Study of the dynamic structure factor of strong glasses

The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm⁻¹. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q² dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.     

Slow dynamic properties of molten silver halides

The sound absorption has been measured for molten AgCl–AgI mixtures to investigate the relation between the dynamic properties and chemical bonding. The sound absorption for molten AgI shows a large value just above the melting temperature, and decreases with increasing temperature. The sound absorption for molten mixtures around the eutectic composition increases with temperature from the melting point, showing maximum value around 580 °C, and decreases with increasing temperature. The frequency dependence of the sound absorption for molten AgI and mixtures are also measured, and the relaxation time and relaxation strength are evaluated from the observed data by assuming single Debye-type relaxation process. The evaluated relaxation time for molten AgI is about 9.0 ns just above melting temperature and decreases slightly with temperature to about 6.4 ns at 750 °C. This relaxation time corresponds to a quite slow motion compared with the ordinary atomic one, and may suggest a structural relaxation of atomic association between ionic and covalent character. In the mixture around the eutectic region, evaluated relaxation time shows a rather complicated temperature dependence. Those values decreases slightly with temperature from the melting temperature, and have a minimum value around 600 °C, and then slightly increase with the temperature.     

Spectroscopy of transition metal ions in inorganic glasses

Transition metal (TM) ions have been used as colouring agents in the glass industry for a long time. Recently, great attention has been paid to the TM ion doped glasses for the development of new lasers or luminescence materials. The absorption spectra of TM ions in different kinds of glasses have been studied extensively, but little work has been done for fluorescence and relaxation spectra. In this paper emphasis is laid on analysing the influence of chemical bond characteristics of the base glass on the spectra and the site structure of transition metal ions in glasses. Recent experimental results about the luminescence characteristics of low valence ions (such as Ti³⁺, Cr³⁺, V²⁺, Mn²⁺, Cu⁺, Mo³⁺) in glasses are also reported.     

Acidity and ionic structure of molten alkali silicates

An attempt has been made to identify the structural elements in binary lithium, sodium, potassium and cesium silicate melts by voltammetry. In the lithium melts g-SiO₂ or quartz-like clusters are proposed, although other interpretations are possible. Further, a second voltammetric wave may be an indication of peroxo bonds in lithium melts. In sodium melts, anions derived from cristobalite have been confirmed by comparison with information from Raman spectra, as have the tridymite derivatives in potassium melts. The structural elements in cesium silicate melts resemble those of potassium. The influence of the alkali ion on the acidity of the melt is calculated and is found to increase in the order Cs < K < Na < Li.     

The influence of polyvalent metal cations on the corrosion rate of molybdenum in molten glass

The corrosion behavior of molybdenum in a silicate melt has been studied using electrochemical techniques. The aim was to examine whether or not polyvalent metal ions such as iron (Fe³⁺), nickel (Ni²⁺) and cobalt (Co²⁺) are able to influence the corrosion behavior of molybdenum and, if so, to which extent. In addition the impact of the temperature of the melt on the corrosion rate has been addressed.     

Electric field gradient in transition metal amorphous alloys

The mean atomic configurations in Hf- and Zr-based amorphous alloys prepared by different routes were studied using the perturbed angular correlations (PAC) technique. For all the as-prepared amorphous samples, the PAC spectra are well described by a wide distribution of frequencies revealing atomic configurations around the probes compatible with those in a dense random packing of ions (DRP). The measured electric field gradient strength V_zz seems to be independent of the preparation method and of the addition of second phase nanoparticles (CaO and ZrC) for the same early transition metal content. The V_zz values observed in Hf based alloys are systematically higher than that in Zr-based alloys of similar composition and show a greater variation with composition. These behaviors are discussed in terms of the different contributions to the EFG, its dependence with the density of states at the Fermi level and in comparison with similar trends observed in binary amorphous alloys. Also, the Ta probe role on the measured V_zz in Hf and Zr systems was analyzed.     

A possibility of the photo-induced semiconductor–metal transition in liquid selenium

We have performed transient absorption measurements of liquid selenium after the illumination of a pulsed laser to clarify the possibility of the photo-induced semiconductor–metal transition. The intensity of the laser was increased up to 23 mJ/pulse and the temperature of the sample was increased up to 600 °C. By increasing the laser intensity and temperature of the sample, quite a large photo-induced change has been observed, which suggests the semiconductor–metal transition.     

