Crystallographic Transition Research Articles

Austenitic-ferritic transition joint cracking under thermal fatigue loading

The use of transition joints is connected with problems caused owing to differences between austenitic-ferritic thermal expansion coefficient, moderate oxidation resistance of the low-alloy ferritic steel, metallurgical deterioration caused by elevated temperature service, and decarburization. Additional factors that come out of pure operational conditions and stochastic parameters deteriorate the problem, which in some cases results in crack formation. The problem can be enhanced when crystallographic transitions occur at high temperatures. The focus of this paper is the study of failure in austenitic-ferritic transition joints in connection with simultaneous cell transition when the ferritic material temperature goes beyond the critical eutectoid temperature of the respective ferritic steel.

High pressure response of nanophase materials

We consider the behaviour of nanophase materials under high pressure thermodynamic conditions along the room temperature isotherm. Concurrent high temperatures exceeding a few hundred degrees at high pressure are avoided to ensure that no grain-coarsening occurs, in which case there may be a reversion back to investigating bulk behavior. The effect of pressure in the bulk is normally to induce a structural transition to a high pressure phase which has the lowest Gibbs free energy in P-T phase space. What are the implications for such phase transitions in ultrafine nanophase materials with grain sizes approaching critical dimensions, d∼10 nm in some systems? More than one quarter of the total atoms in such a nano-grain are constituted in the surface, and the surface energy cost involved in the formation of a new phase is significant. The pressure response of ultrafine nano-materials will be exemplified by the anatase polymorph of TiO2 a highly topical wide band-gapmaterial. Anatase is particularly useful in this respect because the pressure response is not sensitively dependent on the quality (e.g., stoichiometry) of the sample, nor on whether a pressure transmitting medium for quasi-hydrostaticity is used or not. The implications of nanometer size grains on the known pressure-induced phase transitions of anatase have been investigated up to 30-40 GPa, using synchrotron XRD complemented by laser Raman measurements. At room temperature the bulk material undergoes a crystallographic transition from the tetragonal structure to the orthorhombic _-PbO2-type structure at 2-5 GPa, and then to the monoclinic baddeleyite phase at 12-15 GPa. The pressure response of ultrafine nano-anatase of d ( 10 nm grain size, is shown to be radically different to that of its macro-crystalline analog [1,2]. The anatase polymorph is stable to appreciably higher pressure than in bulk. In these ultrafine nano-anatase samples long range order eventually collapses at P > 20 GPa, although signatures of short-range ’’crystallinity’’ of the anatase polymorph are apparent up to the highest pressure of this study. The ’’ultra-stability’’ of nano-anatase is specific to grain sizes of d∼10 nm and below. This may be rationalised in terms of the thermodynamics involved in a mechanism of nucleation and growth of a new phase, and partly by usingMD simulations for the ultrafine samples where computational work is tractable. The effect of grain-size onmechanical properties (e.g., compressibility-ductility) is also deduced from the pressure response (P-V EOS data). The inverse Hall-Petch relationship explains the maximum or plateau in stiffness-hardness at a critical value of small grain size. There will be brief mention of the pressure response of other nanophase systems.

Structural and magnetic properties of Cu–Ni–Cr spinel oxides

The compounds CuCr 2O 4 and NiCr 2O 4 crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I4 1/amd, with axes ratio c/ a<1 and >1, respectively. The distortion is caused by the Jahn–Teller ions Cu 2+ and Ni 2+ which flatten or elongate their surrounding oxygen tetrahedron. CuCr 2O 4 and NiCr 2O 4 form a complete solid solution series Cu 1− x Ni x Cr 2O 4 where for 0.825< x<0.875 members with orthorhombic symmetry were found. Using neutron powder diffraction and thermal analysis methods several members of the solid solution series were investigated. On cooling, all samples showed a temperature-dependent crystallographic phase transition from cubic to tetragonal symmetry between 865 K (CuCr 2O 4) and 310 K (NiCr 2O 4). The phase Cu 0.15Ni 0.85Cr 2O 4 undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T=300 K. The neutron diffraction experiments as well as SQUID measurements reveal magnetic ordering of the ions between 150 and 50 K which partially occurs as a two-step mechanism.

