Giant Unit Cell Research Articles

Geometric origin of magnetic frustration in theμ-Al4Mn giant-unit-cell complex intermetallic

The structurally ordered μ-Al4Mn complex intermetallic phase with 563 atoms in the giant unit cell shows the typical broken-ergodicityphenomena of a magnetically frustrated spin system. The low-field zero-field-cooled andfield-cooled magnetic susceptibilities show splitting below the spin freezing temperatureTf = 2.7 K. The ac susceptibility exhibits a frequency-dependent cusp,associated with a frequency-dependent freezing temperatureTf(ν). The decay of the thermoremnant magnetization is logarithmicallyslow in time and shows a dependence on the aging timetw and thecooling field Hfc typical of an ultraslow out-of-equilibrium dynamics of a nonergodic spinsystem that approaches thermal equilibrium, but can never reach it onthe experimentally accessible time scale. The above features classify theμ-Al4Mn complex intermettalic among spin glasses. The origin of frustration of magneticinteractions was found to be geometrical due to the distribution of a significantfraction of Mn spins on triangles with antiferromagnetic coupling. Theμ-Al4Mn phase is a geometrically frustrated spin glass.

Electrical resistivity of the μ-Al4Mn giant-unit-cell complex metallic alloy

The μ-Al4Mn complex intermetallic phase with 563 atoms in its giant unit cell exhibits a complicated temperature dependence of electrical resistivity that has a broad maximum at about 175 K and a minimum at 13 K. The temperature dependence of the resistivity was reproduced by employing the theory of quantum transport of slow charge carriers, which predicts a crossover from the metallic (Boltzmann-type) positive-temperature-coefficient electrical resistivity at low temperatures to the insulator-like (non-Boltzmann) negative-temperature-coefficient resistivity at elevated temperatures. The low-temperature resistivity minimum was reproduced by considering it as a magnetic effect due to increased scattering of the conduction electrons by the Mn spins on approaching the spin glass phase that develops below the spin freezing temperature Tf = 2.7 K.

Thermal conductivity of Taylor phase T-Al73Mn27 complex metallic alloy

Thermal conductivity coefficient, κ of Taylor phase T-Al73Mn27 complex metallic alloy with giant unit cell has been studied in the temperature interval from 2 K to 300 K. κ is relatively small in magnitude (comparable to that of thermal insulators), have a slope-change at about 50 K and increases up to 300 K. Such a low κ originates in complex structure: aperiodic in short length scale, that leads to frequent electron scattering and consequent low electron contribution to the thermal conductivity, while large lattice constant defines a small Brillouin zone that enhances umklapp scattering of extended phonons, and suppressing lattice thermal conductivity. Above ~100 K hopping of localized lattice vibrations gives an additional heat-carrying channel.

LaMg 11 with a giant unit cell synthesized by hydrogen metallurgy: Crystal structure and hydrogenation behavior

We have successfully applied a hydrogen metallurgy synthesis to yield LaMg 12− x intermetallic at 500 °C. During a vacuum thermal desorption from the LaH 3–MgH 2 nanocomposite prepared by reactive ball milling in H 2, hydrogen desorption from the La dihydride occurred via a mechanism of cooperative phase transformation at temperatures being 400° lower than the desorption from pure LaH 2. The crystal structure of the intermetallic was determined by synchrotron X-ray diffraction (SR XRD). The orthorhombic LaMg 12− x has a giant unit cell, the volume of which exceeds 8000 Å 3. In situ SR XRD studies showed the fine details of the hydrogen desorption process with several parallel and consecutive transformation steps.

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A(3) and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. These cluster-based structures are closely related to the class of Frank-Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase.

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

This is the second of two papers, where we discuss the cluster structures of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A3 and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. The structures can be described as packings of clusters such as fullerenes, dodecahedra, pentagonal bifrusta and Friauf polyhedra. A characteristic feature of the two larger structures are nets of hexagonal bipyramidal Ta clusters (h.b.p.). The extremely short distance of 2.536-2.562 A between their apical Ta atoms indicates unusually strong bonding. The large h.b.p. nets are sandwiched between slabs of Friauf polyhedra resembling the structure of the mu phase.