NMR investigation of rapid structural changes and non-metal-to-metal transition in the molten eutectic Ge–Te alloy

Resonance shift, K¹²⁵, spin-lattice relaxation time, T₁ and spin–spin relaxation time, T₂ for ¹²⁵Te nuclei in molten Ge₁₅Te₈₅ and Te in deeply under-cooled state have been measured as a function of temperature. The temperature dependence of K¹²⁵ is compared with that of the extent of structural change, C, and it is shown that the rapid increase of K¹²⁵ with increasing temperature can be attributed to an increase of the density of states at the Fermi level proportional to the extent of structural change. At high temperatures, the dependences on the temperature of K¹²⁵ and T₁ conform to the Korringa relation with an enhancement factor or the relaxation mechanism expected metallic nature. At low temperatures, in contrast, the Redfield theory assuming localized spins of which the density is proportional to C can explain the overall temperature dependences of T₁ and T₂. These results provide the first clear evidence for a crossover transition from extended to localized states in the electron system accompanying the rapid structural changes.     

Correlations between electronic structure of transition metal atoms and performance of high-k gate dielectrics in advanced Si devices

This paper develops a classification scheme for non-crystalline dielectrics that separates them into three groups with different amorphous morphologies, and identifies a linear scaling relationship between average bond ionicity and oxygen atom coordination. The classification scheme is applied to transition metal silicate and aluminate alloys and provides a structural model for molecular orbital, MO, calculations that are based on the coordination and symmetry of transition metal atoms and the orbital energies of their oxygen neighbors. The MO calculations show that conduction band offset energies with respect to Si scale inversely with the energy difference between transition metal atomic n+1 s- and n d-states providing an important insight into the choice of alternative gate dielectrics for advanced Si devices.     

Crystal structure and infrared and electronic spectra of some divalent first series transition metal pentacyanonitrosylmanganates (I)

Structural and infrared and electronic spectroscopic studies of the coordination polymers corresponding to the compounds of simplest formulas M ₃ ^II [Mn(CN)₅NO]₂·nH₂O (M=Mn, Co, Ni, Cu, Zn, andn=8–16) are reported and discussed in comparison with the corresponding nitroprussides. These polymers have a typical face centered cubic unit cell containing 1 1/3 formula units, except for M=Mn where the crystal system is orthorhombic.     

Structural instability and the metal–non-metal transition in expanded fluid metals

X-ray diffraction, small angle X-ray scattering and inelastic X-ray scattering measurements for fluid Hg in the wide density range up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the static and dynamic structure factors as a function of temperature, pressure and density. Based on the results we found that there exist medium-range fluctuations accompanying the metal–non-metal transition in fluid Hg. The scale of the fluctuations was about 10 A in space and sub-picoseconds in time. We discuss the possibility that such structural instability induces the metal–non-metal transition in expanded fluid Hg.     

Specific features of the crystallization of melts in systems with a transition from syntectic equilibrium to monotectic equilibrium

A technology has been proposed for implementing the computer design of phase diagrams and their investigation with the kinematic specification of surfaces by using the T-x-y diagram with a monovariant syntectic equilibrium and a solid-phase solubility as an example. The specific features of the crystallization upon transition from syntectic equilibrium to monotectic equilibrium have been considered, and surfaces with a sign reversal of the mass increment and material balance for a three-phase region have been constructed.     

On the structure of the molten mixture Ag(I0.3Br0.7)

We present neutron diffraction measurements of the total structure factor of the molten salt mixture Ag(I_0.3Br_0.7) at 650 °C, measured by using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the spallation neutron source of the Neutron Science Laboratory in the High Energy Accelerator Research Organization of Japan. We have compared the experimental data with molecular dynamics simulations using Vashishta–Rahman type potentials. Within the known constraints of pairwise rigid ion potentials, the results of the simulations are in fair agreement with experiment. We also present results using a linear combination of the experimental and molecular dynamics simulations of the structure factors of the AgI and AgBr melts. The latter are in somewhat better agreement with experiment.     

Crystal structure of calcioburbankite and the characteristic features of the burbankite structure type

The crystal structure of calcioburbankite (Na,Ca)₃(Ca,RE,Sr,Ba)₃(CO₃)₅ found in carbonatites from Vuoriyarvi (North Kareliya) was solved by the Rietveld method. The experimental data were collected on an ADP-2 diffractometer (λCuK _α radiation; Ni filter; 16.00° < 2θ < 130.00°; the number of (α₁ + α₂) reflections was 455). All the calculations were performed within the sp. gr. P6₃ mc; a = 10.4974(1) Å, c = 6.4309(1) Å, V = 613.72(1) ų; R _wp = 2.49%. The structure was refined with the use of the anisotropic thermal parameters for the (Na,Ca) and (Sr,Ba,Ce) cations. The comparison of the crystal structures of all of the known hexagonal representatives of the burbankite family demonstrates that the burbankite structure type (sp. gr. P6₃ mc) is stable, irrespectively of the occupancy of the ten-vertex polyhedra predominantly with Ca, Sr, or Ba cations and the occupancies of the positions in the eight-vertex polyhedra.     