Exploration of the conformational space of myosin recovery stroke via molecular dynamics

Muscle contractions are driven by cyclic conformational changes of myosin, whose molecular mechanisms of operation are being elucidated by recent advances in crystallographic studies and single molecule experiments. To complement such structural studies and consider the energetics of the conformational changes of myosin head, umbrella sampling molecular dynamics (MD) simulations were performed with the all-atom model of the scallop myosin sub-fragment 1 (S1) with a bound ATP in solution in explicit water using the crystallographic near-rigor and transition state conformations as two references. The constraints on RMSD reaction coordinates used for the umbrella sampling were found to steer the conformational changes efficiently, and relatively close correlations have been observed between the set of characteristic structural changes including the lever arm rotation and the closing of the nucleotide binding pocket. The lever arm angle and key residue interaction distances in the nucleotide binding pocket and the relay helix show gradual changes along the recovery stroke reaction coordinate, consistent with previous crystallographic and computational minimum energy studies. Thermal fluctuations, however, appear to make the switch-2 coordination of ATP more flexible than suggested by crystal structures. The local solvation environment of the fluorescence probe, Trp 507 (scallop numbering), also appears highly mobile in the presence of thermal fluctuations.

Low‐temperature cooling behavior of single‐domain magnetite: Forcing of the crystallographic axes and interactions

Low‐temperature cycling (LTC) of magnetic remanences carried by rocks has become a standard technique in paleomagnetism, rock magnetism, and environmental magnetism as a means of identifying mineralogy and grain size. LTC usually involves measuring low‐temperature thermomagnetic curves on cooling through crystallographic transitions, such as magnetite's Verwey transition. Historically, it has been assumed that remanence carried by single‐domain (SD) magnetite grains is not affected by cooling through the cubic/monoclinic Verwey transition, whereas larger multidomain (MD) magnetite grains partially demagnetize. However, it has been recently pointed out that the shape anisotropy even for an infinitely long cylinder is approximately 3 times smaller than the monoclinic magnetocrystalline anisotropy along the hardest axis, i.e., SD remanences are not impervious to LTC. Using a micromagnetic algorithm we simulate LTC curves for assemblages of effectively elongated SD magnetite grains and consider the contribution of magnetostatic interactions. Initially, we assume that relationship between the cubic and monoclinic symmetry is chosen randomly; however, there are key experimental features, which this model does not explain. A new “controlled switching” model is developed; the orientation of the low‐temperature monoclinic axes are not chosen randomly, but instead are controlled by the direction of the magnetic moment on cooling through the Verwey transition. This new model correctly predicts experimentally observed low‐temperature trends that the “random” model does not. We therefore propose a new model for the mechanism controlling the behavior of SD grains at the Verwey transition and show that the low‐temperature behavior of SD and MD grains can yield ambiguously similar behavior.

Deformation-induced phase transformation and strain hardening in type 304 austenitic stainless steel

Deformation-induced phase transformation in a type 304 austenitic stainless steel has been studied in tension at room temperature and −50 °C. The evolution of transformation products was monitored using X-ray diffraction (XRD) line profile analysis of diffraction peaks from a single XRD scan employing the direct comparison method. Crystallographic texture transitions due to deformation strain have been evaluated using (111) γ pole figures. The tensile stress-strain data have been analyzed to explain the influence of underlying deformation-induced microstructural changes and associated texture changes in the steel. It is found that the initial stage of rapidly decreasing strain hardening rate in type 304 steel is primarily influenced by hcp ɛ-martensite formation, and the second stage of increasing strain hardening rate is associated with an increase in the α′-martensite formation. The formation of ɛ-martensite is associated with a gradual strengthening of the copper-type texture components up to 15 pct strain and decreasing with further strain at −50 °C. Texture changes during low-temperature deformation not only change the mechanism of ɛ-martensite formation but also influence the strain rate sensitivity of the present steel.

