Phase Transformations In Single Crystals Research Articles

White Laue and powder diffraction studies to reveal mechanisms of HCP-to-BCC phase transformation in single crystals of Mg under high pressure

Mechanisms of hexagonal close-packed (HCP) to body-centered cubic (BCC) phase transformation in Mg single crystals are observed using a combination of polychromatic beam Laue diffraction and monochromatic beam powder diffraction techniques under quasi-hydrostatic pressures of up to 58 ± 2 GPa at ambient temperature. Although experiments were performed with both He and Ne pressure media, crystals inevitably undergo plastic deformation upon loading to 40–44 GPa. The plasticity is accommodated by dislocation glide causing local misorientations of up to 1°–2°. The selected crystals are tracked by mapping Laue diffraction spots up to the onset of the HCP to BCC transformation, which is determined to be at a pressure of 56.6 ± 2 GPa. Intensity of the Laue reflections from HCP crystals rapidly decrease but no reflections from crystalline BCC phase are observed with a further increase of pressure. Nevertheless, the powder diffraction shows the formation of 110 BCC peak at 56.6 GPa. The peak intensity increases at 59.7 GPa. Upon the full transformation, a powder-like BCC aggregate is formed revealing the destructive nature of the HCP to BCC transformation in single crystals of Mg.

Multigrain phase-field simulation in ferroelectrics with phase coexistences: An improved phase-field model

A new 3D phase-field model is developed to investigate the evolution of the domain structure and phase transformation in a ferroelectric material near the morphotropic phase boundary (MPB) under the influence of external stimuli. The model combines the Landau–Ginzburg–Devonshire (LGD) theory with a general multiphase approach and enables to analytically compute the microstructure inside the grain structures with coexisting phases. Together with experimental analysis, a numerical energy overlap was employed to find the phase-dependent phenomenological coefficients. Domain structures and phase transformations in single crystals, as well as in the polycrystalline systems with or without an external electric field have been investigated in a MPB ferroelectric material, by employing phase-field simulation. The simulated results reveal the relation between the polarization switching and the phase transformation under an applied electric field, and the role of the external electric field orientation and amplitude in the domain structure and the phase transformation fraction.

Reversible single-crystal to single-crystal phase transformation between a new Werner clathrate and its apohost.

In this work, we report the synthesis and structural characterisation of the ligand 2-(pyridin-3-yl)-benzo[de]isoquinoline-1,3(2H)-dione, 5, its isostructural Werner complexes ML4(NCS)2 (L = 5; M = Co(ii) and Ni(ii)), and five clathrates with three aromatic guests, ML4(NCS)2·2G (M = Co(ii) and Ni(ii), G = nitrobenzene (NB); M = Co, G = 1,2-dichlorobenzene (1,2-DCB); M = Co(ii) and Ni(ii), G = o-xylene (OX)). 5 was prepared in high yield by condensation in the solid-state (C3S3, Cocrystal Controlled Solid-State Synthesis). The Werner complexes ML4(NCS)2 (M = Co(ii) and Ni(ii)) (apohosts) were prepared by reacting M(NCS)2 (M = Co(ii) and Ni(ii)) and 5 in 1-butanol at 60 °C for 24 h. The Werner clathrates were prepared by reacting M(NCS)2 (M = Co(ii) and Ni(ii)), G and 5 in 1-butanol at 60 °C for 48–96 h. The clathrates were observed to transform to the apohost ML4(NCS)2 upon heating. CoL4(NCS)2·2NB was subsequently regenerated by exposing CoL4(NCS)2 to liquid NB at 60 °C for 48 h. This phase change occurred as a single-crystal to single-crystal phase transformation and was studied by single crystal X-ray diffraction, powder X-ray diffraction and thermal analyses. Structural analyses of the apohost CoL4(NCS)2 and its Werner clathrate CoL4(NCS)2·2NB indicated that rotational freedom of the Co-N bonds together with torsional flexibility of the ligand between the imide bond and the pyridine moiety are key to enabling the structural switching induced by exposure to NB or its removal.