Structure and properties of transition metal complexes of naphthoquinonedithiolate

Naphthoquinonedithiolate (nqdt) is a noninnocent ligand forming dithiolene transition metal complexes. Most [M(nqdt)₂]^–1 and [M(nqdt)₂]^–2 complex ions are planar with each forming an isomorphous series with the (n-butyl)₄N⁺ (tba) counter ion. The cobalt complex is isolated as a step dimer (tba)₂[Co(nqdt)2]₂ where two square-planar monomer units are bonded via Co–S intermolecular interactions.Ab initio andsemi-empirical calculations are used to investigate the complex ions. The complexes form tba single crystals only with difficulty, and an extensive study with ions of various shapes and charges failed to identify a more satisfactory counter ion. The magnetic susceptibilities of four complexes are discussed. The (tba)[Ni(nqdt)₂] complex undergoes a weak ferromagnetic transition at 4.43 K while a mixed oxidation state Mn(II) complex exhibits a weak ferromagnetic transition at 40K. The crystal structures of (tba) [Cu(nqdt)₂], (tba)[Ni(nqdt)₂], (tba)₂[Ni(nqdt)₂], (tba)₂[Pd(nqdt)₂], {(tba)[Co(nqdt)₂]}₂ and four side reaction products 5,7,12,14-tetrahydrodibenzo-[b,i]thianthrene-5,7,12,14-tetraone (2), benzocyclohexa-2,5-diene-1,4-dione-1,3-thiole-2-thione (3), a thiomethyl derivative (4), and an unusual oxidatively coupled product (5) are reported.     

Phase transition and domain structure in iodoform crystals

Phase transition in an iodoform crystal has been established from the temperature dependence of the Raman spectra of “external” crystal vibrations at 260 K. The transformation mechanism is interpreted under the assumption on the existence in the crystal of domains where dipole moments of the molecules have parallel or antiparallel orientations. Below the phase-transition temperature, the structure is more ordered, and the crystal consists mainly of domains with the parallel orientation of dipole moments of the molecules, whereas above this point, the domains with the antiparallel orientation of dipole moments prevail. The crystal as a whole is not a pyroelectric.     

Recent progress in chemical vapor deposition growth of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have received significant attention recently due to their unique properties such as a transition from indirect to direct band gap when thinned down to a monolayer and also valley-dependent photoluminescence. In addition, being a semiconductor with considerable mobility, it has been touted as a candidate in next generation electronics. However, a major hurdle to its implementation is the difficulty in producing large areas of these 2D TMDCs with well-defined thicknesses. In this review, we will first introduce the basic properties as well as the various synthesis methods of 2D TMDCs. Focus will be placed on recent advances in chemical vapor deposition (CVD) growth as they currently yield the largest areas. Obstacles present in CVD growth will be presented and existing solutions to them will be discussed in tandem with current characterization methods for evaluation of crystal quality. Through our presentation on the latest approaches to issues in CVD growth, we hope to present the readers a perspective on recent developments as well as providing an outlook on the future of CVD growth of TMDCs.     

Optical and magnetic properties of first-row transition metal ions in lead-silicate glass

First-row transition metal ions in lead-silicate glasses of composition: 37.9% mol PbO, 61.8% mol SiO₂ have been studied with regard to the oxidation state and the coordination by means of optical and magnetic measurements. Optical spectra have been recorded at 300 and 77 K. All the observed bands are assigned as being due to d-d transitions in T_d and/or O_h fields. Magnetic moments are of the order of the spin-only values. By ESR measurements we reveal and quantify titanium, vanadium and manganese oxidation states not detectable by the other techniques.It turns out that the various metal ions have the following oxidation states: Ti³⁺ and Ti⁴⁺, V⁴⁺ and V⁵⁺, Cr³⁺ and Cr⁶⁺, Mn²⁺ and Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺. The coordination is 4-fold for Ti⁴⁺ and V⁵⁺ and 6-fold for Mn³⁺ and Cu²⁺. Moreover it has been found that Fe³⁺, Co²⁺ and Ni²⁺ are both 4-fold and 6-fold coordinated and that Cr³⁺ and Cr⁶⁺ oxidation states coexist in the same sample.     

Characteristic features of structure formation of nickel monoaluminide formed in a gasless combustion wave

The dynamics of structural transformations in the gasless combustion wave is studied. It is shown that crystalline NiAl starts forming behind the wave front at the first milliseconds of the process. The attained time resolution on the order of 5 ms provides an opportunity to extract fundamentally new data on the phase and structural tranformations in the course of a fast high-temperature reaction.