Isostructural Transformation Research Articles

Explanation of the microscopic mechanism of h-BN isostructural transformation under biaxial strain

Polymorphism and stacking faults can seriously affect the physical properties of two-dimensional materials. What’s important, the biaxial strain has a significant impact on the structural transformation of two-dimensional materials and changes the related path, which ultimately leads to a phase structure different from zero strain. Also, external pressure or biaxial strain can change the crystal lattice without introducing impurities, making it a clean and controllable method for adjusting physical properties. In this paper, the AA2 and AB1 stack structures with relatively small energy are simulated by first-principles calculation to explain the isomorphic phase transition of h-BN under biaxial strain. Energy analysis shows that the AB1 stack is relatively stable without strain, and when the strain increases to 4.295 %, the AB1 stacking will spontaneously rotate into the AA2 stacking, which makes AA2 stable. By analyzing the density of states (DOS) and the electron density difference, the isomorphic phase transition is attributed to the weakening of the B–N bond by electron transfer between layers under strain conditions. The sliding behavior caused by biaxial strain at the atomic scale may be of fundamental importance for understanding the macroscopic friction behavior. In addition, the explanation of the isomorphic phase transition plays a very important role in the h-BN double layer as a future electronic and optoelectronic device material.

On-surface isostructural transformation from a hydrogen-bonded network to a coordination network for tuning the pore size and guest recognition.

Rational manipulation of supramolecular structures on surfaces is of great importance and challenging. We show that imidazole-based hydrogen-bonded networks on a metal surface can transform into an isostructural coordination network for facile tuning of the pore size and guest recognition behaviours. Deposition of triangular-shaped benzotrisimidazole (H3btim) molecules on Au(111)/Ag(111) surfaces gives honeycomb networks linked by double N–H⋯N hydrogen bonds. While the H3btim hydrogen-bonded networks on Au(111) evaporate above 453 K, those on Ag(111) transform into isostructural [Ag3(btim)] coordination networks based on double N–Ag–N bonds at 423 K, by virtue of the unconventional metal–acid replacement reaction (Ag reduces H+). The transformation expands the pore diameter of the honeycomb networks from 3.8 Å to 6.9 Å, giving remarkably different host–guest recognition behaviours for fullerene and ferrocene molecules based on the size compatibility mechanism.

Coupling of organic cation and inorganic lattice in methylammonium lead halide perovskites: Insights into a pressure-induced isostructural phase transition

Organic-inorganic hybrid metal-halide perovskite (OIHP) materials provide a tunable platform for engineering their optoelectronic properties. Although several high-pressure studies have been conducted from the OIHP family of single crystals and films, the exact nature of the dynamic coupling of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}$ (MA) cation with the octahedral lattice framework and the mechanisms responsible for the structural phase transformation under pressure are not well captured. By combined photoluminescence (PL), synchrotron-based x-ray diffraction, and Raman-scattering studies as a function of pressure from methylammonium lead bromide ($\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$), we shed light on an isostructural phase transition due to the coupling of the MA cation and the $\mathrm{Pb}{\mathrm{Br}}_{6}$ lattice through hydrogen bonding. The sharp discontinuities at $\ensuremath{\sim}$ 1 GPa and $\ensuremath{\sim}$ 3 GPa in the PL peak positions correlate with the structural changes observed in high-pressure XRD and Raman-scattering studies. The electronic band edge as a function of pressure is calculated within density-functional theory. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman-scattering studies. The frequencies of the lattice modes and the C--H/N--H bending and stretching modes of the MA cation show anomalous changes and other nuances at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II ($Im\overline{3}$) to orthorhombic ($Pnma$) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this paper highlights the impact of molecular vibrations on the electronic properties of $\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$ under pressure.

Interaction Between Ceramic Molds and High-Speed Steel during Gravity and Vacuum Investment Casting

Ceramic molds containing the ZrSiO4/SiO2 primary face coat with and without addition of cobalt aluminate were produced. After firing, the surface and the cross sections of the face coats were subjected to detailed characterization using XRD, SEM and EDS. The interaction between the mold systems and 0.9C-6W-5Mo-4Cr-2V high-speed steel was studied during the gravity and vacuum investment casting. During casting, the isostructural transformation of the initial ZrSiO4 filler occurred. For the mold without addition of the cobalt aluminate, two different types of interaction products [multicomponent phase containing O, Al, Zr, Fe, W and V and (Fe, W,Mo,Cr)2Al2O6)] were revealed. An interaction layer of a depth in the range of 5-30 μm was thoroughly studied after casting into the mold containing the cobalt aluminate. Addition of the cobalt aluminate into the ceramic system provided significant refinement of the high-speed steel microstructure.

