Pretransitional Phenomena Research Articles

Unconventional short-range structural fluctuations in cuprate superconductors

The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La2-xSrxCuO4 and Tl2Ba2CuO6+δ, two cuprate superconductors with distinct point disorder effects and with optimal superconducting transition temperatures that differ by more than a factor of two. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behaviour. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behaviour to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics.

Structure and optical properties of twist-bend nematic liquid crystals doped with chiral dopants.

Twist-bend nematic liquid crystals (N_{TB} LCs), although consisting of achiral molecules, possess a spontaneous conic helix. They have been intensively studied and utilized in many applications in recent years. Herein we add chiral molecules to N_{TB} LCs and study their effects on the structure of the conic helix. We observe that the system is in the regular chiral nematic phase at high temperature and is still in the twist-bend nematic phase at low temperature. The addition of the chiral molecules does not induce a twist of the conic helical axis. The main effect of the chiral molecules is increasing the cone angle of the conic helix. We show that the structural chirality parameters in the chiral nematic phase and the twist-bend chiral nematic phase can be calculated from the same intrinsic chirality parameter, which only depends on the molecular structure and concentrations of the chiral molecule. We also observe a pretransitional phenomenon that the helical pitch of the chiral nematic phase increases dramatically when temperature is decreased toward the chiral nematic to twist-bend nematic phase transition temperature.

Inside out Approach to Rotator State in Hydrogen-Bonded System-Experimental and Theoretical Cross-Examination in n-Octanol.

The experimental and theoretical description of premelting behavior is one of the most challenging tasks in contemporary material science. In this paper, n-octanol was studied using a multi-method approach to investigate it at macroscopic and molecular levels. The experimental infrared (IR) spectra were collected in the solid state and liquid phase at temperature range from C to −15 C to detect temperature-related indicators of pretransitional phenomena. Next, the nonlinear dielectric effect (NDE) was measured at various temperatures (from −30 C to −15 C) to provide insight into macroscopic effects of premelting. As a result, a two-step mechanism of premelting in n-octanol was established based on experimental data. It was postulated that it consists of a rotator state formation followed by the surface premelting. In order to shed light onto molecular-level processes, classical molecular dynamics (MD) was performed to investigate the time evolution of the changes in metric parameters as a function of simulation temperature. The applied protocol enabled simulations in the solid state as well as in the liquid (the collapse of the ordered crystal structure). The exact molecular motions contributing to the rotator state formation were obtained, revealing an enabling of the rotational freedom of the terminal parts of the chains. The Car–Parrinello molecular dynamics (CPMD) was applied to support and interpret experimental spectroscopic findings. The vibrational properties of the stretching of OH within the intermolecular hydrogen bond were studied using Fourier transformation of the autocorrelation function of both dipole moments and atomic velocity. Finally, path integral molecular dynamics (PIMD) was carried out to analyze the quantum effect’s influence on the bridged proton position in the hydrogen bridge. On the basis of the combined experimental and theoretical conclusions, a novel mechanism of the bridged protons dynamics has been postulated—the interlamellar hydrogen bonding pattern, resulting in an additional OH stretching band, visible in the solid-state experimental IR spectra.

Influence of the initial structural-phase state on inelastic and plastic strains behavior in coarse-grained samples of the Ti49.3Ni50.7 (at.%) alloy

The paper presents the experimental results of studies of the temperature dependence of inelastic and plastic strains during torsion of coarse-grained samples of the Ti49.3Ni50.7 (at.%) alloy. Investigations of the deformation behavior of the test alloy samples in the martensitic, two-phase and high-temperature states have been carried out. It is shown that the value of the summary inelastic strain reaches a maximum value of ∼ 18% under deformation of the samples in the martensitic and two-phase state, as well as in the temperature range of pre-transition phenomena.

Pre-Transition Phenomena in the Temperature Range of Structural Phase Transitions in Perchlorate Crystals

Molecular relaxation processes in sodium perchlorate NaClO4 and potassium perchlorate KClO4 are studied via Raman spectroscopy. It is found that the structural first-order phase transition in crystalline perchlorates NaClO4 and KClO4 is extended (a diffuse phase transition). The existence of a pre-transition region in the studied perchlorates NaClO4 and KClO4 is revealed.

