A Definitive Example of Aluminum-27 Chemical Shielding Anisotropy

Solid-state²⁷Al NMR spectra have been obtained for a crystalline 1:1 complex of AlCl₃and OPCl₃. Aluminum chloride phosphoryl chloride, AlCl₃· OPCl₃(1), is unusual in that the Al–O–P bond angle is close to 180°. From analysis of the²⁷Al MAS NMR spectra, it was determined that the²⁷Al nuclear quadrupole coupling constant is 6.0(1) MHz, the asymmetry in the electric field gradient (efg) tensor is 0.15(2), and the isotropic chemical shift, δ_iso(²⁷Al), is 88(1) ppm. Solid-state²⁷Al NMR of a stationary sample reveals a line shape affected by a combination of anisotropic chemical shielding and second-order quadrupolar interactions. Analysis of this spectrum yields a chemical shift anisotropy of 60(1) ppm and orientations of the chemical shift and electric field gradient tensors in the molecular frame. Experimental results are compared with those calculated usingab initioHartree–Fock and density functional theory.     

1-D and 2-D NMR of broad-line solids: an application to quasicrystals

Nuclear magnetic resonance (NMR) techniques for measuring one-dimensional absorption spectra and two-dimensional exchange spectra of solids with extremely inhomogeneously broadened lines are discussed. Among various “broad-line” solids, quasicrystals represent alloys of metallic elements, the structures of which include “forbidden” symmetry elements. NMR absorption lines of quasicrystals exhibit a strong electric-quadrupole-induced inhomogeneous broadening that originates from the lack of translational periodicity of the otherwise perfectly long-range-ordered quasiperiodic lattice. Recording an NMR spectrum of a quasicrystalline sample requires a magnetic field-sweep technique. The two-dimensional exchange experiment on quasicrystals can be performed on selectively excited portions of the NMR spectrum only. Due to the off-resonance effects in a selective excitation, the use of a simple three-pulse stimulated-echo exchange sequence is preferred. The²⁷Al spectra of the Al-Pd-Mn and Al-Pd-Re families show interesting features like temperature-dependent frequency shifts and exchange effects due to atomic motion.     

Decagonal quasicrystals

Following the discovery of two dimensional quasicrystals in rapidly solidified Al-Mn alloys by us and L. Bendersky in 1985, a number of fascinating studies has been conducted to unravel the atomic configuration of quasicrystals with decagonal symmetry. A comprehensive mapping of the reciprocal space of decagonal quasicrystals is now available. The interpretation of the diffraction patterns brings out the comparative advantages of various indexing schemes. In addition, the nature of the variable periodicity can be addressed as a form of polytypism. The relation between decagonal quasicrystals and their crystalline homologues will be explored with emphasis on Al₆₀Mn₁₁Ni₄ and ‘Al₃Mn’. It will also be shown that decagonal quasicrystals are closely related to icosahedral quasicrystals, icosahedral twins and vacancy ordered phases.     

<Superscript>27</Superscript>Al NMR Study of the Structure and Dynamics of Natrolite

The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of ²⁷Al nuclei in natrolite (Na₂Al₂Si₃O₁₀· 2H₂O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the ²⁷Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T₁ of ²⁷Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the ²⁷Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H₂O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the ²⁷Al nuclei only at low temperatures (<270 K).     

Symmetry-dependent Mn-magnetism in Al<Subscript>69.8</Subscript>Pd<Subscript>12.1</Subscript>Mn<Subscript>18.1</Subscript>

We investigated the stability of magnetic moments in Al_69.8Pd_12.1Mn_18.1. This alloy exists in both, the icosahedral (i) and the decagonal (d) quasicrystalline form. The transition from the i- to the d-phase is achieved by a simple heat treatment. We present the results of measurements of the ²⁷Al NMR-response, the dc magnetic susceptibility, and the low-temperature specific heat of both phases. In the icosahedral compound, the majority of the Mn ions carries a magnetic moment. Their number is reduced by approximately a factor of two by transforming the alloy to its decagonal variety. For both compounds, we have indications for two different local environments of the Al nuclei. The first reflects a low density of states of conduction electrons and a weak coupling of the Al nuclei to the Mn-moments. The second type of environment implies a large d-electron density of states at the Fermi level and a strong coupling to the magnetic Mn moments. Spin-glass freezing transitions are observed at T^deca_f=12 K for the decagonal, and T^ico_f=19 K for the icosahedral phase.     

