Structure,ionic conduction,and phase transformations in lithium titanate Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spinel structure of lithium titanate Li₄Ti₅O₁₂ is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li₄Ti₅O₁₂ compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li₄Ti₅O₁₂ compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)_8a[Li_0.33Ti_1.67]_16dO₄ → [Li□]_16c[Li_1.33Ti_1.67]_16dO₄ → [Li_1.33□_0.67]_16c[Ti_1.67□_0.33]_16dO₄. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....     

The influence of phase transitions in the Rb<Subscript>2</Subscript>CdI<Subscript>4</Subscript> ferroelastic on the exciton absorption spectrum

The parameters of the long-wavelength exciton band for Rb₂CdI₄ films are investigated in the temperature range 90–410 K. It is found that the Rb₂CdI₄ films undergo a sequence of phase transitions at temperatures T_c1=380 K (paraphase → incommensurate phase), T_c2=290 K (incommensurate phase → ferroelastic phase I), and T_c3 = 210 K (ferroelastic phase I → ferroelastic phase II). The parameters of the exciton band (such as the spectral position and the half-width) measured during heating and cooling of the Rb₂CdI₄ film differ significantly. This is especially true for the incommensurate phase. Upon heating of the incommensurate phase, the domain boundaries become frozen, whereas the cooling of this phase is accompanied by the generation of solitons and their pinning, which, in turn, results in a first-order phase transition at the temperature T_c2. It is revealed that the oscillator strength of the exciton band anomalously increases in the range of existence of commensurate phase I (T_c3<-T<-T_c2) due to ordering of the Rb₂CdI₄ crystal lattice.     

Investigation of the conditions of efficient I*<Subscript>2</Subscript> (342 nm) luminescence in a barrier discharge in a Kr-I<Subscript>2</Subscript> mixture

The energy and spectral characteristics of a barrier discharge in a mixture of iodine vapor with krypton have been investigated theoretically and experimentally. The emission spectrum consists of the single I*₂ molecular band D′ → A′ peaking at 342 nm, the iodine resonance line at 206.2 nm, and the group of bands corresponding to iodine emission. The highest intensity of the I*₂ (342 nm) band was obtained at a mixture pressure near 450 Torr. A mean output power and an efficiency of 550 mW and 1.6%, respectively, have been achieved. It is shown that, for the I₂ barrier-discharge excilamp, the homogeneous, rather than filamentary form of discharge glow is optimal from the viewpoint of the highest mean output power. The maximal calculated value of the emission efficiency for the 342-nm band was 5%. The main processes determining energy losses in plasma have been found, and ways to increase the efficiency of emission in the D′ → A′ band of the I*₂ molecule have been proposed.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Magnetic circular dichroism and optical absorption in TmAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The polarized spectra of absorption and magnetic circular dichroism in a TmAl₃(BO₃)₄ single crystal are studied in the region of ³ H ₆ → ³ F ₄, ³ H ₆ → ³ F ₃, and ³ H ₆ → ³ F ₂ electronic transitions in the Tm³⁺ ion. The structure of the spectra is interpreted qualitatively. It is shown that the magnetic circular dichroism of the ³ H ₆ → ³ F ₄ transition is determined by the contribution from the splitting of the ground state, whereas the magnetic circular dichroism of the ³ H ₆ → ³ F ₃ transition is governed by the contribution from the splitting of an excited state in a trigonal crystal field.     

Electronic and optical properties of Fe<Subscript>2</Subscript>SiO<Subscript>4</Subscript> under pressure effect: ab initio study

We report first-principles studies the structural, electronic, and optical properties of the Fe₂SiO₄ fayalite in orthorhombic structure, including pressure dependence of structural parameters, band structures, density of states, and optical constants up to 30 GPa. The calculated results indicate that the linear compressibility along b axis is significantly higher than a and c axes, which is in agreement with earlier work. Meanwhile, the pressure dependence of the electronic band structure, density of states and partial density of states of Fe₂SiO₄ fayalite up to 30 GPa were presented. Moreover, the evolution of the dielectric function, absorption coefficient (α(ω)), reflectivity (R(ω)), and the real part of the refractive index (n(ω)) at high pressure are also presented.     

Magnon satellite bands in the optical spectrum of antiferromagnetic Rb<Subscript>2</Subscript>MnCl<Subscript>4</Subscript>

The evolution of optical absorption in a two-dimensional antiferromagnet is investigated in the range of the transition ⁶A_1g → ⁴A_1g, ⁴E _g (⁴G) observed in manganese ions in an external magnetic field inducing noncollinearity of the magnetic structure. It is revealed that hot and cold satellites of the exciton-magnon bands appear in the optical absorption spectrum and then increase in intensity. The shapes of the magnon satellite bands corresponding to a two-dimensional magnetic structure are calculated. It is demonstrated that magnons at the inner points of the Brillouin zone appreciably contribute to the absorption. The zero-point magnetic oscillations play a decisive role in the absorption associated with the magnon decay at low temperatures.     