Magnetocaloric effect of Gd 5Si xSn 4−x Alloys

The crystal structure and magnetocaloric effect of Gd 5Si x Sn 4− x (with x = 2.4 , 2.6 and 2.8) alloys were studied by means of X-ray power diffraction (XRD) and magnetic measurements. From the XRD results, these alloys adopt a Gd 5Si 4-type structure for x = 2.8 , Gd 5Si 4-type and Gd 5Si 2Ge 2-type mixed structures for x = 2.4 and 2.6, while some minor phases can also be found. The Curie temperatures of the Gd 5Si x Sn 4− x increases gradually when x increases from 276 K for x = 2.4 , to 301.5 K for x = 2.8 . Magnetic entropy changes of these alloys at a magnetic field change of 0–1.8 T are 1.88, 2.26 and 1.69 J/kg K for x = 2.4 , 2.6 and 2.8, respectively. The temperature-dependent XRD analysis shows that there is no crystallographic transition for these alloys, which can explain their low magnetic entropy changes.

Characterization of the phase transitions of ethyl substituted polyhedral oligomeric silsesquioxane

This study describes the synthesis and molecular mobility of both partially deuterated and fully protonated ethyl polyhedral oligomeric silsesquioxane (POSS) crystals. The primary phase transitions were identified with differential scanning calorimetry at ∼257 and ∼253 K for partially deuterated and fully protonated ethyl POSS, respectively. A change in entropy between ∼47 and 28 ± 2 J mol −1 K −1 was observed for these transitions. At high temperature the unit cells are rhombohedral, while triclinic unit cells are observed at temperatures below the phase transition point. The crystallographic transition to the low temperature phase, 110 K, is marked by an abrupt increase in density (1.31 to 1.43 ± 0.05 g cm −3) and decrease in symmetry ( R-3 to P-1). Additionally, the crystallographic transition results in abrupt changes in the spin lattice relaxation and linewidth as detected with solid-state proton nuclear magnetic resonance (NMR) spectroscopy. This NMR behavior suggests a transition in molecular mobility of both ethyl derivatives. Both POSS derivatives exhibit an increase in correlation time and activation energy. For deuterated ethyl POSS, the motions became increasingly anisotropic after the temperature is lowered past its transition point.

A crystallographic orientation transition and early stage growth characteristics of thin Bi films on HOPG

We report on the growth of ultra-thin bismuth films on the basal plane of highly ordered pyrolitic graphite (HOPG) substrates. Scanning electron microscopy and atomic force microscopy have been used to characterize the morphology, and the crystallographic orientation was obtained using electron back scatter diffraction. Low coverage films are comprised of islands with a striped surface morphology, and show the orientation relationship { 0 1 1 ¯ 2 } Bi ∥ { 0 0 0 1 } HOPG with preferred in-plane orientations 〈 1 1 2 ¯ 0 〉 Bi ∥ 〈 1 0 1 ¯ 0 〉 HOPG . With increasing film thickness, we identify an unusual orientation transformation to the commonly found Bi{0 0 0 1} (trigonal) orientation.

Mo¨ssbauer study of iron sulfides doped with 3d-transition metals

It is found by Mo¨ssbauer measurements on M 0.025Fe 0.975S (M=Sc, Ti, V, Cr, Mn, Co, Ni, Cu) that the 3d-transition metal impurities profoundly affect both the crystallographic and spin rotation transitions of iron sulfide. It is noteworthy that both V 0.025Fe 0.975S and Co 0.025Fe 0.975S have Morin transition temperatures T M which are distinctly different from that of FeS; furthermore, the directions of changes of T M are opposite for V 0.025Fe 0.975S and Co 0.025Fe 0.975S. A vanadium impurity of 2.5% of the metal atoms in the iron sulfide makes the crystallographic transition take place rapidly in a narrow temperature region of about 15 K, while the α transition in FeS takes place over a wide temperature range of about 200 K. It is also found that the α transition for V 0.025Fe 0.975S has a hysteresis width of 5 K.

