry are described. Differential-difference equations describing plane oscillations of a two-dimensional lattice of material structures are derived in a harmonic approximation, and dispersion dependences of acoustic and optical phonons are calculated. Branches of optical oscillations and intervals of forbidden frequencies are shown to appear when a lattice consists of two types of atoms.     

Structure and soft mode behavior of two chalcogenide crystals

The crystal structures of T?₃PSe₄ and T?₃AsS₄ have been determined by x-ray diffraction techniques. These isomorphous materials are layered, with orthorhombic Pcmn symmetry. Ultrasonic measurements have shown that the $\text{a}$$\text{b}$ shear mode velocity is anomalously low for both materials. Measurements at high pressure on T?₃PSe₄ have shown that the anomalous shear velocity softens to zero at 1.40 ± 0.01 GPa, indicating a second- order structural transition at that pressure.     

Finite-temperature thermophysical properties of fcc-Ca

At an extreme environment, such as high-temperature and high-pressure, harmonic theory has obvious limitations, where the anharmonic effects are influential in determining bulk properties of the materials. In this regard, necessity for incorporating anharmonicity through vibrational contribution and thermally excited electrons to the total free energy at finite temperatures is illustrated taking an example of divalent fcc-Ca. In this regard, we have employed a coupling scheme of combining recently proposed mean-field potential (MFP) with the local pseudopotential to obtain vibrational contribution to the total free energy. To access the applicability of the present coupling scheme, we have calculated temperature variation of several thermodynamical properties. Static EOS, shock Hugoniot and temperature along principal Hugoniot are also estimated. Results are satisfactorily compared with the other theoretical and experimental data and the use of local pseudopotential in conjunction with the MFP approach is justified.     

The properties of two-dimensional photonic crystals bandgap structure with rhombus lattice

The relative bandgap ω_R of photonic crystals structure with rhombus lattice for both TE and TM modes is studied by the plane wave expansion method. Based on the analysis, a bending waveguide of U type can be easily obtained. The steady-state distribution of the electric field with ω = 0.4765(a/λ) of TM mode is given by FDTD method and the transmission is obtained by FFT of the time domain field. All of the sufficient analysis shows the conceptual framework of the proposed rhombus lattice PCs and its great potential application in photonic integration circuit.     

Dielectric properties of NaCl single crystals X-irradiated in presence of DC fields-time effects

The time dependence of the dielectric properties of quenched NaCl single crystals X-irradiated in the presence of moderately large d.c. fields is presented. It is observed that the dipoles formed during X-irradiation (with field) are broken into smaller size with time. High field strenghts (8.66 kV/cm) during irradiation gave rise to dipoles only initially but are dissociated completely with time. Results are discussed qualitatively.