Structure and dynamics of the Lu<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> lattice: Ab initio calculation

The ab initio calculations have been carried out for the crystal structure and Raman spectrum of a single crystal of lutetium pyrosilicate Lu₂Si₂O₇. The types of fundamental vibrations and their frequencies and intensities in the Raman spectrum for two polarizations of the crystal have been determined. The calculations have been performed within the framework of the density functional theory (DFT) using the hybrid functionals. The ions involved in the vibrations have been identified using the method of isotopic substitution. The results of the calculations are in good agreement with the experiment.     

Structure and lattice dynamics of PrFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>: Ab initio calculation

The crystal structure and phonon spectrum of PrFe₃(BO₃)₄ are ab initio calculated in the context of the density functional theory. The ion coordinates in the unit cell of a crystal and the lattice parameters are evaluated from the calculations. The types and frequencies of the fundamental vibrations, as well as the line intensities of the IR spectrum, are determined. The elastic constants of the crystal are calculated. A “seed” frequency of the vibration strongly interacting with the electron excitation on the praseodymium ion is obtained for low-frequency A₂ mode. The calculated results are in agreement with the known experimental data.     

Photoactive lead ions in ferroelectric Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> and paramagnetic resonance

The recharging of lead matrix ions upon exposure to light in pure, doped, and nonstoichiometric lead germanate crystals has been studied using electron paramagnetic resonance. It has been shown that the maximum concentration of metastable Pb³⁺ ions is achieved in crystals doped with chlorine, fluorine, titanium, and in samples with excess lead oxide. The annealing activation energy and the parameters of the superhyperfine interaction of Pb³⁺ paramagnetic centers have been determined.     

Ab initio calculation of the structure and optical properties of lead oxyhalides Pb<Subscript>3</Subscript>O<Subscript>2</Subscript><Emphasis Type="Italic">X</Emphasis><Subscript>2</Subscript> (<Emphasis Type="Italic">X</Emphasis> = Cl,Br, I)

Using ab initio methods, we obtained information on the crystal structure, calculated the phonon spectra, and determined the optical properties of oxyhalides Pb₃O₂Br₂ and Pb₃O₂I₂. A compound of Pb₃O₂Br₂ is synthesized, and its phonon spectra are recorded. The experimentally observed vibrational bands are assigned. A comparative analysis of the crystal lattice parameters, the phonon spectra, and the anisotropy parameters of lead oxyhalides Pb₃O₂ X ₂ (X = Cl, Br, I) is performed.     

Infralow-frequency dielectric response of Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> polycrystalline layers

This paper reports on the results of the investigation into the frequency dispersion of the capacitance and dielectric loss in capacitor structures based on red lead Pb₃O₄. It is established that, in the range of frequencies f = 1.50 × 10^?3?0.25 Hz, the capacitance decreases and the dielectric loss tangent increases with increasing frequency. The frequency dependence of the electrical conductivity in an alternating-current electric field indicates the applicability of the hopping model of charge transfer under normal conditions. The role of a lone electron pair of Pb²⁺ cations in dielectric polarization is discussed.     

Impedance spectroscopy study of Pb<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

The lead pyrophosphate, Pb₂P₂O₇, compound was prepared by conventional solid-state reaction and identified by X-ray powder diffractometer. Pb₂P₂O₇ has a triclinic structure whose electrical properties were studied using impedance spectroscopy technique. Both impedance and modulus analysis exhibit the grain and grain boundary contribution to the electrical response of the sample. The temperature dependence of the bulk and grain boundary conductivity were found to obey the Arrhenius law with activation energies E _g = 0.66 eV and E _gb = 0.67 eV, respectively. The scaling behavior of the imaginary part of the complex impedance suggests that the relaxation describes the same mechanism at various temperatures.     

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.     

Vibrational spectrum and elastic properties of KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystals

The Raman spectra and elastic moduli of KPb₂Cl₅ crystals were studied experimentally. The results are interpreted using a parameter-free model of the crystal lattice dynamics with inclusion of the multipole moments of the electron shells of ions. The calculated and experimental results are in good agreement. It is shown that not only the halogen ions but also the heavy cations make a significant contribution to the eigenvectors of high-frequency lattice vibration modes, which accounts for the relatively low frequencies of these modes.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

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.     

Analysis of the exchange magnetic structure in Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The indirect-coupling model is used to analyze the exchange magnetic structure of Pb₃Mn₇O₁₅ in the hexagonal setting. The ratios of manganese ions Mn⁴⁺/Mn³⁺ in each nonequivalent position are determined. Pb₃(Mn_0.95Ge_0.05)₇O₁₅ and Pb₃(Mn_0.95Ga_0.05)₇O₁₅ single crystals are grown by the solution–melt method in order to test the validity of the proposed model. The structural and magnetic properties of the single crystals are studied. The magnetic properties of the grown single crystals are compared with those of nominally pure Pb₃Mn₇O₁₅.     

