Twinning of quasicrystals and related crystals

Electron microscopy and electron diffraction have been employed to bring out the intimate relationship between icosahedral quasicrystal and various types of crystalline phases with emphasis on aluminium transition metal alloys. The relationship between quasicrystal and twinned rational approximant structures will be treated in the context of irrational twinning. The twinned aggregates show an icosahedral like symmetry in electron diffraction patterns. The crystallography of irrational twins has been discussed in terms of various ways of representing a twin relationship in a cubic lattice.     

Phason strains on growth, stability and structure of icosahedral phases

Growth, stability and structure of icosahedral (i-) phases have been studied by electron diffraction and X-ray diffraction in relation with the phason strains. Three alloy systems; AlPdCr, AlPdMn and AlCuV were chosen in this study. An i- Al₇₂Pd₂₅Cr₃ grain has been analysed to have a phason matrix toward to a tetragonal or orthorhombic structure. Stability of the i-phases correlated with the phason relaxation was discussed in Al-Pd-Mn system. A quenched i-Al₇₄Pd₁₇Mn₉ close to but containing significant phason strains, revealed that the phason relaxation induced by precipitation of crystalline phases upon annealing. Phason disorder dominated by the chemical composition was evidenced in a composition study in Al-Pd-Mn system. A high density random phasons characterized the icosahedral glass phase was observed in AlCuV alloys.     

Structural, microstructural and electronic characteristics of quasicrystalline and carbon containing fullerene (C60) and graphitic tubule, solids

The quasicrystalline and recently discovered carbon bearing exotic solids- the fullerenes (e.g. C₆₀) and their tubule relatives Nano-tubules have a common linkage in the sense that they have built in icosahedral symmetry either at the level of lattice — as in QC materials, or motif e.g. in fullerene or tubule (caps). It is, therefore, of interest to explore the structural features and properties (e.g. electronic behaviour) of the solids on a comparative basis. The basic theme of this paper centres around this aspect. We have discussed and described the structural/microstructural characteristics and properties (with emphasis on electronic behaviour) of the QC, C₆₀ and Nano-tubule. Attempts have then been made to bring out their comparisons for these type of icosahedron bearing solids.

The case of comparison of structural behaviours and grass root electronic characteristic e.g. Brillouin zones and density of states of the QC and C₆₀ solid — the special carbon bearing material — have been taken first. Thereafter, the case of tubule-solids which are related to C₆₀ have been taken up. Then inter-relationship between tubule and C₆₀ and the exotic electronic characteristics of the tubules have been discussed and described. Finally the similarities and dissimilarities of the three types of solids in regard to their structural and electronic behaviours have been outlined.     

The role of fuzzy interfaces in the nucleation, growth and agglomeration of aluminum hydroxide in concentrated caustic solutions

Fundamental crystal growth theory relies on classical concepts of monomeric addition at step sites on crystal surfaces. The nucleation and growth of crystalline aluminium hydroxide from concentrated caustic solutions does not follow classical crystal growth mechanistic pathways. Numerous techniques including static and dynamic light scattering, small angle X-ray and neutron scattering, cryovitirification transmission electron microscopy, rheology and atomic force microscopy have been employed in the study of aluminium hydroxide crystallisation from concentrated caustic solutions. The observations from these techniques have been interpreted on the basis polymer crystal growth theory, thermodynamic phase inversions analysis and entropically driven insolubility.

The experimental observations can be interpreted on the basis that aluminium hydroxide nuclei and crystals are surrounded by a diffuse interface which grades in density from the crystalline aluminium hydroxide particle core to the surrounding solution. A mechanism for the nucleation and growth mechanisms of aluminium hydroxide has been proposed: initial solution formation of a loose polymeric network; clustering of this network followed by gradual densification to form amorphous nuclei; further densification of the core of the nuclei to form crystallites and gradual densification but not crystallisation of the still amorphous diffuse interface.

