Isosymmetric Phase Research Articles

Pressure-induced shape and color changes and mechanical-stimulation-driven reverse transition in a one-dimensional hybrid halide

Dynamic crystals with directional deformations in response to external stimuli through molecular reconfiguration, are observed predominantly in certain organic crystals and metal complexes. Low-dimensional hybrid halides, resemble these materials due to the presence of strong hydrogen bonds between bulky organic moieties and inorganic units, whereas their dynamic behavior remains largely unexplored. Here we show that a one-dimensional hybrid halide (MV)BiBr5 (MV = methylviologen) undergoes an isosymmetric phase transition at hydrostatic pressure of 0.20 GPa, accompanied by a remarkable length expansion of 20–30% and red to dark yellow color change. Unexpectedly, the backward transition can be fully reversed by mechanical stimulation rather than decompression. In the high-pressure phase, the coexistence of strong Bi3+ lone pair stereochemical activity and large reorientations of the planar MV2+ cations, together with the newly formed CH···Br hydrogen interactions, are the structural features that facilitate microscopic changes and stabilize the metastable high-pressure phase at ambient conditions.

Giant Negative Linear Compressibility, Isosymmetric Phase Transition, and Breathing Effect in a 3D Covalent Organic Framework

Giant Negative Linear Compressibility, Isosymmetric Phase Transition, and Breathing Effect in a 3D Covalent Organic Framework

Effect of isosymmetric phase transition in MIEC perovskite on the kinetic parameters of its interaction with oxygen

Oxygen partial pressure relaxation technique was used to study transient kinetics of re-equilibration in the reaction of nonstoichiometric oxide Ba0.5Sr0.5Co0.75Fe0.2Mo0.05O3-δ with oxygen within a region of 3-δ stoichiometry close to the nominal oxidation state 3+ for cobalt where an isosymmetric phase transition cubic perovskite P1 – cubic perovskite P2 is observed. The relaxation data were analyzed using a macrokinetic model that explicitly takes into account the power-law dependence of the surface reaction rate on the oxygen partial pressure. A twofold decrease in the power-law exponent (n = 0.7 to n = 0.3) was found at the point of the P1-P2 phase transition, which indicates a change in the mechanism of the rate-determining step of the reaction caused by the transition. At the same time, the functional dependence of the diffusion mobility of oxygen vacancies on the equilibrium oxygen pressure does not change at the transition point, which means that the transition does not affect the mechanism of bulk vacancy mobility.

Unveiling the multifaceted properties of a 3d covalent-organic framework: Pressure-induced phase transition, negative linear compressibility and auxeticity

High-pressure behavior and mechanical properties of a three-dimensional covalent-organic framework (NPN-1) were investigated by using different types of first principles molecular simulations. An irreversible pressure-induced first-order isosymmetric phase transition was predicted at 0.14 GPa. The subunit of NPN-1 retains its rigidity under pressure thanks to the strong covalent bonds. However, compression leads to significant tilting of the nitrophenyl groups. The mechanical properties of frameworks are highly anisotropic. Remarkably, both phases exhibit not only negative linear compressibility along the c-axis but also negative Poisson's ratio in certain directions. Detailed structural analysis revealed that the origin of the phase transition and anomalous mechanical properties of both phases are the wine-rack motif and strut-hinge mechanism. To the best of our knowledge, this study is the first report of such behavior in COFs, opening up new avenues for the exploration of COFs as materials for many promising applications.

