Synthesis of a bulk crystalline phase of carbon nitride

A crystalline phase, whose lattice parameters are close to those of theoretically predicted carbon nitride, is synthesized by a high-pressure technique from a precursor containing carbon, nitrogen, and hydrogen. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 466–468 (25 September 1998)     

Homogeneous bulk, surface, and edge nucleation in crystalline nanodroplets

The birth of a crystal is initiated by a nucleus from which the crystal grows--a dust grain in a snowflake is a familiar example. These nuclei can be heterogeneous defects, like the dust grain, or homogeneous nuclei which are intrinsic to the material. Here we study homogeneous nucleation in nanoscale polymer droplets on a substrate which itself can be crystalline or amorphous. We observe a large difference in the nucleating ability of the substrate. Furthermore, the scaling dependence of nucleation on the size of the droplets proves that the birth of the crystalline state can be directed to originate predominantly within the bulk, at the substrate surface, or at the droplets' edge, depending on how we tune the substrate.     

Cation disordering in Ag2HgI4 and Cu2HgI4

The cation order-disorder transitions in Ag₂HgI₄ and Cu₂HgI₄ are first order. This is unusual, since in other superionic conductors the cation disordering is gradual with temperature if there is no structural phase transition. These two materials are also unique in that they have two disordering cations rather than one. A study of a two species lattice gas model shows that this extra degree of freedom is responsible for the first order nature of this transition on the fcc lattice.     

Phases and phase transformations in nanocrystalline ZrO2

Starting from results from He-pycnometry, electron diffraction, Extended X-ray Absorption Fine Structure Spectroscopy and Perturbed Angular Correlation Spectroscopy the phase transformations and structures of zirconia are described. From a comparison of these results with those obtained on other oxide nanoparticles it is concluded that the phases and structure of nanoparticles are different compared to those of coarse-grained material. The difference of the transformation temperature of bare and coated nanoparticles was used to estimate enthalpy and entropy of the tetragonal → monoclinic transformation for nanoparticulate zirconia. By comparison with results obtained from other nanocrystalline oxides, the following rules were derived: Provided the particles are sufficiently small, particles made of materials showing phase transitions crystallize in the high temperature structure. However, compared to coarse-grained materials of the same structure, the density of nanoparticles is reduced. A first estimation limits this phenomenon to particle sizes well below 10 nm. Those nanoparticles follow the generalized phase diagram postulated by Tammann.     

Raman and optical absorption spectroscopic study of the thermochromic phase transition of Ag2HgI4.

The thermochromic transition of Ag₂HgI₄ has been studied by optical absorption and Raman spectroscopy. In the absorption spectra, the shift of the absorption edge towards lower energies at high temperatures is responsible for the yellow to red colour change of Ag₂HgI>₄; this absorption edge is associated with a charge transfer process between electronic levels of Hg and I. The resulting hysteresis loop is similar to those found by reflectance measurements in Cu₂HgI₄. The Raman spectra show hysteresis loops for the position and half width of the ν₁(HgI₄²⁻, Td). There is a different behaviour between the first and subsequent heating cycles. The observed changes in the phase transition can be explained by assuming an increase of the interaction between the I and Ag atoms above the phase transition, and a consequent decrease of the IHg interaction. These changes would be induced by a decrease of the AgI distance above the phase transition, as a consequence of a structural distortion; consequently the HgI distance would increase and lead to the observed changes in the absorption spectrum.     

Visualization of inverse phase velocity surfaces of bulk and surface acoustic waves

Diffraction of light by ultrasound has been used to study complete cuts of the inverse phase velocity surfaces for both bulk and surface acoustic waves in lithium niobate.     

Formation of surface roughness on nanocrystalline aluminium samples under straining by molecular dynamics studies

The surface roughening of nanocrystalline aluminium samples was investigated by molecular dynamics simulations. Attention was focused on the fact that roughness increases with the grain size and the strain. The elastic–plastic transition was found at around 3.5% strain and a reverse Hall–Petch effect was observed under straining conditions. Then, different strain distributions in grains and grain boundaries at the sample surface were highlighted, yielding to the formation of local roughness. Finally, a linear relationship between the magnitude of roughness and the out-of-plane strain component was found.     

Magnetism on the surface of the bulk paramagnetic intermetallic compound YCo2

Using full-potential electronic structure calculations, we predict that the (111) surface of the cubic Laves phase Pauli paramagnet YCo2 is ferromagnetic. The magnetism of the (111) surface is independent of the termination of the surface, does not extend beyond two Co layers, and is related to the field-induced metamagnetism of the bulk. YCo2 appears to be a prominent candidate to demonstrate the phenomenon of surface-induced itinerant magnetism localized in two dimensions.     

Size-quantization effects in the optical spectra of PbI2 and HgI2 nanocrystals

Size-quantization effects (short-wavelength shift of the structure and broadening) in low-temperature absorption spectra of PbI₂ and HgI₂ nanocrystals embedded in porous sodium borosilicate glass matrix with pores ranging in size from 2 to 50 nm have been observed and studied. The nanocrystal size was evaluated in the strong quantization approximation. It is shown that the size does not depend on the pore diameter of the matrices used, but is determined by the concentration of the solution introduced into the pores. The absorption and luminescence spectra of the systems thus obtained are shown to evolve as the substance emerges from the pores out onto the surface. The dynamics of formation and the phase composition of the PbI₂ and HgI₂ crystals on the surface are studied. Fiz. Tverd. Tela (St. Petersburg) 39, 468–473 (March 1997)     

Influence of Ni addition on the process of phase formation in MgB2 bulk

Nickel is an important sheath material for the fabrication of MgB₂ wires. However, the effects of Ni doping on the phase formation and superconducting properties of MgB₂ remain controversial. In this work, Ni powder is selected for doping in MgB₂ bulk in order to examine the corresponding changes. Combining with the DSC analysis and in-situ XRD results, we find indications that the Ni powder reacted with Mg and B, forming MgNi_2.5B₂ at 600°C. The ternary compound began to decompose at a temperature above 800°C. The reactive phase, MgNi_2.5B₂, acted as an obstacle to the supercurrent flow, creating weak links among the MgB₂ grain boundaries. However, it is found that the added Ni formed a eutectic liquid phase with Mg at 506°C. The liquid phase helps the formation of MgB₂ at low temperature, which not only increases the density of the sample, but also improves the grain connectivity. Consequently, the presence of Ni in the MgB₂ sample is not necessarily a disadvantage; it depends on the desired application.     