The presence of this diffuse interface enables the slow agglomeration behaviour of aluminium hydroxides particles in concentrated caustic liquors to be explained. In liquors of very high ionic strength (in this case up to 6 M NaOH) particulate agglomeration would be expected to be rapid due to the small double layer thickness as predicted by DLVO theory. During rapid growth the diffuse interface inhibits the sufficiently close approach of the dense part of the particles to the point where attractive inter-particulate van der Waals forces would dominate and agglomeration would take place. As supersaturation is depleted and the growth rate of the diffuse interface decreases but densification is still occurring the particles can approach more closely and agglomeration will occur. Thus it is probable that the observed agglomeration behaviour is supersaturation and growth rate related.     

Induced SA phases and reentrant nematic phases in binary systems of homologous terminal-nonpolar compounds

The isobaric phase diagrams of binary systems are presented in which one component is represented by members of the homologous series of swallow tailed compounds. The occurrence of induced S_A phases and reentrant nematic phases was studied in dependence on the length of the alkyl chain of the swallow tailed compound. In some selected binary systems X-ray investigations and calorimetric measurements were performed. An attempt has been made to interprete the results by a simple packing model.     

Formation of a new F.C.C. τ2 phase in rapidly quenched Al55Cu35V10 alloy

X‐ray diffraction and transmission electron microscopy experiments were carried out to study the structure of rapidly solidified as‐cast and annealed Al₅₅Cu₃₅V₁₀ alloy. The as‐cast Al₅₅Cu₃₅V₁₀ alloy shows the presence of a new f.c.c. τ2 phase (a=0.58nm) along with a b.c.c. (a = 0.89 nm) phase which after subsequent annealing transforms into single f.c.c. phase (a = 0.58 nm). In this paper, it is also reported that these phases are crystalline approximants to an icosahedral phase on the basis of e/a (valence electron per atom) constant line. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Acoustic properties of aspirin in its various phases and transformation stages studied by Brillouin scattering

The acoustic behaviors of aspirin were studied in its crystal, liquid, glass states as well as transformation stages between them. The Brillouin doublet of the longitudinal acoustic (LA) mode propagating to the [100] direction of aspirin crystals showed a splitting in a temperature range of 393-413 K below the melting temperature, which indicated a softening and distortion of the layered aspirin crystal in its premelting stage. One LA mode was observed in the liquid and glassy phases of aspirin, consistent with Brillouin selection rule for isotropic materials. The acoustic damping in the glassy aspirin was systematically higher than that in the crystal aspirin, which was in contrast to ibuprofen materials. These suggested different activity of remnant local dynamics that could couple to the acoustic waves, persistent in the glassy state of these two representative pharmaceuticals. In the supercooled liquid phase, the Brillouin spectrum displayed a new spectral feature characterized by a high-frequency cutoff and a distributed low-frequency contribution other than the Brillouin peak of the amorphous aspirin. This could be ascribed to the formation of crystalline grains in the glassy matrix due to cold crystallization.     

Optical absorption spectra and related parameters of ammonium zinc chloride crystal in the antiferroelectric and commensurate phases

Optical absorption along the three crystallographic axes of (NH₄)₂ZnCl₄ single crystals is reported over a temperature range from 276 K to 350 K. Our results clearly confirm the antiferroelectric to commensurate phase transition at 319 K. Analysis reveals that at the absorption edge the type of transition is the indirect allowed one. The indirect band gaps at various temperatures are determined and their temperature dependence is estimated in the antiferroelectric and commensurate phases. Optical parameters related to the temperature dependence of the energy gap are evaluated. The single‐effective oscillator model was used to describe the imaginary part of the dielectric constant, which facilitate calculation of the dipole moment parameters. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap and the result is compared with that calculated graphically. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach characteristic parameters are determined and their temperature dependence is investigated. Correlation between different parameters is considered. Most of the results confirm the optical isotropic nature of AZC crystals. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of Ga incorporation on photoinduced phenomena in Ge–S based glasses

To study the influence of Ga addition on photoinduced effect, GaGeS glasses with constant atomic ratio S/Ge = 2.6 have been prepared. Using Raman spectroscopy, we have reported the effect of Ga on the structural behavior of these glasses. An increase of the glass transition temperature T_g, the linear refractive index and the density have been observed with increasing gallium content. The photoinduced phenomena have been examined through the influence of time exposure and power density, when exposed to above light bandgap (3.53 eV). The correlation between photoinduced phenomena and Ga content in such glasses are shown hereby.     

