A model of photostructural changes in chalcogenide vitreous semiconductors: 1. Theoretical considerations

Light-induced processes in chalcogenide vitreous semiconductors are considered in the framework of a configurational model of two stable states (ground and metastable) of atomic units with optical and thermal transitions between these states. Probabilities of these transitions as functions of photon energy and temperature are calculated. an expression for the metastable state population kinetics is given.     

New model of the bond diffractometer for precise determination of lattice parameters and thermal expansion of single crystals

A new model of diffractometer for very precise measurements (1 ppm accuracy) of lattice parameters and thermal expansion is presented. Special attention is paid to the temperature control and stability in a wide range from liquid nitrogen up to 800 K. Two examples of the application of the diffractometer in a study of solid state phase transitions are given.     

Structural transformations in solid and liquid n-butanol from FTIR spectroscopy

Abstract

FTIR spectra of n-butanol were registered for temperatures from ?100 °С to 40 °С during heating of the sample and from 40 °С to ?170 °С during its cooling. Structural changes at phase transitions solid–liquid and liquid–solid were detected. At n-butanol cooling below the melting point the initial crystalline structure was not achieved. Instead of it, a super-cooled liquid or amorphous phase was obtained. Quantum-chemical simulation of 27 n-butanol rotational conformers were provided via ROCBS-QB3 method. According to the calculations, the most stable conformer is TG′t. Comparison of calculated in the anharmonic approximation IR spectra of possible monomers and dimers of n-butanol with experimentally registered spectra showed that there are no monomers and dimers in the studied sample, and the structure of n-butanol in the condensed state is formed by bigger clusters.     

An operational scheme to determine the locally preferred structure of model liquids

We present an operational method to determine the ‘locally preferred structure’ of model liquids, a notion often put forward to explain supercooling of a liquid and glass formation. The method relies on finding the global minimum in the (free) energy landscape of clusters of atoms or molecules embedded in a liquid-like environment. We propose a more systematic approach of the external potential mimicking the influence of the surrounding bulk liquid on the cluster than in our previous work. The procedure is tested on the one-component Lennard-Jones liquid and we recover, without a priori input, that the locally preferred structure is an icosahedral arrangement of 13 atoms.     

Relationship between viscous dynamics and the configurational thermal expansion coefficient of glass-forming liquids

We propose a model to describe the relationship between the viscosity of a glass-forming liquid and its configurational contribution to liquid state thermal expansion. The viscosity of the glass-forming liquids is expressed in terms of three standard parameters: the glass transition temperature (T_g), the liquid fragility index (m), and the extrapolated infinite temperature viscosity (η_∞), which are obtained by fitting of the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) expression to measured viscosity data. The model is tested with experimental data for 41 different glass-forming systems. A good correlation is observed between our model viscosity parameter,h(T_g, m, η_∞), and the configurational coefficient of thermal expansion (i.e., the configurational CTE). Within a given class of glass compositions, the model offers the ability to predict trends in configurational CTE with changes in viscosity parameters. Since viscosity is governed by glass network topology, the model also suggests the role of topological constraints in governing changes in configurational CTE.     

Glass formation and glass transition in supercooled liquids,with insights from study of related phenomena in crystals

We divide glass and viscous liquid sciences into two major research areas, the first dealing with how to avoid crystals and so access the viscous liquid state, and the second dealing with how liquids behave when no crystals form. We review some current efforts to elucidate each area, looking at strategies for vitrification of monatomic metals in the first, and the origin of the property ‘fragility’ in the second. Essential in the first is the ‘ideal glassformer’ concept. Essential in the second is the non-trivial behavior of the glassformer thermodynamics. We explore the findings on non-exponential relaxation and dynamic heterogeneities in viscous liquids, emphasizing the way in which direct excitation of the configurational modes has helped differentiate configurational from non-configurational contributions to the excess heat capacity. We then propose a scheme for understanding the relation between inorganic network and non-network glassformers which includes the anomalous case of water as an intermediate. In a final section we examine the additional insights to be gained by study of the glass-like, ergodicity-breaking transitions that occur in disordering crystals. Here we highlight systems in which the background thermodynamics is understood because the ergodic behavior is a lambda transition. Water, and the classical network glassformers, appear to be attenuated versions of the latter type of cooperative transition.     

