Vibrations in icosahedral boron molecules and in elemental boron solids

The molecular vibrations of the B₁₂H^?₁₂ molecule are studied in the first part. At the beginning, the symmetry properties of the icosahedral modes are discussed. Then, the Raman and i.r. data for B₁₂H^?₁₂ are analyzed using various valence force field model potentials. With a six-parameter model we obtain a mean deviation of ~2 per cent for the nine measured frequencies. We find strong and angular-dependent forces pointing outward from the icosahedron; however, the intra-icosahedral forces are relatively weak and much less angle-dependent.We also investigate the ?q = 0 phonons of elemental boron crystals. For α-rhombohedral boron, we obtain good agreement with the measured Raman lines; we also are able to reproduce the main features of the one-phonon density of states of β-rhombohedral boron. We find that the coupling constants of the strong inter-icosahedral bonds are 2̃5% larger than the intra-icosahedral ones. The latter change very little as compared to B₁₂H^?₁₂. Altogether our results support the idea that within the icosahedron, the binding is metal-like, whereas the outward bonds are more covalent.     

The formation and stability of quasicrystal in Al80Mn14Si6 under high static pressure

Abstract

The method of quenching in fusing state under high static pressure (MQFSHP) was applied for the first time to prepare the quasicrystal icosahedral phase of Al₈₀Mn₁₄Si₆ alloy. The pressure was from 2.8GPa to 3.1GPa and the cooling rate during quenching was of about 100°C/s. Some sharp electron diffraction spots showing an arrangement with a five-fold symmetry axis and noncrystalline ring have been observed in electron diffraction experiment.

The crystallization temperature of I phase obtained from high pressure(HP) is close to that of rapid cooling ribbon, but the cooling rate of the sample obtained is lower than that of rapid cooling ribbon.     

Is the established structure of α-rhombohedral boron correct? Comparative study of IR-active phonons with B6O, B4.3C and β-rhombohedral boron

The established structure of α-rhombohedral boron, based on one B(12) icosahedron per unit cell only, is put in question. A careful evaluation of the IR-active phonons in comparison with B(6)O, B(4.3)C and β-rhombohedral boron makes it evident that-aside from the B(12) icosahedra-the α-rhombohedral boron structure also contains single boron atoms. We assume these single atoms replace the currently assumed inter-icosahedral three-centre-bonds by covalently saturating the outward directing bonds of the equatorial atoms of the three adjacent icosahedra. Indeed, the implied structure formula B(12)B(2) is not supported by previous density measurements. The IR-phonon spectra of the related structures are correlated and merely shifted relative to one another; significant features depend quantitatively on the actual structure, but they can be easily allocated.     

Magnetic properties of a monocrystalline quasicrystal in the Al-Pd-Mn system

Magnetic susceptibility of a monocrystalline icosahedral Al_70.2Pd_21.3Mn_8.5 quasicrystal was measured over the temperature range from 4 to 1100 K. The susceptibility was found to include the temperature-independent diamagnetic contribution, the temperature-dependent Curie’s contribution, and the contribution from the Pauli paramagnetism of an electron system with energy gap. An analysis of the low-temperature susceptibility revealed the presence of about 0.008% of ions with magnetic moment 4µ_B in the quasicrystal at 4 K. It is assumed that the ions with uncompensated magnetic moments appear near the structural vacancies in the quasicrystal lattice. The energy gap between the valence and conduction bands is estimated at Δ=0.64 eV, and the effective mass of charge carriers is equal to approximately 70 electron masses.     

Decagonal quasicrystal in the Al–Pd–Mn system

Abstract

A stable decagonal quasicrystal in Al₇₀Pd_30?xMn_x alloys (x = 10–20) was examined by electron diffraction and high-resolution electron microscopy. The decagonal quasicrystalline grains are formed with definite crystallographic relationships to adjacent icosahedral and Al₃Mn crystalline grains. The structure of the decagonal phase, which is formed as the main phase at near Al₇₀Pd₁₀Mn₂₀ composition, is a mixture of decagonal quasicrystalline regions with some linear phason strain and microcrystalline regions. The structures of both regions may be interpreted in terms of quasiperiodic and periodic tilings, constructed with two types of bond lengths, S (about 2 nm) and L (= τ · S, where τ is the Golden ratio), of the same atom cluster with decagonal symmetry.     

Formation and stability of large B 6 O clusters with icosahedral structure

In this paper, the formation and the stability of large B₆O icosahedral particles was discussed on the basis of elastic deformation theory. Our calculation illustrate the stability of macroscopic Mackay packing B₆O icosahedral particles at high pressure. The transition pressure from rhombohedral structure of B₆O particles to macroscopic B₆O icosahedral ones was calculated to be 6 GPa, which is in good agreement with the experimental data (4.0-5.52 GPa). The maximum diameter of B₆O icosahedral particles at low pressure is estimated to be 200-300 nm. Received 30 November 2000     

High pressure X-ray diffraction study on icosahedral boron arsenide (B12As2)

The high pressure properties of icosahedral boron arsenide (B₁₂As₂) were studied by in situ X-ray diffraction measurements at pressures up to 25.5 GPa at room temperature. B₁₂As₂ retains its rhombohedral structure; no phase transition was observed in the pressure range. The bulk modulus was determined to be 216 GPa with the pressure derivative 2.2. Anisotropy was observed in the compressibility of B₁₂As₂—c-axis was 16.2% more compressible than a-axis. The boron icosahedron plays a dominant role in the compressibility of boron-rich compounds.     

