Bct Phase Research Articles

3-d Model for strain ordering in steel: II Relaxational effects

The spring-defect model developed by us in the accompanying paper I to discuss ferroelasticity, exhibited in the BCT phase of α-iron (BCC metals), is used to analyse anelastic relaxation across the paraelastic to ferroelastic phase transition. The kinetics of the underlying Hamiltonian representing strain-strain interactions is treated within mean-field theory. The relaxation-response relation of the linear response theory is employed to derive explicit expressions for the anelastic strain, the frequency-dependent compliance and the internal friction in terms of the basic parameters of the spring-defect model.

Structural and magnetic properties of Fe/Rh(001) sputter deposited multilayers

We present details of the crystal structure and magnetic behavior of Fe/Rh (001) sputter deposited multilayer thin films. We find that the films undergo a structural phase transformation from a body-centered-tetragonal (bct) to a face-centered-tetragonal (fct) lattice which is concomitant with a magnetic transformation from a ferromagnetic to a nonferromagnetic state. We identify the Rh spacer thickness where both bct and fct phases first coexist as the critical thickness (tcrit). In the region prior to the critical thickness, both Fe and Rh are ferromagnetic with Rh moments of at least 1μB and Fe moments enhanced as much as 30% relative to their bulk value. In the postcritical region, we find the films to be nonferromagnetic. The experimental results are compared with theoretical calculations on similar structures.

Growth of pseudomorphic body centered tetragonal Mn films with an abrupt interface on Ag(001)

Mn films with a thickness of 2–15 monolayers (ML) have been deposited, in ultra-high vacuum, on a Ag(001) single crystal and analyzed by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). When a 12 ML film is deposited at room temperature (RT), a ∼3 ML-thick diffuse interface is formed, and Ag segregation occurs on top of an epitaxial body centered tetragonal (bct) Mn layer. If the same amount of Mn is deposited at 80 K in order to block interdiffusion, the film grows in a polycrystalline α-Mn phase. Annealing the sample at 300 K does not markedly change its structure. However, when a 3 ML template of Mn is first deposited on the substrate kept at 80 K, and then 9 ML at 300 K, the data show that the film is pseudomorphic and adopts an epitaxial bct phase, and that the system exhibits an abrupt interface.

Researching Face Centered Cubic Cs3C60

The possible formation of a fcc Cs3C60 phase which should have a high critical temperature is critically reviewed. Temptative synthesis of this derivative has been realized by playing on the kind of defects present in solid C60 and also by reacting C60 either with cesium vapor or with cesium dissolved in liquid ammonia. Whatever the experimental conditions, the fcc phase was never formed. Moreover, we could not observe the formation of bcc Cs3C60. Opposite to many statments, temperature does not modify drastically the nature of the phases formed, and even heat treatment up to 750°C leads, after cooling down, to a mixture of A15 and bct phases, whatever the method of preparation.

Pressure-induced phase transitions in Pa metal from first-principles theory

Protactinium metal is shown to undergo a phase transition to the {alpha}-U orthorhombic structure below 1 Mbar pressure. At higher pressures, the bct phase reenters in the phase diagram and at the highest pressures, an ideal hcp structure becomes stable. Hence, Pa undergoes a sequence of transitions; bct{r_arrow}{alpha}-U{r_arrow}bct{r_arrow}hcp, with the first transition taking place at 0.25 Mbar and the subsequent ones above 1 Mbar. The bct{r_arrow}{alpha}-U transition is triggered by the pressure-induced promotion of the spd valence states to 5f states. In this regard, Pa approaches uranium which at ambient conditions has one more 5f electron than Pa at similar conditions. At higher compression of Pa, the 5f band broadens and electrostatic interactions in combination with Born-Mayer repulsion become increasingly important and this drives Pa to gradually more close-packed structures. At ultrahigh pressures, the balance between electrostatic energy, Born-Mayer repulsion, and one-electron band energy stabilizes the hcp (ideal packing) structure. The electrostatic energy and Born-Mayer repulsion rule out open crystal structures under these conditions in Pa and between the close-packed structures, the hcp structure is shown to be stabilized by filling of the 5f band. {copyright} {ital 1997} {ital The American Physical Society}

Ball milling induced bct phase formation in iron and iron alloys

A variety of attritor ball milled Fe and Fe alloy powders, processed in an argon or a nitrogen environment, were studied using X-ray diffraction techniques. Broadened XRD spectra were obtained due to severe plastic deformation during ball milling. The broadening was non-uniform, displaying a systematic deviation from the average, with the (310) reflection peak being the broadest, and the (222) reflection peak being the sharpest. The systematic variation in broadening is interpreted as a tetragonal distortion. The tendency towards a tetragonal distortion occurred in all ball-milled samples.