Structural and vibrational properties of GdTaO4 under compression: An insight from experiment and first principles simulations

Polycrystalline GdTaO4, synthesized by solid state reaction route at 1300°C, adopts an M′ fergusonite crystal structure (space group P2/c) with GdO8 and TaO6 as constituent units. The compression behavior of the compound has been investigated in a diamond anvil cell by powder x-ray diffraction and Raman spectroscopic techniques. Both the techniques indicate pressure driven first order isosymmetric phase transition in the compound around 19 GPa. X-ray diffraction data show nearly 6% volume discontinuity at the phase transition and a change in oxygen coordination around the Ta atom from six in the ambient phase to eight in a high pressure phase. Experimental data collected in the process of decompression confirm the reversible nature of phase transition. Bulk modulus obtained by fitting the pressure–volume data to the 3rd-order Birch–Murnaghan equation of state shows a higher value of bulk modulus for the high pressure phase compared to the low pressure phase, which is consistent with increased density due to volume collapse at the phase transition. The pressure dependence of unit cell parameters and Raman active modes along with Grüneisen parameters are also reported. Density functional theory based first principles simulations performed on compound corroborate the experimental findings. In low pressure phase, the simulated volumes of the constituent polyhedra under pressure indicate that the major contribution in the bulk modulus comes from lower valence rare earth polyhedra; however, for a high pressure phase, both the polyhedra units (GdO8 and TaO8) have almost similar contribution to the bulk modulus of the compound.

Voronoi Tessellations and the Shannon Entropy of the Pentagonal Tilings.

We used the complete set of convex pentagons to enable filing the plane without any overlaps or gaps (including the Marjorie Rice tiles) as generators of Voronoi tessellations. Shannon entropy of the tessellations was calculated. Some of the basic mosaics are flexible and give rise to a diversity of Voronoi tessellations. The Shannon entropy of these tessellations varied in a broad range. Voronoi tessellation, emerging from the basic pentagonal tiling built from hexagons only, was revealed (the Shannon entropy of this tiling is zero). Decagons and hendecagon did not appear in the studied Voronoi diagrams. The most abundant Voronoi tessellations are built from three different kinds of polygons. The most widespread is the combination of pentagons, hexagons, and heptagons. The most abundant polygons are pentagons and hexagons. No Voronoi tiling built only of pentagons was registered. Flexible basic pentagonal mosaics give rise to a diversity of Voronoi tessellations, which are characterized by the same symmetry group. However, the coordination number of the vertices is variable. These Voronoi tessellations may be useful for the interpretation of the iso-symmetrical phase transitions.

Biaxial Hard Compression, Anisotropic Elastic Property, and Pressure-Induced Isosymmetric Phase Transition in Ammonium Bicarbonate

The elastic property, exceptional anisotropic compression, and pressure-induced isosymmetric phase transition in ammonium bicarbonate (AB) are studied by in situ synchrotron X-ray diffraction, Raman spectroscopy, and computational methods. The exceptional anisotropic compression as a result of biaxial hard compression is induced by stiff hydrogen-bonding “double-wine-rack” geometric motifs. The large values of elastic anisotropy such as Young’s moduli, Shear moduli, and Poisson’s ratio verify the anisotropic nature in AB. The biaxial hard compression further induces an isosymmetric phase transition at 2 GPa through the generation of new N–H···O hydrogen bonds. Our work first demonstrates the stiff mechanical study of “double-wine-rack” geometric hydrogen-bonding networks and its influence on the phase transition, which can be used as a model to investigate the robust nature of hydrogen bonds and the structural origin of isosymmetric phase transition. Moreover, the hydrogen-bonding “double-wine-rack” geometric mechanism also provides an inspiration to construct a covalent-bonding “double-wine-rack” geometric model favoring anomalous biaxial mechanical/thermal responses. The determination of the high-pressure phase of AB provides new information to understand the composition of the ternary system H2O–CO2–NH3.