Dynamic Resolution of Piezosensitivity in Single Crystals of π-Conjugated Molecules.

Targeted synthesis of piezoresponsive small molecules and in-depth understanding of their mechanism is of utmost importance for the development of smart devices. This work reports the synthesis, structure and piezosensitivity of a bola-amphiphile 1,4-bis(pentyloxy)-2,5-bis(2-pyridineethynyl)-benzene (C5-PPB). Depending on the rate of compression, two different phases in C5-PPB can be generated. The ambient-pressure α-phase is stable up to 0.8 GPa, beyond which it undergoes an isostructural transformation to β-phase, accompanied by a clearly visible elongation of the crystal. This α-to-β phase transition requires the sample to be compressed slowly. When quickly compressed, phase α persists to about 1.5 GPa, beyond which its amorphization starts, accompanied by the appearance of irregular grooves on the largest faces. Mechanical pressure also affects the optical property of C5-PPB, which shows reversible mechanochromism with a green to cyan transformation in the emission, associated with a 15 nm shift in the maxima. The conductivity of C5-PPB as a direct outcome of its crystal packing has also been studied.

Effect of a core-softened O–O interatomic interaction on the shock compression of fused silica

Isotropic soft-core potentials have attracted considerable attention due to their ability to reproduce thermodynamic, dynamic, and structural anomalies observed in tetrahedral network-forming compounds such as water and silica. The aim of the present work is to assess the relevance of effective core-softening pertinent to the oxygen-oxygen interaction in silica to the thermodynamics and phase change mechanisms that occur in shock compressed fused silica. We utilize the MD simulation method with a recently published numerical interatomic potential derived from an ab initio MD simulation of liquid silica via force-matching. The resulting potential indicates an effective shoulder-like core-softening of the oxygen-oxygen repulsion. To better understand the role of the core-softening we analyze two derivative force-matching potentials in which the soft-core is replaced with a repulsive core either in the three-body potential term or in all the potential terms. Our analysis is further augmented by a comparison with several popular empirical models for silica that lack an explicit core-softening. The first outstanding feature of shock compressed glass reproduced with the soft-core models but not with the other models is that the shock compression values at pressures above 20 GPa are larger than those observed under hydrostatic compression (an anomalous shock Hugoniot densification). Our calculations indicate the occurrence of a phase transformation along the shock Hugoniot that we link to the O–O repulsion core-softening. The phase transformation is associated with a Hugoniot temperature reversal similar to that observed experimentally. With the soft-core models, the phase change is an isostructural transformation between amorphous polymorphs with no associated melting event. We further examine the nature of the structural transformation by comparing it to the Hugoniot calculations for stishovite. For stishovite, the Hugoniot exhibits temperature reversal and associated phase transformation, which is a transition to a disordered phase (liquid or dense amorphous), regardless of whether or not the model accounts for core-softening. The onset pressures of the transformation predicted by different models show a wide scatter within 60-110 GPa; for potentials without core-softening, the onset pressure is much higher than 110 GPa. Our results show that the core-softening of the interaction in the oxygen subsystem of silica is the key mechanism for the structural transformation and thermodynamics in shock compressed silica. These results may provide an important contribution to a unified picture of anomalous response to shock compression observed in other network-forming oxides and single-component systems with core-softening of effective interactions.

High Pressure Effects on Zwitterionic and Thione Mesomeric Contributions in 2-Benzimidazole-2-Thione

High pressure reduces the zwitterionic mesomeric contribution and increases the thione contribution in 2-benzimidazole-2-thione. These mesomeric changes are manifested in the shortened bond S–C and elongated bond C–N in the S–C–N moiety. These transformations are consistent with the le Chatelier law, as they counteract the increase of electrostatic interactions when the intermolecular distances between electronegative sulfur atoms and arene π-electrons are compressed. The changing interactions affect the crystal strain and its structural transformations. Consequently, the crystal compression and thermal expansion initially, until about 1.0 GPa, are inconsistent with the inverse relationship rule of pressure and temperature effects. Some anomalous features of the thermal expansion can be associated with isostructural transformations of the crystal.

Thermal Transformation Arrest Phenomena in Mn2Sb0.9Sn0.1

Magnetic and structural properties of Sn-substituted Mn 2 Sb were investigated. A first-order magnetic transition (FOMT) from a ferrimagnetic (FRI) to an antiferromagnetic state occurred between 100 and 200 K, which was accompanied by an iso-structural transformation. Upon cooling in 5 T, the residual FRI phase was observed even at 10 K, and the residual FRI phase slightly decreased with increasing time. The results indicate that the FOMT of Mn 2 Sb 0.9 Sn 0.1 is arrested by applying a magnetic field.