Non-Topotactic Phase Transformations in Single Crystals of β-Glycine

The metastable β polymorph of glycine exhibits a single-crystal-to-single-crystal transformation (SCSCT) to either the α or γ phase with retention of the crystal habit of the parent β phase. X-ray diffraction and optical microscopy reveal that of 51 single crystals of the β parent phase, 24 form single domain crystals of α or γ with a single orientation matrix, confirming an SCSCT. The remaining 27 β parent crystals transform to crystals with a few domains of either α and/or γ, each domain arising from a single nucleation event. In three cases the β → α and β → γ transformations occurred within the same glycine single crystal. In all cases, regardless of the number of domains, the transformation occurred with retention of the original habit of the parent phase. Both β → α and β → γ transformations proceed in a non-topotactic manner, as evident from the random orientations of the daughter phases in the crystallographic reference frame of the parent β phase. The transformation rates, as measured by advancem...

Modulation of Thermal Expansion by Guests and Polymorphism in a Hydrogen Bonded Host

Thermal expansions of the three isostructural solvates of an “X”-shaped host molecule, 1,1,4,4-tetrakis(4-hydroxyphenyl)cyclohexane (1), with phenol (1-ph), p-cresol (1-pc), and m-cresol (1-mc), two polymorphic solvates of 1 with o-cresol (1-oc-1D and 1-oc-2D), and a guest-free form of 1 (1-gf) have been studied. This work demonstrates that isostructural 1-ph and 1-pc show uniaxial negative thermal expansion (NTE), but along different directions and only the 1-ph system undergoes single-crystal to single-crystal phase transformation. On the other hand, the isostructural 1-mc host lattice experiences a normal positive thermal expansion (PTE). The one-dimensional hydrogen bonded triclinic form of the o-cresol solvate (1-oc-1D) exhibits a biaxial NTE, whereas the two-dimensional hydrogen bonded monoclinic form (1-oc-2D) exhibits a normal PTE. The 1-gf form experiences only a weak uniaxial NTE and smaller volumetric expansion compared to all the solvates. The uniaxial NTE that occurred in some of these materials is caused by sliding of the layers, made of hydrogen bonded tapes of the host molecules. The guest molecules experience stronger thermal vibration than the host molecules. Systems with a higher guest to the host ratio experience larger thermal expansions.

Nonequilibrium Pathways during Electrochemical Phase Transformations in Single Crystals Revealed by Dynamic Chemical Imaging at Nanoscale Resolution

The energy density of current batteries is limited by the practical capacity of the positive electrode, which is determined by the properties of the active material and its concentration in the composite electrode architecture. The observation in dynamic conditions of electrochemical transformations creates the opportunity of identifying design rules toward reaching the theoretical limits of battery electrodes. But these observations must occur during operation and at multiple scales. They are particularly critical at the single‐particle level, where incomplete reactions and failure are prone to occur. Here, operando full‐field transmission X‐ray microscopy is coupled with X‐ray spectroscopy to follow the chemical and microstructural evolution at the nanoscale of single crystals of Li1+xMn2–xO4, a technologically relevant Li‐ion battery electrode material. The onset and crystallographic directionality of a series of complex phase transitions are followed and correlated with particle fracture. The dynamic character of this study reveals the existence of nonequilibrium pathways where phases at substantially different potentials can coexist at short length scales. The results can be used to inform the engineering of particle morphologies and electrode architectures that bypass the issues observed here and lead to optimized battery electrode properties.

Reversible single-crystal to single-crystal polymorphic phase transformation of an organic crystal

Two polymorphs of a substituted dihydroanthracene derivative were prepared by crystallization from different solvents. Each polymorph is reversibly interconvertible to the other by means of a single-crystal to single-crystal phase transformation.

Extra-strong wear-resistant materials based on nanostructured crystals of partially stabilized zirconium dioxide

The preparation, properties and applications of nanostructured materials, namely, crystals of partially stabilized zirconium dioxide (PSZ), are summarised. PSZ is formed due to the self-organization of the nanostructure during the cubic-to-tetragonal phase transformation of single crystals, and as a result the bulk of the material is built of densely and orderly packed domains with characteristic sizes of 10–20 nm. Unique mechanical and physicochemical properties of PSZ crystals, such as extremely high impact strength and wear resistance combined with low friction factor, thermal stability, bioinertness, and biocompatibility, make them very attractive for many technical and medical applications.

Why do polymorphs form? A single crystal phase transformation from weak dipolar interactions to a sixfold phenyl embrace

The crystal structures of the α- and β-polymorphs of GeHPh3 are reported along with mechanistic studies of the transformation by DSC and optical microscopy. DSC results also reveal new metastable phases for XHPh3 (X = Si, Sn). The results highlight the significance of the sixfold phenyl embrace (6PE) as an organising motif.

