Detecting quasi-periodic {11<Emphasis Type="Italic">n</Emphasis>} (<Emphasis Type="Italic">n</Emphasis> = 7–11) faces in samples with Ge/Si quantum dots by grazing X-ray reflectometry

High-resolution grazing X-ray reflectometry is used to obtain experimental and theoretical data on the intensity of specular and diffuse reflection from self-organized structures grown by molecular beam epitaxy with single-layer (non-overgrown) and multilayer (overgrown) Ge/Si quantum dots (QDs). Using the positions of diffuse scattering peaks in the direct space, the slopes of quasi-periodic faces have been determined to within ±0.1° by a method employed previously for the investigation of In(Ga)As/GaAs quantum dots. Finding the quasi-periodic {11n} (n = 7−11) faces (typical of the growth of ordered QDs) in the samples with disordered Ge/Si quantum dots is evidence of the generality of mechanisms of QD formation in different systems.     

Superconducting and Charge Ordering Phases of Two-Dimensional Organic Conductors

In order to investigate relationships between superconducting phase and charge ordered one, the uniaxial strain method is applied to the superconducting salt α-(BEDT-TTF)₂NH₄Hg(SCN)₄ and the organic conductor α-(BEDT-TTF)₂CsCd(SCN)₄. Both salts preserve metallic states under a-axial strain and show insulating behaviours under c-axial strain. These results suggest that i) c-axial strain controls the electronic properties of the α-type salts, and ii) the insulating state in the vicinity of the superconducting phase of α-(BEDT-TTF)₂NH₄Hg(SCN)₄ is a charge ordered phase, as expected from the comparison in the lattice parameters with the charge ordered salt θ-(BEDT-TTF)₂RbZn(SCN)₄.     

The ubiquitous icosahedral B12 in boron chemistry

Though boranes exhibit a wide variety of polyhedral structures, all the three polymorphs of elemental boron essentially contain icosahedral B₁₂ units as the predominant building block in their unit cell. Theoretical and experimental studies on boranes show that the icosahedral arrangement leads to most stable boranes and borane anions. This paper attempts to explain the phenomenal stability associated with the icosahedral B₁₂ structure. Using fragment molecular orbital theory, the remarkable stability of B₁₂H ₁₂ ²⁻ amongcloso boranes are explained. The preferential selection icosahedral B₁₂ unit by elemental boron is explained by improvising a contrived B84 sub-unit of the β-rhombohedron, the most stable polymorph. This also leads to a novel covalent way of stuffing fullerenes with icosahedral symmetry.     

Electron diffraction and HREM image characteristics from quasicrystalline phases in alloys of Al-Mn and Al-Mn-Si

An investigation of the quasicrystalline phases in rapidly cooled alloys of Al₈₆Mn₁₄ and Al₇₄Mn₂₀Si₆ is carried out. The Al-Mn-Si alloy shows three different kinds of phase, the icosahedral phase, the crystalline α-type phase and a third phase which resembles the decagonal T phase found in alloys of Al-Mn. Both kind of compounds (Al-Mn and Al-Mn-Si) have icosahedral diffraction patterns along similar zone axes whose lowest order diffracted vectors are different in magnitude. In the past, these extra rings of reflections were associated with the presence of superstructures. The HREM images obtained along the five-fold axis display image contrast features of similar characteristics when the specimen thickness is increased. These kind of images also show high atomic density features which are curved and modulated in intensity. These features can be seen along the five-fold, three-fold and two-fold axis. Diffraction patterns along the main icosahedral axis give rise to spots with characteristic morphology. A qualitative insight on the nature of these effects can be obtained from simulated images of density waves in a field of phasons and phonons. The Fourier transform of these images can also give a qualitative understanding of some features in the experimental diffraction patterns.     

