Vibrational spectra of monoisotopic SiH<Subscript>4</Subscript> and GeH<Subscript>4</Subscript> in low-temperature matrices

IR absorption spectra of monoisotopic ²⁸SiH₄ and ⁷⁶GeH₄ are studied in Ar and N₂ matrices at 10 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the stretching ν₃ and bending ν₄ vibrations. Four groups of bands can be separated out in the spectra of each molecule: (1) narrow bands characteristic of the matrix isolation studies, (2) broad bands, (3) diffuse absorption with a large value of the spectral moment M ₂^* the intensity of which increases upon annealing, and (4) bands of dimers the intensity of which increases quadratically with concentration. The spectra of ²⁸SiH₄ and ⁷⁶GeH₄ in nitrogen matrices contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry from T _d to C _3V on passage from the gas phase to solid nitrogen.     

Electric field-induced formation of a new commensurate phase of the [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystal

The formation of a new commensurate phase with a dimensionless wave number q = 1/3 in an [N(CH₃)₄]₂CuCl₄ crystal under the action of an external electric field is analyzed theoretically. The phase diagram is constructed on a plane specified by two coefficients of the thermodynamic potential in the presence and in the absence of an external electric field.     

Rotation of the inner shell in a C<Subscript>20</Subscript>@C<Subscript>80</Subscript> nanoparticle

The stability of a C₂₀@C₈₀ nanoparticle and the rotation of its inner shell are studied theoretically within the tight-binding approximation. It is found that the C₂₀ skeleton in the free state is described by space group D_3d; in the case where C₂₀ is placed into the C₈₀(I _h ) fullerene field, the space group of C₂₀ is raised to I _h due to isomerization. The total energy surface of the C₂₀@C₈₀ compound is scanned over two rotation angles. Based on an analysis of the surface relief and energy isoline map, orientational melting of the nanoparticle is predicted. A nanoparticle gyroscope—C₂₀ rotating in the field of C₈₀ at a certain relative orientation and energy supply—is also predicted to exist.     

Sub-unit cell layer-by-layer growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>, MgO,and Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> thin films

The use of oxide materials in oxide electronics requires their controlled epitaxial growth. Recently, it was shown that Reflection High Energy Electron Diffraction (RHEED) allows the growth of oxide thin films to be monitored, even at high oxygen pressures. Here, we report the sub-unit cell molecular or block layer growth of the oxide materials Sr₂RuO₄, MgO, and magnetite using Pulsed Laser Deposition (PLD) from stoichiometric targets. Whereas a single RHEED intensity oscillation is found to correspond to the growth of a single unit cell for perovskites such as SrTiO₃ or doped LaMnO₃, in materials where the unit cell is composed of several molecular layers or blocks with identical stoichiometry, sub-unit cell molecular or block layer growth is established, resulting in several RHEED intensity oscillations during the growth of a single unit cell. PACS 61.14.Hg; 74.76.Db; 75.70.-i; 81.15.Fg     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

Band structure of a new layered La<Subscript>3</Subscript>Ni<Subscript>4</Subscript>P<Subscript>4</Subscript>O<Subscript>2</Subscript> superconductor

The ab initio FLAPW-GGA calculations of the band structure of a new layered low-temperature (T _C ～ 2.2 K) La₃Ni₄P₄O₂ superconductor are presented. The energy bands, distributions of the densities of electron states, charge states of the atomic layers, low-temperature electron specific heat, and molar Pauli paramagnetic susceptibility for La₃Ni₄P₄O₂ have been determined. They are discussed compared to the existing experimental data.     

Retardation of polarization in mixed K<Subscript>0.88</Subscript>(NH<Subscript>4</Subscript>)<Subscript>0.12</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystal

The time dependences of polarization of K_0.88(NH₄)_0.12H₂PO₄ mixed crystal have been studied within the temperature range of 74–100 K. Two mechanisms of polarization relaxation were found. The first mechanism is caused by domain walls lateral motion and their interaction with point lattice defects. The second one supposedly is due to polar regions infiltration through the regions of frustrated paraelectric phase.     

Synthesis of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al₂O₃ substrate, ZnAl₂O₄/α-Al₂O₃ complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl₂O₄ changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl₂O₄ transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl₂O₄ prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl₂O₄/α-Al₂O₃ substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl₂O₄ layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl₂O₄ layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.     

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.     

Electronic excitations in BeAl<Subscript>2</Subscript>O<Subscript>4</Subscript>, Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>, and Be<Subscript>3</Subscript>Al<Subscript>2</Subscript>Si<Subscript>6</Subscript>O<Subscript>18</Subscript> crystals

Low-temperature (T = 7 K) time-resolved selectively photoexcited luminescence spectra (2–6 eV) and luminescence excitation spectra (8–35 eV) of wide-bandgap chrysoberyl BeAl₂O₄, phenacite Be₂SiO₄, and beryl Be₃Al₂Si₆O₁₈ crystals have been studied using time-resolved VUV spectroscopy. Both the intrinsic luminescence of the crystals and the luminescence associated with structural defects were assigned. Energy transfer to impurity luminescence centers in alexandrite and emerald was investigated. Luminescence characteristics of stable crystal lattice defects were probed by 3.6-MeV accelerated helium ion beams.     