Effect of solid-solid phase transformations on elastic constitutive relations

A two-dimensional (2D) Ginzburg-Landau (GL) theory for temperature induced phase transformations in single crystals is extended to incorporate the effects of mechanical and thermal (expansion or dilatational) strain effects. This extension of the GL theory allows the single crystal GL model to be implemented into polycrystalline homogenization schemes. The proposed model is capable of examining the interactions of applied mechanical loading states and phase transformation processes within single crystals.

Slit-collimated Laue diffraction using synchrotron radiation: an examination of the paraelectric-to-ferroelectric phase transformation in ammonium sulfate

The use of the synchrotron Laue diffraction technique with a slit-collimated incident beam as a structural probe to examine solid-state phase transformations in single crystals is presented. The utility of the method is demonstrated through a study of the paraelectric-to-ferroelectric phase transition in ammonium sulfate at 223 K.

Experimental dynamic metamorphism of mineral single crystals

This paper is a review of some of the rich and varied interactions between non-hydrostatic stress and phase transformations or mineral reactions, drawn mainly from results of experiments done on mineral single crystals in our laboratory or our co-authors. The state of stress and inelastic deformation can enter explicitly into the equilibrium phase relations and kinetics of mineral reactions. Alternatively, phase transformations can have prominent effects on theology and on the nature of inelastic deformation. Our examples represent five types of structural phase changes, each of which is distinguished by particular mechanical effects. In increasing structural complexity, these include: (1) displacive phase transformations involving no bond-breaking, which may produce anomalous brittle behavior. A primary example is the a- β quartz transition which shows anomalously low fracture strength and tertiary creep behavior near the transition temperature; (2) martensitic-like transformations involving transformation strains dominated by shear deformation. Examples include the orthoenstatite → clinoenstatite and w u ̈ rtzite → sphalerite transformations; (3) coherent exsolution or precipitation of a mineral solute from a supersaturated solid-solution, with anisotropy of precipitation and creep rates produced under nonhydrostatic stress. Examples include exsolution of corundum from MgO · nAl 2O 3 spinels and Ca-clinopyroxene from orthopyroxene; (4) order-disorder transformations that are believed to cause anomalous plastic yield strengthening, such as MgO - nAl 2 O 3 spinels; and (5) near-surface devolatilization of hydrous silicate single-crystals that produces a fundamental brittleness thought to be connected with dehydration at microcracks at temperatures well below nominal macroscopic dehydration temperatures. As none of these interactions between single-crystal phase transformations and non-hydrostatic stress is understood in detail, this paper serves as a challenge to field structural geologists to test whether interactions of these types occur in nature, and to theoreticians to reach a deeper understanding of the complex relations between phase transformations, the local state of stress and associated deformation and deformation rates.

Raman spectroscopic study of phase transformation in single crystals of cadmium fluorosilicate hexahydrate

AbstractA detailed temperature‐dependent Raman study of oriented single crystals of CdSiF6 · 6H2O down to 10 K was made in the region of internal vibrations and lattice modes. The frequency shifts, Iinewidths and intensities of some of the modes associated with the [Cd(OH2)6]2+ and (SiF6)2− ions show abrupt changes at temperatures around 220 K and some doubly degenerate modes split into two components. These studies suggest that the system undergoes a structural phase transition at ca. 220 K.

X‐ray investigation of the mechanisms of phase transformations in single crystals of ZnS, ZnxCd1−xS, and ZnxMn1−xS (II). Comparison of observed and calculated diffraction effects

AbstractThe experimentally observed intensity profiles recorded from ZnS, ZnxCd1−xS and ZnxMn1−xS crystals at different stages of the 2H‐3C and 2H‐6H transformations are compared with those computed theoretically from the three parameter model in the previous paper. It is found that the 2H‐3C and 2H‐6H transformations occur by the nonrandom nucleation of deformation faults. The probability of the growth of a nucleus is much greater than that of the creation of a fresh nucleus. At least two different fault probabilities have therefore to be employed in computing the diffraction effects in each case. The transformation behaviour of ZnxCd1−xS and ZnxMn1−xS crystals is strongly influenced by variations in stoichiometry (x). Large values of x (x > 0.95) favour the 2H‐3C transformation, whereas smaller values (x ≦ 0.94) favour the formation of the 6H phase. A comparison of the calculated and observed intensity distributions indicates that in some crystals the 2H‐3C and 2H‐6H transformations occur simultaneously in different regions of the same single crystal.