Fe/Rh (100) multilayer magnetism probed by x-ray magnetic circular dichroism

We report the layer-averaged magnetic moments of both Fe and Rh in sputtered Fe/Rh (100) multilayer thin films as measured by x-ray magnetic circular dichroism. We observe two distinct regimes in these films. The first is characterized by Rh moments of at least 1${\ensuremath{\mu}}_{B},$ Fe moments enhanced as much as 30% above bulk, and a bct crystal structure. The second regime is distinguished by sharp declines of both Fe and Rh moments accompanied by a transition to an fct crystal lattice. The demarcation between the two regions is identified as the layer thickness for which both bct and fct phases first coexist, which we term the critical thickness ${t}_{\mathrm{crit}}.$ We attribute the change in magnetic behavior to the structural transformation.

Identification of metastable phases: face-centred cubic Ti

First-principles total-energy electronic structure calculations are used to find the epitaxial line of tetragonal Ti, i.e., the line of body-centred tetragonal (bct) structures produced by isotropic epitaxial strain on the (001) plane of tetragonal phases of Ti. An fcc phase and a higher-energy noncubic bct phase are found at minima of the total energy with respect to tetragonal deformations, but the bcc structure is found to be a saddle point of energy. The calculation yields the strained structures, the corresponding stresses, and the elastic stiffness coefficients on the epitaxial line. A segment of the epitaxial line between the two phases is shown to be inherently unstable, so unique meaning can be given to designating a structure as strained fcc Ti. The bulk structure of a film of Ti on Al{001} is shown to be strained fcc Ti, a new phase not on the phase diagram.

Evidence for a strain-stabilized bct phase of cobalt deposited on Pd{100}: An x-ray photoelectron diffraction study.

The structure of cobalt layers grown on a Pd{100} single-crystal surface has been studied by scanned angle x-ray photoelectron diffraction. At room temperature the cobalt layers grow in a structure related to the bulk cobalt fcc phase characterized by a large expansion of the in-plane lattice parameters and a corresponding contraction of the interlayer spacing (\ensuremath{\sim}10%). This structure persists to coverages greater than 30-layer equivalents, and can be interpreted as a metastable body-centered-tetragonal (bct) phase. \textcopyright{} 1996 The American Physical Society.

First-principles theory of iron up to earth-core pressures: Structural, vibrational, and elastic properties.

Ab initio electronic-structure calculations, based on density-functional theory and a full-potential linear-muffin-tin-orbital method, have been used to predict crystal-structure phase stabilities, elastic constants, and Brillouin-zone-boundary phonons for iron under compression. Total energies for five crystal structures, bcc, fcc, bct, hcp, and dhcp, have been calculated over a wide volume range. In agreement with experiment and previous theoretical calculations, a magnetic bcc ground state is obtained at ambient pressure and a nonmagnetic hcp ground state is found at high pressure, with a predicted bcc \ensuremath{\rightarrow} hcp phase transition at about 10 GPa. Also in agreement with very recent diamond-anvil-cell experiments, a metastable dhcp phase is found at high pressure, which remains magnetic and consequently accessible at high temperature up to about 50 GPa. In addition, the bcc structure becomes mechanically unstable at pressures above 2 Mbar (200 GPa) and a metastable, but still magnetic, bct phase ($\frac{c}{a}\ensuremath{\simeq}0.875$) develops. For high-pressure nonmagnetic iron, fcc and hcp elastic constants and fcc phonon frequencies have been calculated to above 4 Mbar. These quantities rise smoothly with pressure, but an increasing tendency towards elastic anisotropy as a function of compression is observed, and this has important implications for the solid inner-core of the earth. The fcc elastic-constant and phonon data have also been used in combination with generalized pseudopotential theory to develop manybody interatomic potentials, from which high-temperature thermodynamic properties and melting can be obtained. In this paper, these potentials have been used to calculate full fcc and hcp phonon spectra and corresponding Debye temperatures as a function of compression.

Structural properties of bulk copper: Pseudopotential plane-wave-basis study

We have examined the structural properties of bulk copper within the body-centered-tetragonal (bct) crystal system using the pseudopotential plane-wave-basis scheme. By calculating the total energy as a function of $\frac{c}{a}$ and volume, where $a$ and $c$ are in-plane and vertical lattice constants, we show that the bcc double minima and metastable bct phase, which were reported in the previous pseudopotential mixed-basis studies, are absent in the energy surface. Based on the calculated results, we suggest an alternative interpretation of a recent scanning tunneling microscopy experiment on Cu/Pd(100).