Comparative Study of the Magnetoelectric Effect in HoAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and HoGa<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Single Crystals

The comparative study of the magnetoelectric properties and magnetostriction of HoGa₃(BO₃)₄ and HoAl₃(BO₃)₄ single crystals has been carried out. The investigated compounds exhibit qualitatively similar magnetodielectric and inverse magnetoelectric ME _E effects with the close absolute values, which is indicative of the weak effect of a nonmagnetic metal ion. On the contrary, the magnetostriction of the galloborate has been found to be threefold higher than that of the alumoborate. In addition, the difference between the qualitative behaviors of magnetostriction has been established: the magnetic-field dependence of magnetostriction for the alumoborate has the maximum near 70 kOe at T = 4.2 K, while the galloborate magnetostriction has no maximum and does not saturate in a field of 140 kOe.     

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.     

Quantum oscillations in dual-layered quasi-two-dimensional organic metal (ET)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>4</Subscript>Cl<Subscript>2</Subscript>)

We study the effect of interionic anisotropy on the phase states of a non-Heisenberg ferromagnet with magnetic ion spin S = 1. It is shown that depending on the relation between the interionic anisotropy constants, uniaxial and angular ferromagnetic and nonmagnetic phases exist in the system. We analyze the dynamic properties of the system in the vicinity of orientational phase transitions, as well as a phase transition in the magnetic moment magnitude. It is shown that orientational phase transitions in ferromagnetic and nematic phases can be first- as well as second-order.     

Cluster formation in LiNi<Subscript>0.4</Subscript>Fe<Subscript>0.6</Subscript>O<Subscript>2</Subscript>

The structure of an LiNi_0.4Fe_0.6O₂ cubic solid solution is determined using magnetic measurements and electron diffraction. It is found that this solid solution has a microinhomogeneous structure due to the formation of superparamagnetic clusters. The electron diffraction analysis of LiNi_0.4Fe_0.6O₂ samples has revealed diffuse scattering characteristic of the substitutional short-range order in ordered solid solutions with a B1-type structure. It is shown that the short-range order is associated with the LiNiO₂-type rhombohedral superstructure (space group \(R\bar 3m\)), i.e., with the redistribution of lithium and nickel atoms in the (111)_B1 alternating planes. The short-range order is observed in regions with a nickel content higher than the mean nickel content corresponding to the macroscopic composition.     

Entanglement in the linear-chain Heisenberg antiferromagnet Cu(C<Subscript>4</Subscript>H<Subscript>4</Subscript>N<Subscript>2</Subscript>)(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>

Quantum correlations are generally impossible to address directly in bulk systems. Quantum measures extended only to a few number of parties can be discussed in practice. In the present work we study a cluster of spins belonging to a compound whose structure is that of a quantum magnet. We reproduce at a much smaller scale the experimental outcomes and then we study the role of quantum correlations there. A macroscopic entanglement witness has been introduced in order to reveal quantum correlations at nonzero temperatures. The critical point beyond which entanglement is zero is found at T _c = 15 K.     

Kinetics of electron tunneling transfer in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript> and NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH₂PO₄ (KDP) and NH₄H₂PO₄ (ADP) on a wide temporal scale (10⁻⁸–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.     

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.     

Effect of pressure on the structure and the electronic properties of LiClO4, NaClO<Subscript>4</Subscript>, KClO<Subscript>4</Subscript>, and NH<Subscript>4</Subscript>ClO<Subscript>4</Subscript>

The effect of pressure on the structural and electronic properties of lithium, sodium, potassium, and ammonium perchlorates have been studied in terms of the density functional theory with allowance for the Van der Waals dispersion interaction. The pressure dependences of the geometric parameters, the band gaps, the densities of states, the charge distributions, and the atomic charges are calculated. The compressibilities of the perchlorates are found to be anisotropic, which is due to the differences of the lattice parameters and the nature of interatomic bonds. Ammonium cation is rotated under pressure around axis b at an angle of ~9°. The band gaps of the perchlorates are ~4.5–4.7 eV and increase with pressure.     

Raman and FTIR spectroscopy study of LiFeTiO<Subscript>4</Subscript> and Li<Subscript>2</Subscript>FeTiO<Subscript>4</Subscript>

We report the Fourier transform infrared (FTIR)–Raman spectroscopy study of spinel Li–Fe–Ti–O oxides viz., LiFeTiO₄ and Li₂FeTiO₄ in order to probe structural details such as type of bonding networks viz., octahedral and tetrahedral, and type of different atomic bonds present in those materials. Both the samples were prepared through solid-state reaction route prior to high-energy ball-milling. All the phases prepared through solid-state reaction and ball-milled were probed using X-ray diffraction, field emission scanning electron microscopy, and FTIR–Raman spectroscopy. X-ray diffraction study indicates spinel phase formation with Fd3m space group symmetry for both LiFeTiO₄ and Li₂FeTiO₄. However, pure phase of Li₂FeTiO₄ was not achieved in these preparation routes, rather mixed phases of Li₂FeTiO₄ and Fe₂TiO₄ were achieved. Field emission scanning electron microscopy (FESEM) analysis indicated porous microstructure for LiFeTiO₄ while more agglomerated microstructure for Li₂FeTiO₄. Ball-milling reduces the grain size partly for both the samples. FTIR–Raman spectroscopy indicates the presence of LiO₄ tetrahedral, LiO₆ and TiO₆ octahedral in the spinel network. Presence of Li–Li–O type bonding was also indicated from spectroscopy analysis. Existence of Fe₂TiO₄ phase with Li₂FeTiO₄ was also identified from both FTIR and Raman spectrum. Effect of ball-milling on the spectrum has been exhibited by broadening and peak shifting the FTIR–Raman spectrum, arising from the enhanced lattice strain and structural disorder.