On the energy scale involved in the metal to insulator transition of quadruple perovskite EuCu3Fe4O12: infrared spectroscopy and ab-initio calculations.

Optical measurements were carried out by infrared spectroscopy on AA′3B4O12 A-site ordered quadruple perovskite EuCu3Fe4O12 (microscopic sample) as function of temperature. At 240 K (=TMI), EuCu3Fe4O12 undergoes a very abrupt metal to insulator transition, a paramagnetic to antiferromagnetic transition and an isostructural transformation with an abrupt large volume expansion. Above TMI, optical conductivity reveals a bad metal behavior and below TMI, an insulating phase with an optical gap of 125 meV is observed. As temperature is decreased, a large and abrupt spectral weight transfer toward an energy scale larger than 1 eV is detected. Concurrently, electronic structure calculations for both high and low temperature phases were compared to the optical conductivity results giving a precise pattern of the transition. Density of states and computed optical conductivity analysis identified Cu3dxy, Fe3d and O2p orbitals as principal actors of the spectral weight transfer. The present work constitutes a first step to shed light on EuCu3Fe4O12 electronic properties with optical measurements and ab-initio calculations.

Elastic strain energy induced split during precipitation in alloys

The elastic strain energy induced split during the precipitation of the isostructural transformation in the modeled A–B alloys is simulated by the phase field method. The simulation implies that for the single precipitating particle in the modeded alloy, the particle with initial scale in the range from 80l to 320l (l=12.18 Å) is to split during aging. For the modeled alloy, the γ′ particle is to split at about 5.0×103τ (τ=4.45 s) after the split incubation period. At the end of split the total interfacial energy obtains a maxima and the total elastic energy holds a minima at about 2.4×104τ. The requirement of the four-blocks split may be given by the relationship between the elastic strain energy coefficient, the positive gradient energy coefficient and the solute concentration inhomogeneity. In multi-particles systems, the particles split into other appearances due to the additional interaction energy induced by the non-symmetrical arrangement of the particles, The increment of the interaction elastic energy has little effect on the interfacial energy during precipitation in multi-particles systems.

X-Ray Diffraction Measurements of the Fe-Rh Alloy Under High Magnetic Fields and at High Temperatures

The first-order antiferromagnetic-ferromagnetic phase transition in B2 FeRh was studied by magnetization and X-ray diffraction measurements in magnetic fields up to 5 T at high temperatures, in order to investigate the relationship between magnetic and structural properties affected by the field. It turned out that the compound shows the field-induced isostructural transformation by applying a magnetic field, accompanied by the first-order magnetic transition from the antiferromagnetic to the ferromagnetic state.

The low-temperature thermodynamic properties of lithium hydride isotopomers

The thermodynamic properties of lithium hydride isotopic forms at low temperatures were studied in the light of the hypothesis of the existence of the temperature Tλ between 10 and 14 K at which these crystals experience isostructural transformation. Experimental data processing gave Tλ = 11.1 K for LiH and Tλ = 12.85 K for LiD. An analytic description of the low-temperature behavior of heat capacity, volumetric thermal expansion coefficient, and heat conductivity of LiA crystals was suggested.

Structure and magnetic properties of stress-induced martensites in ferromagnetic shape memory alloy Mn2NiGa

In-situ high pressure X-ray diffraction experiments on Heusler-type ferromagnetic shape memory alloy Mn2NiGa under pressure up to 40 GPa have been carried out by using diamond anvil cell device with synchrotron radiation source， and magnetic measurements have also been performed on uninstalled samples by using vibration sample magnetometer. At ambient pressure Mn2NiGa has body centered cubic structure. Two phase transitions in Mn2NiGa were observed during high pressure experiments. The first at about 0.77 GPa is belong to martensitic transformation, and the second at about 20 GPa is isostructural transformation between two different martensites. Meanwhile, a lot of defects and distortion of lattice were generated in martensites due to pressurization. Consequently, coercive force of stress-induced martensites rises up to 204 kA/m and it is almost ten times larger than that of thermo-induced martensites. Pressure processes also make saturation magnetization of martensites decrease rapidly, which reveals the obvious defect effect.

