Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Grain size effects on dynamics of Li-ions in Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic nanocomposites

Li₃V₂(PO₄)₃ glass-ceramic nanocomposites, based on 37.5Li₂O-25V₂O₅-37.5P₂O₅ mol% glass, were successfully prepared via heat treatment (HT) process. The structure and morphology were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD patterns exhibit the formation of Li₃V₂(PO₄)₃ NASICON type with monoclinic structure. The grain sizes were found to be in the range 32–56 nm. The effect of grain size on the dynamics of Li⁺ ions in these glass-ceramic nanocomposites has been studied in the frequency range of 20 Hz–1 MHz and in the temperature range of 333–373 K and analyzed by using both the conductivity and modulus formalisms. The frequency exponent obtained from the power law decreases with the increase of temperature, suggesting a weaker correlation among the Li⁺ ions. Scaling of the conductivity spectra has also been performed in order to obtain insight into the relaxation mechanisms. The imaginary modulus spectra are broader than the Debye peak-width, but are asymmetric and distorted toward the high frequency region of the maxima. The electric modulus data have been fitted to the non-exponential Kohlrausch–Williams–Watts (KWW) function and the value of the stretched exponent β is fairly low, suggesting a higher ionic conductivity in the glass and its glass-ceramic nanocomposites. The advantages of these glass-ceramic nanocomposites as cathode materials in Li-ion batteries are shortened diffusion paths for Li⁺ ions/electrons and higher surface area of contact between cathode and electrolyte.     

Impurity bands in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

The absorption spectra and photo-and dark conductivity of the CdI₂-PbI₂ system are investigated in the temperature range 4.2–300 K. The temperature dependence of the photoluminescence and the IR stimulation spectra of luminescence and induced photoconductivity at 4.2 K are also studied. The results are interpreted by considering microphase inclusions of PbI₂ into the CdI₂ crystal lattice as activator centers. An energy-level diagram in which the narrow band of the PbI₂ microphase genetically related to the metastable ³P₀ level of lead plays a significant role is presented for this crystal system.     

High-resolution spectroscopy of HoFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal: a study of phase transitions

The transmission spectra of HoFe₃(BO₃) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm^–1 with a resolution up to 0.1 cm^–1. A new first-order structural phase transition close to the second-order transition is recorded at T_c = 360 K by the appearance of a new phonon mode at 976 cm^–1. The reasons for considerable differences in T_c for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho³⁺ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at T_N = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at T_SR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to T_SR = 4.0 K.     

Electron and electron-phonon effects in the quasi-two-dimensional molecular conductor θ-(BETS)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl): Optical studies at 300–15 K

This paper reports on a study of the polarized reflectance and optical conductivity spectra of the quasi-two-dimensional molecular conductor θ-(BETS)₄HgBr₄(C₆H₅Cl) within the 700–6500-cm^?1 region at 300–15 K and within the 9000–40 000 cm^?1 region at 300 K performed along two principal directions in the crystal plane parallel to the conducting layers of the BETS molecules. The IR spectra obtained at 300 K follow a close-to-Drude behavior, with strong broad features (1200–1400 cm^?1) due to electron-vibrational (vibronic) coupling (VC) superposed on the high Drude background. As the temperature is lowered in the range 180–80 K, in the spectra there appears a Lorentz term with ω_t=2900 cm^?1, as well as three additional VC-induced bands in the 800–1180-cm^?1 region, which disappear as the temperature is decreased further. The results obtained indicate the existence of unstable structural distortions along the two principal directions in the crystal, which are accompanied by the formation of a commensurate charge-density wave.     

An Electron Paramagnetic Resonance Study of the Orbital Ordering Compound MgTi<Subscript>2</Subscript>O<Subscript>4</Subscript>

We report the electron paramagnetic resonance (EPR) studies of MgTi₂O₄ in the 300–140 K range. Above the transition temperature T _t (~258 K), the EPR results indicate that MgTi₂O₄ is paramagnetic. The parameters of the EPR spectra show an anomalous change at T _t. The clear EPR lines can be observed in temperature between T _t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi₂O₄ system are formed gradually with decreasing temperature at low temperature range.     

Synthesis,thermal and electrical behavior of the superprotonic conductor phase in Rb<Subscript>3</Subscript>(HSeO<Subscript>4</Subscript>)<Subscript>2.5</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>0.5</Subscript> compound

The Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound was prepared and its thermal behavior and electric properties were investigated. The thermogravimetry (TGA) analysis and the differential scanning calorimetric (DSC) show the presence of a structural phase transition of the title compounds at 374 K which is confirmed by the variation of fp and σ_dc as a function of temperature. The complex impedance of the Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound has been investigated in the temperature range of 295–453 K and in the frequency range 209 Hz–1 MHz. The impedance plots show semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance Rp and constant phase elements CPE₁ in series with fractal capacity CPE₂. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of modulus, conductivity data, and circuit equivalent confirm that the transport is through the ion hopping mechanism, dominated by the motion of the H⁺ proton in the structure of the investigated materials.     

