Relaxor properties of Fe-doped TlInS<Subscript>2</Subscript>

The relaxor properties of Fe-doped TlInS₂ crystals are investigated. It is shown that Fe-doped TlInS₂ compounds exhibit all features inherent in relaxor ferroelectric materials. The temperature range of existence of the microdomain (relaxor) state and the temperature of the transition from this state to a macro-domain state are determined.     

Relaxor properties and the mechanism of conduction in TlInS<Subscript>2</Subscript> crystals exposed to gamma irradiation

The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS₂ crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS₂ crystal.     

Photodielectric effect in TlInS<Subscript>2</Subscript> activated by the La impurity

The influence of light on the low-frequency dielectric properties of the TlInS₂ layered crystal is investigated for the first time. It is established that the illumination of the crystal during measurements leads to a substantial change in the behavior of the temperature dependence of the permittivity ɛ of the TlInS₂ compound doped with the lanthanum impurity in the range of the existence of the incommensurate phase. The temperature dependences of the permittivity ε of the TlInS₂ compound doped with the lanthanum impurity after preliminary cooling of the crystal in constant electric fields with different strengths are studied for the first time. The inference is made that the experimentally observed photodielectric effect is associated with the localization of charge carriers at defect levels in the band gap of the crystal with the formation of local polarized states.     

Chromium-and manganese-doped TlInS<Subscript>2</Subscript>—A new relaxor ferroelectric

The dielectric, pyroelectric, and polarization properties of TlInS₂ crystals doped with 0.1 mol % Cr and Mn are investigated. It is shown that TlInS₂ compounds doped with chromium and manganese can be assigned to the class of relaxor ferroelectric materials. The temperature range of existence of the stable relaxor (nanodomain) state and the temperature of the phase transition from this state to the ferroelectric (macrodomain) state are determined. The temperature dependence of the pyroelectric current is characterized by an anomaly in the range of the phase transition.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Luminescent and photochemical processes in CdBr<Subscript>2</Subscript> : AgCl crystals

This paper reports on the results of the investigation into the optical luminescence properties of photochromic crystals CdBr₂ : AgCl grown by the Bridgman-Stockbarger method. It has been shown that, under X-ray, optical, and N₂-laser excitations of the grown crystals, there occurs emission due to Ag⁺ impurities in addition to emission from centers characteristic of CdBr₂. The photostimulated chemical reactions occurring in CdBr₂ : AgCl lead to a weakening of the luminescence and to a change in its spectral composition. Models of photosensitive centers and centers of photochemical coloring have been proposed. The mechanisms of the photochromic effect have been considered. The nature of luminescent and trapping centers has been discussed.     

Low-frequency relaxation processes in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric crystals

Measurements and analysis of the temperature and frequency dependences of permittivity and losses and of the electrical resistivity of Pb₅Ge₃O₁₁ ferroelectric crystals at temperatures of 100 to 600 K and frequencies of 0.1 to 100 kHz are reported. The dielectric characteristics of the crystals exhibit, in addition to clearly pronounced anomalies near the Curie point T_C=450 K, less distinct anomalous features of the relaxation character in the range 230–260 K. The data obtained on the effect of various factors (degree of crystal polarization, crystal annealing at different temperatures and in different environments, etc.) on the low-temperature anomalies serve as a basis for discussing the possible mechanisms responsible for these anomalies. It is concluded that the low-temperature dielectric anomalies originate from thermal carrier localization in defect levels in the band gap, which entail the formation of local polarized states.     

Electrical instability of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu₂O₂ phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T₀/T)^1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Evolution of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> crystals in Al-Mg-SiO<Subscript>2</Subscript> composites

The present study analyzes the morphological transformations of reaction products i.e., MgO, MgAl₂O₄ occurring during the reaction between SiO₂ and Al-Mg alloy in Al-Mg-SiO₂ composite processed by the liquid metallurgy technique. Different phases of platelet and hexagonal morphologies are detected and their composition analysis by EDS has confirmed them as being transition phases existing between MgO, MgAl₂O₄ and Al₂O₃. This study has also revealed the gradual transformation of (i) MgO needles to octahedral MgAl₂O₄ through Mg-Al-Si-O and Mg-Al-O transition phases having platelet morphologies and (ii) MgAl₂O₄ to Al₂O₃ through hexagonal transition phases on holding of Al-5Mg-SiO₂ and Al-1Mg-SiO₂ composites respectively at 1023K. Fully developed α-Al₂O₃ crystals are not observed under the present experimental conditions, wherein the Mg content is well above the equilibrium Mg content required for the formation of stable Al₂O₃ (<0.05 wt. %). PACS 05.70.Np     

