Absorption spectra of thin layers of solid solutions Cs<Subscript>2</Subscript>(Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)I<Subscript>4</Subscript>

The excitonic absorption spectra of thin films of ferroelectric Cs₂CdI₄ and Cs₂ZnI₄ solid solutions are studied for the first time. It is found that, within the whole range of molar concentrations x, the spectra of Cs₂(Cd_{1 − x} Zn _x )I₄ comprise two bands that originate from the bands of the two compounds. It is shown that the exciton transfer occurs most efficiently between the tetrahedrons CdI₄ and ZnI₄ along the b axis of the crystal. Unusual behavior of the concentration with regards to the maxima of long-wavelength excitonic bands E _m (x) agrees well with the developed theory that takes into account dependence on x of the matrix element of the intertetrahedron exciton transfer and that is similar to the theory of Davydov’s splitting in molecular crystals.     

UV absorption spectra of thin films of Cs<Subscript>2</Subscript>CdI<Subscript>4</Subscript> and Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelectrics

Thin films of M₂CdI₄ ferroelectrics (M=Cs, Rb) of orthorhombic structure were synthesized, and their electronic optical spectrum was studied. It was established that both compounds belong to direct-gap dielectrics and that their low-frequency excitons are localized on a sublattice made up of (CdI₄)^2? tetrahedra. The temperature dependence of the exciton band parameters was studied for Cs₂CdI₄ within the temperature interval 90–420 K. The phase transitions occurring in this interval manifest themselves as breaks in the temperature behavior of the band spectral positions and weak peaks in the halfwidth and oscillator strength.     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Spectral and kinetic characteristics of CdI<Subscript>2</Subscript> and CdI<Subscript>2</Subscript>:Pb scintillators

We have studied the effect of lead dopant on the optical absorption, photoluminescence, and x-ray luminescence spectra, and the scintillation characteristics of CdI₂ at room temperature. The crystals for the study were grown by the Stockbarger-Bridgman method. Activation of CdI₂ from the melt by the compound PbI₂ leads to the appearance in the absorption spectra in the near-edge region of an activator band at 395–405 nm, which is interpreted as an A band connected with electronic transitions from the ¹S₀ state to the ³P₁ levels in the Pb²⁺ ion. For x-ray excitation, CdI₂:Pb²⁺ crystals with optimal dopant concentration (∼1.0 mol%) are characterized by a light yield with maximum in the 570–580 nm region that is an order of magnitude higher than for CdI₂ crystals in the 490–500 nm band. For α excitation, the radioluminescence kinetics for cadmium iodide is characterized by a very short (∼0.3 nsec) rise time and fast decay of luminescence, with τ₁ ≈ 4 nsec and τ₂ = 10–76 nsec. Depending on the conditions under which the crystals were obtained, the fast component fraction is 95%–99%. The crystal is characterized by a similar scintillation pulse in the case of excitation by x-ray pulses. The radioluminescence pulse shape for CdI₂:Pb in the decay stage is predominantly exponential, with luminescence decay time constants τ₁ ≈ 10 nsec and τ₂ = 200–250 nsec. This system is characterized by low afterglow, at the level for the Bi₄G₃O₁₂ scintillator. We have demonstrated the feasibility of using CdI₂:Pb as a scintillator for detecting α particles. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 6, pp. 825–830, November–December, 2008.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

<Superscript>29</Superscript>Si MAS-NMR study of oxide conductors derived from the apatite-type La<Subscript>9.33</Subscript>Si<Subscript>6</Subscript>O<Subscript>26</Subscript> compound

The preparation of (La_9.33−2x/3Sr _x □_0.67−x/3)Si₆O₂₄O₂ (0 ≤ x ≤ 2) samples with different amounts of cation vacancies is reported. Structure and unit-cell parameters were deduced by Rietveld analysis of XRD patterns. Structural features that enhance oxygen conductivity in Sr-doped apatites are discussed. Up to three components were detected in ²⁹Si MAS-NMR spectra which change with the amount and distribution of cation vacancies. In general, oxygen conductivity increases with the amount of vacancies at La1 (6h) sites, passing through a maximum for x = 0.4. In the case of activation energy, a minimum is detected near x = 1.2, indicating that entropic and enthalpic change in different ways. The presence of cation vacancies should enhance oxygen hopping along c-axis; however, the analysis of the frequency dependence of conductivity suggests that oxygen motions are produced along three axes.     

Anisotropy of amorphous nanogranular composites CoNbTa-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> and CoFeB-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

A comparative investigation of the magnetic properties of amorphous nanogranular composites (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} and (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} has been performed in the subpercolation region at temperatures in the range 4.2–300 K. The thermomagnetic dependences in the range 4.2–300 K and the processes of magnetization reversal and remanent magnetization relaxation at liquid-helium temperatures have been studied. It has been established that the average anisotropy constants of amorphous nanograins are equal to 3.6–7.0 kJ/m³ for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites and 5–8 kJ/m³ for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites. The fundamental differences in the concentration dependences of the anisotropy constant K _eff and the coercive force H _C have been revealed for the two systems under investigation. It has been demonstrated that, as the concentration of the metal phase increases, the quantities K _eff and H _C increase for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites and decrease for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites.     

Specific features of structural states in the BiFeO<Subscript>3</Subscript>-YMnO<Subscript>3</Subscript> solid solutions

The (1 − x)BiFeO_3−x YMnO₃ solid solutions have been found to undergo the following sequence of phase transformations with increasing x: R3c → Pbnm → C2 → Pnma → P6₃ cm. It has been established that the Pbnm and Pnma phases have different orientations of atomic displacements and can exhibit antiferroelectric properties.     

