Articles de revues sur le sujet « Pretransitional phenomena »

Structural and dynamic properties and molecular relaxation processes in crystalline sodium NaClO4 perchlorate in the temperature range from 300 K to 650 K were studied by Raman spectroscopy. The temperature dependences of the position of the maximum v (frequency), the width w and the intensity I of the spectral band, corresponding to the fully symmetric vibration v1(A) of the ClO4–1 perchlorate ion, in the spectral range from 933 cm–1 to 944 cm–1 were analyzed. The frequency v and intensity I decrease, and the width w increases with the increasing temperature. It is shown that these temperature dependences have certain features at a temperature of 460 K. With a further increase in temperature, the frequency v decreases more rapidly, the width w increases, and the intensity I decreases more intensively. In the temperature range from 460 K to the temperature Tstr = 581 K of the first order structural phase transition, we observe a deviation of the temperature dependence of the frequency and width from the linear dependences characteristic of lower temperatures. These deviations appear at a temperature of 460 K and increase with the increasing temperature and approaching the phase transition temperature. It has been established that in the crystalline sodium perchlorate NaClO4 a structural first-order phase transition is stretched. At the phase transition temperature (Tstr = 581 K), the width increases sharply, and the frequency decreases sharply, decreasing with a further increase in temperature. The existence of a pretransitional region in the studied crystalline sodium perchlorate NaClO4 was found. This pre-transition region occurs in the temperature range from 460 K to Tstr = 581 K.

The nature of precursor phenomena in the paraelectric phase of ferroelectrics is one of the main questions to be resolved from a fundamental point of view. Barium titanate (BaTiO3) is one of the most representative perovskite-structured ferroelectrics intensively studied until now. The pretransitional behavior of BaTiO3 single crystal grown using a solid-state crystal growth (SSCG) method was investigated for the first time and compared to previous results. There is no melting process in the SSCG method, thus the crystal grown using a SSCG method have inherent higher levels of impurity and defect concentrations, which is a good candidate for investigating the effect of crystal quality on the precursor phenomena. The acoustic, dielectric, and piezoelectric properties, as well as birefringence, of the SSCG-grown BaTiO3 were examined over a wide temperature range. Especially, the acoustic phonon behavior was investigated in terms of Brillouin spectroscopy, which is a complementary technique to Raman spectroscopy. The obtained precursor anomalies of the SSCG-grown BaTiO3 in the cubic phase were similar to those of other single crystals, in particular, of high-quality single crystal grown by top-seeded solution growth method. These results clearly indicate that the observed precursor phenomena are common and intrinsic effect irrespective of the crystal quality.

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 −84∘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.

Методами спектроскопии комбинационного рассеяния света исследованы структурно-динамические свойства и процессы молекулярной релаксации в кристаллическом сульфате калия K2SO4 в интервале температур от 293 до 900 К. Проанализированы температурные зависимости положения максимума v (частоты), ширины w и интенсивности I спектральной полосы, отвечающей полносимметричному колебанию v1(A) сульфат-иона SO4 2–, в спектральном интервале от 963 до 976 см–1. С ростом температуры частота колебания уменьшается. Примерно при 650 K имеют место определённые особенности температурной зависимости v(T). При дальнейшем увеличении температуры частота продолжает уменьшаться. В точке структурного фазового перехода первого рода (Ts = 854 K)уменьшение частоты приостанавливается. С ростом температуры ширина возрастает, а интенсивность уменьшается. Примерно при 650 K имеют место определённые особенности температурных зависимостей w(T) и I(T). Уменьшение интенсивности приостанавливается, и в интервале температур 650–850 K интенсивность остаётся почти постоянной. При структурном фазовом переходе первого рода (Ts = 854 K) интенсивность уменьшается. Рост ширины при температуре T ≈ 650 K приостанавливается, а затем снова ширина начинает увеличиваться. Ближе к структурному фазовому переходу первого рода (Ts = 854 K) рост ширины замедляется и в точке структурного фазового перехода первого рода (Ts = 854 K) имеет место уменьшение ширины. Установлено, что в кристаллическом сульфате калия K2SO4 структурный фазовый переход первого рода носит растянутый характер. При температуре фазового перехода (Ts = 854 К) ширина резко возрастает, а частота резко уменьшается, уменьшаясь и при дальнейшем увеличении температуры. Обнаружено существование предпереходной области в исследованном кристаллическом сульфате калия K2SO4. Эта предпереходная область имеет место в интервале температур от 650 К до Ts = 854 К. REFERENCES Ivanova E. S., Petrzhik E. 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