Composition rule for Al–transition metal binary quasicrystals

Compositions of Al–transition metal (TM) binary quasicrystals are revisited using the cluster-plus-glue-atom model. These compositions all conform to a unified cluster formula [icosahedron](glue)₁, where the icosahedron is taken from a crystalline approximant. In a given binary system, icosahedral and decagonal quasicrystals are expressed by the same icosahedron but glued, respectively, by one Al and one TM, and their e/a ratios fall close to 1.86 and 1.71–1.79, respectively. Their relative stabilities are discussed by referring to compositions satisfying the fully filled TM-3d states according to Häussler's resonance model.     

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.     

Correlation functions and the dynamical structure factor of quasicrystals

The icosahedral or decagonal symmetry of quasicrystals is well described by a periodic structure in higher dimensions. One consequence is the existence of dynamic phason modes in addition to the phonon modes. In an atomistic model phasons show up as correlated atomic jumps. We detect the phasons by the calculation of correlation functions and the dynamical structure factor in molecular dynamics simulations similar to the procedure used for phonons. In the simulations it is also possible to observe atomic jump processes directly. The models studied here represent icosahedral AlCuLi and decagonal AlCuCo quasicrystals. Ring processes are observed in the icosahedral case, and flips in the decagonal model. Received 17 March 2000 and Received in final form 8 June 2000     

Recent advances in the study of quasicrystals

Some recent advances in the study of quasicrystals at Ames Laboratory are reviewed. In particular, growth from the melt of large, high-quality, single-grain quasicrystals is described in detail for icosahedral R-Mg-Zn and decagonal Al-Ni-Co. In addition, the magnetic properties of the rare-earth-containing icosahedral R-Mg-Zn quasicrystals are discussed.     

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.     

<Superscript>23</Superscript>Na and <Superscript>27</Superscript>Al NMR Study of Structure and Dynamics in Mordenite

Temperature dependencies of ²⁷Al and ²³Na nuclear magnetic resonance spectra and spin–lattice relaxations in mordenite have been studied in static and magic angle spinning regimes. Our data show that the spin–lattice relaxations of the ²³Na and ²⁷Al nuclei are mainly governed by interaction of nuclear quadrupole moments with electric field gradients of the crystal, modulated by translational motion of water molecules in the mordenite channels. At temperatures below 200 K, the dipolar interaction of nuclear spins with paramagnetic impurities becomes an important relaxation mechanism of the ²³Na and ²⁷Al nuclei.     

TRAPDOR double-resonance and high-resolution MAS NMR for structural and template studies in zeolite ZSM-5

A combination of ²⁷Al magic-angle spinning (MAS)/multiple-quantum (MQ) MAS, and ²⁷Al–{¹⁴N} TRAnsfer of Population in DOuble-Resonance (TRAPDOR) nuclear magnetic resonance (NMR) was used to study aluminium environments in zeolite ZSM-5. ²⁷Al–{¹⁴N} TRAPDOR experiments, in combination with ¹⁴N NMR were employed to show that the two tetrahedral peaks observed in the ²⁷Al MAS/3Q-MAS spectra of as-synthesized ZSM-5 are due to aluminium atoms occupying crystallographically inequivalent T-sites. A ¹³C–{²⁷Al} TRAPDOR experiment was used to study the template, tetrapropyl ammonium bromide (TPABr), in the three-dimensional pore system of ZSM-5. The inequivalency of the methyl groups of TPA was observed in the ¹³C–{²⁷Al} TRAPDOR spectra of as-synthesized ZSM-5 and the motion of the methyl end of the propyl chain appeared to be more restricted in the sinusoidal channel than in the straight channel.     

Disorder diffuse scattering from quasicrystals

Very sharp Bragg reflections accompanied by diffuse scattering phenomena are typical for most stable quasicrystals. The correlation length of the quasiperiodic average structure can reach several micrometers as proved by high-resolution X-ray diffraction experiments. This corresponds to a structural perfection of some quasicrystals similar to that of silicon. Nevertheless, the omnipresent diffuse scattering indicates significant deviations from a strictly ordered quasiperiodic structure especially in the case of decagonal phases. These structural deviations may be caused by phason fluctuations, by disorder in the packing of the basic atomic clusters, by the formation of nanodomains, by chemical disorder, or by superstructure formation on a short-range scale. Characteristic examples of different types of structural disorder present in icosahedral and decagonal quasicrystals are reported. The diffuse scattering phenomena in decagonal Al-Co-Ni as a function of composition and temperature are discussed in more detail.     

