Approximant Crystal Research Articles

Formation and hydrogen adsorption properties of Ti-Hf-Ni quasicrystals and crystal approximants

The formation of a high-order rational approximant (RA) phase in rapidly quenched Ti-Hf-Ni alloys and the hydrogen absorption properties of that phase are reported. Electron diffraction patterns show systematic shifts of the diffraction spots from their expected positions for the icosahedral phase (i-phase); the diffraction patterns are consistent with those expected for a 3/2 RA phase RA-(Ti-Hf-Ni). Based on differential scanning calorimetry studies, RA-(Ti-Hf-Ni) is metastable, transforming between 350 and 500°C to a quasicrystal with strong phason disorder. This crystallizes at 620°C to a Ti 2 Nitype phase. RA-(Ti-Hf-Ni) readily absorbs hydrogen, up to 1.2 hydrogen atoms per metal atom ([H][M] =1.2). Pressure-composition isotherm studies for gasphase loading show that the pressure plateau for RA-(Ti-Hf-Ni) is similar to that observed for the i-(Ti-Zr-Ni) phase, although it occurs at as lightly higher pressure and extends over a smaller range of hydrogen concentrations. Unlike i-(Ti-Zr-Ni), no irreversible hydride phase forms with hydrogen loading at 250°C, suggesting that RA-(Ti-Hf-Ni) may have superior cycling properties, of interest for hydrogen storage applications.

Extended X-ray absorption fine-structure study of the hydrogen location in a Ti–Zr–Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Abstract Extended X-ray absorption fine-structure measurements have been performed on Ti-Zr-Ni alloys using the Dispositif de Collision dans l'Igloo synchrotron radiation facility. Both a multigrained icosahedral quasicrystal and a 1/1 bcc polycrystalline approximant were studied. Alloys hydrogenated to a hydrogento-metal ratio of 1.7 for the icosahedral phase and 1.4 for the crystalline phase were also studied. The local atomic structures, which are different around the different types of atom, titanium, zirconium and nickel, are very similar in the non-hydrogenated alloys, crystal or quasicrystal. Drastic changes are observed in the local order in both hydrogenated alloys with modifications of distances and an increase in the disorder. Moreover, these modifications are slightly different in the 1/1 and icosahedral alloys after hydrogenation.

Extended X-ray absorption fine-structure study of the hydrogen location in a Ti–Zr–Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Extended X-ray absorption fine-structure study of the hydrogen location in a Ti–Zr–Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

High-temperature specific heat of quasicrystals and a crystal approximant

Specific heat measurements have been performed by DSC in the temperature range from 430 to 1080 K for an icosahedral quasicrystal of Al–Pd–Mn, a decagonal quasicrystal of Al–Cu–Co and a crystal approximant of Al–Pd–Fe. The measured specific heat per atom at constant pressure c p( T) has been converted to that at constant volume c v( T) by use of the data of bulk modulus previously reported and the data of the thermal expansion coefficient which we measured by dilatometry. c v( T) of all three samples stay at around 3 k B up to about 700 K, as expected from Dulong–Petit's law. For the two quasicrystals, however, c v( T) increases dramatically and reaches 4.7 k B at 1080 K. In contrast, the deviation in c v( T) of the crystal approximant is small; it is about 3.6 k B at 1080 K. The origin of the large deviations in c v( T) of the quasicyrstals from Dulong–Petit's value is discussed in light of thermal excitation of phasons.

Cluster structure and hydrogen in Ti–Zr–Ni quasicrystals and approximants

Elastic neutron diffraction data from icosahedral Ti45Zr38Ni17 are presented and analyzed using information from the 1/1 approximant Ti50Zr35Ni15. These data indicate that similar clusters exist in the approximant and the i-phase. This is shown to be consistent with simulated diffraction from an icosahedral glass model of the quasicrystal, placing a Bergman cluster on the glass sites. An electrochemical method was used to hydrogenate Ti-based quasicrystals and their crystal approximants. This technique gives a consistently high hydrogen to metal atom ratio of 1.9, without crystal hydride formation in the quasicrystal.