Influence of surface on the nanocrystalline structure formation

In contrast to the common Fe-B-base amorphous alloys, Mössbauer transmission and CEMS studies of the early stages of the nanocrystalline structure formation in the new magnetically soft FeCuNbSiB alloys produced via the amorphous way, revealed that the primary crystallization of the Fe₃Si-like phase is inhibited by the surface. This result supports the role of copper as nucleation agent in the amorphous structure.     

Effects of bulk charged impurities on the bulk and surface transport in three-dimensional topological insulators

In the three-dimensional topological insulator (TI), the physics of doped semiconductors exists literally side-by-side with the physics of ultrarelativistic Dirac fermions. This unusual pairing creates a novel playground for studying the interplay between disorder and electronic transport. In this mini-review, we focus on the disorder caused by the three-dimensionally distributed charged impurities that are ubiquitous in TIs, and we outline the effects it has on both the bulk and surface transport in TIs. We present self-consistent theories for Coulomb screening both in the bulk and at the surface, discuss the magnitude of the disorder potential in each case, and present results for the conductivity. In the bulk, where the band gap leads to thermally activated transport, we show how disorder leads to a smaller-than-expected activation energy that gives way to variable-range hopping at low temperatures. We confirm this enhanced conductivity with numerical simulations that also allow us to explore different degrees of impurity compensation. For the surface, where the TI has gapless Dirac modes, we present a theory of disorder and screening of deep impurities, and we calculate the corresponding zero-temperature conductivity. We also comment on the growth of the disorder potential in passing from the surface of the TI into the bulk. Finally, we discuss how the presence of a gap at the Dirac point, introduced by some source of time-reversal symmetry breaking, affects the disorder potential at the surface and the mid-gap density of states.     

Amorphous,nanocrystalline and crystalline calcium carbonates in biological materials

Raman spectroscopy is a powerful tool in identifying different calcium carbonate polymorphs. Here, the method is applied to cultured pearls from freshwater (genus Hyriopsis) and marine bivalve species (Pinctada maxima) as well as to shells of Diplodon chilensis patagonicus bivalves. Raman spectra for vaterite, detected for the first time in an adult shell, and amorphous calcium carbonate (ACC) are discussed. Results for ACC are compared with those of synthetically produced ACC and with the Raman spectroscopic features of stable biogenic ACC from the crustacean Porcellio scaber. Decomposition of the most intense signal of all calcium carbonate polymorphs—the ν₁ symmetric stretching mode of the carbonate ion—leads to the identification of two polymorphs within the ACC areas: a mixure of an amorphous and a crystalline fraction. The amorphous phase is characterised by a broad peak in the region of the lattice modes, which is composed of two distinct lattice modes with very high full‐widths at half‐maximum (FWHMs). The FWHMs of most of the crystalline fractions (in the range of 6.3–10.7 cm⁻¹) are too high for well‐crystallised materials and support reports of nanocrystalline calcium carbonate polymorph clusters in ACC. Crystallinity indices of different samples are calculated and found to be useful to describe roughly the state of crystallisation in the ACC areas. Copyright © 2010 John Wiley & Sons, Ltd.     

Grain-boundary dislocations and enhanced diffusion in nanocrystalline bulk materials and films

A theoretical model is suggested which describes the transformations of grain-boundary dislocation walls and their influence on diffusion processes in nanocrystalline materials fabricated under highly non-equilibrium conditions. It is shown that the decay of boundary dislocation walls of finite extent, occurring via the climb of boundary dislocations and the corresponding emission of vacancies, is capable of highly enhancing the grain-boundary diffusion in nanocrystalline materials. The enhanced diffusion, in turn, strongly affects the deformation behaviour of nanocrystalline materials. In the case of nanocrystalline films deposited on to substrates, the effects of misfit stresses on the transformations of boundary dislocation walls and the diffusion are analysed. It is demonstrated that the mean diffusion coefficient in a nanocrystalline film may increase by approximately several orders of magnitude owing to misfit stresses.     

Acousto-optical effect in the luminescence excitation spectrum of HgI2

A study of the photoluminescence excitation spectrum in a crystal of mercury diiodide is reported. Each of the two luminescence bands peaking at 543 and 572–575 nm investigated was found to have its own excitation spectrum. The excitation spectrum of the 575-nm line in the long-wavelength doublet band has been observed to be sensitive to strong ultrasound vibrations and preliminary irradiation of the sample by 590-nm light. This line is associated with radiative recombination of photocarriers at intrinsic point defects (vacancies or iodine and mercury interstitials) located close to dislocations. The binding energy of the photosensitive center to a dislocation was estimated from the change in the excitation spectrum. Fiz. Tverd. Tela (St. Petersburg) 41, 1965–1968 (November 1999)     

Magnetic states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe₁₂O₁₉) are performed in the phase transition region around the Curie temperature (T _c). The method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe₁₂O₁₉ the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below T _c. It is established that the transition temperature T _c(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value T _c at the lower boundary of the “critical” surface layer. Therefore, near T _c a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Néel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.