Crystal growth of copper-rich ytterbium compounds: The predicted giant unit cell structures YbCu 4.4 and YbCu 4.25

Two new phases YbCu 4.4 and YbCu 4.25 are found as a result of careful phase diagram investigations. Between the congruent and peritectic formation of YbCu 4.5 and YbCu 3.5, respectively, the phases YbCu 4.4 and YbCu 4.25 are formed peritectically at 934(2) °C and 931(3) °C. Crystal growth was realised using a Bridgman technique and single crystalline grains of about 50–100 μm were analysed by electron diffraction and single crystal X-ray diffraction. Due to the only slight differences in both compositions and formation temperatures the growth of larger single crystals of a defined phase is challenging. The compounds YbCu 4.4 and YbCu 4.25 fit in Černýs [J Solid State Chem 2003;174:125] building principle {(RECu 5) n ·(RECu 2)} where RE = Yb with n = 4 and 3. YbCu 4.4 and YbCu 4.25 are based on AuBe 5/MgCu 2-type substructures and contain approximately 4570 and 2780 atoms per unit cell. The new phases close the gap in the series of known copper-rich rare-earth compounds for n = 1, 2 (DyCu 3.5, DyCu 4.0) and n = 5 (YbCu 4.5, DyCu 4.5).

Thermal and electrical conductivities in Al-based complex metallic alloys

A study is reported of the heat transport of AlPd(Mn, Fe, Co, Rh) complex metallic alloys that belong to an interesting class of Al-based alloys characterized by giant unit cells with quasicrystal-like cluster substructure. The electrical and thermal conductivity were investigated in the temperature range 8–300 K, in order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the transport properties of these materials. The alloys have electrical conductivity typical of metallic alloys, and low thermal conductivity which is at room temperature comparable to that of thermal insulators, e.g. amorphous SiO2 and Zr/YO2 ceramics.

Diffusion processes during heat treatment of Al–Cr–Fe thin films

Complex metallic alloys (CMAs) are a group of materials that have a giant unit cell and a high density of point defects. Among the more interesting phases is the hexagonal Al 4Cr, containing 574 atoms, which can also be alloyed with iron to form a ternary Al 4(Cr, Fe). One possibility to deposit such thin films is by sequential deposition of constituent elements and subsequent heat treatment in order to homogenize the depth profile. For this purpose bi- and trilayers were deposited in a triode sputtering apparatus. Heat treatment was performed in vacuum in the temperature range of 300–600 °C and annealing times up to 5 h. The evolution of depth profiling was evaluated by Auger electron spectroscopy (AES). It was also observed by indirect methods, such as X-ray diffraction (XRD), nanoindentation and electric resistivity measurement. A major change in depth profile was found to be accompanied by a change of present phases, a change in nanohardness and in electrical resistivity. These indirect methods are simple yet effective ways to follow the heat treatment. In simple bilayer systems, the quickest process is the diffusion of chromium into aluminum (occurs already at 400 °C), to be followed by the couple Al/Fe at 500 °C.

Magnetic and transport properties of the giant-unit-cell Al 3.26Mg 2 complex metallic alloy

The β-Al 3Mg 2 complex metallic alloy comprises about 1168 atoms in the giant-unit cell, making this material excellent candidate to investigate how the exceptional structural complexity and the coexistence of two different length scales – one defined by the unit-cell parameters and the other by the cluster substructure – affect physical properties of a metallic material. We have investigated magnetic, electrical, thermal transport and thermoelectric properties of a monocrystalline and a polycrystalline Al 3.26Mg 2 sample in a mixed β–β′ phase, grown by the Czochralski technique. Electrical resistivity is in the range ρ ≈ 30–40 μΩ cm and exhibits T 2 dependence at low temperatures and T at higher temperatures, resembling nonmagnetic amorphous alloys. Magnetic susceptibility χ measurements revealed that the samples are Pauli paramagnets with significant Landau diamagnetic orbital contribution. The susceptibility exhibits a weak increase towards higher temperature. Combined analysis of the ρ( T) and χ( T), together with the independent determination of the Pauli susceptibility via the NMR Knight shift suggests that the observed temperature dependence originates from the mean-free-path effect on the orbital susceptibility. The electronic density of states (DOS) at the Fermi energy E F was estimated by NMR and was found to amount about 90% of the DOS of the fcc Al metal. Thermal conductivity contains electronic, Debye and hopping of localized vibration terms, whereas the thermopower is small and negative. High structural complexity of the β-Al 3Mg 2 complex metallic alloy does not result in high complexity of its electronic structure. We found no evidence for the existence of a pseudogap in the DOS at E F.