Study of the anisotropy of the dielectric response of Na<Subscript>1/2</Subscript>Bi<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript> relaxor ferroelectric

The dielectric response, conductivity, and domain structure of (Na_1/2Bi_1/2)TiO₃ single crystals are studied in the temperature range of 290–750 K for the [100], [110], and [111] crystallographic directions. It is shown that the region of optical isotropization is observed in polarized light in the temperature range of 570–620 K. In this case, the birefringence (Δn) decreases and disappears (together with the image of the domain structure) for the [100] directions. The region of optical isotropization in the [111] directions is characterized by the disappearance of the image of the domain structure and by the existence of individual regions with partial quenching. The domain structure in the [110] directions remains distinguished against the background of a significant decrease in Δn in the indicated temperature range. The region of isotropization is also manifested in the temperature dependence of the imaginary part of the dielectric response and is determined by the isotropic character of the conductivity in the range of 570–620 K. The bulk conductivity has a thermally activated character with activation energies E _a = 50?60 meV at T < 500 K and E _a = 700?900 meV for T > 620 K. The low-frequency dispersion of the dielectric response is determined by the Maxwell–Wagner mechanism and is due to an increase in the ionic conductivity at temperatures above 620 K. The anisotropy of the susceptibility holds in the entire studied ranges of frequencies (25 Hz–1 MHz) and temperatures.     

Structure,the lattice dynamic,and the dielectric characteristics of Sr<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> films

Solid solution Sr_0.5Ba_0.5Nb₂O₆ films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.     

Impedance spectroscopy analysis of Li<Subscript>2</Subscript>SnO<Subscript>3</Subscript>

In this work, Li₂SnO₃ has been synthesized by the sol–gel method using acetates of lithium and tin. Thermogravimetric analysis (TGA) has been applied to the precursor of Li₂SnO₃ to determine the suitable calcination temperature. The formation of the compound calcined at 800 °C for 9 h has been confirmed by X-ray diffraction (XRD) analysis. The Li₂SnO₃ is then pelletized and electrically characterized by using electrochemical impedance spectroscopy (EIS) in the frequency range from 50 Hz to 1 MHz. The complex impedance spectra clearly show the dominating presence of the grain boundary effect on electrical properties whereas the complex modulus plots reveal two semicircles which are due to the grain (bulk) and grain boundary. The spectra of imaginary parts of both impedance and modulus versus frequency show the existence of peaks with the modulus plots exhibiting two peaks that are ascribed to the grain and grain boundary of the material. The peak maximum shifts to higher frequency with an increase in temperature and the broad nature of the peaks indicates the non-Debye nature of Li₂SnO₃. The activation energy associated with the dielectric relaxation obtained from the electrical impedance spectra is 0.67 eV. From the electric modulus spectra, the activation energies related to conductivity relaxation in the grain and grain boundary of Li₂SnO₃ are 0.59 and 0.69 eV, respectively. The conductivity–temperature relationship is thermally assisted and obeys the Arrhenius rule with the activation energy of 0.66 eV. The conduction mechanism of Li₂SnO₃ is via hopping.     

Tunneling conductivity in lithiated transition metal oxide cathode Li<Subscript>0.9</Subscript>[Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>]O<Subscript>1.95</Subscript>

Electrical complex ac conductivity of the compound Li_0.9[Ni_1/3Mn_1/3Co_1/3]O_1.95 has been studied in the frequency range 10 Hz–2 MHz and in the temperature range 93–373 K. It has been observed that the frequency dependence of the ac conductivity obeys a power law and the temperature dependence of the ac conductivity is quite weak. The experimental data have been analyzed in the framework of several theoretical models based on quantum mechanical tunneling and classical hopping over barriers. It has been observed that the electron tunneling is dominant in the temperature range from 93 K to 193 K. A crossover of relaxation mechanism from electron tunneling to polaron tunneling is observed at 193 K. Out of the several models discussed, the electron tunneling and the polaron tunneling models are quite consistent with the experimental data for the complex ac conductivity. The various parameters obtained from the fits of the experimental results for the real and imaginary parts of the conductivity to the predictions of these models are quite reasonable.     