Optical spectra of lithium tantalate

The optical spectra of lithium tantalate in the ferroelectric phase are investigated in the visible and near-IR ranges. Specific features in the optical spectra of lithium tantalate at temperatures of 120 and 180°C, namely, minima in the parameter σ of the Urbach rule and the degree of polarization of the luminescence band, indicate a rearrangement of the electronic subsystem and the corresponding isostructural transformations of the crystal lattice. In the near-IR range, the optical spectra contain bands at energies of 1.51, 1.30, and 1.75 eV, which, according to the shape and temperature behavior, are attributed to polarons of large radius with binding energies of 0.36, 0.31, and 0.42 eV, respectively.

Optical spectra of ammonium sulfate in the paraelectric phase

The optical spectra of ammonium sulfate in the paraelectric phase are investigated in the visible and near-IR ranges. The temperature dependences of the parameter σ of the Urbach rule and the degree of depolarization of the luminescence spectrum exhibit a minimum in the temperature range 120–140°C. This minimum can be associated with the rearrangement of the electronic subsystem and the corresponding isostructural transformations of the lattice. In the near-IR range, the absorption spectrum contains two bands with maxima at energies of 0.50 and 0.75 eV. The shape of the bands and the temperature dependence of the conductivity allow us to interpret these bands as the result of dissociation of polarons of a large radius with binding energies of 0.12 and 0.17 eV.

Pressure-induced phase transformations in theBa8Si46clathrate

The nature of isostructural transformations of a type-I ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ clathrate has been studied by in situ high-pressure angle-dispersive x-ray powder diffraction using liquid He as pressure transmitting medium. The good quality of the diffraction data permitted refinement of structural and thermal parameters from Rietveld analysis. The results show that the first transition at $7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is caused by the displacement of the Ba atoms in the ${\mathrm{Si}}_{24}$ cages. The cause of the second transition at $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, characterized by a dramatic reduction of cell volume, is not as clear. Theoretical calculations predicted an electronic topological transition of Fermi surface at this pressure. Analysis of the anomalously large Si thermal parameters suggested a highly disordered Si framework. This disordering is probably static and may be due to the presence of Si vacancies. The latter hypothesis is supported by electronic calculations on model disordered systems.

Collapsed hexagonalωphase in a compressedTiZralloy: Angle-dispersive synchrotron-radiation x-ray diffraction study

An in situ high-temperature high-pressure angle-dispersive synchrotron radiation diffraction study has revealed the $\ensuremath{\omega}$-to-${\ensuremath{\omega}}^{\ensuremath{'}}$ isostructural transformation in the equiatomic $\mathrm{TiZr}$ alloy. Volume collapse across the phase transformation is about 14%. On further compression the collapsed ${\ensuremath{\omega}}^{\ensuremath{'}}$ structure transforms to a noncollapsed cubic $\ensuremath{\beta}$ structure with the partial atomic volume increase about 13%. Observed effects are discussed in terms of the $s\text{\ensuremath{-}}d$ electron transfer and nonadiabatic electron-phonon interactions.

Isostructural transformation and polymorphism of thiourea dioxide at high pressure

Isostructural transformation and polymorphism of thiourea dioxide at high pressure

The effect of pressure, and formation of new polymorphs of the amino acids L-cysteine and L-serine

With the notable exceptions of water, ammonia and a very limited number of other examples, there have been relatively few highpressure structural studies of simple molecular compounds. Such studies can provide fascinating information about intermolecular interactions and pressure-induced phase transition, in particular hydrogen-bonding interactions. Recent results for thiourea and urea obtained from the experiments on neutron beamlines at the UK ISIS facilities have shown a very rich high-pressure behaviour [1]. In the light of these results, we have undertaken neutron and synchrotron studies on thiourea dioxide, which has been selected by virtue of its interesting ambient pressure structure [2]. We report a phase transition between a powder sample of orthorhombic phase I of thiourea dioxide to a new monoclinic phase II at a pressure of 0.54 GPa. This transition has also been observed in a single crystal sample at a pressure of 0.45 GPa. We also report an unusual isostructural transformation in thiourea dioxide at 6.8 GPa that involves the formation of a new hydrogen bond.

Theoretical study of the isostructural transformation in ice VIII

We present a first-principles study of the structural and vibrational properties of ${\mathrm{D}}_{2}\mathrm{O}$-ice VIII. We do not confirm the existence of the structural discontinuity that was proposed on the basis of a neutron diffraction study and of a first principles calculation, although we reproduce well other physical properties, including the nonlinear behavior of phonon frequencies below $3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This study brings about the necessity of further experiments to investigate the structure of ice VIII at high pressure. The relationship between phonon frequencies and structural parameters is clarified in detail.