V. I. Torgashev

Concentration phase diagram of Ba x Sr 1 − x Ti O 3 solid solutions

Method of derivation of phenomenological thermodynamic potential of solid solutions is proposed in which the interaction of the order parameters of constituents is introduced through the account of elastic strain due to misfit of the lattice parameters of the end-members. The validity of the method is demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of ferroelectric solid solution. Its phase diagram is determined using experimental data for the coefficients in the phenomenological potentials of SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for small Ba concentration, there are a tricritical point and two multiphase points one of which is associated with up to 6 possible phases.

Observation of an intersublattice exchange magnon in CoCr2O4 and analysis of magnetic ordering

= 250÷1230 GHz = 1÷5meV)andattemperaturesbetween4and300K.Theradiationwaslinearlypolarized.Theabsolutevaluesofopticaltransmis-sion (normalized to the empty-channel measurements) wereobtained in essentially the same way as described, e.g., inRef. 19.A commercial split-coil magnet, embedded into an opticalcryostat, was utilized for measurements in magnetic fieldsup to 8 T. In these measurements, we used three differentgeometries:

Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstones.

When water is confined to nanocavities, its quantum mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.

Optical properties of BiFeO3 ceramics in the frequency range 0.3–30.0 THz

Reflection and transmission infrared spectra of BiFeO3 ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm−1 at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm−1 by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm−1. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.

B-Tphase diagram of CoCr2O4in magnetic fields up to 14 T

We have measured the magnetization and specific heat of multiferroic CoCr2O4 in magnetic fields up to 14 T. The high-field magnetization measurements indicate a new phase transition at T* = 5 - 6 K. The phase between T* and the lock-in transition at 15 K is characterized by magnetic irreversibility. At higher magnetic fields, the irreversibility increases. Specific-heat measurements confirm the transition at T*, and also show irreversible behavior. We construct a field-temperature phase diagram of CoCr2O4.

Magnetic and dielectric response of cobalt-chromium spinel CoCr2O4 in the terahertz frequency range

The nature of the phonon and magnon modes in the CoCr2O4 multiferroic with a cubic spinel structure has been studied using submillimeter spectroscopy and infrared Fourier spectroscopy. This paper reports on the first measurement of the evolution with temperature of the exchange optical magnon in the ferrimagnetic (TC = 94 K) and two low-symmetry (TS ≈ 26 K, Tlock-in = 14.5 K) phases of CoCr2O4 down to T = 5 K in zero magnetic field. It has been shown that the detected magnon is not a ferrimagnetic order parameter and originates, most probably, from spin precession in the cobalt sublattices. At the points of the magnetic phase transitions, the oscillator parameters of the two lowest-frequency phonon modes reveal an anomalous temperature behavior, thus evidencing the presence of significant interaction between the magnetic and phonon subsystems. The increase by 25% of the damping parameter of the phonon mode originating from vibrations of the CoO4 tetrahedra during the transition of CoCr2O4 to the multiferroic state (T < TS) suggests structural changes in the lattice involving loss of spatial central symmetry of the medium.

Incipient ferroelectricity of water molecules confined to nano-channels of beryl

Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole–dipole coupling and suppress the ferroelectric order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H2O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole–dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie–Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices.

Boson peak in overdoped manganites La1−xCaxMnO3

In the charge-ordered phase of strongly doped manganites La

Dielectric spectra of Bi0.98Nd0.02FeO3.00 multiferroic thin films in the terahertz frequency range

_{1-x}

Soft polar modes and phase states of Ca1−xPbxTiO3 solid solutions

Ca