Tracking structural phase transitions via single crystal x-ray diffraction at extreme conditions: advantages of extremely brilliant source

Highly brilliant synchrotron source is indispensable to track pressure-induced phenomena in confined crystalline samples in megabar range. In this article, a number of experimental variables affecting the quality high-pressure single-crystal x-ray diffraction data is discussed. An overview of the recent advancements in x-ray diffraction techniques at extreme conditions, in the frame of European Synchrotron Radiation Facility (ESRF)- Extremely Bright Source (EBS), is presented. Particularly, ID15b and ID27 beamlines have profited from the source upgrade, allowing for measurements of a few-micron crystals in megabar range. In case of ID27, a whole new beamline has been devised, including installation of double-multilayer mirrors and double crystal monochromator and construction of custom-made experimental stations. Two case studies from ID27 and ID15b are presented. Hypervalent CsI3 crystals, studied up to 24 GPa, have shown a series of phase transitions: Pnma → P-3c1→ Pm-3 n. First transition leads to formation of orthogonal linear iodine chains made of I3 -. Transformation to the cubic phase at around 21.7 GPa leads to equalization of interatomic I–I distances and formation of homoleptic In m- chains. The second study investigates elastic properties and structure of jadarite, which undergoes isosymmetric phase transition around 16.6 GPa. Despite a few-micron crystal size, twinning and dramatic loss of crystal quality, associated with pressure-induced phase transitions, crystal structures of both compounds have been determined in a straightforward matter, thanks to the recent developments within ESRF-EBS.

Ultrahigh elasticity and anomalous softening of α-Ag2S under pressure

Inorganic semiconductors are often brittle at room temperature, but α-Ag2S was recently observed to own extraordinary metal-like ductility and reversible pressure-induced isosymmetric phase transitions. Here, we report findings from first-principles calculations that α-Ag2S exhibits ultrahigh elasticity and anomalous softening under pressure. Such unusual mechanical behaviors have their underlying origins. Subsequent experimental measurements confirm that α-Ag2S is very soft with a large elastic strain limit of ∼1.7%. This study provides crucial insights into the understanding of high elasticity in semiconducting materials, which is beneficial for the design of semiconductors for flexible and stretchable electronic devices.

Origin of supertetragonality in BaTiO3

Understanding ferroelectricity is of both fundamental and technological importance to further stimulate the development of new materials designs and manipulations. In this respect, supertetragonal (ST) phases with high $c/a$ ratio of $\ensuremath{\sim}1.3$ have recently resulted in outstanding ferroelectric polarization values, which urges for a microscopic origin of these novel structural phases. Here, we perform an in-depth first-principle study on the well-known ferroelectric barium titanate ${\mathrm{BaTiO}}_{3}$ under a hydrostatic negative pressure, showing an isosymmetric phase transition to such a ST phase. The chemical origin and driving mechanisms of this phase transition are identified as a drastic change of the covalently $\ensuremath{\pi}$-bonded electrons. These findings provide guidance in the search for new ST phases, with great opportunities for novel multiferroic materials, and can be generalized in the understanding of other isosymmetric phase transitions.

Structural Phase Transitions between Layered Indium Selenide for Integrated Photonic Memory.

The primary mechanism of optical memoristive devices relies on phase transitions between amorphous and crystalline states. The slow or energy-hungry amorphous-crystalline transitions in optical phase-change materials are detrimental to the scalability and performance of devices. Leveraging an integrated photonic platform, nonvolatile and reversible switching between two layered structures of indium selenide (In2 Se3 ) triggered by a single nanosecond pulse is demonstrated. The high-resolution pair distribution function reveals the detailed atomistic transition pathways between the layered structures. With interlayer "shear glide" and isosymmetric phase transition, switching between the α- and β-structural states contains low re-configurational entropy, allowing reversible switching between layered structures. Broadband refractive index contrast, optical transparency, and volumetric effect in the crystalline-crystalline phase transition are experimentally characterized in molecular-beam-epitaxy-grown thin films and compared to ab initio calculations. The nonlinear resonator transmission spectra measure of incremental linear loss rate of 3.3GHz, introduced by a 1.5µm-long In2 Se3 -covered layer, resulted from the combinations of material absorption and scattering.