Acoustic attenuation in ferroelectric Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals

The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn₂P₂S₆, which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of Sn₂P₂S₆ crystals.     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Spectral characteristics and force constants of irradiated α-SiO<Subscript>2</Subscript> single crystals

The IR-radiation reflectance spectra of α-SiO₂ single crystals exposed, in a reactor, to fast neutrons with fluences changing in a wide range have been investigated. The regularities of the change in spectral characteristics of the antisymmetric and degenerate vibrations of the bridge bonds of the crystals studied and in the dynamic parameters of these crystals in the process of their irradiation have been determined. The critical dynamics of the crystal lattice in the region of antisymmetric stretching and degenerate modes and the extremum radiation kinetics of the ion-polarizability parameters and the force constants of the indicated crystals were detected. The changes in some characteristics of these crystals, arising on their irradiation as a result of the damage of the substance, the rupture and extension of some silicon—oxygen bonds, and the transformation of the structure of α-SiO₂ single crystals by two mechanisms, have been considered. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 773–777, November–December, 2005.     

Optical phonons in Rb<Subscript>2</Subscript>TeBr<Subscript>6</Subscript> and Cs<Subscript>2</Subscript>TeBr<Subscript>6</Subscript> crystals

Raman scattering in Rb₂TeBr₆ and Cs₂TeBr₆ crystals is studied. The phonon spectra of the crystals are calculated using the factor group method. The number of Raman-active modes, their symmetries, and selection rules are found. Observed Raman spectrum lines are identified with atomic vibration modes of the crystal.     

Polarization processes in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics in infralow-frequency fields

The dielectric nonlinearity in BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ layered ceramics was studied by measuring their polarization loops and reverse dependences of permittivity. It was shown that the features of the dielectric response of BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ in strong fields can be explained by glass-like properties and the contribution of the domain structure of the ferroelectric material to repolarization processes, respectively.     

Luminescence and electronic excitations in KBe<Subscript>2</Subscript>BO<Subscript>3</Subscript>F<Subscript>2</Subscript> crystals

This paper reports on a study of the dynamics of electronic excitations in KBe₂BO₃F₂ (KBBF) crystals by low-temperature luminescent vacuum ultraviolet spectroscopy with nanosecond time resolution under photoexcitation by synchrotron radiation. The first data have been obtained on the kinetics of photoluminescence (PL) decay, time-resolved PL spectra, time-resolved PL excitation spectra, and reflection spectra at 7 K; the estimation has been performed for the band gap E _g = 10.6−11.0 eV; the predominantly excitonic mechanism for PL excitation at 3.88 eV has been identified; and defect luminescence bands at 3.03 and 4.30 eV have been revealed. The channels of generation and decay of electronic excitations in KBBF crystals have been discussed.     

Effect of lattice deformation and phase transitions on the electronic spectra of TlGaS<Subscript>2</Subscript>, TlGaSe<Subscript>2</Subscript>, and TlInS<Subscript>2</Subscript> layered semiconductors

The effect of the lattice deformation on the electronic spectra of TlGaS₂, TlGaSe₂, and TlInS₂ layered semiconductor crystals is analyzed. It is shown that changes in the band gap of these semiconductors due to thermal expansion and a change in the composition under hydrostatic or uniaxial pressure can be described within a unified model of the deformation potential. The main feature of this model is the inclusion of deformation potentials with different signs, which is characteristic of other semiconductors with a layered structure. An analysis of the lattice deformation of the studied semiconductors in terms of the proposed model of the deformation potential has revealed that, in the immediate vicinity of the phase transitions, the crystal lattice under pressure undergo an unusual deformation.     

Regular hexagonal MoS<Subscript>2</Subscript> microflakes grown from MoO<Subscript>3</Subscript> precursor

Regular hexagonal MoS₂ microflakes with high yield were grown from MoO₃ precursor by a sulfurization process using S powders as sulfuration reducer. The precursors, long and smooth MoO₃ microbelts, were synthesized through a direct oxidation reaction of Mo plates in air. X-ray powder diffraction and scanning electron microscopy revealed that the sulfurized products were hexagonal MoS₂ with regular hexagonal flake-like morphology. The results of transmission electron microscopy examinations demonstrated that the microflakes were single crystalline MoS₂. Elemental analysis by EDAX and XPS showed that the microflakes consist of Mo and S with the atomic ratio near to 0.5. Factors influencing the formation of the product were systematically studied. PACS 81.15.Gh; 81.15.Kk; 81.05.Hd; 78.67.Pt; 82.40.Ck     

Luminescence and electronic excitations in crystals K<Subscript>2</Subscript>Al<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K₂Al₂B₂O₇ crystals have been discussed.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.