Strengthening in magnesium alloys by icosahedral phase

Strengthening effects of quasicrystalline icosahedral phase has been studied in two alloys Mg₉₅Zn_4.2Y_0.8 and Mg_92.5Zn_6.5Y extruded at 250 and 400 °C. The quasicrystal particles are facetted and show definite orientation relationships with the matrix. Due to its high symmetry and quasiperiodicity, the icosahedral phase can form strong interfaces with the matrix in various orientations. The icosahedral phase particles have a strong pinning effect on the grain boundaries, which stabilizes grain size. The icosahedral particles are resistant to coarsening, and remain hard at higher temperatures, imparting good strength with ductility at 200 °C. Very few deformation structures such as high dislocation density and twins are observed after extrusion or tensile tests. Dislocations commonly observed are c-type. Due to the stability of microstructure, various post-extrusion treatments are possible. In the Mg_92.5Zn_6.5Y alloy upon annealing at 400 °C the icosahedral phase transforms to a hexagonal Mg₂₅Zn₅₈Y₁₇ phase. The icosahedral phase then reprecipitates on its interface, forming a nano-composite. Effects of microstructural features on the deformation behavior are described.     

Phase formation in rapidly quenched Cu-based alloys

In the present investigation, we report the formation of γ-brass type phase in the rapidly quenched Cu₅₀Ga₃₀Mg₅Ti₁₅ and Cu₅₀Al₃₀Mg₅Ti₁₅ alloys. Rapid solidification of Cu₅₀Ga₃₀Mg₅Ti₁₅ alloy shows the formation of simple cubic γ-brass type phase (a = 0.863 nm), which on annealing at 1,023 K for 60 h transforms to disordered type γ-brass phase (a = 0.879 nm). It has been observed that intensity modulation of electron diffraction spots corresponding to simple cubic γ-brass phase is similar to the intensity modulation observed in the mirror orientation of icosahedral quasicrystalline phase. Contrary to the crystalline phase formation in Cu–Ga–Mg–Ti alloy, rapid solidification of Cu₅₀Al₃₀Mg₅Ti₁₅ shows the formation of amorphous and nanocrystalline bcc γ-brass type phase (a = 0.870 nm), which on annealing transforms to ordered γ-brass type phase (a = 0.872 nm). The structural and microstructural characterization was done through X-ray diffraction and transmission electron microscopy.     

Calculation of phase diagrams of binary fcc and ideal hcp alloys undergoing ordering transitions

The tetrahedron approximation of the cluster variation method (CVM) has been employed to investigate phase diagrams having fcc-based ordered and disordered phases. This approximation is also applicable to the binary hcp ordered structures with ideal axial ratio. The CVM developed by Kikuchi consists of calculating approximate expressions for the number of configurations and hence entropy of a crystal lattice having definite distribution of clusters (points, pairs, triangles, tetrahedra, etc.) of lattice points which in general may be occupied by one of a given set of atomic species. Tetrahedral multi-atom interactions denoted by α and β are utilized for expressing the configurational energy. The equilibrium cluster distribution is then found by minimizing the free energy by utilizing the natural iteration method developed by Kikuchi. The effect of α and β parameters on the topology of the resulting phase diagrams is observed by assigning several negative and positive values to them. The invariant reactions were also determined in each case. Finally a study was made on the Cd-Mg diagram.     

Microstructures and calorimetric evaluation of non-equilibrium states in rapidly solidified Al-Mn alloys containing less than 7 at % Mn

Morphology and enthalpy differences between a non-equilibrium state of as-melt spun ribbons and the equilibrium state of fully annealed ribbons were investigated by TEM observation, X-ray analysis and differential scanning calorimetry (DSC) measurements for Al-1.66, 2.56, 3.95, 5.37 and 6.98 at % Mn alloys. The icosahedral phase was observed with α-Al solid solution in as-melt spun 3.95, 5.37 and 6.98 at % Mn ribbons. The morphology of the icosahedral phase was complex; small particles, petal-like or coral-like branches and eutectic structure within the cell and small particles, films and networks along cell boundaries were observed depending on the manganese content. The differential enthalpy, d(ΔH _o ^ne )/dT, versus T curve obtained by characteristic DSC measurements normally showed three peaks. The peak P₁ at 520–560 K corresponds to the precipitation and growth of the icosahedral phase, the peak P₂ at about 600–800 K to the transformation from the icosahedral phase to the equilibrium Al₆Mn phase, and the peak P₃ at about 700–880 K to the precipitation of Al₆Mn from α-Al solid solution. The temperatures of the peaks P₂ and P₃ shift to the lower side and the height of the peaks becomes higher with increasing manganese content. The enthalpy difference due to the fine microstructure in as-melt spun ribbons was also estimated.     