A study on the formation and stability of quasicrystal in Al4Mn and Al6Cr

Abstract

The condition of the formation of quasicrystal in Al₄Mn and Al₆Cr under high static pressure has been investigated for the first time. I-phase and T-phase have been observed in electron diffraction experiment. The structures of Al₄Mn quenched at about 100 K/s are different under various pressure from 0.95GPa to 4.45GPa. The phase transition from I- and T-phase to crystal phase has also been investigated.     

First principles calculations of the vibrational properties of icosahedral solid boron oxygen B12O2

In this paper, the geometrical structure and vibrational properties of icosahedral solid boron oxygen B₁₂O₂ have been calculated using first principles calculations. The symmetry of crystal vibration modes at the center of Brillouin zone is analyzed based on our numeric results and structure symmetry. The calculated results are compared with available experimental investigations.     

The electronic spectrum of a three-dimensional quasicrystal

The electronic spectrum of an icosahedral quasicrystal with a central-atom decoration of the Amman-Mackay network is investigated in the tight-binding approximation. The quasicrystal is described as a structural limit of the optimal cubic approximants with increasing period. The electronic spectra for the first four optimal cubic approximants do not contain the hierarchical gap structure which is typical for the Cantor set of the spectrum of a one-dimensional quasicrystal. At the same time, as the order of the approximant increases, the spectrum becomes singular throughout the entire energy scale. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 557–562 (25 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

First principles study of α -boron: can the B12 cage host hetero-atoms?

The peculiar electronic and structural properties of elemental boron are studied with first principles calculations including both density functional theory and wave-function based approaches. Interaction between boron and atomic hydrogen and lithium, is considered. In particular trapping a hetero-atom, such as hydrogen or lithium in a B₁₂ icosahedral cage, proceeds by a highly endothermic reaction and induces structural and bonding rearrangement of the host boron lattice. The peculiar charge distribution of the B₁₂ icosahedron favours bonding that points radially outwards from the icosahedral centre.     

Interface of quasicrystal and crystal

A model on the coexisting phase of quasicrystal-crystal is proposed,with which we concretely investigate the interface effects for coexisting phases of one-dimensional orthorhombic quasicrystal-isotropic crystal and three-dimensional icosahedral quasicrystal-cubic crystal.The phason strain fields which play an important role in some processes are determined.Some factors affecting the strain fields,e.g.,the material constants of phonon,phason,phonon-phason coupling of the quasicrystal and the elastic modulus and the size of the crystal are also explored.     

B12@B12@B60 and B12@(B12)12 giant clusters with doping atoms observed by high-resolution electron microscopy

B₁₂@B₁₂@B₆₀ and B₁₂@(B₁₂)₁₂ giant clusters consisting of B₁₂-based icosahedral clusters in B₁₀₅Al_2.6Cu_1.8 and B₅₆Y crystals were directly observed by high-resolution electron microscopy, image calculation and crystallographic image processing. Doping atom positions such as Al, Cu, and Y were also detected in the high-resolution images. The present work indicates that the cluster arrangements with light element of boron could be detected by using the crystallographic image processing technique.     

Mössbauer effect study of the decagonal quasicrystal Al65Co15Cu20

The ⁵⁷Fe Mössbauer effect study at 5.0 K and in an external magnetic field of 9.0 T on a high-quality stable decagonal quasicrystal Al₆₅Co₁₅Cu_19.9Fe_0.1 is presented. It is shown that the iron atoms are located in two distinct classes of sites. The values of the principal component of the electric field gradient tensor and the asymmetry parameter at these sites are, respectively, ?1.90(10)?×?10²¹ V/m², 0.97(15) and ?3.95(12)?×?10²¹ V/m², 0.00(17).     

Atomic dynamics of a <Emphasis Type="Italic">d</Emphasis>-AlNiFe decagonal quasicrystal

The atomic dynamics of an Al_71.3Ni₂₄Fe_4.7 decagonal quasicrystal has been investigated using the isotopic contrast method for inelastic neutron scattering. The partial vibrational spectra of the Ni, Fe, and Al atoms and the spectrum of the thermal vibrations of the alloy have been reconstructed directly from the experimental data without any model assumptions. The cutoff energies and the positions of the main features of the spectra have been determined. It has been revealed that the average binding energy of the nickel atoms in the quasicrystal under investigation is lower than that of the iron atoms and the vibrational spectrum of the aluminum atoms is noticeably harder than the spectrum of the pure metal. The results obtained for the d-AlNiFe decagonal quasicrystal have been compared with the previously published data for an i-AlCuFe icosahedral quasicrystal.     

Amorphization and a Polymorphic Transformation of Boron Stimulated by High Dynamic Pressures

Structural transformations of polycrystalline boron have been studied under megabar multiple-shock compression. Shock-wave experiments on the compression and subsequent recovery of polycrystalline samples of β-rhombohedral boron β-B₁₀₆ have been performed. Thermodynamic states of boron under the conditions of the performed experiments have been calculated. According to these calculations, the maximum pressure in the samples is 115 GPa. X-ray diffraction analysis of the stored boron samples has been performed. It has been shown that the dynamic compression of polycrystalline β-B₁₀₆ to 115 GPa results in its partial amorphization and transformation to the tetragonal modification of boron T-B₁₉₂.     

High pressure phases of different tetraboranes

High pressure phases of boron hydrides B₄H₁₀, B₄H₈ and B₄H₆ and their stability against dissociation into H and smaller B–H units are reported. Structure predictions based on particle swarm optimization reveal that all the boron hydrides studied show a tendency to separate into smaller structural units at low pressure. Under high pressure, the three-dimensional network in all the stoichiometries selected seems to be the most favourable arrangement. A study of the dissociation of B₄H₁₀ reflects an affinity to dissociate into B₄H₈ and H₂ in all the studied pressure range. Nevertheless, B₄H₈ does not seem to segregate in the studied pressure range and B₄H₆ may dissociate at 150 GPa.