Formation and composition of the clathrate phase in the H<Subscript>2</Subscript>O-H<Subscript>2</Subscript> system at pressures to 1.8 kbar

The transition of the hexagonal ice phase I_h to the clathrate phase sII has been found in the H₂O-H₂ system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the I_h → sII and sII → I_h transitions have been determined over a temperature range from ?36 to ?18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the I_h and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → I_h transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.     

Zeeman effect and stark splitting of the electronic states of the rare-earth ion in the paramagnetic terbium garnets Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Tb<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

The Zeeman effect in the ⁷ F ₆ → ⁵ D ₄ absorption band of the Tb³⁺ ion in the paramagnetic garnets Tb₃Ga₅O₁₂ and Tb₃Al₅O₁₂ was studied. The field dependences of the Zeeman splitting of some absorption lines are found to exhibit unusual behavior: as the magnetic field increases, the band splitting decreases rather than increases. Symmetry analysis relates these lines to 4f → 4f electron transitions of the doublet-quasi-doublet or quasi-doublet-doublet type, for which the field dependences of the splitting differ radically from the well-known field dependences of the Zeeman splitting for quasi-doublet-quasi-doublet or quasi-doublet-singlet transitions in a longitudinal magnetic field.     

Stabilization of Ferromagnetism in BiFeO<Subscript>3</Subscript>:Ho at Hydrostatic Pressure

The isothermal magnetization of the Bi_{1 – x}Ho _x FeO₃ (x = 0?0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at P_C ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO₃ (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: P_C decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi_{1 – x}Ho _x FeO₃ with x = 0.2, the transition pressure P_C = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition.     

Phase transitions and metastable states in stressed SrTiO<Subscript>3</Subscript> films

The sequence of the ground states in SrTiO₃ films subjected to epitaxial strain and fixed mechanical stress in the [001] and [110] directions is calculated from first principles within the density functional theory. Under the fixed-strain conditions, an increase in the substrate lattice parameter results in the following sequence of the ground states: I4cm → I4/mcm → Ima2 → Cm → Fmm2 → Ima2(II). When moving to the fixed-stress conditions, the phase sequence changes significantly and depends on how the stress is applied. It is revealed that the simultaneous presence of two types of the lattice instability (the ferroelectric and structural ones) in strontium titanate leads to the formation of a whole system of metastable phases whose number increases abruptly under the fixed-stress conditions. The stability of these phases changes with pressure and phase transitions occur between them. The appearance of broad bistability regions in certain parts of the phase diagram enables the use of this phenomenon for developing nonvolatile phase-change memory.     

Refractometry of mechanically compressed (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δn _i and refractive indices n _i of (NH₄)₂SO₄ crystals has been investigated. It is shown that the dispersion of n _i (λ) and Δn _i (λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dn _i /dλ and dΔn _i /dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum.     

Specific features of optical absorption in rare-earth orthoaluminate DyAlO<Subscript>3</Subscript>

Polarization spectra of optical absorption of the 4f-4f transition ⁶ H _15/2 → ⁶ F _3/2 in the rare-earth orthoaluminate DyAlO₃ are theoretically and experimentally studied at the temperature T=78 K. It is shown that the nontrivial character of the anisotropy of the polarization absorption spectra at low temperatures can be explained by the J-J mixing of excited multiplets of the 4f ⁹ configuration of Dy³⁺ ions in a low-symmetry crystal field of the orthoaluminate structure. The energy and wave functions of the Stark sublevels within the excited ⁶ F _5/2 multiplet in the 4f ⁹ configuration of the Dy³⁺ rare-earth ion in the crystal field of C _s symmetry are numerically calculated.     

A theoretical investigation of structural,mechanical, electronic and thermoelectric properties of orthorhombic CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>PbI<Subscript>3</Subscript>

The structural, mechanical, electronic and thermoelectric properties of the low temperature orthorhombic perovskite phase of CH₃NH₃PbI₃ have been investigated using density functional theory (DFT). Elastic parameters bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt–Reuss–Hill averaging scheme. Phonon dispersions of the structure were investigated using a finite displacement method. The relaxed system is dynamically stable, and the equilibrium elastic constants satisfy all the mechanical stability criteria for orthorhombic crystals, showing stability against the influence of external forces. The lattice thermal conductivity was calculated within the single-mode relaxation-time approximation of the Boltzmann equation from first-principles anharmonic lattice dynamics calculations. Our results show that lattice thermal conductivity is anisotropic, and the corresponding lattice thermal conductivity at 150 K was found to be 0.189, 0.138, and 0.530 Wm^?1K^?1 in the a, b, and c directions. Electronic structure calculations demonstrate that this compound has a DFT direct band gap at the gamma point of about 1.57 eV. The electronic transport properties have been calculated by solving the semiclassical Boltzmann transport equation on top of DFT calculations, within the constant relaxation time approximation. The Seebeck coefficient S is almost constant from 50 to 150 K. At temperatures 100 and 150 K, the maximal figure of merit is found to be 0.06 and 0.122 in the direction of the c-axis, respectively.     