Quasicrystal–crystal structural transformation in Al–5 wt.% Mn alloy

The atomic structure of Al–5 wt.%Mn (Al–5Mn) alloy, prepared by rapid solidification, and pre-annealed at 623 and 773 K for 5 and 1 h, respectively, were characterized by X-ray powder diffraction (XRD) and extended X-ray absorption fine structure (XAFS) techniques. The sample in as-quenched stage was found crystalline, consisting of metastable α-Al (Al–Mn solid solution) and icosahedral quasicrystalline I-Al₆Mn phases. Five hours annealing at 623 K proved thermal stability of both the phases. Pre-annealing at 773 K/1 h on the other hand leads to α-Al phase decomposition and structural transformation of metastable I-Al₆Mn to stable orthorhombic Al₆Mn phase. The EXAFS results indicate that Mn atoms are located preferably on the outer shell of icosahedrons. During the I-Al₆Mn→o-Al₆Mn transformation the total Al atoms coordinating one Mn were found to be constant (∼10). Based on the results, only distance/symmetry changes in atomic arrangement around Mn atoms were suggested.     

High-performance bismuth-telluride compounds with highly stable thermoelectric figure of merit

The p-type (Bi_0.25Sb_0.75)₂Te₃ ingot doped with 8 wt% excess Te alone and the n-type Bi₂ (Te_0.94Se_0.06)₃ ingot codoped with 0.068 wt% I and 0.017 wt% Te were grown by the Bridgman method and annealed at 673 K for 5 h in a hydrogen stream. The electrical resistivity ρ, Seebeck coefficient α and thermal conductivity κ before and after annealing were measured at 298 K, so that the annealing degraded significantly ZT of the p-type specimen but enhanced remarkably that of the n-type one. The temperature dependences of ρ, α and κ of the as-grown p-type and annealed n-type specimens with higher ZT were investigated in the temperature range from 200 to 360 K. As a result, ZT values of the as-grown p-type and annealed n-type specimens have a broad peak and reached great values of 1.19 and 1.13 at approximately 320 K, respectively. The present materials were thus found to be far superior to any other bismuth-telluride compound in the thermal stability of energy conversion efficiency in addition to the high performance.     

Enhancement of the Seebeck coefficient in touching M/Bi–Te/M (M = Cu and Ni) composites

The resultant Seebeck coefficient α of the touching p- and n-type M/Bi–Te/M (M = Cu and Ni) composites was measured as a function of z at a scan step of 0.5 mm using thermocouples set at three different intervals of s = 4, 6.5 and 8 mm, where s is the interval between two probes and z is the distance from the center of Bi–Te compound to the middle of two thermocouples. Bi–Te compounds have a thickness of t _Bi–Te = 6 mm but the thickness t _M of both end metals sandwiching their compounds was varied from 0.5 mm to 6 mm. The composites were compacted tightly at a force of about 10 N by a ratchet. When two probes are placed on both end metals, the resultant α was significantly enhanced and exhibited a tendency to increase as s approaches t _Bi–Te, like the welded composites. The enhancement in α is attributed to the contribution from the barrier thermo-emf generated near the interface. When the thickness t ₀ of metal outside two probes set at s = 6.5 mm was increased from 0.25 mm to 5.75 mm, the averaged α for M = Cu and Ni was increased by 3.8% in the p-type composite, while reversely it was decreased by 4.8% in the n-type one. It was first observed that t ₀ also has a significant influence on the resultant α. The maximum α of the p- and n-type Ni/Bi–Te/Ni composites then reached great values of 264 μV/K at t _M = 6 mm (corresponding to t ₀ = 5.75 mm) and −280 μV/K at t _M = 1.2 mm (corresponding to t ₀ = 0.95 mm), respectively, which are 29% and 23% larger in absolute value than their intrinsic α values. These maximum α were barely changed with time. It was thus found that the barrier thermo-emf is generated steadily even in touching composites and the resultant α is highly sensitive to the position of leads connected to the metal electrode of a thermoelement.     