Pre-Transition Phenomena During Phase Transition in Potassium Carbonate

Molecular relaxation processes in potassium carbonate (K2CO3) are investigated using Raman-scattering spectroscopy. It is found that the first-order phase transition is temperature-expanded, and the pre-transition phenomena are observed in crystalline K2CO3. The paper gives theoretical grounds for pre-transition phenomena using the theoretical model of tunneling.

NMR relaxation and self-diffusion in aqueous micellar gels of pluronic F-127

Abstract We studied the transverse NMR relaxation of protons of different chemical groups and diffusion of molecules in an aqueous pluronic F-127 system. The system was studied at concentrations of 15, 21 and 28 wt% and in the temperature range of 293–333 K. The dynamic peculiarities in different phases were analyzed based on the NMR relaxation and diffusion data. Our study demonstrated a correlation between phase states, T2 relaxation times of “solid-like” and “liquid” protons of selected chemical groups of polypropylene oxide (PPO) and polyethylene oxide (PEO) blocks, and diffusion coefficients. Transverse NMR relaxation established that the presence of a “solid-like” component of CH3 protons of PEO blocks is observed only at temperatures and concentrations corresponding to the rigid-gel phase, due to entanglements between micellar coronas. At all temperatures and concentrations, some CH3 protons of PPO blocks and some CH2 protons of PEO blocks show “liquid-like” transverse NMR relaxation. Under the conditions corresponding to formation of the rigid-gel phase of the pluronic, relaxation of the “liquid-like” protons additionally accelerates due to entanglements of micellar coronas. Transition of the system to the rigid-gel phase is accompanied by decrease of diffusion coefficients by a factor of 10–104 relative to the diffusivity of free-moving pluronic micelles in the sol phase. Diffusion measurements show that there are pre-transition phenomena, which are characteristic for temperatures and concentrations near sol-gel phase boundaries.

Pretransition Phenomena in the Vicinity of Structural Phase Transition in Crystalline Sodium Carbonate

Molecular relaxation processes in crystalline sodium carbonate Na2CO3 have been investigated using Raman spectroscopy. It is established that the structural first-order phase transition in Na2CO3 is diffuse. The existence of a pretransition region is found.

Предпереходные явления в области структурного фазового перехода в карбонате калия

Предпереходные явления в области структурного фазового перехода в карбонате калия

Pretransition Phenomena in the Region of a Structural Phase Transition in Potassium Perchlorate

Molecular relaxation processes in potassium perchlorate KClO4 are studied using Raman spectroscopy. The first-order structural phase transition in crystalline perchlorate KClO4 is shown to be temperature-extended. The presence of a pretransition region in the studied perchlorate KClO4 is reported.

Pretransitional phenomena in the region of structural phase transition in sodium sulfate

Pretransitional phenomena in the region of structural phase transition in sodium sulfate

Optical and calorimetric studies of K2TaF7

Optical and calorimetric experiments on K2TaF7 are performed in a wide temperature range. No features were found in the behavior of the birefringence Δnb(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔCp(T) except for those associated with the Pnma↔P21/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pre-transition phenomena in Δnb(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS = 22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase.

Предпереходные явления в области структурного фазового перехода в перхлорате калия

Методами спектроскопии комбинационного рассеяния исследованы процессы молекулярной релаксации в перхлорате калия KClO4. Установлено, что в кристаллическом перхлорате KClO4 структурный фазовый переход первого рода носит растянутый характер. Обнаружено существование предпереходной области в исследованном перхлорате KClO4.

