Anomalous states of the crystal structure of (Rb0.1(NH4)0.9)2SO4 solid solutions in the temperature range 4.2–300 K

Crystals of (Rb_0.1(NH₄)_0.9)₂SO₄ solid solutions are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameter a exhibits an anomalous behavior, namely, the “invar effect” at temperatures above the ferroelectric phase transition point T _c and an anomalous increase in the temperature range from T _c to the liquid-helium temperature. An anomalous increase in the lattice parameter a and an increase in the intensity of Bragg reflections with a decrease in the temperature are interpreted within the model of the coexistence of two sublattices hypothetically responsible for the ferroelectric phase transition. A series of superstructure reflections observed along the basis axes corresponds to a sublattice formed in the matrix of the host structure. Analysis of the ratio between the lattice parameters of these structures allows the inference that, in the temperature range 4.2–300 K, the structure of the crystal under investigation can be considered an incommensurate single-crystal composite.     

Emergence of the sub‐THz central peak at phase transitions in artificial BaTiO3/(Ba,Sr)TiO3 superlattices

A prominent central peak in the sub‐THz frequency range was observed in the Raman spectra of BaTiO₃/(Ba,Sr)TiO₃ (BT/BST) superlattice grown on (001)MgO substrate. Both soft and central mode show an anomaly around 200 K and 280 K, which can be correlated with orthorhombic to monoclinic phase transition of BST and BT, respectively. The observed temperature dependence of the central mode enabled us to explain rather broad temperature dependence of the dielectric permittivity previously observed in BT/BST superlattices. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Polar phonons and specific features of the ferroelectric states in cadmium titanate

Polarized spectra of reflectance R(ν) and transmittance Tr(ν) of single-crystal CdTiO₃ samples have been obtained in the frequency range 7 < ν < 1000 cm^?1 and for temperatures from 5 to 300 K using IR Fourier spectroscopy and submillimeter-range techniques. Dispersion analysis was carried out in terms of the additive-oscillator model, and dielectric responses ε(ν) and ε″(ν) were calculated. The polar modes were assigned to particular symmetry types, and their oscillator parameters (dielectric contributions, normal frequencies, damping constants) were determined. The numerical values of the components of the static permittivity tensor, ε₁₁ and ε₃₃, are shown to be almost fully determined by the total dielectric contributions due to the B_3u and B_1u phonons, respectively. In the low-frequency domain, lines showing anomalous behavior of the oscillator parameters, which is characteristic of soft ferroelectric modes, were observed. It is shown that, in CdTiO₃ at cryogenic temperatures, there exist several different polar states with switching in the direction of the spontaneous polarization vector.     

Tilting structures in perovskites

Thirty five low-symmetry tilting phases of the octahedra in which the atoms located at the initial octahedral position remain equivalent are derived for the perovskite structure at the k₁₀(X), k₁₁(M), and k₁₃(R) points of the Brillouin zone. For each low-symmetry phase, structural data are presented and a relationship between the atomic displacements and the order parameters is deduced. All the low-symmetry phases can be obtained by considering only one or two order parameters.     

Raman spectra of cadmium titanate

Polarized Raman spectra of CdTiO₃ single crystals are recorded for the first time over the frequency range 5 < ν < 1000 cm^?1 at temperatures of 10 to 1200 K. The emphasis was on the low-frequency range, where an anomalous temperature dependence of a few phonon modes was observed. At high temperatures, four phonon modes exhibiting a behavior typical of soft modes were found to exist. These phonon modes are assumed to restore the cubic symmetry of the lattice. Their extrapolated temperature dependences suggest that there exists a sequence of three hypothetical high-temperature phase transitions analogous to those observed in the genuine perovskite CaTiO₃. At temperatures below 78 K, the Raman spectrum exhibits new lines associated with polar distortions of the unit cell. At low frequencies, three lines are observed whose parameters exhibit an anomalous behavior typical of soft modes in a ferroelectric phase. Several different polar states are assumed to exist at low temperatures.     

Dielectric spectra of Bi<Subscript>0.98</Subscript>Nd<Subscript>0.02</Subscript>FeO<Subscript>3.00</Subscript> multiferroic thin films in the terahertz frequency range

Transmission and reflection spectra of Bi_0.98Nd_0.02FeO_3.00 multiferroic thin films on MgO single-crystal substrates have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 8 to 1000 cm⁻¹ at room temperature. The complex permittivity spectra of the films have been calculated in terms of the layered medium model. It has been revealed that a decrease in the film thickness leads to a considerable increase in the losses in a range of 30 cm⁻¹ and the corresponding fivefold increase in the static permittivity (to 500 for a film 32 nm thick). This phenomenon has been discussed in the framework of the phenomenological theory of phase transitions.     

Nature of Local Symmetry Violations of Ions in the Magnetic Subsystem of Magnetoplumbite Crystal,According to Raman Scattering Spectroscopy Data

The structure of an M-type lead hexaferrite (magnetoplumbite) crystal is studied via the diffraction of synchrotron radiation. The crystal model contains two iron ions positions with dynamic disorder. The dynamics of the crystal lattice in the temperature range of 80 to 775 K is studied by Raman scattering. Particular attention is given to the discussing the nature of the local disorder of iron ions in the trigonal bipyramidal environment of oxygen ions.     

Nitro azines. 21. Reactivities and electron structures of 6-nitroazolo[1,5-a]pyrimidines

6-Nitroazolopyrimidines react with the acetonyl anion to give 6-nitro-7-acetonyl-4,7-dihydroazolo[1,5-a]pyrimidines. The reactivity of this class of compounds with respect to charged and uncharged nucleophiles is determined by their aromatic character and the deficit of electron density in the pyrimidine ring, while the direction of attack is determined by the overall charge on fragments of the valence-bonded atoms.For Communication 20 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 807–813, June, 1993.     

Low-frequency dynamic response of the bismuth strontium ferrite (Bi,Sr)FeO3−x

Broad-range measurements of the dynamic response of polycrystalline samples of the (Bi,Sr)FeO_3?x perovskite-like solid solution are performed over a frequency range from 10 Hz to 1 GHz at temperatures of 100–300 K for the first time. The colossal dielectric constant effect and the influence of electric contacts on the results of measurements are considered. It is shown that the frequency dependences of the permittivity and dynamic conductivity of (Bi,Sr)FeO_3?x samples can be described within the universal dielectric response model.