Structural modelling of the Ti–Zr–Ni quasicrystal

The atomic structure of the icosahedral Ti–Zr–Ni quasicrystal is studied using a canonical cell tiling approach. An investigation of the previously proposed structure for W-Ti–Zr–Ni, the 1/1 crystal approximant to the Ti–Zr–Ni icosahedral quasicrystal, has revealed improbable occupation of sites in the region between the Bergman clusters and at the cluster centre. The 1/1 structure is refined for a better-ordered sample of the approximant, using X-ray and neutron diffraction measurements and ab initio relaxation studies. Based on this refined structure, an atomic decoration model for large canonical cell tilings is developed. Calculated diffraction patterns from the generated structures are in reasonable agreement with those measured for the icosahedral quasicrystal.

Ultrasonic study of hydrogen motion in a Ti-Zr-Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Ultrasonic attenuation measurements have been performed on the Ti-Zr-Ni alloy system over the temperature range 35--335 K. Both a multigrained icosahedral quasicrystal and a 1/1 bcc polycrystalline approximant were studied. The alloys were hydrogenated to a hydrogen to metal ratio of 0.79 for the icosahedral phase and 0.20 for the crystalline phase. Temperature-dependent attenuation peaks were observed in the alloys loaded with hydrogen while the hydrogen-free materials showed no unusual features. For measurement frequencies near 1 MHz the attenuation maxima occurred near a temperature of 250 K for the crystalline phase and 220 K for the icosahedral phase. The results imply that the hydrogen motion is about one order of magnitude faster in the icosahedral phase than in the approximant phase. Whether this result is due to an intrinsic difference between the two Ti-Zr-Ni phases or to a dependence on hydrogen concentration cannot be determined from the present measurements.

Containerless solidification studies of the α–1/1 crystal approximant in Ti–Cr–Si–O alloys

The nucleation behavior of α(TiCrSiO), a 1/1 Fibonacci crystal approximant phase, was investigated in alloys made near the stoichiometric composition. Containerless solidification studies were made with electromagnetic radio-frequency levitation and the 105-m NASA Drop Tube. The solidification microstructures indicate that the α–Ti hexagonal solid solution was the primary crystallizing phase in these alloys, growing dendritically. The α(TiCrSiO) phase nucleated in the remaining liquid. The competition between these two phases resulted from the high oxygen concentration needed to form α(TiCrSiO), which also stabilized the hexagonal-close-packed α–Ti phase.

Oxygen in Ti-(Cr, Mn)-Si icosahedral phases and approximants

Abstract Nearly single-phase icosahedral quasicrystals can be obtained in Ti-Cr and Ti-Mn alloys by rapid quenching, when the amounts of silicon and oxygen are carefully controlled. The role of phase stabilization by these elements, however, is unknown. Here, the essential role of both oxygen and silicon for the formation of phases with local icosahedral symmetry is demonstrated in these alloys. Rapidly quenched and as-cast samples of Ti-Cr-Si and Ti-Mn-Si alloys made without oxygen form primarily the β solid solution phase. In contrast as-cast alloys of Ti-Cr-Si containing more than 10 at.% oxygen and Ti-Mn-Si alloys made with more than 6at.% oxygen contain significant quantities of the 1/1 crystal approximant. Correspondingly, as-quenched alloys of these compositions form the icosahedral phase. Annealing studies of samples containing the 1/1 phase demonstrate that the oxygen has a thermodynamic rather than a kinetic role in phase stabilization. It is likely that the oxygen atoms bind with Ti atoms, form...

Neutron diffraction determination of hydrogen atom locations in theα(TiCrSiO)1/1 crystal approximant

Titanium/zirconium-based quasicrystals and their related crystal approximants have been identified as potential new materials for hydrogen storage applications. To better understand the local chemistry and atomic ordering in these phases, preferential interstitial sites for hydrogen/deuterium were determined for $\ensuremath{\alpha}(\mathrm{TiCrSiO}).$ This is a bcc 1/1 crystal approximant to the icosahedral quasicrystal phase that contains a two-shell, Mackay-icosahedral cluster of atoms at each bcc site. It absorbs hydrogen or deuterium, without formation of other hydride phases, to a maximum hydrogen to metal atom ratio $(H/M)$ of 0.26. For fully deuterated samples, both tetrahedral and octahedral interstitial sites are occupied with fractions of 0.14 and 0.12, respectively. Here, the hydrogen/deuterium sites are determined from a Rietveld analysis of x-ray and neutron powder diffraction data taken from samples of $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ loaded with deuterium. Only the octahedral sites are occupied in the partially deuterated samples $(D/M=0.11).$ A decrease in the oxygen concentration below the stoichiometric value for $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ leads to an increase in the total amount of hydrogen that can be absorbed, suggesting that these interstitial atoms are competing for the same octahedral interstitial sites.