Туннельный эффект и оценка вероятности предпереходных явлений в области структурных фазовых превращений в кристаллах

Туннельный эффект и оценка вероятности предпереходных явлений в области структурных фазовых превращений в кристаллах

Optical anisotropy, order parameter and its critical behavior in temperature-dependent refractive indices of nematic liquid crystals

The temperature dependence of optical birefringence, order parameter, effective geometry parameter, and normalized polarizability was determined for nematic liquid crystal mixtures by measuring the refractive indices. The effects of the adjustable parameters obtained from temperature-dependent refractive indices were analyzed appropriately for the crossover temperature and order parameter values. Besides, a four-parameter power law expression method was used for the optical birefringence measurement to address inappropriate fits in the weak first-order phase transition region. From this method, the effective second order phase transition temperature T⁎⁎ was determined by showing the slightly higher values than phase transition temperature. The value of the order parameter critical exponent λ obtained from this method showed a good agreement with the mean field theory by demonstrating closeness to the tricritical hypothesis (β = 0.25). As a consequence, the measurement of the temperature-dependent optical birefringence was successfully verified the pre-transitional phenomena accurately, and it can facilitate an insightful description of that region.

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is the most spectacular manifestation of electronic ferroelectricity, in connection with a high degree of covalency between alternating donor and acceptor molecules along stacks. In addition, a charge-transfer instability from a quasi-neutral to a quasi-ionic state takes place concomitantly with the stack dimerization, which breaks the inversion symmetry. Moreover, these systems exhibit exceptional one-dimensional fluctuations, with an enhancement of the effects of electron-lattice interaction. This may lead to original physical pictures for the dynamics of pre-transitional phenomena, as the possibility of a pronounced Peierls-type instability and/or the generation of unconventional non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linear responsiveness. These different aspects will be discussed through the structural and dynamical features of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and conclude by their interplay with the photo-induced dynamics.

Frustrated pretransitional phenomena in aperiodic composites

This paper reports on symmetry breaking in the aperiodic inclusion compound $n\text{-octadecane/urea}$ and its isotopomer $n\text{-octadecane/urea-}{d}_{4}$. The high-symmetry phase is described by a hexagonal rank-4 superspace group. Pretransitional phenomena in this crystallographic superspace reveal competing short-range-ordering phenomena within the high-symmetry phase. Very high-resolution diffraction data show that critical scattering appears at inequivalent points within the four-dimensional Brillouin zone, although the first phase transition at ${{T}_{c}}_{1}$ near 158 K implies the condensation at only one of those points. The resulting superspace group remains of dimension 4. Two other phase transitions are reported at ${{T}_{c}}_{2}\phantom{\rule{4pt}{0ex}}=152.8(4)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${{T}_{c}}_{3}=109(4)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ in $n\text{-octadecane/urea-}{d}_{4}$. The two low-symmetry phases that arise are described by rank-5 superspace groups.

Rheological evidence for the existence of subphases in the liquid crystalline 4-n-alkoxybenzoic acids

ABSTRACTThe rheological behaviour of the homologous series, the alkyloxybenzoic acids, has been studied at steady flow and small amplitude oscillatory shear. The temperature dependencies of the viscosity were compared with differential scanning calorimetry data in order to estimate pretransitional phenomena in mesophases. The values of the viscosity and flow activation energy (E) can be indicative of the mesophase state. 4-n-pentyloxybenzoic acid yields a classic nematic (N) phase while the next homologues show a cybotactic nematic phase having a higher E value. The N phase is a Newtonian fluid of lower viscosity than that of the isotropic phase. The cybotactic nematic phase by its rheological properties takes an intermediate position between the N phase and a smectic C (SmC) phase. The SmC phase is a viscoelastic and viscoplastic medium. The SmC phase of 4-n-octyloxy-, 4-n-nonyloxy-, 4-n-decyloxy- and 4-n-dodecyloxybenzoic acid can exist in high- and low-ordered states. The more ordered smectic phase is characterised by higher stiffness, viscosity and yield stress than those of the less ordered. The SmC phase of 4-n-hexadecyloxybenzoic acid is characterised by the only state with unstable value of apparent viscosity. The phase state and the transition temperature can be dependent on the thermal history of the sample and a mechanical shear.