Pressure-induced phase transitions in the ZrXY (X = Si, Ge, Sn; Y = S, Se, Te) family compounds

Pressure is an effective and clean way to modify the electronic structures of materials, cause structural phase transitions and even induce the emergence of superconductivity. Here, we predicted several new phases of the ZrXY family at high pressures using the crystal structures search method together with first-principle calculations. In particular, the ZrGeS compound undergoes an isosymmetric phase transition from P4/nmm-I to P4/nmm-II at approximately 82 GPa. Electronic band structures show that all the high-pressure phases are metallic. Among these new structures, P4/nmm-II ZrGeS and P4/mmm ZrGeSe can be quenched to ambient pressure with superconducting critical temperatures of approximately 8.1 K and 8.0 K, respectively. Our study provides a way to tune the structure, electronic properties, and superconducting behavior of topological materials through pressure.

A first-order phase transition in Blatter's radical at high pressure.

The crystal structure of Blatter's radical (1,3-diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) has been investigated between ambient pressure and 6.07 GPa. The sample remains in a compressed form of the ambient-pressure phase up to 5.34 GPa, the largest direction of strain being parallel to the direction of π-stacking interactions. The bulk modulus is 7.4 (6) GPa, with a pressure derivative equal to 9.33 (11). As pressure increases, the phenyl groups attached to the N1 and C3 positions of the triazinyl moieties of neighbouring pairs of molecules approach each other, causing the former to begin to rotate between 3.42 to 5.34 GPa. The onset of this phenyl rotation may be interpreted as a second-order phase transition which introduces a new mode for accommodating pressure. It is premonitory to a first-order isosymmetric phase transition which occurs on increasing pressure from 5.34 to 5.54 GPa. Although the phase transition is driven by volume minimization, rather than relief of unfavourable contacts, it is accompanied by a sharp jump in the orientation of the rotation angle of the phenyl group. DFT calculations suggest that the adoption of a more planar conformation by the triazinyl moiety at the phase transition can be attributed to relief of intramolecular H...H contacts at the transition. Although no dimerization of the radicals occurs, the π-stacking interactions are compressed by 0.341 (3) Å between ambient pressure and 6.07 GPa.

Where the extraordinaries meet: a cascade of isosymmetrical superionic phase transitions and negative thermal expansion in a novel silver salt-inclusion borate halide

A new silver iodide borate, Ag3B6O10I, is presented as a promising solid-state electrolyte with a δ ↔ γ ↔ β ↔ α cascade of the isosymmetric superionic phase transitions.

Crystal structure of the high-P polymorph of Ca2B6O6(OH)10·2(H2O) (meyerhofferite)

The crystal structure of the high-pressure polymorph of meyerhofferite, ideally Ca2B6O6(OH)10·2(H2O), has been determined by means of single-crystal synchrotron X-ray diffraction data. Meyerhofferite undergoes a first-order isosymmetric phase transition to meyerhofferite-II, bracketed between 3.15 and 3.75 GPa, with a large volume discontinuity. The phase transition is marked by an increase in the coordination number of the boron B1 site, from III to IV, leading to a more interconnected and less compressible structure. The main structural differences between the two polymorphs and the P-induced deformation mechanisms at the atomic scale are discussed.

Can We Predict the Isosymmetric Phase Transition? Application of DFT Calculations to Study the Pressure Induced Transformation of Chlorothiazide.

Isosymmetric structural phase transition (IPT, type 0), in which there are no changes in the occupation of Wyckoff positions, the number of atoms in the unit cell, and the space group symmetry, is relatively uncommon. Chlorothiazide, a diuretic agent with a secondary function as an antihypertensive, has been proven to undergo pressure-induced IPT of Form I to Form II at 4.2 GPa. For that reason, it has been chosen as a model compound in this study to determine if IPT can be predicted in silico using periodic DFT calculations. The transformation of Form II into Form I, occurring under decompression, was observed in geometry optimization calculations. However, the reverse transition was not detected, although the calculated differences in the DFT energies and thermodynamic parameters indicated that Form II should be more stable at increased pressure. Finally, the IPT was successfully simulated using ab initio molecular dynamics calculations.