Elastic moduli of a Ti-Zr-Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Elastic moduli of a Ti-Zr-Ni icosahedral quasicrystal and a 1/1 bcc crystal approximant

Magnetic susceptibility in Ti(Cr, Mn)SiO [formula omitted] crystal approximant as a function of oxygen concentration — no evidence for a minimum in N( EF)

To examine the possible roles for oxygen in the formation of the icosahedral and crystal approximant phases in Ti-TM-Si alloys (TM = V, Cr, Mn, Fe), the magnetic susceptibility was measured in the α(TiCrSiO) and α(TiMnSiO) 1 1 crystal approximant phases. The measured magnetic susceptibility, ξ, for α(TiCrSiO) is nearly independent of temperature over the range 6–300 K, showing no measurable Curie-Weiss component, The susceptibility of α(TiMnSiO) contains a small Curie-Weiss component corresponding to a local-moment ( j = 5 2 ) from only a small fraction (∼0.2%) of the Mn. The electronic density of states at the Fermi energy, N( E F ), estimated from the susceptibility contribution of the conduction electrons, changes monotonically with increasing oxygen in both alloys. This suggests that phase stabilization by oxygen does not arise from a Hume-Rothery-type mechanism in these alloys.

Hydrogen Sites In The α(TiCrSiO) 1/1 Approximant Phase

AbstractCrystal approximant phases are important because they are believed to have a similar local atomic structure to corresponding quasicrystals. Interstitial hydrogen is used as a probe of the local structures of the Ti-based quasicrystals and crystal approximants. Phase pure samples of ct(TiCrSiO), a Mackay type 1/1 crystal approximant, which were plasma etched and coated with a thin layer of Pd, were loaded with deuterium from the gas phase to a maximum deuterium atom to metal atom ratio (D/M) of 0.26. After deuteration, the sample becomes a fine powder and remains single-phase. A Rietveld structural refinement of powder x-ray and neutron diffraction data was made to determine the location of the interstitial sites. For the fully deuterated sample the deuterium atoms sit at both octahedral and tetrahedral sites. The octahedral sites are formed by six Ti atoms in the first and second shells of the icosahedra. Deuterium atoms are also located at the tetrahedral interstitial sites formed between the clusters along <100> directions. In the partially loaded samples with DiM = 0.11, the deuterium atoms occupy only octahedral sites, demonstrating a preference for these sites.

Ti/Zr-Based Quasicrystals - Formation, Structure And Hydrogen Storage Properties

AbstractMost common in Al-3d transition metal(TM) alloys, icosahedral quasicrystals (i-phase) are also found frequently in Ti/Zr-TM alloys. The Ti-TM-Si-O quasicrystals are metastable, produced only by rapid quenching, and are extremely disordered. In contrast, Ti-Zr-Ni i-phases are well ordered and stable, produced by relatively low temperature annealing, nearly 300°C below the melting temperature. Important for both basic and practical reasons, Ti/Zr-based quasicrystals can absorb significant quantities of hydrogen, up to two hydrogen atoms for each metal atom. The formation and structures of these quasicrystals and closely related complex crystal phases are discussed. The hydrogenation properties of these quasicrystals are presented and shown to be competitive with materials in current use for hydrogen storage and battery applications. Results from recent studies using hydrogen as a probe of the local structure of the quasicrystal and crystal approximant are presented.

Structural Phase Transitions Of Decagonal Quasicrystals

AbstractA two step model for the transformation of decagonal quasicrystals (DQC) to their crystalline approximants is presented. First, a partially disordered approximant domain structure results rapidly from small atomic displacements. Subsequently, atomic diffusion leads very slowly to an ordered approximant crystal (AC) structure. The model is discussed on atomic scale. Its basic idea is to describe the structure of a quasicrystal (QC) as incommensurately modulated structure (IMS). In that description, the average structure (AS) of a QC is at the same time the AS of all its irrational and rational approximants. In this approach, the positional quasicrystal- to-approximant-crystal (QC-AC) transformation can be performed by atomic displacements smaller than any interatomic distance. This mechanism, however, leads inherently to to a certain amount of chemical disorder. In most cases also some positional disorder will result. The tools developed for the description of phase transitions of IMS can analogously be used for QC. Examples for one- and two-dimensional quasiperiodic structures are discussed.