High-resolution electron microscopy of nanocrystalline Ni-Al alloys: instability of ordered structure and dynamic behaviour of grain boundaries

High-resolution transmission electron microscopy was performed on vacuum-deposited nanocrystalline nickel aluminide films. Several nickel aluminide ordered structures, i.e. L1₂(Ni₃Al)-, B2(NiAl)-, D5₁₃(Ni₂Al₃)- and D0₂₀(NiAl₃)-type structures, were observed in the deposited films. The L1₂ and B2 ordered structures became unstable with decreasing grain sizes. The critical grain size on transformation from the L1₂- and B2-type ordered structures into disordered structures was ca. 5 nm at ambient temperatures. High atomic diffusion, sufficient for grain growth, and an increase in the ordering occurred just above 400°C in the nanocrystalline Ni-Al films with L1₂- and B2-type structures. The diffusion bonding process, at ambient temperatures, between Ni-Al nanocrystallites with an L1₂-type structure was observed dynamically at atomic resolution under strong electron irradiation. It was found that the nanocrystallites rotated and slid without crack generation, and neck-growth proceeded even at ambient temperatures.     

Effect of titanium on the structure and mechanical properties of Cu-Al-Ti alloys

The structure and mechanical properties of Cu10 wt% Al base alloys with 0–2.5 wt% Ti additions were investigated using transmission electron microscopy, optical microscopy and tensile tests. Addition of titanium has a decreasing effect on the grain size after quenching fromα + β region and causes significant strengthening of alloys. Alloy containing 1 wt%Ti quenched from 900° C shows mixture ofα, retainedβ (DO₃), disorderedβ′ (3R) and orderedβ′ ₁ (18R) martensites. Alloy with 2.5 wt% Ti addition after quenching containsα, retainedβ (DO₃), ordered T₁ phase of L2₁ superlattice and orderedβ′ ₁ martensite with either R18 or L1₀ structure indicating different stacking of ordered planes as the effect of titanium addition.     

Microstructure evolution and ordering in commercial Mg-PSZ

Sintered commercial ZrO₂-9 mol % MgO (PSZ) alloy was heat-treated at different temperatures in the range 900 to 1400° C. The microstructure was studied using transmission electron microscopy (TEM). The as-sintered material was characterized by either fine tetragonal precipitation in cubic matrix grains, or coarser precipitates which had transformed martensitically to the monoclinic symmetry. The diffuse scattering intensity (DSI) was observed to originate from the cubic lattice, and was correlated with the short-range ordering of the oxygen vacancies present in the cubic matrix. However, by annealing at temperatures below 1100° C for relatively short times, long-range ordering occurred in the cubic matrix. The ordered phase was-Mg₂Zr₅O₁₂ with a rhombohedral symmetry, which belongs to the homologous series of M _n O _2n-2 (M₇O₁₂) defect structures derived from the CaF₂-type structure. The ordering process is characteristic only for the cubic regions between the fine-tetragonal precipitates. This microstructure is considered to be a pseudo-equilibrium state and is related to the limited extent of diffusion.     

Testing metal-dielectric-semiconductor tunnel structures on p-silicon as nuclear particle detectors

Metal-dielectric-semiconductor (MDS) structures with aluminum nitride (AlN) as a tunnel dielectric based on high-ohmic p-type silicon substrates have been studied. The samples were characterized with respect to the charge collection efficiency and energy resolution on probing with 5.4-MeV α particles. In addition, the nature of noises and the state of the AlN-p-Si interface were investigated. It is established that the parameters of these MDS structures as radiation detectors are close to those of widely used Schottky-barrier detectors based on n-Si (Au-n-Si). A decrease in the concentration of deep centers at the AlN-p-Si interface allows the proposed MDS structures to compete successfully with n-Si based detectors, which is due to a higher purity of the initial material.     