Magnetism of Body-Centered Tetragonal FeRh1-xPdxAlloys (I) Magnetic Properties

Crystalline structures and magnetic properties of the FeRh 1- x Pd x system were investigated. Substitution of Pd for Rh stabilized body-centered tetragonal (bct) structure of the ordered CuAu-type with increasing axial ratio c / a from 1.18 to 1.36. The bct alloys in the range x =0.530∼0.61 exhibited a first-order phase transition from antiferromagnetic to ferromagnetic state, while the bct alloys with x <0.530 exhibited a first-order phase transition from antiferromagnetic to Curie paramagnetic state. Both the axial ratio and the volume increased abruptly at the transitions. A new ferromagnetic phase with orthorhombic structure with the space group C m m m was found in the bct alloys with x =0.525∼0.61 below room temperature. This ferromagnetic phase possesses a larger magnetic moment than that of the bct phase. Reduction of the crystalline symmetry may have enhanced the magnetic moment.

Structural Phase Transformation and Phonon Softening in Iron-Based Alloys

The interrelation of Invar and Martensite in iron-based alloys like Fe 3 Ni is discussed on the basis of ab initio calculations for different crystal structures. Inelastic neutron scattering and ultrasonic experiments have shown that a soft transverse phonon mode in [110] direction acts as a precursor in the close packed fcc phase of these systems. Unfortunately there are no experimental results for phonon modes in the transformed bct phase. We have examined in detail the changes in the phonon dispersions of these systems by varying the ratio of the lattice constants along the Bain path. In recent calculations we have used the method of Varma and Weber in combination with the full potential LAPW (FLAPW) method to calculate frozen phonons for the tetragonal phase. Our new numerical results reproduce the experimental findings for the fcc phase and give an overview of the changes of the phonon energies when varying the c/a ratio towards the bct structure. In particular we observe that phonon softening is caused by the long-range electron-phonon interaction in systems with magnetovolume instabilities, and vanishes after the martensitic transformation.

Theoretical investigation of the high-pressure crystal structures of Ce and Th.

The high-pressure crystal structures of Ce and Th are studied theoretically by means of a full potential linear muffin-tin orbital technique, where the generalized gradient correction to density-functional theory has been implemented. A crystallographic phase transition from fcc to bct is found for u-Ce at about 10 GPa, in good agreement with experiment. The calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For n-Ce a subsequent transition to the hcp crystal structure is predicted at about 1600 GPa. For Th a continuous transformation from fcc to bct at 45 GPa is found, confirming recent experimental findings. Also for this material the calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For Th we also predict a transformation to the hcp crystal structure at even higher pressures. The metals Ce (Z= 58) and Th (Z= 90) are located at the beginning of the 4f (lanthanides) and the Sf (actinides) transition-metal series, respectively. Ce is the first lanthanide metal that has an appreciable occupation of the 4f states, and is a known subject for controversy. Primarily the debate concerns the localized versus itinerant character of the 4f electron. The electronic structure of Ce has been reported earlier. As regards the 5f population at ambient conditions, calculations show that the actinide metal Th has a smaller f occupation than ct Ce Se-ver.al recent investigations of these two metals, both experimental as well as theoretical, have been motivated by their crystallographic phase transitions under compression. In the lanthanide series localized 4f states are progressively being filled and their inhuence on the chemical bonding is negligible. In the actinide series, however, the earlier metals have delocalized 5f electrons that contribute to the chemical bonding. The increasing 5f occupation as a function of atomic number results in increasingly distorted (complex) crystal structures. This is understood from the increased contribution to the bonding from the narrow 5f bands. ' Particularly interesting in the present context is the occurrence of the bct (c/a =0.825) structure at ambient conditions for Pa (Z=91). The increased 5f occupation in Pa, in comparison to Th, results in a bonding which favorizes the bct crystal structure over the fcc structure (which is found in Th at ambient pressure). The effect of pressure upon an itinerant 5f metal is first a lowering of the energy of the 5f bands relative to the other valence states and second a broadening of the Sf band as well as a broadening of the other bands. The relative lowering of the Sf states results in an increased Sf population. Therefore, in view of the known crystal structure of Pa (bct), it is not surprising to find a pressure-induced onset of a bct crystal structure in Th. Since u-Ce from certain aspects can be regarded as an analog element to Th, a similar behavior (stabilization of the bct structure) of Ce under pressure is not surprising either. However, for Ce, in contrast to Th, there is an intermediate bodycentered-monoclinic (bcm) phase found experimentally in the interval 6— 12 GPa. This bcm structure can be seen as a small distortion of the bct crystal structure, and can therefore be, approximately, described as a bct structure. In fact both the bcm and the bct phases of Ce at moderate compressions can be viewed as more or less distorted fcc structures. We note here that the experimental structure is not quite clear for Ce since it has been reported that an orthorhombic n-U type of structure is stabilized, instead of the bcm structure. In the present report we take the view point that the bcm and the u-U structures are very close in energy and that the u-U

Magnetism of bct Fe in (100) FeIr superlattices

We have shown in a previous paper that iron can be grown in a low-spin ferromagnetic bct phase in (100) FeIr superlattices. Additional results on this system are given in this paper. The very good quality of the superlattices is shown to be due to the two-dimensional growth of both Fe on Ir and Ir on Fe. The ferromagnetic order is confirmed by Arrott plot experiments. Two magnetically dead layers are observed, which makes theoretical analysis of the magnetic properties difficult.