Strain behavior and lattice dynamics in Ni50Mn35In15

The lattice dynamics in the polycrystalline shape-memory Heusler alloy Ni50Mn35In15 have been studied by means of resonant ultrasound spectroscopy (RUS). RUS spectra were collected in a frequency range 100–1200 kHz between 10 and 350 K. Ni50Mn35In15 exhibits a ferromagnetic transition at 313 K in the austenite phase and a martensitic transition at 248 K accompanied by a change of the magnetic state. Furthermore it displays a paramagnetic to ferrimagnetic transition within the martensitic phase. We determined the temperature dependence of the shear modulus and the acoustic attenuation of Ni50Mn35In15 and compared it with magnetization data. Following the structural softening, which accompanies the martensitic transition as a pretransitional phenomenon, a strong stiffening of the lattice is observed at the martensitic magneto-structural transition. Only a weak magnetoelastic coupling is evidenced at the Curie temperatures both in austenite and martensite phases. The large acoustic damping in the martensitic phase compared with the austenitic phase reflects the motion of the twin walls, which freezes out in the low temperature region.

Thermoelastic Martensitic Transitions and Shape Memory Effects: Classification, Crystal and Structural Mechanisms of Transformations, Properties, Production and Application of Promising Alloys

In the article there is presented a brief overview of combined systematic investigations of the alloys exhibiting thermoelastic martensitic transformations (TMTs). As is known, such alloys are distinguished by a whole number of specific one-and multi-fold-reversibility shape-memory (SM) effects occurring upon changing temperature, pressure, magnetic field, at the background of superplasticity and highly reversible deformation of transformation under a load or its relieving.There have been considered the alloys classification, the thermodynamical and kinetic aspects of TMTs, the origin and structural mechanisms of realization of multivarious pre-transition phenomena and TMTs proper, the influence of complex alloying, the peculiar features of a structure, the physico-mechanical properties and methods of production of the alloys most promising from the viewpoint of application. For this purpose there were employed structural methods of the X-ray diffraction (XRD) structure-phase analysis, neutron diffraction analysis, transmission and scanning electron microscopy techniques of high resolution, together with studying texture and atomic composition, as well as measuring a number of physical properties and SM-effect characteristics.The effect of the alloying and external actions of different origin on the magnetic and structural phase transformations and properties of the studied alloys with the magnetically, thermally, and mechanically controlled manifestations of the shape memory is discussed. A sequence of the TMTs with the formation of different martensitic phases, as well as the crystallographic structure and crystal-geometry specific features of the formation of these phases is described. The phase diagrams of the magnetic and martensitic transformations in a number of binary, ternary, and quaternary alloys synthesized via different schemes of alloying are presented.On the example of the binary and doped alloys TiNi it has been shown that an employment of thermo-mechanical treatments of the alloys via multiple or repeated torsion under pressure, rolling or drawing leads to their high-level strengthening and grain refinement (up to amorphization). In this case the use of low-temperature annealing provides for both the creation in the alloys of the homogeneous nanostructured state with a controllable grain size already in the interval of 50–200 nm and efficient regulation of physico-mechanical properties with retaining comparably high values of the parameters of SM, including deformation-, temperature-, and force-related.There has been revealed the effect of the grain size on the critical temperatures of TMTs and, as a consequence, on the TMT-stipulated SMEs. On this basement, in dependence of the grain size, the poly-packet, mono-packet twinned or single-crystal structures of martensite can be realized.There are discussed an experimentally revealed deformation-induced atomic disordering in Heusler alloys with self-forming a nanocrystalline fcc (A1) structure and amorphization in the alloys of titanium nickelide, as well as long-range order recovery in them taking place in the course of low-temperature annealing in conditions of retaining of the nanostructured state of austenite and at feasibility of cascade occurrence of TMTs and SMEs. It has been shown that the use of super-rapid quenching (SRQ) via employment of a melt spinning technique makes it possible to produce ductile submicrocrystalline ribbons of the Heusler Ni2MnGa-based alloys with magnetically controllable TMTs. For a number of alloys based on titanium nickelide the effect of SRQ on the internal structure, grain refinement, and amorphization has been considered. The reasons of the amorphization and ways of subsequent nanostructurization of the alloys under investigation are discussed.