Superposition of M<Subscript>5</Subscript>X<Subscript>5</Subscript> superstructures and X-ray diffraction in TiO<Subscript>1.0</Subscript> titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO_1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3?m) M₅X₅ superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M₅X₅ cubic superstructure. The transition state model can be considered as an M_(5–i)X_(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).     

On the correlation between supercooling,superheating and kinetic arrest in a magnetic glass Pr<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>Mn<Subscript>0.975</Subscript>Al<Subscript>0.025</Subscript>O<Subscript>3</Subscript>

We report a quantitative investigation of the magnetic field-temperature phase diagram by taking into account a simple phenomenological model arising out of the interplay of kinetic arrest and thermodynamic transitions in a magnetic glass Pr_0.5Ca_0.5Mn_0.975Al_0.025O₃, through magnetization measurements. Such studies are necessary as kinetic arrest plays an important role in the formation of “magnetic glasses”, which has been observed in systems undergoing first order magnetic phase transitions. It has been shown that disorder in a system results in the formation kinetic arrest (H _K ,T _K ) band, like supercooling (H ^*,T ^*) and superheating (H ^**,T ^**) band. Quantitative proofs are given to show that (H _K ,T _K ) band is anticorrelated with (H ^*,T ^*) and (H ^**,T ^**) bands, while the later two are correlated among themselves. Analysis of time dependence of magnetization at different temperatures is carried out to establish the fact that the kinetic arrested state is different from the supercooled state.     

Ab initio calculations of B<Subscript>2</Subscript> type RHg (R = Ce,Pr, Eu and Gd) intermetallic compounds

Spin polarized ab initio calculations have been carried out to study the structural, electronic, elastic and thermal properties of RHg (R = Ce, Pr, Eu and Gd) intermetallic compounds in B₂ structure. The calculations have been performed by using both generalized gradient approximation (GGA) and local spin density approximation (LSDA). The calculated value of lattice constant (a ₀) for these compounds with GGA is in better agreement with the experimental data than those with LSDA. Bulk modulus (B), first-order pressure derivative of bulk modulus and magnetic moment (μ _B ) are also presented. The energy band structure and electron density of states show the occupancy of 4f states for light as well as heavy rare earth atom. The elastic constants are predicted from which all the related mechanical properties like Poisson’s ratio (σ), Young’s modulus (E), shear modulus (G _H ) and anisotropy factor (A) are calculated. The ductility or brittleness of these compounds is predicted from Pugh’s rule (B/G _H ) and Cauchy pressure (C ₁₂ ? C ₄₄). The Debye temperature (θ _D ) is estimated from the average sound velocity, which have not been calculated and measured yet.     

Electronic Structure,Optical Properties,and Pressure Behavior of the CdB<Subscript>4</Subscript>O<Subscript>7</Subscript> and HgB<Subscript>4</Subscript>O<Subscript>7</Subscript> Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB₄O₇ and HgB₄O₇ tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2₁ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2₁ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Optical characteristics of radiation from N<Stack><Subscript>2</Subscript><Superscript>*</Superscript></Stack> dimers in a barrier discharge

The spectral and power characteristics of radiation of the second positive system of nitrogen (C ³Π _u → B ³Π _g ) in Ar-N₂ and Ar-N₂-Cl₂ mixtures excited by barrier discharge have been studied experimentally. Addition of argon to N₂ increased the radiation power by sixfold. In the triple mixture Ar-N₂-Cl₂ = 210/0.5/0.005, minor chlorine additions increased the intensity of the C ³Π _u → _B ³Π _g transition by 26% compared to Ar-N₂ mixtures. Radiation power density of 2.7 mW/cm² has been achieved. In both binary and triple mixtures, the second positive system of nitrogen was the major contributor to radiation, while the contributions of the fourth positive system of N ₂ ^* (D ³Σ _u ⁺ → B ³Π _g ), the Vegard-Kaplan transition of N ₂ ^* (A ³Σ _u ⁺ → X ¹Σ _g ⁺ ), and the D′ → A′ band of Cl ₂ ^* were negligibly small.