Pressure-concentration (P-X) diagram for the Ce-Th alloy system

Pressure is an important variable in the phase stability of rare earth and actinide metals and alloys. This is largely due to the enormous change of the f-electron character induced by high pressure. Under high pressure, the increased f-electron overlap and interactions between the f-electrons and the spd conduction electrons are the main causes of the change of f-electron behavior and the relevant phase transformations. Diamond anvil cell techniques now allow for pressures over 300 GPa and this has given new impetus to the field of structural phase transformations. In this paper, the authors report their high pressure measurements on a series of Ce{sub 1{minus}x}Th{sub x} alloys performed at ambient temperature in a diamond anvil cell using synchrotron x-ray source. In particular, the authors focus their attention on the {gamma}(fcc) to {alpha}(fcc) and {alpha} to bct (body centered tetragonal) phase transformations caused by application of high pressure. The data is combined with the earlier measurements to construct a P-X diagram for the Ce-Th Alloy system.

Magneto-Volume and Tetragonal Elongation Effects on Magnetic Phase Transitions of Body-Centered Tetragonal FeRh1-xPtx

Magnetic properties of FeRh 1- x Pt x system with body-centered cubic and body-centered tetragonal (bct) structures were studied by means of X-ray diffractometry, magnetometry and Mössbauer spectroscopy, and the phase diagram in a temperature-composition plane was determined. With substituting Pt for Rh, the axial ratio c / a was increased and the bct phase was stabilized. With increasing temperature, the bct alloys with 0.72< x <0.81 underwent a first-order phase transition from the antiferromagnetic state to the ferromagnetic state, while the alloys with 0.3< x <0.72 exhibited a first-order phase transition from the antiferromagnetic state to the paramagnetic state with a volume expansion and a tetragonal lattice distortion. The mechanism of those phase transitions was discussed in terms of the magneto-volume and the tetragonal elongation effect.

Magnetism in FeRh Alloys

The magnetic properties of Fei_,Rh, ordered and disordered alloys were studied in the last years [1-8]. Recently, Miyajima and Yuasa [2] have found that the fcc FeRh alloy was paramagnetic and stable at the room temperature. The change of bcc structure to a mixture phase composed of fcc and bct phases was performed by an application of uniaxial pressure of 2.5 x 10 4 kg/cm2 to the bcc FeRh alloy. The experimental lattice constant for bcc FeRh alloy was a = 2.981 A, and for the disordered fcc FeRh a = 3.731 A. In the bct FeRh Miyajima and Yuasa [2] found a = 2.81 A and c = 3.38 A (c/a = 1.203). In this paper we study the change of the electronic structure and magnetic moment in FeRh ordered alloy during the structural transformation from the CsCl to CuAu type structure. The method of calculations is presented in Sec. 2 and the main results are discussed in Sec. 3. Finally we give some conclusions.

Structure Sequence and Physical Properties of Rubidium Fulleride C70Rbx

ABSTRACTA structure sequence of doped fcc, bct, bcc and Rb-saturated fcc phases was observed in C70Rbx with increasing Rb concentration using x-ray-diffraction measurements. The composition of the Rb-saturated phase was estimated to be C70Rb9.4±0.4 from a weight measurement. A model for the structure of C70Rb9 is proposed. Superconductivity in each phase was checked using ac magnetic-susceptibility measurements. All these phases were nonsuperconducting down to 1.3 K.

Structure sequence and possible superconductivity in potassium-doped fullerene C70Kx.

A sequence of doped fcc or doped rhombohedral, bct, bcc, and K-saturated fcc phases was observed in ${\mathrm{C}}_{70}$${\mathrm{K}}_{\mathit{x}}$ with increasing K concentration using x-ray-diffraction measurements. A model for the structure of the K-saturated fcc phase ${\mathrm{C}}_{70}$${\mathrm{K}}_{9}$ is proposed. Superconductivity in each phase was checked using ac magnetic-susceptibility measurements. A small diamagnetic transition was observed below 12 K in the doped rhombohedral phase. All other phases were nonsuperconducting down to 1.3 K.