Features Of Structural Phase Transitions Research Articles

Possibilities of a Simplified Approach to Studying the Features of Structural Phase Transitions in H-Bonded Ferroelectrics with the use of Ab Initio Calculations

Possibilities of a Simplified Approach to Studying the Features of Structural Phase Transitions in H-Bonded Ferroelectrics with the use of Ab Initio Calculations

Features of Structural Phase Transitions of Aluminum Hydroxides under Laser Processing

The results of the studies of laser processing of alkoxide aluminum hydroxides with micrometer and nanometer particle sizes are presented. It is shown that the pseudo-boehmite processing process and phase composition of formed oxides are controlled by particle packing, laser radiation propagation in powder, and specific energy deposition. The main phases formed upon laser heating are γ, α-Al2O3; the content of δ, θ-Al2O3 is low. The minimum corundum crystallite size is ∼50 nm.

Electron and phonon spectra dynamics and features of phase transitions in sodium at P=0-100 GPa

An electron and phonon spectra dynamics, features of structural phase transitions, and melting of sodium under the pressure ranging from 0 to 100 GPa are investigated. The electron and the phonon spectra of crystal sodium are calculated ab initio within the density functional theory by means of the software package LmtART-7 (see [1-3] and references herein), allowing the fully potential method of linear muffin-tin orbitals (FP-LMTO method). Earlier, this method was used in papers [4-6] for the calculation of a metals band structure within an atomic-spheres approximation (LMTO-ASA method). Using the Lindemann measure and the calculated phonon spectra, the theoretical values of melting points corresponding to the experimental data are obtained. Features of the electron and the phonon spectra dynamics in the melting curve maximum vicinity and within the structural transition range are discussed cI2 → cF4.

Deformation behavior of layered amorphous-crystalline Ti-Ni-Cu composite under different conditions of torsion in a bridgman chamber

Features of structural phase transitions under the high-pressure torsion of layered amorphouscrystalline Ti50Ni25Cu25 composite with 3 : 1 ratios of the layer thicknesses of the amorphous and crystalline phases, respectively, are analyzed by means of X-ray diffraction and transmission electron microscopy. The applied pressure is varied from 2 to 8 GPa. It is found that the structure of the layered Ti50Ni25Cu25 composite evolves in different ways in different parts of the material.

Structural phase transitions of the B1–B2 type in small-sized ionic crystals

The features of structural phase transitions that occur under high pressure are studied. The density functional theory is used to calculate the B1–B2 phase transition pressure as a function of the crystal size for small-sized alkali halide crystals. A size effect (an increase in the phase transition pressure as the crystal grain radius decreases) is revealed for the B1–B2 transitions in all halogen compounds except for lithium fluoride, for which the dependence is inverse.

Effects of temperature and pressure on phase transitions in a ternary microemulsion system

Temperature variation experiments of small angle neutron scattering (SANS) and neutron spin echo (NSE) were carried out in order to compare effects of temperature and pressure on a structural formation in a ternary microemulsion system composed of AOT (Aerosol-OT; dioctyl sulfosuccinate sodium salt), D2O, and n-decane. From SANS measurements, a phase transition from one-phase dense water-in-oil droplet to two-phase coexistence with a lamellar and a disordered structure was observed with increasing temperature, similar to the case of pressure variation. Another phase transition was observed at a higher temperature above the lamellar phase, although such a subsequent phase transition has not been observed at higher pressure. The characteristic features of structural phase transitions by temperature and by pressure were compared by introducing a reduced temperature and pressure. The dynamical property observed from the NSE measurement was different between the high-temperature phase and the high-pressure phase. These results indicate that the mechanism of the phase transition induced by temperature is different from that by pressure.

Features of structural phase transitions in Ca 2−xSm xMnO 4

Features of structural transitions in Ca 2− x Sm x MnO 4 have been investigated by means of electron diffraction. When the temperature is lowered from a high-temperature tetragonal (HTT) phase in x = 0.3, there are successive transitions from the HTT phase to a low-temperature orthorhombic (LTO) one at 300 K and further to a low-temperature tetragonal (LTT) one at 100 K. In electron diffraction patterns, satellite spots with q ̄ = 1 8 〈110〉 also appear below about 290 K. Features of the satellite spot arc that it is observed in 0.1 ≤ x ≤ 0.35 and its location depends on x.

Green's-function theory of ferroelectric phase transitions in hydrogen-bonded triglycine sulfate with the pseudospin-lattice coupled-mode model: A unified theory of structural phase transitions. I. Static and dynamic properties.

A pseudospin-lattice coupled-mode (PLCM) model and the statistical Green's-function theory have been used to explain the static as well as the dynamic properties of ferroelectric phase transitions in triglycine sulfate (TGS) and in its deuterated and undeuterated families. An attempt to settle the long-standing controversy over the coexistence of both order-disorder and displacive phase transitions in TGS and in many other H-bonded crystals (as observed from the studies of different static and dynamic properties) has been made by expressing the transition mechanism in these crystals as a ``mixed'' type rather than a purely order-disorder or a purely displacive type. This mixed behavior is due to the importance of the pseudospin-phonon and the higher-order anharmonic phonon-phonon interaction terms. The pseudospins are considered to be associated with the local ordering (rotation, displacements, etc.) of some ions or groups in the lattice. This makes the mechanism of structural phase transitions, in general, very complicated.As a consequence, the calculated expression for the total phonon self-energy is found to contain an extra term in addition to that which appears in the existing theoretical calculations by Cowley and others describing purely displacive phase transitions with phonon-phonon interaction only. This additional self-energy term is found, in principle, to be responsible for the observed deviations of some thermal and dielectric properties of TGS from the Landau theory, the existence of the ``double peak'' in the NMR or NQR relaxation times in TGS and in other crystals, and the appearance of both order-disorder and the displacive type of behavior in many crystals. This additional part of the self-energy is also found to be related to the ``central peak'' phenomenon. Depending on the strengths of the pseudospin-phonon interaction terms, this part of the self-energy might also be responsible for the crossover from order-disorder to displacive behavior observed in some structural transitions. This is also supported by recent EPR and other experimental observations made by M\"uller. The theoretical expressions derived for the transition temperature (${T}_{c}$), dielectric constant (\ensuremath{\epsilon}'), spontaneous polarization (${P}_{s}$), Curie-Weiss constant (C), renormalized phonon frequency, etc., have been fitted with the corresponding experimental data to find a single set of Blinc--de Gennes model parameters for the TGS family. Finally, the unified character of the PLCM model in explaining various salient features of structural phase transitions in crystals has been discussed, and supports the conclusion of our previous work.