Orientation-dependent NMR study of the giant-unit-cell intermetallicsβ−Al3Mg2, Bergman-phaseMg32(Al,Zn)49, andξ′−Al74Pd22Mn4

We present a $^{27}\mathrm{Al}$ NMR study of three giant-unit-cell complex metallic compounds, $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Al}}_{3}{\mathrm{Mg}}_{2}$, the ``Bergman-phase'' ${\mathrm{Mg}}_{32}{(\mathrm{Al},\mathrm{Zn})}_{49}$, and ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$, which contain some hundreds up to more than a thousand atoms in the unit cell. The NMR spectra of monocrystalline samples are strongly inhomogeneously broadened by the electric quadrupole interaction and the line shapes are featureless and powderlike, but still exhibit significant orientation-dependent variation of the intensity on the satellite part of the spectrum in the magnetic field. Measuring orientation-dependent satellite intensity in appropriate frequency windows yields rotation patterns that can be related to the structure and symmetry of the giant unit cells. For a theoretical reproduction of the rotation patterns, we derived a distribution of the electric-field-gradient (EFG) tensors for each of the investigated compounds from existing structural models using point-charge and ab initio calculations. The EFG distribution yields important structural information on the manifold of different local atomic environments in the unit cell and distinguishes crystallographically inequivalent lattice sites from the equivalent ones. The distribution of the EFGs for the 1168-atom unit cell of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Al}}_{3}{\mathrm{Mg}}_{2}$ was successfully determined by a point-charge calculation, whereas the ab initio approach was successful for the 160-atom unit cell of the ${\mathrm{Mg}}_{32}{(\mathrm{Al},\mathrm{Zn})}_{49}$ Bergman phase. For the 258-atom unit cell of ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$, the experimental rotation patterns revealed a pseudotenfold symmetry, whereas the theoretical point-charge calculation revealed predominant twofold symmetry with traces of tenfold symmetry, so that no quantitative matching between the theory and experiment could be obtained.

Nuclear magnetic resonance reveals ‘forbidden’ symmetries in quasicrystals and related metallic alloys with giant unit cells

We report on measurements of angular dependences of the selectively excited NMR intensity in a decagonal AlNiCo quasicrystal, icosahedral AlPdMn quasicrystal and orthorhombic ξ′-AlPdMn single crystal with approximately 258 atoms in the unit cell. In all three cases, we found 10-fold rotation patterns, in agreement with structural properties of the investigated samples. However, in the orthorhombic ξ′-AlPdMn complex metallic alloy, a non-perfect 10-fold symmetry was observed, which is a consequence of periodicity that strictly forbids non-crystallographic rotational symmetries. These measurements represent a starting point for testing structural models of quasicrystals and related metallic alloys with giant unit cells by means of NMR.

Complex ɛ-phases in the Al–Pd-transition–metal systems: Towards a combination of an electrical conductor with a thermal insulator

ɛ-Phases in the Al–Pd–(Mn,Fe,Co,Rh,…) alloy systems form in wide compositional ranges and belong to the interesting class of complex intermetallics that are characterized by giant unit cells with quasicrystals-like cluster substructure. In order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the physical properties of the material, we performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the ɛ-phases in the Al–Pd–Fe, Al–Pd–Co and Al–Pd–Rh systems. Magnetic measurements reveal that the materials are diamagnetic, containing tiny fractions of magnetic transition–metal atoms (of the order 10–100 ppm). Electrical resistivity is moderate, of the order 10 2 μΩ cm, and shows weak temperature dependence (in most cases of a few %) in the investigated temperature range 4–300 K. An interesting feature of the ɛ-phases is their low thermal conductivity, which is at room temperature comparable to that of thermal insulators amorphous SiO 2 and Zr/YO 2 ceramics. While SiO 2 and Zr/YO 2 are also electrical insulators, ɛ-phases exhibit electrical conductivity typical of metallic alloys, so that they offer an interesting combination of an electrical conductor with a thermal insulator. The reason for the weak thermal conductivity of the ɛ-phases appears to be structural: large and heavy atomic clusters of icosahedral symmetry in the giant unit cell prevent the propagation of extended phonons, so that the lattice can no more efficiently participate in the heat transport. The thermoelectric power of the investigated ɛ-phase families is small, so that these materials do not appear promising candidates for the thermoelectric application.