Phase separation in high-T c superconductors, copper oxides, and related antiferromagnetic phases CuO and Y2BaCuO5

The acoustic properties and crystal structure of high-T _c superconducting cuprates and the related antiferromagnetic phases CuO and Y₂BaCuO₅ exhibit similar properties at a temperature of about 160 K and 240 K. These properties can be associated with the formation of inhomogeneous state of phase separation. Analysis of the magnetic properties of Y₂BaCuO₅ shows that these effects are of a nonmagnetic nature. The results of EXAFS data for the high-T _c superconducting compound Hg_0.8Tl_0.2Ba₂Ca₂Cu₃O_8.10 show that the phenomenon of phase separation is suppressed by superconductivity.     

Correlations between electric,magnetic, and galvanomagnetic quantities in stable icosahedral phases based on aluminum

On the basis of the results of the experimental investigation of the correlations between the electric, magnetic, and galvanomagnetic properties, the nature of the electronic phenomena leading to the difference of icosahedral quasicrystals from typical metals and insulators (low conductivity and negative temperature coefficient of resistance, diamagnetism of the ground state and its decrease with increasing temperature, and existence of residual and thermally induced charge carriers) is discussed.     

Filled SA phases in binary systems of terminal-nonpolar compounds (IV). Mixtures of biswallow-tailed compounds and nematogenic twin compounds

The phase diagrams of three binary systems of liquid crystals are presented whereby one component is a biswallow-tailed compound and the second component is a member of homologous twin compounds. The most important feature of the diagrams is the occurrence of induced S_A phases and reentrant nematic phases. The existence of the induced S_A phase can be interpreted by the special molecular structure of the mixing components.     

On the role of rotating tetrahedra for generating auxetic behavior in NAT and related systems

Systems with negative Poisson’s ratios (auxetic) exhibit the unexpected feature of expanding laterally when uniaxially stretched and becoming narrower when compressed. Here, we examine the role of the tetrahedra found in the frameworks of the predicted auxetic zeolites natrolite (NAT), thomsonite (THO) and edingtonite (EDI) for generating negative Poisson’s ratios in an attempt to relate the auxeticity in the (0 0 1) plane to rotations of the tetrahedra in the zeolite framework. The behavior of the tetrahedra is then examined in terms of their 2D projections in the (0 0 1) plane and we show that in the extreme scenario, where the three-dimensional tetrahedra in the zeolite framework are perfectly rigid and simply rotate relative to each other, then their 2D projected behavior in the (0 0 1) plane becomes equivalent to the idealized two-dimensional ‘rotating squares model’ with a Poisson’s ratio of ?1.     

Metastable microheterogeneity of melts in eutectic and monotectic systems and its influence on the properties of the solidified alloy

Experimental results that are the basis for the existence of metastable microheterogeneities in melts of eutectic and monotectic systems are summarized. Among these, recent results of small angle neutron scattering experiments are discussed in some detail. It is shown that the influence of a homogenizing heat treatment of a melt on the structure of crystalline castings and amorphous ribbons obtained after its solidification is very important.     

Inelastic neutron and low-frequency Raman scattering in a niobium-phosphate glass for Raman gain applications

We present measurements of the vibrational spectrum of a binary niobium-phosphate glass in the THz frequency range using inelastic neutron and Raman scattering. The spectra of these glasses show a low-frequency enhancement of the vibrational density of states (“boson peak”). Using a recently developed theory of vibrational excitations in disordered solids we are able to reconcile the measured neutron and Raman spectra using fluctuating elastic and Pockels constants as a model concept. As the spontaneous Raman susceptibility is a key parameter for Raman amplification our results suggest a significant gain profile for application of niobium-phosphate glasses in Raman amplifiers.     