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

It has been shown using atomic-force microscopy that the PbI₂ impurity is embedded in the CdI₂ crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the ³ P ₂ state of a free Pb²⁺ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI₂ crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI₂ in the CdI₂ crystal lattice.     

Conditions favoring the polar weak ferromagnetic state in BiFeO<Subscript>3</Subscript>-type multiferroics

The crystal structure and magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A = Ca, Sr, Pb, Ba), Bi_{1 − x} A _x (Fe_{1 − x} Ti _x )O₃, and Bi_{1 − x} A _x (Fe_{1 − x/2}Nb _x/2)O₃ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.     

Phase segregation in Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> for large Na contents

We have investigated a set of sodium cobaltates (Na _x CoO₂) samples with various sodium content (0.67 ≤ x ≤ 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of ⁵⁹Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.     

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature T _c and the second critical magnetic field H_c2, in the (Pb_0.3Sn_0.7)_0.95In_0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at T _c ∼ 2.3 K, i.e., close to the maximum value T _c ∼ 2.9 K found for the (Pb _z Sn_{1 − z} )_0.95In_0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence T _c (P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level E _F with increasing pressure within the impurity band of the In quasi-local states. In this case, E _F passes through a maximum in the density of impurity states at P = 3–5 kbar.     

Large-scale potential fluctuations caused by SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compositional inhomogeneity

The short-range order in amorphous SiO _x (0 ≤ x ≤ 2) films has been studied by high-resolution X-ray photoelectron spectroscopy. Both the random bonding and random mixture models do not describe experimental photoelectron spectra of SiO _x (x ≤ 2). An intermediate model of the SiO _x structure has been proposed. The measured photoelectron spectra of the SiO _x (x ≤ 2) valence band indicate the presence of the silicon phase and silicon oxide.     

Electrical characterization of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>][N(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>4</Subscript>]ZnCl<Subscript>4</Subscript> compound

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound has been synthesized by a solution-based chemical method. The X-ray diffraction study at room temperature revealed an orthorhombic system with P2₁2₁2 space group. The complex impedance has been investigated in the temperature and frequency ranges 420–520 K and 200 Hz–5 MHz, respectively. The grain interior and grain boundary contribution to the electrical response in the material have been identified. Dielectric data were analyzed using the complex electrical modulus M ^* for the sample at various temperature. The modulus plots can be characterized by full width at half height or in terms of a non-exponential decay function ϕ(t) = exp[(−t/τ) ^β ]. The detailed conductivity study indicated that the electrical conduction in the material is a thermally activated process. The variation of the AC conductivity with frequency at different temperatures obeys the Almond and West universal law.     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.     

Electronic structure of defective lithium cobaltites Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript>

X-ray absorption, resonant X-ray emission, and X-ray photoelectron spectroscopical methods have been applied for the study of the electronic structure of defective lithium cobaltites Li _x CoO₂ (0.6≤x≤1.0). Resonant O K α X-ray emission spectra of LiCoO₂ showed localized excitonic states due to a dd transition between occupied and unoccupied Co 3d states. On the base of measurements of Co 3s X-ray photoelectron, Co 2p, and O 1s X-ray absorption spectra, it was established that in defective cobaltites the electronic holes are localized mainly in O 2p states. An evidence of phase separation in Li _x CoO₂ has been found. It was shown that the semiconductor-to-metal transition in Li _x CoO₂ (x<0.76) at about 160 K is not accompanied by changes in the Co 3d electronic configuration which remains 3d ⁶.     

Electronic spectra and phase transitions in thin [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> microcrystals

Optical absorption spectra in thin [N(CH₃)₄]₂CuCl₄ crystals in the thickness range 10 μm ≤ d < 100 μm have been studied. Strengthening of the crystal field has been found with a decrease in the [N(CH₃)₄]₂CuCl₄ crystal size. The reasons for absorption band shifts in the visible region depending on the [N(CH₃)₄]₂CuCl₄ crystal thickness and the manifestation of a size effect in crystals with an incommensurate superstructure are discussed.     

Formation of ceramic solid solutions in a SrTiO<Subscript>3</Subscript>-BiScO<Subscript>3</Subscript> system

X-ray structural, X-ray phase, and dilatometric analyses were used to explore specific features of the formation of solid solutions in the (1 − x)SrTiO₃−xBiScO₃ system with x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5. It was found that the synthesis of solid solutions from the initial Bi₂O₃, Sc₂O₃, TiO₂, and SrCO₃ components is accompanied by a considerable increase in the linear dimensions of the samples, depending on their composition. Solid solutions with x > 0.1 are formed through intermediate phases (Sr₅Bi₆O₁₄ at x = 0.2; Sr_0.78Bi_2.22O₄ at x = 0.3; and Sr_2.25Bi_6.75O_12.38 at x = 0.4 and 0.5). It was shown that the samples with x = 0.2, 0.3, 0.4, and 0.5 have two phases: one with a cubic Pm3m structure and one with a tetragonal I4/mcm structure.     

Thermal stability and electrical conductivity of multiferroics BiFeO<Subscript>3</Subscript>/REEs

Ceramic samples of bismuth ferrite and solid solutions of Bi_{1 − x} A _x FeO₃ type (where A = Lu, Yb, Tm, Er, Ho, Dy, Tb, Gb, Eu, Sm, Nd, Pr, La; 0.05 ≤ x ≤ 0.20; Δx = 0.05) were prepared. Spectra of the real part of electrical conductivity were studied within the range 10⁻⁴–10⁻⁶ Hz. The dependence of the samples’ thermal stability and electrical conductivity on the size of the substituting ions was established.     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.