Magnetic properties of iron sulfides doped with 3d transition-metals studied by Mo˝ssbauer spectroscopy

It is found by Mőssbauer measurements on M 0.025Fe 0.975S (M=Sc, Ti, V, Cr, Mn, Co, Ni, Cu) that the 3d transition-metal impurities influence the spin reorientation transitions of iron sulfide. It is noteworthy that both V 0.025Fe 0.975S and Co 0.025Fe 0.975S have Morin transition temperatures T M which are distinctly different from that of FeS; furthermore, the directions of the T M changes are opposite for V 0.025Fe 0.975S and Co 0.025Fe 0.975S. However, it is found that both the Néel temperature T N and the α-transition temperature T α are almost unaffected by the 3d transition-metal impurities. A vanadium impurity of 2.5% of the metal atoms in the iron sulfide makes the crystallographic transition take place rapidly in a narrow temperature region of about 15 K, while the α transition in FeS takes place over a wide temperature range of about 200 K. It is also found that the α transition for V 0.025Fe 0.975S has a hysteresis width of 5 K.

Size-induced stability and structural transition in monodispersed indium nanoparticles

The present study reports the stability and the physical significance of the size-induced crystallographic structural transition in the gas-phase synthesized monodispersed indium nanoparticles. Transmission electron microscopy and x-ray photoelectron spectroscopy studies reveal that the formation of a thin oxide shell results in enhanced stability of indium nanoparticles. These results also show a size-induced structural transition from the bulk tetragonal to face-centered-cubic structure, which is attributed to an increase in the binding energy of core electrons of indium nanoparticles due to quantum confinement effects and the presence of a thin oxide shell.

Evidence for a coupled magnetic-crystallographic transformation inNd5(Si0.6Ge0.4)4

We have performed a detailed study of the magnetic and structural properties of the compound ${\mathrm{Nd}}_{5}{({\mathrm{Si}}_{0.6}{\mathrm{Ge}}_{0.4})}_{4}$ by means of neutron powder diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. A coupled magnetic and crystallographic first-order transformation from a room-temperature monoclinic ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$-type paramagnetic state to a low-temperature orthorhombic ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{4}$-type ferromagnetic structure takes place on cooling at $\ensuremath{\sim}68\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This magnetostructural transition shifts towards higher temperatures by the application of an external magnetic field at a rate of $0.11(1)\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kOe}$, which is lower than the $\ensuremath{\sim}0.6\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{kOe}$ observed in the ${\mathrm{Gd}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ and ${\mathrm{Tb}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ compounds. The magnetostructural transformation cannot be completed in the isothermal regime by a $120\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ magnetic field. It is suggested that the single-ion anisotropy of the Nd ions hinders the completion of the field-induced transformation. The existence of a simultaneous magnetic and crystallographic transformation in this alloy, which is in sharp contrast with other previously studied ${\mathrm{Nd}}_{5}{({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x})}_{4}$ alloys, is discussed in terms of the role of interstitial impurities in triggering and coupling/decoupling the crystallographic transition.

Discrepancy between the Spin Distribution and the Magnetic Ground State for a Triaminoxyl Substituted Triphenylphosphine Oxide Derivative

The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.

Crystallographic transitions related to magnetic and electronic phenomena under high pressure in iron oxides and compounds

The main objective of this paper is the study of structural aspects of strongly correlated systems in conjunctions with magnetic/electronic properties in a regime of matter under very high density. Synchrotron X-ray diffraction (XRD) measurements were carried out to 130 GPa to probe structural features specifically related to pressure-induced (PI) magnetic/electronic transitions in selected transition metal (TM) compounds, emphasizing those with geophysical interest. The types of electronic transitions discussed are the Mott transition (MT), and high-spin (HS) to low-spin (LS) crossover. In these cases the electronic transition may induce or be a consequence of structural alterations. For instance a first-order PI spin-crossover will be accompanied by an abrupt reduction of the TM ionic-radius, hence by a discontinuous volume decrease. On the other hand a “sluggish” or second-order spin-crossover may result in non-linear crystal elastic constants which will affect the equation of state. In case of a MT, a fundamental electronic process in which a strongly correlated magnetic-insulating system becomes non-magnetic and metallic, one expects major modifications of the lattice parameters or even structural symmetry changes. As example-cases we present results in Fe2O3, Fe3O4, FeI2, FeCr2S4, and the RFeO3 orthoferrites (R=rare earth).