Formation of nanocrystalline and amorphous phases in Cu80P20, Ni82P18 and Cu87Ni13 mechanically alloyed systems

Variations in the crystallite size and stored strain caused by mechanical milling of binary alloy systems of copper, nickel and phosphor were compared. During mechanical alloying process, an amorphous phase is partially produced in CuP and NiP alloy systems and only a nanoscale crystalline phase in CuNi alloy system. The CuP alloy system first forms a Cu₃P intermetallic compound and then the partial amorphous phase by the reaction of primary copper phase and Cu₃P intermetallic compound, while the NiP alloy system directly forms a partial amorphous phase by the reaction of primary nickel and phosphor phases. When the crystallite sizes of primary copper and nickel phases become of the order of a few nanometers in the CuP and NiP alloy systems, they are in an almost strain-free state.     

Lattice vibration and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate

Raman scattering and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate were examined. The A₁(TO) modes appear in E symmetry spectrum of the doped lithium niobate. Their intensities vary with different dopings. In the spectrum of z(yy)x geometry, the properties of the lowest-frequency E(TO) mode of the Cr-doped lithium niobate are different from those of pure, Co- and Fe-doped lithium niobate. The intensity of the A₁(TO) mode at 637cm^-1 I is decreased in doped lithium niobate compared with the pure crystal. We attribute these properties to both the photorefractive effect which is enhanced by dopants and to the different occupation of the doping ions. A light climbing effect was observed in Co- and Cr-doped lithium niobate for the first time. A higher photodamage threshold and quicker light climbing speed were found in Co- and Cr-doped lithium niobate in comparison with the light climbing effect in the Fe-doped lithium niobate. The results from the photoinduced light scattering experiments were compared with those from a Raman spectroscopic study.     

Estimation of inter-atomic force constants and phonon dispersion using correlation effects among thermal displacement of atoms in Ge

Neutron diffraction measurements have been performed on powder Ge at 6, 150 and 300 K. Oscillatory diffuse scattering intensity is clearly observed at 150 and 300 K. The diffuse scattering theory including correlation effects among thermal displacements of atoms is applied to background function in the Rietveld analysis. The inter-atomic distance and temperature dependence of the values of correlation effects are discussed. The force constants among the first, second and third nearest neighboring atoms in Ge are obtained from the values of correlation effects and Debye-Waller temperature parameters. A phonon dispersion spectrum in Λ [111] direction for Ge crystal has also been calculated at 80 K using inter-atomic force constants and the crystal structure.     

The selenium based chalcogenide glasses with low content of As and Sb: DSC, StepScan DSC and Raman spectroscopy study

Thermal properties and structure of As_xSe_100−x and Sb_xSe_100−x glass-forming systems (x = 0, 1, 2, 4, 8 and 16) were studied by conventional and StepScan DSCs and Raman spectroscopy. Compositional dependence of the glass transition temperature, T_g, was determined from reversible part of StepScan DSC records and discussed. The attention was also focused on the crystallization of undercooled melts of these systems. It was found that only selenium crystallizes from undercooled melts of As–Se system and its tendency to crystallize decreases markedly with increasing As content, for arsenic content higher than 4 at.% no crystallization was observed. In the case of Sb–Se system Sb₂Se₃ crystallizes in the first step followed by trigonal selenium crystallization from non-stoichiometric undercooled melt. Sb₂Se₃ crystallizes from incongruent melt with crystallization enthalpy ΔH_c(Sb₂Se₃) = −52 ± 2 J/(g of Sb₂Se₃), Johnson–Mehl–Avrami kinetics of crystallization and kinetic exponent close to 3 was found. Raman spectra were measured to obtain basic information on the structure of both glassy systems.