An apparent critical point in binary mixtures of m-nitrotoluene with n-alkanes; experimental and simulation study

We report a simulation study of the system m-nitrotoluene–n-decane, showing an apparent critical point, which lies in their metastable, experimentally inaccessible state, below their melting point, affecting physical and chemical properties of this systems in the stable liquid phase. The presence of the apparent critical point in this mixture has been experimentally observed by the non-linear dielectric effect (NDE) as an anomalous increase in the NDE values typical of critical concentrations. The phase diagrams of this mixture have evidenced that the system freezes in the homogenous phase and its melting point is higher than its critical temperature [M. Śliwińska-Bartkowiak, B. Szurkowski, T. Hilczer, Chem. Phys. Lett. 94 (1983) 609, M. Śliwińska-Bartkowiak, Ber. Bunsengess. Phys. Chem. 94 (1990) 64, M. Śliwińska-Bartkowiak, Phys. Lett. A 128 (1988) 84]. For such a system, we performed Monte Carlo simulations aimed at analyzing the kind of phase transition observed, and their conditions of their occurrence in a Lennard-Jones mixture. The enthalpy, configurational energy and radial distribution function have been estimated by the MC simulation method in the NPT system. Immiscibility conditions according to Hoheisel [M. Schoen, C. Hoheisel, Mol. Phys. 57 (1986) 65] approach have also been discussed.     

Computer simulation of GeO2 liquid

The local structure and dynamic of liquid GeO₂ have been investigated by mean of molecular dynamics simulation. We have prepared 13 systems upon pressure ranging from −0.41 to 48.9 GPa. The local structure was analyzed through the pair radial distribution function, coordination number and bond-angle statistics, the characteristics of voids and void aggregations. Two kinds of void aggregations have been examined: void clusters and void tubes. It was found a large number of vacancy-like voids and microscopic cavities in low-density system. Furthermore, there is a large void tube consisting of 94% oxygen vacancy-like voids and it spreads over whole system. The considerable changes in dynamic and local structure between high-density and low-density systems indicated the possibility of the liquid–liquid phase transition.     

The configurational entropy of glass

Configurational entropy is of fundamental importance to understanding the universal nature of the glassy state. According to the conventional view, the configurational entropy of a liquid is ‘frozen’ below the glass transition temperature (T_g) all the way to absolute zero. Contrary to this, we claim that: (i) there is an entropy loss associated with the liquid to glass transition, and (ii) the configurational entropy in the glassy state vanishes at absolute zero. This paper contrasts our arguments (called the ‘entropy loss’ view) and the conventional view.     

Self-organization model for a cell system: Ferroelectric, ferroelastic, and magnetic states and related phase transitions

A model is proposed to explain the stability, phase state transformations, and coexistence of different phases for fungi cell ensembles (in particular, dimorphism and linear-to-spiral structure transitions with the Earth’s magnetic field screened). This model is based on (i) cell-connected soft polarization modes induced by charge compensation and related ferroelectric and ferroelastic phase transitions and (ii) intracell mobile orbit-spin-lattice clusters with competitive ferromagnetic-diamagnetic behavior and with orbitlattice and spin-lattice interactions. This model makes it possible to explain the structural and magnetic properties of the systems under consideration. In particular, the Lifshitz invariants in the free energy explain the formation of orbit-lattice and spin-lattice spiral and ring-type structures that are formed when the Earth’s magnetic field is effectively screened. The model proposed is not restricted to mitochondria, containing orbit-spin-lattice clusters based on the Fe³⁺/Fe²⁺ states (considered here).     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature-dependent structural changes in liquid Ge15Te85

The structure of liquid Ge₁₅Te₈₅ has been studied with neutron diffraction in the liquid state up to 740 °C and in the supercooled liquid state down to 345 °C. The temperature dependences of the diffraction patterns are analyzed. It is shown that the liquid Ge₁₅Te₈₅ can be described by the model of heterogeneous structure, which assumes that the melt consists of atoms joined in clusters and a proportion of atoms with higher mobility that fill the space in between clusters. The number and the size of clusters decrease while the volume fraction of ‘free’ atoms increases under heating.     

Composition and polyamorphism in supercooled yttria-alumina melts

By extending recent work on liquid-liquid transitions in supercooled yttria-alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.     

Phase transitions in solid and liquid silicon

It has been established that continuous variations of impurity concentration and temperature in silicon and other semiconductors give rise to phase transitions in the range of existence of solid and liquid solutions. The specific feature of these phase transformations is the occurrence of a phase transition proper in the range of the varying parameter of state, which is accompanied by a thermal effect. In such ranges, all the electrophysical and thermodynamic properties oscillate. It has also been shown that these oscillations result from the equilibrium self-organization of the material. The beginning of phase transitions with the change of the temperature depends on the concentration of defect in the initial material. The “driving force” of the phase transformations is the instability arising in the material with the variations in the temperature and the component concentrations.     