Physical properties of the complex metallic alloy phases in the Al–Pd–Mn system

The Al–Pd–Mn system of intermetallics contains complex metallic alloy (CMA) phases, whose crystal structures are based on giant unit cells comprising up to more than a thousand atoms per cell. We performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the ξ′ phase and the related Ψ phase on single-crystalline samples.

Magnetic, electrical, thermal transport, and thermoelectric properties of theξ′andΨcomplex metallic alloy phases in the Al-Pd-Mn system

The Al-Pd-Mn system of intermetallics contains complex metallic alloy (CMA) phases, whose crystal structures are based on giant unit cells comprising up to more than a thousand atoms per cell. We performed investigation of the magnetic, electrical, and thermal transport and thermoelectric properties of the ${\ensuremath{\xi}}^{\ensuremath{'}}$ phase and the related $\ensuremath{\Psi}$ phase on single-crystalline samples grown by the Bridgman technique. The samples are diamagnets with a tiny paramagnetic Curie-like magnetization and an estimated fraction of magnetic Mn atoms about 100 ppm. The electrical resistivity between 300 and 4 K exhibits a temperature variation of less than 2%. The origin of this temperature-compensated resistivity is analyzed in terms of the spectral conductivity model. The thermal conductivity of the samples is small and can be described by the sum of the electronic and lattice contributions, which are of comparable size at room temperature. The lattice contribution can be reproduced by the sum of the Debye term (long-wavelength phonons) and the term due to hopping of localized vibrations. The thermoelectric power is small and negative, compatible with a low concentration of electrons as the majority charge carriers. The studied physical properties of the giant-unit-cell CMA phases in the Al-Pd-Mn system are in many respects intermediate between those of metals or simple intermetallics and quasicrystals, suggesting that both the polytetrahedral local atomic order and the large-scale periodicity influence the physical properties of the material.

Structurally complex alloy phases

Structurally complex alloy phases are exceptional metallic systems based on crystal structures with giant unit cells comprising up to more than a thousand atoms per cell. They have since long been studied in crystallography. Nevertheless, they represent interesting new materials, since until recently hardly any studies have been carried out on their physical properties. An overview is presented offering first a characterization of these materials with respect to their salient structural features. Subsequently, the results of recent experimental and theoretical work on plastic deformation and electronic properties are reviewed.

ChemInform Abstract: Dy117Co57Sn112, a New Structure Type of Ternary Intermetallic Stannides with a Giant Unit Cell.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The new ternary intermetallide with a giant unit cell in the Nd–Ru–Sn system

The new ternary intermetallide with a giant unit cell in the Nd–Ru–Sn system

Crystallographic characteristics of the Al-Co decagonal quasicrystal and its monoclinic approximant τ2-AI13Co4

The binary Al-Co decagonal quasicrystal (DQC) is stable between 800 °C and 900 °C and transforms to a primitive monoclinic phase (a = 3.984 nm,b = 0.8148 nm,c = 3.223 nm, and β = 107.97 deg) above 950 °C or below 750 °C. The composition of this crystalline approximant is similar to theC-centered monoclinic Al13Co4 but witha andc parameters τ2 (τ = (1 + √5)/2 = 1.61803. . .) times larger. This τ2-A113Co4 occurs frequently as (001) and (100) twins. The transformation of the Al-Co DQC to (100) twins of τ2-A113Co4 has been followed by selected-area electron diffraction. With reference to the main zonal electron diffraction patterns and the X-ray diffractogram of the Al-Co DQC, the X-ray powder diffractogram of τ2-A113Co4 with a giant unit cell can be successfully indexed.

Indexing of X-ray powder diffractograms of phases with giant unit-cells with the aid of electron diffraction patterns

Indexing of X-ray powder diffractograms of phases with giant unit-cells with the aid of electron diffraction patterns