Structure and composition of cleaved and heat-treated tenfold surfaces of decagonal Al-Ni-Co quasicrystals

We investigated cleavage surfaces perpendicular to the tenfold direction of as-grown decagonal Al-Ni-Co quasicrystals by scanning tunneling microscopy, Auger electron spectroscopy, and scanning electron microscopy. The cleavage surface is determined by a cluster-subcluster structure. The image contrast of the smallest features, 1-2 nm in diameter, is related to the columnar atom arrangements extending perpendicular to the cleavage plane, which are predicted by current models of the decagonal quasicrystal structure. No voltage dependence of the STM images is observed. The presence of surface states and an enhanced density of states are discussed. Heat-treatments of the cleaved Al-Ni-Co quasicrystal surfaces show nearly no changes in chemical composition and structure up to about 750 °C. This is correlated with a much lower concentration of vacancies in as-grown decagonal Al-Ni-Co quasicrystals as compared to that in as-grown icosahedral Al-Pd-Mn quasicrystals.     

Crossover between a displacive and an order-disorder phase transition

The phase transition in a three-dimensional array of classical anharmonic oscillators with harmonic nearest-neighbor coupling (discrete straight phi(4) model) is studied by Monte Carlo (MC) simulations and by analytical methods. The model allows us to choose a single dimensionless parameter a determining completely the behavior of the system. Changing a from 0 to +infinity allows to go continuously from the displacive to the order-disorder limit. We calculate the transition temperature T(c) and the temperature dependence of the order parameter down to T=0 for a wide range of the parameter a. The T(c) from MC calculations shows an excellent agreement with the known asymptotic values for small and large a. The obtained MC results are further compared with predictions of the mean-field and independent-mode approximations as well as with predictions of our own approximation scheme. In this approximation, we introduce an auxiliary system, which yields approximately the same temperature behavior of the order parameter, but allows the decoupling of the phonon modes. Our approximation gives the value of T(c) within an error of 5% and satisfactorily describes the temperature dependence of the order parameter for all values of a.     

Importance of annealing on the order-disorder phase transition in alkali cyanides

Abstract

Contrary to monocrystals powdered KCN displays, after grinding and without annealing, an intermediate monoclinic phase between the high-temperature pseudocubic and the low-temperature orthorhombic phases. The consequence of annealing is the total disappearance of this intermediate phase. A further effect of the annealing process is the reduction of the cubic distortion at the transition. In this paper, we analyse this effect on the intermediate rhombohedral phase observed in the (KCN) _x (KBr)_1?x mixed system.     

Solid-phase reactions and the order-disorder phase transition in thin films

A comparative analysis was carried out of the initiation temperatures of solid-phase reactions in bilayer solid films and the Kurnakov temperatures of the phases forming in the reaction products. It has been shown that in superstructures where ordering is usually observed, the Kurnakov temperature coincides with the initiation temperature of the solid-phase reactions if no other solid-phase structural transformation precedes the order-disorder phase transition in the state diagram. A rule was proposed by which pairs of films capable of entering into solid-phase reactions and their initiation temperatures can be determined.     

Specific features of the strain-induced order-disorder phase transition and its mechanisms

In the present paper, the methods of x-ray diffractometry are used to investigate the influence of plastic deformation on the long-range atomic order in alloys with L1₂, L1₂(M), and L1₂(MM) superlattices. The influence of ordering energy, initial values of the long-range order parameter, and average sizes of antiphase domains on the specific features of the dependence of long-range order parameter on the strain degree is considered. It is demonstrated that the wetted strain-induced order-disorder phase transition occurs with rapid accumulation of antiphase boundaries that play an important role in the destruction of long-range atomic order.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 21–32, September, 2004.     

Special features of the deformation-induced order-disorder phase transition and its modeling

Methods of x-ray diffractometry are used to investigate the influence of plastic deformation on the state of the long-range order in alloys with the L1₂, L1₂(M), and L1₂(MM) superstructure. It is demonstrated that the heterogeneous order-disorder phase transition accompanied by intensive accumulation of antiphase boundaries occurs under plastic deformation. The antiphase boundaries play an important role in the destruction of long-range order. Based on the available experimental results, a model of destruction of long-range order is constructed that takes into account the dislocation mechanisms of accumulation of antiphase boundaries under deformation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 9–24, January, 2006.     

Interface of transition metal oxides at the atomic scale

Remarkable phenomena arise at well-defined heterostructures, composed of transition metal oxides, which is absent in the bulk counterpart, providing us a paradigm for exploring the various electron correlation effects. The functional properties of such heterostructures have attracted much attention in the microelectronic and renewable energy fields. Exotic and unexpected states of matter could arise from the reconstruction and coupling among lattice, charge, orbital and spin at the interfaces. Aberration-corrected scanning transmission electron microscopy (STEM) is a powerful tool to visualize the lattice structure and electronic structure at the atomic scale. In the present study some novel phenomena of oxide heterostructures at the atomic scale are summarized and pointed out from the perspective of electron microscopy.     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

An example of a phase transition in an open pseudobimolecular system

A phase transition at microscopic level is exhibited for an open bimolecular chemical system. We also give another interpretation of an apparent incompatibility between the microscopic and the macroscopic analysis shown by J. Keizer.     

Electromagnetic coupling on an atomic scale

Light emission from a scanning tunneling microscope is used to investigate the electromagnetic coupling (EMC) of a metal tip and a metal sample. Subatomic scale modifications of the tip-sample region cause spectral shifts of the fluorescence as demonstrated for a monatomic step and by variation of the tip-sample distance. For sharp tips the EMC is confined to a lateral range of a few nm. The results are consistent with model calculations of the electromagnetic response of an appropriate tip-sample geometry.     

Plastic analysis of the crack problem in two-dimensional decagonal Al-Ni-Co quasicrystalline materials of point group 10,10

The fundamental plastic nature of the quasicrystalline materials remains an open problem due to its essential complicacy. By developing the proposed generalized cohesive force model, the plastic deformation of crack in point group 10, 10 decagonal quasicrystals is analysed strictly and systematically. The crack tip opening displacement （CTOD） and the size of the plastic zone around the crack tip are determined exactly. The quantity of the crack tip opening displacement can be used as a parameter of nonlinear fracture mechanics of quasicrystalline material. In addition, the present work may provide a way for the plastic analysis of quasicrystals.     

原子尺度上的异质催化

现实中的催化剂是个相当复杂的系统体系,且为粉末状,限制了多种表面科学表征手段的应用.科学家通过建立简化的催化剂模型,充分利用目前多种有力的科技分析手段,如扫描隧道显微镜(STM)、透射电镜(TEM)、光电子能谱(XPS)、傅里叶变换红外吸收谱(FTIR)、电子能量损失谱(EELS)等,直观地在原子尺度上研究催化反应的机理,从而使得人们能够设计出选择性能更高、催化性能更好的催化剂.     

原子尺度上的异质催化

现实中的催化剂是个相当复杂的系统体系，且为粉末状，限制了多种表面科学表征手段的应用,科学家通过建立简化的催化剂模型，充分利用目前多种有力的科技分析手段，如扫描隧道显微镜（STM）、透射电镜（TEM）、光电子能谱（XPS）、傅里叶变换红外吸收谱（FTIR）、电子能量损失谱（EELS）等，直观地在原子尺度上研究催化反应的机理，从而使得人们能够设计出选择性能更高、催化性能更好的催化剂．