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl₂ are investigated by magnetic susceptibility measurements and ²⁷Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The ²⁷Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the ²⁷Al nuclear spins at low temperature. The ²⁷Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the ²⁷Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μ_B. The spin-lattice relaxation rate 1/T₁ is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.     

High-field27Al mas NMR studies of the formation of metakaolinite by flash calcination of kaolinite

Flash calcination was used to dehydroxylate kaolinite and produced calcines kinetically frozen at various stages during metakaolinite formation. High speed magic-angle spinning²⁷Al NMR, in magnetic fields of 14.1 and 11.8 T, enabled new features to be observed in the metakaolinite spectrum. The dependence on magnetic field of simulated and experimental spectra is discussed, and the effects of a distribution of electric field gradients is examined. The evolution of the metakaolinite peaks is followed during dehydroxylation.     

Experimental evaluation of phonon–phason coupling in icosahedral quasicrystals

By measuring phonon strain introduced in crystal approximants, the sign and magnitude of the phonon–phason coupling constant have been evaluated for icosahedral quasicrystals of Mg–Ga–Al–Zn and Al–Cu–Fe systems. The evaluated coupling constants are approximately ?0.04μ and 0.004μ (μ?=?shear modulus) for the former and the latter, respectively. They are in good agreement with the results of a previously reported theoretical calculation. Possible effects of phonon–phason coupling on the onset of phasonic elastic instability in icosahedral quasicrystals are discussed.     

Electron spectrum and wave functions of icosahedral quasicrystals

Electron spectra and wave functions of icosahedral quasicrystals have been investigated in the tight-binding approximation using the two-fragment structural model (the Amman-MacKay network) with “central” decoration. A quasicrystal has been considered as a limiting structure in a set of optimal cubic approximants with increasing lattice constants. The method of level statistics indicates that the energy spectrum of an icosahedral quasicrystal contains a singular (nonsmooth) component. The density of electron states has been calculated for the first four optimal cubic approximants of the icosahedral quasicrystal, and the respective Lebesgue measures of energy spectra of these approximants have been obtained. Unlike the case of a one-dimensional quasiperiodic structure, the energy spectrum of an icosahedral quasicrystal does not contain a hierarchical gap structure typical of the Cantor set of measure zero in a one-dimensional quasicrystal. Localization of wave functions in an icosahedral quasicrystal has been studied, and their “critical” behavior has been detected. The effect of disorder due to substitutional impurities on electron properties of icosahedral quasicrystals has been investigated. This disorder makes the electron spectrum “smoother” and leads to a tendency to localization of wave functions. Zh. éksp. Teor. Fiz. 113, 1009–1025 (March 1998)     

Parallelizing acquisitions of solid-state NMR spectra with multi-channel probe and multi-receivers: Applications to nanoporous solids

A five-channel (¹H, ¹⁹F, ³¹P, ²⁷Al, ¹³C) 2.5 mm magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) probe is used in combination with three separate receivers for the parallel acquisitions of one (1D) and two-dimensional (2D) NMR spectra in model fluorinated aluminophosphate and porous Al-based metal-organic framework (MOF). Possible combinations to record simultaneously spectra using this set-up are presented, including (i) parallel acquisitions of quantitative 1D NMR spectra of solids containing nuclei with contrasted T₁ relaxation rates and (ii) parallel acquisitions of 2D heteronuclear NMR spectra. In solids containing numerous different NMR-accessible nuclei, the number of NMR experiments that have to be acquired to get accurate structural information is high. The strategy we present here, i.e. the multiplication of both the number of irradiation channels in the probe and the number of parallel receivers, offers one possibility to optimize this measurement time.     

Dissolution patterns caused by chemical etching of Al–Co–Cu and Al–Co–Ni decagonal quasicrystals with a HF–HNO3–H2O solution

Dissolution patterns essential for Al–Co–Cu and Al–Co–Ni decagonal quasicrystals (d-QCs) have been investigated in detail by chemical etching using a HF–HNO₃–H₂O solution followed by scanning electron microscopy (SEM) observations. The chemical etching of definite surface areas of the samples, which are either normal or parallel to the tenfold axes, using a solution with HF–HNO₃–H₂O (1?:?5?:?4 in volume ratio; 0°C; 5–10?min), produces a large number of microfacet pits of decagonal prismatic shape. In addition, the same etching test in combination with SEM observations was carried out on a deformed sample of the Al–Co–Ni d-QC, which had been subjected to concentrated mechanical stress at an elevated temperature of 850°C by means of the Vickers indentation technique. The morphological features of the resulting micropits and their possible origins are discussed on the basis of results obtained by SEM observations.