To the Unified Model of Condensed State

The problem of unified model of condensed state can be solved by using solid-like model of liquids. A liquid arises when in a heated crystal a universal process of microcrack formation takes place that leads to originating a great number of very small crystals: microcrystallites. In the paper the two basic notions: the effective temperature of crystal surfaces and the internal thermal stresses in crystals are discussed. These notions make possible the microcrystallite model of liquids and the unified model of condensed state. Thus, the microcracks, resulted from thermal stresses, below the melting point lead only to decreased strength of crystals but above melting point the microcracks are spread all over the crystal bulk and cause splitting of the crystal into microcrystallites that are moving freely, in short, making up a liquid. The simple relations for these processes are given. In the end of the paper the identical features of the condensed states, solid and liquid, has been considered.     

Vibrational spectra of the YbB<Subscript>12</Subscript> Kondo insulator

The density of phonon states for the YbB₁₂ Kondo insulator is calculated from the inelastic neutron scattering spectra of this compound. It is established that thermal vibrations of rare-earth atoms predominantly occur in the low-energy range. These atoms are most weakly bound in the crystal structure of the YbB₁₂ Kondo insulator. The high-energy part of the vibrational spectrum is determined by thermal vibrations of the boron atoms forming a rigidly connected structure of the compound. It is revealed that the temperature dependence of the intensity of the phonon peak attributed to thermal vibrations of the ytterbium atoms exhibits an anomalous behavior. This circumstance suggests that the magnetoelastic coupling occurring in the structure of the YbB₁₂ Kondo insulator is relatively strong and can contribute to the magnetic excitation spectrum of this compound.     

Indications of the magnetic state in the charge distributions in MnO, CoO, and NiO. I: Para-and antiferromagnetism of MnO

X-ray diffraction intensities from MnO and CoO were measured above and below their Néel temperatures and from NiO, below the Néel temperature. To detect possible characteristics of the paramagnetic and antiferromagnetic states of the crystals, the data were subjected to direct multipole analysis of the atomic-charge densities. For MnO, both spherical and nonspherical accumulation-of-charge densities indicate the exchange of the roles played by manganese and oxygen in the magnetic phase transition. Both spherical and nonspherical features characteristic of the ionic nature are inherent in both states. The electron counts of the density peaks correspond to Mn²⁺ and O^1?, with the tenth electron of O2? being distributed in a wider region. In the paramagnetic state, there is an electronic Mn-Mn bond which seems to be formed due to coupling with the tenth electron of O^2? and builds up a three-dimensional net of the charge density with the “cages” surrounding oxygen atoms. In the antiferromagnetic state, some Mn-Mn bonds disappear, while the preserved nonspherical ionic features enhance the role of oxygen atoms in electronic coupling.     

Structural and physical properties of cobalt nanocluster composite glasses

Composite systems with metallic nanoparticles embedded in dielectrics present peculiar physical properties which are attractive in several application fields. In the case of transition elements, the magnetic properties of the metal clusters embedded in a dielectric matrix mainly depend on the particle size and structure. In this work, silica films containing cobalt atoms were synthesized by RF magnetron co-sputtering deposition technique, with cobalt concentration of a few atomic percent. X-ray diffraction and transmission electron microscopy showed the presence of hcp cobalt nanoclusters in the as-deposited sample with the highest cobalt concentration. After deposition, thermal treatments were performed to promote cobalt compounds or nanoparticle formation. The thermal treatments were able to change the oxidation state of cobalt atoms, as well as the structure of metallic cobalt nanoclusters (from hcp to fcc), their final size depending on both the preparation parameters and the subsequent annealing atmospheres.     

The neutron diffraction study of liquid GeTe and As2Te3

The radial distribution analyses for GeTe and As₂Te₃ are made at temperatures above the melting point in the range of momentum transfer between 0.7 and 10.0 Å^?1 by the neutron diffraction technique. Furthermore, the local order in amorphous GeTe is determined by analyzing the intensity data of the electron diffraction of its thin film. The result for the amorphous film indicates that the local distribution of atoms in amorphous GeTe is not characteristic of the structure of its crystalline state. The shape of the peaks of the intensity curve for liquid GeTe differs from that for the amorphous and crystalline states. However, the short bond length and the small coordination number determined from liquid RDF suggest that the covalent-like bonding between nearest-neighbor atoms remains unbroken when melting. The general form of the structure factor for liquid As₂Te₃ is similar to that for the amorphous material reported previously. The position of the first peak of RDF in the liquid state is observed to be shifted to a shorter distance than the average of nearest-neighbor atoms in crystalline As₂Te₃. The structure of GeTe differs considerably between the crystalline, amorphous and liquid states, whereas the local order in the liquid As₂Te₃ is similar to that in the amorphous state but not in the crystalline state.