X‐ray investigation of the mechanism of phase transformation in single crystals of ZnS, ZnxCd1−xS and ZnxMn1−xS (I). Calculation of diffraction effects by a three parameter model

AbstractHexagonal close‐packed (2H) single crystals of ZnS, ZnxCd1−xS and ZnxMn1−xS are known to undergo solid state transformation to the cubic close‐packed (3C) and 6H‐structures on annealing at elevated temperatures. The transformations occur by the non‐random nucleation of stacking faults on individual close‐packed layers parallel to (0001). The nature of the faults involved and their probability distribution during transformation determine the diffraction effects produced along the 10. L reciprocal lattice row by a crystal quenched in an intermediate state of transformation. We have investigated the mechanism of the transformation by comparing the diffraction effects recorded from such crystals on a single crystal diffractometer, with those calculated for an assumed model of the transformation.It is known that in these materials the faults involved in the transformation are deformation faults. To explain the observed diffraction effects we develop a three parameter theoretical model employing a fult probability α for the radom insertion of a deformation fault in the 2H structure, a fault probability β for the deformation faults to occur at three layer separations and a fault probability γ of their occcurrence at 2‐layer separations. The probability α corresponds to the development of a fresh nucleus, the probability β to the growth of the 6H nucleus and the probability γ to the growth of a 3C nucleus. This paper develops the necessary theory of X‐ray scattering for such a model of the transformation and predicts the diffraction effects for different values of α, β, and γ. The next paper compares these results with experimental observations.

Laser Raman study of phase transformation in single crystals of cadmium fluorotitanate hexahydrate

AbstractA detailed temperature‐dependent Raman study of oriented single crystals of CdTiF6·6H2O down to 10 K was carried out in the regions of internal vibrations and lattice modes. The line width, frequency shift and intensities of the modes associated with the [Cd(H2O)6]2+ and [TiF6]2− octahedra show abrupt changes at temperatures around 200 and 80 K, respectively, and many doubly degenerate modes split into two components. These studies suggest that this system undergoes two structural phase transitions at 200 and 80 K. A mechanism for these phase transitions is suggested.

An unusual photo-induced conformational polymorphism: a crystallographic study of bis(p-methoxy)-trans-stilbene

A novel example of a photo-induced single-crystal to single-crystal phase transformation is described for title compound; the accompanying change in molecular conformation is rationalised in terms of an intermediate excimeric state.

Reversibility and mechanism in a single-crystal phase transformation

Optical and x-ray diffraction studies of p-di-iodobenzene single crystals undergoing polymorphic phase transformation are strongly indicative of the existence of heterophase domains at temperatures well away from the classical phase equilibrium temperature. Domains of either polymorphic form can exist in a crystal of the other form. These domains seem to have a significant effect on the course of the transformation. Annealing the crystals at temperatures far from the transformation temperatures diminishes or eliminates these domains.

X-ray study of the crystallographic transformation in hydrogen below 1.5 K

The fcc to hcp and hcp to fcc phase transformations in single crystals of hydrogen have been studied using x-ray diffraction techniques. The transformation temperature T* has been mapped as a function of orthohydrogen concentration chi in the region of 0.17 < T* < 1.5 K and 0.48 < chi < 0.7. The fcc to hcp transformation temperature extrapolates to zero near a concentration of 0.49. The transformation temperatures and widths agree with other experimental results at higher temperatures, but exclude low-concentration NMR data in the literature from being associated with the crystallographic transformation in bulk samples.

A probable crystallographic path for phase transformations in single crystals of ammonium nitrate

Assuming that the shuffles of a solid-state single-crystal phase transformation are least-motion processes consistent with the symmetry of the phases and with the accepted interatomic distances, it follows that the structure of the higher-symmetry phase will be predictable solely from its space-group symmetry and unit-cell dimensions if the complete crystal structure of the low-symmetry phase has already been determined [Swaminathan & Srinivasan, Acta Cryst. (1975). A31, 628-634]. The possibility has been here verified successfully in the sequence V → IV → II of the phase transformations in ammonium nitrate.