A. P. Kiselev

Rayleigh wave with a transverse structure

A new simple closed–form solution describing a surface wave propagating along a free, plane surface of a homogeneous solid half–space is presented. The solution, in which in–plane and antiplane motions are combined, linearly depends on the transverse variable. The wave speed is the same as that of the classical Rayleigh wave.

Effect of solid-phase amorphization on the spectral characteristics of europium-doped gadolinium molybdate

A method is proposed for detecting spectral characteristics of optically inactive molybdates of rare-earth elements by their doping with rare-earth ions whose luminescence lies in the transparency region of all structural modifications of the sample. Gadolinium molybdate is chosen as the object of investigations, while europium ions are used as an optically active and structurally sensitive admixture. It is shown that after the action of a high pressure under which gadolinium molybdate passes to the amorphous state, the spectral characteristics of Gd1.99Eu0.01(MoO4)3 (GMO:Eu) change radically; namely, considerable line broadening is observed in the luminescence spectra and the luminescence excitation spectra, while the long-wave threshold of optical absorption is shifted considerably (by approximately 1.1 eV) towards lower energies. It is found that by changing the structural state of GMO:Eu by solid-state amorphization followed by annealing, the spectral characteristics of the sample can be purposefully changed. This is extremely important for solving the urgent problem of designing high-efficiency light-emitting diodes producing “white” light.

Modifications of Light Emission Spectra and Atomic Structure of Europium Molibdate Bulk Crystals Caused by High Pressure and Heat Treatment

Essential enhancement of light emission ability of europium molybdate - Eu2(MoO4)3 (EMO) crystals has been achieved by combined application of high pressure (9 GPa) and heat treatment (a procedure was repeated from 3 to 25 times for different samples). Multifold increase in light emission of EMO single crystals can be achieved, without deterioration of the optical transparency of the material. Simultaneously the light emission spectra change substantially. Nature of this phenomenon is explained by the modifications of the crystal field caused by structural transformations induced by the treatments. These modifications of the crystal fields bring to increase in probability of light emission optical transitions between 4f sublevels of europium ions.

A Ray Description of All Wavefields Generated by a High-Frequency Point Source near an Interface

We consider the wavefield of a point source situated near an interface between two homogeneous acoustic half-spaces and placed in the faster medium. A high-frequency asymptotic description of all waves present is given by an elementary approach avoiding the explicit solution. The basic tool is the ray method based on real rays though some of the waves (e.g., the so-called P* wave) have complex eikonals. For a description of field in the vicinity of the limiting ray, a parabolic-equation approach is employed.

Evolution of the spectral characteristics upon annealing of lithium borate glasses containing europium and aluminum

The spectral and structural characteristics of lithium borate glasses containing europium and aluminum have been investigated upon annealing at different temperatures. It has been found that the spectral characteristics of the studied system change nonmonotonically with an increase in the annealing temperature. After annealing at a temperature of 600°C, the luminescence spectra of the glasses exhibit broad structureless bands that are specific for the amorphous phase containing Eu3+ ions. Then, after annealing at T = 700°C, narrow lines appear in the wavelength ranges 585–595 and 610–620 nm, which correspond to the luminescence of the Eu(BO2)3 and EuAl3(BO3)4 borates. A further increase in the annealing temperature (T = 800–900°C) leads to the disappearance of europium aluminum borate. In the luminescence spectra of these samples, there are narrow bands in the wavelength range λ = 585–595 nm, which are typical of europium metaborate. Finally, at a temperature of 1050°C, these bands disappear and narrow lines appear again in the wavelength range 610–620 nm, which are characteristic of the EuAl3(BO3)4 borate. Thus, the temperature annealing makes it possible to purposely change the spectral characteristics of the studied system in the wavelength range 590–615 nm.

Spectral Characteristics of Different Structural Modifications of Lu 1- x Eu x BO 3

The spectral and structural characteristics of polycrystals of Eu3+-doped lutetium borates Lu1 − xEuxBO3) annealed at different temperatures have been investigated over a wide range of europium concentrations. The conditions for the preparation of Lu1 − xEuxBO3 in the calcite and vaterite phases have been determined. It has been found that there is a radical difference between the excitation spectra of the main emission bands of the calcite and vaterite phases of the Lu1 − xEuxBO3 borates. The influence of the europium concentration on the structure of Lu1 − xEuxBO3 has been analyzed. It has been established that, at europium concentrations of higher than 15 at %, only the vaterite structure is formed independently of the annealing temperature. Thus, by varying the Eu3+ concentration and the annealing temperature of Lu1 − xEuxBO3, it is possible to directionally synthesize a specific structural modification and, consequently, to control the spectral characteristics of this compound.