Simulation of the magnetic properties of manganese oxide Pb<Subscript>3</Subscript>Mn<Subscript>7</Subscript>O<Subscript>15</Subscript>

The magnetic susceptibility, heat capacity, and spin-spin correlation functions of manganese oxide Pb₃Mn₇O₁₅ are calculated by the Monte Carlo method. Two critical temperatures are determined: T ₁ ≈ 20 K, above which a modulated structure along the hexagonal axis is formed, and T ₂ ≈ 70 K, at which the long-range magnetic order disappears. The antiferromagnetic exchange interaction constant in a hexagonal plane is estimated to be J ₁ ～ 7 K, and the antiferromagnetic and ferromagnetic exchange interaction constants between hexagonal planes are calculated to be J ₂ ～ 3 K and K ～ 50 K, respectively.     

Interaction of carbon monoxide with In<Subscript>2</Subscript>O<Subscript>3</Subscript> and In<Subscript>2</Subscript>O<Subscript>3</Subscript>-Au nanocomposite

This is an IR spectroscopic study of the interaction of CO with In₂O₃ and the nanocomposite In₂O₃-Au. A mechanism for low-temperature detection of CO on nanocomposite In₂O₃-Au can be determined from these data. This process includes catalytic oxidation of CO through formation of intermediate complexes involving hydroxyl groups of In₂O₃.     

Magnetic structure of the Sr<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>4</Subscript>Cl<Subscript>2</Subscript> two-subsystem antiferromagnet according to nuclear quadrupole resonance data

The magnetic structure of the Sr₂Cu₃O₄Cl₂ two-subsystem antiferromagnet is studied by the nuclear quadrupole resonance (NQR) method on the ^{63, 65}Cu and ³⁵Cl nuclei. The resonance spectrum above T _N2 = 40 K is determined by the Zeeman splitting of the levels of the ^{63, 65}Cu nuclei of the copper atoms at the Cu1 site with the first-order quadrupole perturbation. The magnetic field on the copper nuclei is equal to 93 kOe. The spectrum below n is significantly different: it includes a low-frequency part, which is associated with the ordering of the second magnetic subsystem Cu2. The splitting of the NQR lines of ³⁵Cl is observed above and below T _N2. This fact indicates the ferromagnetic ordering of the moments of the Cu1 subsystem, which are located along the c axis of the crystal, and makes it possible to determine the direction of the magnetic field on Cu1 copper as (110).     

Electromechanical properties of Pb<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> piezoelectric crystals grown from solution in a melt

Single crystals of lead gallium germanate Pb₃Ga₂Ge₄O₁₄ are grown from their own solution melts. The propagation of bulk acoustic waves is investigated, and the elastic, piezoelectric, and dielectric constants are calculated. The temperature dependences of the dielectric constants of this compound are analyzed.     

Conductivity and spectroscopic studies of Li<Subscript>2</Subscript>O-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

Lithium vanadium-borate glasses with the composition of 0.3Li₂O–(0.7-x)B₂O₃–xV₂O₅ (x?=?0.3, 0.325, 0.35, 0.375, 0.4, 0.425, 0.45, and 0.475) were prepared by melt-quenching method. According to differential scanning calorimetry data, vanadium oxide acts as both glass former and glass modifier, since the thermal stability of glasses decreases with an increase in V₂O₅ concentration. Fourier transform infrared spectroscopy data show that the vibrations of [VO₄] structural units occur at V₂O₅ concentration of 45 mol%. It is established that the concentration of V⁴⁺ ions increases exponentially with the growth of vanadium oxide concentration. Direct and alternative current measurements are carried out to estimate the contribution both electronic and ionic conductivities to the value of total conductivity. It is shown that the electronic conductivity is predominant in the total one. The glass having the composition of 0.3Li₂O-0.275B₂O₃-0.475V₂O₅ shows the highest electrical conductivity that has the value of 7.4?×?10^?5 S cm^?1 at room temperature.     

NMR spectrum in a Mn<Subscript>3</Subscript>Al<Subscript>2</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> noncollinear antiferromagnet

The dependence of the NMR frequencies on the external magnetic field in a Mn₃Al₂Ge₃O₁₂ non-collinear 12-sublattice antiferromgnet is calculated using the exchange approximation for the spin dynamics.     

Ab initio study of La-doped BaSnO<Subscript>3</Subscript> proton conductor

The hydration properties of Lanthanum-doped barium stannate have been studied using density functional calculations. The interaction energy between elementary defects (proton-dopant and oxygen vacancy-dopant) have been calculated in charged states corresponding to p-type conditions. Surprisingly, the most attractive energies are not found for first-neighbor relative positions. The geometric distortions and the electronic densities of state in the different configurations involved are presented. With these data, two simple models are used to estimate the energy of the hydration reaction. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.