Anisotropic clusters with itinerant charge carriers in the layered EuBaCo<Subscript>1.9</Subscript>O<Subscript>5.36</Subscript> cobaltite

The anisotropy of reflection spectra is studied for the single crystals of layered EuBaCo_1.9O_5.36 cobaltite within the temperature range of 80–295 K. The results involving the comparison with the magnetic and transport characteristics are analyzed. In the reflection spectra from the (001) and (120) planes measured at T = 295 K (below the temperature corresponding to the transition to the semiconducting state, T_MI = 345 K), a contribution from itinerant charge carriers has been revealed. This contribution is associated with the existence of an inhomogeneous charge state. In the reflection spectrum from the (120) plane, the contribution from itinerant charge carriers holds down to T = 80 K. The difference between the reflection spectra from different planes and different characters of their changes with the temperature are attributed to the anisotropy of the clusters with itinerant charge carriers.     

Phase transition and electrical investigation in lithium copper pyrophosphate compound Li<Subscript>2</Subscript>CuP<Subscript>2</Subscript>O<Subscript>7</Subscript> using impedance spectroscopy

Lithium pyrophosphate compound Li₂CuP₂O₇ has been synthesized through solid state reaction method. FTIR and XRD results, realized at room temperature, indicate respectively the dominant feature of pyrophosphate anion (P₂O₇)^4? and a pure monoclinic phase with I2/a space group. Electrical and dielectric properties have been studied using impedance spectroscopy complex over a wide temperature (576–710 K) and frequency (209 Hz–1 MHz) range. From the direct and alternative conductivities (DC and AC), electrical conduction is found to be thermally activated process. The frequency-dependent AC conductivity obeys Jonscher’s universal power law σ_AC~Aω^s. The differential scanning calorimetry spectrum discloses phase transition at 622 K.     

Transient optical absorption induced by an electron pulse in KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystals

The efficiency of formation and time evolution of radiation-induced structural defects and pulsed luminescence in KPb₂Cl₅ crystals under the action of a single electron pulse (E = 250 keV, τ = 20 ns) have been investigated. The spectra (1.1–3.8 eV) and relaxation kinetics (time interval 5 × 10^?8?5 s) of transient optical absorption and the pulsed cathodoluminescence spectra and decay kinetics (1.4–3.1 eV) have been measured in the temperature range 80–300 K. It is revealed that the induced optical density and its time evolution depend strongly on temperature, and the absorption relaxation time contains several components and reaches several seconds at T = 300 K. The decay kinetics of transient absorption and pulsed cathodoluminescence kinetics have different orders and are controlled by different relaxation processes.     

Conductivity behavior of the new pyrophosphate NaNi<Subscript>1.5</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

NaNi_1.5P₂O₇ compound was obtained by the classic ceramic method at high temperature and was characterized by XRD. It was found to crystallize in the triclinic symmetry with the P-1 space group. The electrical conductivity and modulus characteristics of the system have been investigated in the temperature and the frequency range 586–723 K and 200 Hz–1 MHz, respectively, by means of impedance spectroscopy. The ac conductivity for grain contribution was interpreted using the universal Jonscher’s power law. The exponent s decreased with increasing temperature revealing that the conduction inside the studied material is insured by the correlated barrier hopping (CBH) model. The conduction mechanism was explained with the help of Elliot’s theory, and the Elliot’s parameters were determined. Thermodynamic parameters such as the free energy for dipole relaxation ΔG, the enthalpy ΔH, and the change in entropy ΔS have been calculated.     

Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

Acoustical investigations of a La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> single crystal

The acoustical, resistive, and magnetic properties of a La_0.75Sr_0.25MnO₃ lanthanum manganite single crystal are investigated in the temperature range involving the second-order magnetic phase transition. The acoustical measurements are performed by the pulse-echo method in the frequency range 14–90 MHz. It is found that, as the temperature decreases, the velocity of a longitudinal acoustic wave propagating along the [111] axis in the single crystal drastically increases at temperatures below the critical point of the magnetic phase transition. No dispersion of the acoustic velocity is revealed. A sharp increase in the acoustic velocity is accompanied by the appearance of an acoustical absorption peak. The observed effects are discussed with due regard for the interaction of acoustic waves with the magnetic moments of the manganese ions.     

AC conductivity and mechanism of conduction study of lithium barium pyrophosphate Li<Subscript>2</Subscript>BaP<Subscript>2</Subscript>O<Subscript>7</Subscript> using impedance spectroscopy

The Li₂BaP₂O₇ compound has been obtained by the conventional solid-state reaction and characterized by X-ray powder diffraction. The title material crystallizes in the monoclinic system with C2/c space group. Electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 589–724 K. Temperature dependence of the DC conductivity and modulus was found to obey the Arrhenius law. The obtained values of activation energy are different which confirms that transport in the titled compound is not due to a simple hopping mechanism. AC conductivity measured follows the power-law dependence σ _AC?～?ω ^s typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process.