A new 1/1 crystal approximant to the stable Ti-Zr-Ni icosahedral quasicrystal

We report here the discovery of a new bcc 1/1 crystal approximant (W-phase) to the stable icosahedral quasicrystal (i-phase) discovered recently in Ti-Zr-Ni alloys. As-cast Ti-Zr-Ni ingots, which originally contain only the C14 Laves and the hcp solid-solution phases, transform almost completely to the i-phase after annealing at 570oC for 64 h. Anneals at higher temperature (610oC) lead to the formation of the bcc W-phase, with a = 1.432 nm. Preliminary X-ray 0 diffraction studies indicate that the structure of the W-phase is similar to that of the Al-Mg-Zn Bergman phase, or the isostructural Al-Li-Cu R-phase, supporting a previous proposal that i-(Ti-Zr-Ni) is structurally different from i-(Ti-Cr-Si-O) or i-(Ti-Mn-Si-O), but similar to i-(Al-Li-Cu) or i-(Al-Mg-Zn).

Electronic and vibrational properties of an Al-Cu-Ru icosahedral quasicrystal deduced from heat capacities

Heat capacity measurements have been made in the temperature range 1-310 K for a phasonless perfect quasicrystal Al65Cu20Ru15, an Al65Cu20Ru15 quasicrystal with phason disorder, and a crystal approximant Al70Cu20Ru10. The electronic heat capacity of the perfect quasicrystal was found to be essentially zero. The existence of some excitation associated with an energy of about 1.6 meV is proposed. The vibrational properties of the quasicrystal are distinctly different from those of the crystal approximant in the region T < ThetaD/10, indicating that the densities of states are significantly different at the corresponding energies.

Hydrogenation of titanium-based quasicrystals.

Measurements of hydrogen loading and unloading in a quasicrystal are reported. The icosahedral phase (i phase) in a Ti-Zr-Ni alloy demonstrates hydrogen absorption from the gas phase at a temperature of 260 \ifmmode^\circ\else\textdegree\fi{}C and a pressure of 40 atmospheres. The incorporation of hydrogen causes the quasilattice to expand, with an increase in quasilattice constant of almost 7%, from ${\mathit{a}}_{\mathit{i}}$=5.18 to 5.52 \AA{}. Similar abilities for storing hydrogen are found in high-order crystalline approximant and amorphous phases of like composition. Of the phases examined, the i phase absorbs the most hydrogen, giving a hydrogen atom to metal atom ratio ([H]/[M]) of 1.6. Initial studies indicate that hydrogen desorption is hindered by phase transitions to stable hydride phases. Hydrogen loading at lower temperatures with higher ${\mathrm{H}}_{2}$ pressures favors higher [H]/[M] ratios for the i phase and the crystal approximant, and also minimizes the growth of more stable crystalline hydride phases.

The role of oxygen in the formation of titanium-based icosahedral phases and crystal approximants

Abstract The first report of an oxygen-stabilized quasicrystal is presented. Oxygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys. Recent X-ray and neutron diffraction structural studies by us of the 1/1 approximant phase reveal specific sites for the oxygen in the Mackay icosahedral clusters. On the basis of these data, possible explanations for the role of oxygen in quasicrystal formation are considered. These include firstly the creation of a nucleation barrier for competing crystalline phases, secondly the stabilization of the fundamental Mackay icosahedral cluster and thirdly an adjustment of the electron-to-atom ratio to shift the Fermi surface into a minimum in the density of states.

Occupancy of central sites in the Zn49Mg32 quasicrystal from total-energy computations on its crystal approximant.

First-principles electronic-structure and total-energy calculations related to the 1/1 crystal approximant to the Al-Zn-Mg quasicrystal are carried out with different possible atoms at the center of the packing unit. The linear muffin-tin-orbital (LMTO) method with the atomic-sphere approximation and the muffin-tin correction to the Coulomb energy is employed. The density of states shows a very shallow minimum near the Fermi level, supporting the view that the pseudogap exists in the crystal approximants to quasicrystals even in the absence of a d-state contribution to the valence electrons. It is shown that packing units with Al or Zn at the centers are more stable than those with empty centers, supporting the recent positron-annihilation data and pseudopotential calculations, but disagreeing with qualitative conclusions from some LMTO calculations for crystal approximants to other quasicrystals.