Structure and thermal evolution of Mg–Al layered double hydroxide containing interlayer organic glyphosate anions

Layered double hydroxide (LDH) with the Mg2+/Al3+ molar ratio of 2.0 containing interlayer organic pesticide glyphosate anions (MgAl–Gly–LDH) has been synthesized by the use of anion exchange and coprecipitation routes. Intercalation experiments with glyphosate (Gly) reveal a correlation between the temperatures for thermal treatments and the types of reaction it undergoes with Gly. The grafting of the Gly anion onto hydroxylated sheets of LDH by moderate thermal treatments (hydrothermal treatments and calcinations) was confirmed by a combination of several techniques, including powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA–DTG), and 31P nuclear magnetic resonance (NMR). The thermal decomposition of MgAl–Gly–LDH results in the removal of loosely held interlayer water, grafting reaction between the interlayer anions and hydroxyl groups on the lattice of LDH, dehydroxylation of the lattice and decomposition of the interlayer species in succession, thus leading to a variety of crystallographic transitions.

Thermal Evolution of the Crystallographic and Magnetic Structure in LuVO3: A Neutron Diffraction Study.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A novel reduction–hydrolysis method of preparing VO 2 nanopowders

A novel and convenient reduction–hydrolysis method was developed to make high-purity VO 2 nanopowders. The products have been studied with XRD, IR, XPS, and DTA. The nanopowders exhibited a strong crystallographic transition to the rutile-type structure around 68–72 °C. The XRD patterns and TEM images of products showed that the average size of samples was in the range of 30–40 nm. The XPS results demonstrated that the vanadium exists as V 4+ ions in the products. We also found that the phase transition temperature of products decreased by tungsten doping.

Thermal Evolution of the Crystallographic and Magnetic Structure in LuVO3: A Neutron Diffraction Study

Polycrystalline LuVO 3 perovskite has been studied by neutron powder diffraction (NPD), specific heat, and magnetization measurements. Below T N 107 K, LuVO 3 undergoes magnetic ordering according to a canted antiferromagnetic structure. Neutron diffraction data show that the magnetic structure is characterized by the propagation vector k = 0 and the spin arrangement for the V atoms below T N is given by the basis vector (0,A y ,F z ). The magnetic structure remains stable down to 2 K and the ferromagnetic component is very weak, in good agreement with the magnetic measurements. The ordered magnetic moment at T = 2.4 K for the V atoms is 1.33(6)μ B . A few degrees below T N , a crystallographic transition is observed, from the orthorhombic room-temperature structure (Pnma space group) to monoclinic (P2 1 /n space group) below the transition. The changes in the crystallographic structure occur between 94 and 82 K. Despite the fact that below 82 K there are two sites for the V atoms, the arrangement of the V-O bonding lengths is the same in the y = 0 plane and in the y = 1/2 layer, so the (010) planes are in phase along the b-direction. This suggests an in-phase orbital ordering of the V t 2g orbitals compatible with the A-type magnetic structure.

A neutron diffraction study of the crystallographic and magnetic structure of LuVO3

A study of the crystallographic and magnetic structure evolution of LuVO 3 has been carried out from neutron powder diffraction data and magnetic susceptibility measurements. A few degrees below T N ≈107 K a crystallographic transition from orthorhombic (Pnma) to monoclinic (P2 1 /n) takes place; the antiferromagnetic structure, stable down to 2.4 K , is given by the basis vectors (0, A y , F z ) (Pnma setting; A = m 1 =− m 2 =− m 3 = m 4 ).