Structural disorder and solid state transformations in single crystals of Zn x Cd1−x S and Zn x Mn1−x S

Single crystals of Zn _x Cd_1−x S and Zn _x Mn_1−x S were grown from the vapour phase at 1100°C in the rangex=0·9 to 1. X-ray characterization shows that polytypes and disordered structures occur in Zn _x Cd_1−x S forx ≥ 0·94, whereas Zn _x Mn_1−x S displays disordered and polytype structures in the entire rangex=0·9 to 1. It is observed that Zn _x Cd_1−x S and Zn _x Mn_1−x S undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β ≫ α approximates the actual mechanism of transformation in these crystals.     

In situ X-ray study of order–disorder phase transitions in Cu–Al–Be melt spun ribbons

The disorder–order phase transitions in Cu–Al–Be shape memory alloys were studied by in-situ X-ray diffraction. Isothermal measurements in an inert gas chamber were made on ribbons of Cu–Al–Be alloys, obtained by melt spinning. A position sensitive detector was used for a fast record of the X-ray signal on a θ–θ geometry diffractometer. Each ribbon was used as the heating element, avoiding the need of a heating sample holder and obtaining a better measurement of the temperature with a thermocouple attached directly to the ribbon. The order transition was followed at different temperatures after a high temperature annealing (700 °C) in the disordered β-phase region. A first order transition was observed from disordered β (A2) to ordered β ₁ (DO₃), without prior β (B2) ordering. No precipitation was detected during this fast measuring procedure.     

Electrical,thermal and infrared studies of cadmium metavanadate

Cadmium(II) metavanadate has crystal structure related to brannerite (ThTi₂O₆) structure. The high temperatureβ-CdV₂O₆ phase isn-type semiconductor between 185 and 750°C. The electrical conduction in theβ-CdV₂O₆ occurs due to deviation from oxygen stoichiometric composition of the lattice. The seebeck coefficient (α) of the sample is negative and constant in the entire range of investigation. The mechanism of transport in cadmium metavanadate lattice is via thermally activated hopping of localized electrons on vanadium (V⁵⁺) sites of the lattice. The DTA result indicated that CdV₂O₆ undergoes phase transition at 185°C and not at 670°C as reported earlier. There is no DTA evidence to show the possibility ofβ →α phase reverse transition. The XRD powder patterns of the two modifications are nearly similar indicating brannerite related structures. The infrared absorption band of vanadium-oxygen stretching vibration modes of distorted VO₆ octahedra ofβ-CdV₂O₆ is exhibited at 855 cm⁻¹.     

Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in Ni-Al system and theα, β, γ, ɛ and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al₃Ni and NiAl were always ordered, while Ni₃Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni₃Al phase fields, particularly towards Al-rich compositions. Al₃Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni₃Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at.% Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In case of NiAl and Al₃Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔH _f) is large (> 120 kJ/mol), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases, irrespective of the magnitude of ΔH _f, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.     

Conduction in Mn substituted Ni-Zn ferrites

The d.c. electrical resistivity ‘ρ’ and thermoelectric power ‘α’ are studied as a function of temperature for Mn substituted ferrites with general formula Zn_0·3Ni_0·7+x Mn _x Fe_2−2x O₄. At lower Mn concentrations, the increase in d.c. resistivity is attributed to the hindering of Verwey mechanism Fe²⁺ ⇌ Fe³⁺ due to stable bonds of Mn³⁺ + Fe²⁺ pair. The decrease in resistivity at higher Mn concentrations (i.e. whenx > 0·15) is attributed to the formation of Mn³⁺ cluster and Ni²⁺ ⇌ Ni³⁺. The activation energy values show one to one correspondence with resistivity values. The compositional variation of thermoelectric power showsn-type behaviour for the samples withx < 0·2 whereasp-type behaviour for the samples withx ⩾ 0·2. Thep −n transition is attributed to the formation of Ni³⁺, Fe²⁺ + vacancies which act asp-type carriers. The temperature dependences ofα, ρ, and mobility clearly confirm the conduction mechanism to be due to polaron hopping.