Spectral and structural features of Lu1 − xRExBO3 compounds

The luminescence spectra, luminescence excitation spectra, IR absorption spectra, and crystal structure of orthoborates Lu1 − xRExBO3 (RE = Eu, Gd, Tb, Y, Dy) have been investigated. It has been found that the solid solution consisting of a LuBO3 orthoborate, which has two stable structural modifications (calcite and vaterite), and an REBO3 orthoborate, which has one structural modification (vaterite), crystallizes only in the vaterite structure when the concentration of a rare-earth ion substituting for lutetium exceeds 15–20 at %. The investigation of the photoluminescence spectra has demonstrated that, for rare-earth ions Lu3+, Eu3+, Y3+, and Gd3+ in the vaterite modification of Lu1 − xRExBO3 orthoborates, there are at least two positions that are not equivalent in the symmetry of the local environment. It has been established that the maximum intensity of the luminescence of the vaterite modification of Lu1 − xTbxBO3 compounds synthesized at 970°C, which is observed at a terbium concentration of 15 at %, is several times higher than the maximum intensity of the luminescence of the calcite modification.

Spectral characteristics and energy transfer from Ce3+ to Tb3+ in compounds Lu1 – x – yCexTbyBO3

The structure, IR absorption spectra, morphology, and spectral characteristics of compounds Lu1 – x – yCexTbyBO3 have been investigated. It has been shown that the Tb3+ luminescence excitation spectrum of the Lu1 – x – yCexTbyBO3 compounds is dominated by a broad band coinciding with the excitation band of Ce3+ ions, which clearly indicates energy transfer from the Ce3+ ions to the Tb3+ ions. The spectral position of this band depends on the structural state of the sample: in the structures of calcite and vaterite, the band has maxima at ~339 and ~367 nm, respectively. By varying the ratio between the calcite and vaterite phases in the sample, it is possible to purposefully change the Tb3+ luminescence excitation spectrum, which is important for the optimization of the spectral characteristics of Lu1 – x – yCexTbyBO3 when it is used in light-emitting diode sources. An estimate has been obtained for the maximum distance between Ce3+ and Tb3+ ions, which corresponds to electronic excitation energy transfer. It has been shown that the high intensity of Tb3+ luminescence in these compounds is due to the high efficiency of electronic excitation energy transfer from the Ce3+ ions to the Tb3+ ions as a result of the dipole–dipole interaction.

Structural ordering upon annealing of europium molybdate subjected to pressure treatment

It has been found that treatment of europium molybdate single crystals under high uniform pressure leads to radical changes of the IR reflectance spectra of the samples. Instead of a series of narrow lines, which are characteristic of the spectrum of the initial crystal, the spectrum of the sample subjected to pressure treatment exhibits broad bands. As follows from the results of X-ray diffraction measurements, this transformation of the IR spectra is associated with the fact that, after pressure treatment, the sample represents an amorphous matrix with inclusions of nanocrystals of the high-pressure phase. In the IR spectra, the high-pressure phase manifests itself in the form of a new line at approximately 600 cm−1. After annealing of the sample with an increase of the temperature from 100 to 400°C, the high-pressure phase disappears. A further increase of the annealing temperature to 550°C leads to the recovery of the crystal structure of the initial β′ phase of europium molybdate and to the appearance of lines corresponding to the α phase.

Spectroscopy and X-ray diffraction analysis of europium molybdate single crystals subjected to different thermobaric treatments

Europium molybdate (EMO) single crystals subjected to high uniform pressures (Pβ’-EMO) (exposure under 9 GPa at 300 K for 7 days) have been investigated by optical spectroscopy and X-ray diffraction. It is shown that Pβ’-EMO and amorphous europium molybdate have similar luminescence and transmission spectra. Annealing of EMO single crystals subjected to thermobaric treatments exhibits the same sequence of phase transitions as in the case of annealing of amorphous europium molybdate. It is shown that an EMO single crystal in the high-pressure state is a structurally inhomogeneous material consisting of two domains: a dominant amorphous-like part, in which the long-range order is completely absent, and a minor crystalline part.