Vibrational and conformational analysis of structural phase transition in Estradiol 17β valerate with temperature

Estradiol 17β valerate (E2V) is a hormonal medicine widely used in hormone replacement therapy. E2V undergoes a reversible isosymmetric structural phase transition at low temperature (̴ 250 K) which results from the reorientation of the valerate chain. The reversible isosymmetric structural phase transition follows Ehrenfest’s classification when described as first-order and Buerger’s classification when classified as order–disorder. The conformational difference also induces changes in molecular torsional angles and on the hydrogen bond pattern. In combination with density functional theory (DFT) calculations, vibrational spectroscopy has been used to correlate the valerate chain modes with the modifications of the dihedral angles on phase transition. We are expecting improvement in our understanding of the phase transition mechanism driven by the temperature. The Conformational analysis reveals the feasible structures corresponding to changes in the dihedral angles associated with the valerate chain. The infrared spectra of calculated conformers are in good agreement with the experimental spectra of E2V structure recorded at room temperature revealing that the changes in valerate chain modes at 1115 cm−1, 1200 cm-1and 1415 cm−1 fingerprint the molecular conformation.An investigation made to determine the ligand–protein interaction of E2V through docking against estrogen receptor (ER) reveals the inhibitive and agonist nature of E2V.

Comparison of the temperature- and pressure-dependent behavior of the crystal structure of CrAs

The crystal structure of CrAs was investigated using synchrotron X-ray single-crystal diffraction for separate dependences on temperature (30–400 K) and on pressure (0–9.46 GPa). The isosymmetrical magnetostructural phase transition at T N = 267 K can induce a change in the microstructure by twinning due to a crossing of the orthohexagonal setting of the unit-cell parameter ratio c/b. Within the crystal structure, one particular Cr–Cr distance exhibits anomalous behavior in that it is nearly unaffected by temperature and pressure in the paramagnetic phase, which is stable above 267 K and at high pressures. The distinction of this shortest Cr–Cr distance might be of importance for the superconducting properties of CrAs.

Mechanism of Unusual Isosymmetric Order-Disorder Phase Transition in [Dimethylhydrazinium]Mn(HCOO)3 Hybrid Perovskite Probed by Vibrational Spectroscopy

[DMHy]Mn(HCOO)3 (DMHy+ = dimethylhydrazinium cation) is an example of an organic–inorganic hybrid adopting perovskite-like architecture with the largest organic cation used so far in the synthesis of formate-based hybrids. This compound undergoes an unusual isosymmetric phase transition at 240 K on heating. The mechanism of this phase transition has a complex nature and is mainly driven by the ordering of DMHy+ cations and accompanied by a significant distortion of the metal–formate framework in the low temperature (LT) phase. In this work, the Density Functional Theory (DFT) calculations and factor group analysis are combined with experimental temperature-dependent IR and Raman studies to unequivocally assign the observed vibrational modes and shed light on the details of the occurring structural changes. The spectroscopic data show that this first-order phase transition has a highly dynamic nature, which is a result of balanced interplay combining re-arrangement of the hydrogen bonds and ordering of DMHy+ cations. The tight confinement of organic cations forces simultaneous steric deformation of formate ions and the MnO6 octahedra.

Phase transition and high-pressure behavior of ulexite, a potential aggregate in radiation-shielding concretes

The high-pressure behavior of ulexite [ideally NaCaB5O6(OH)6·5(H2O)], a B-bearing raw material (with B2O3 ≈ 43 wt%) and a potential B-rich aggregate, has been studied by in-situ single-crystal synchrotron X-ray diffraction up to 9.5 GPa, in order to explore its stability field at high pressure, the anisotropy of its compressional behavior and the deformation mechanisms at the atomic scale. Ulexite undergoes a first-order iso-symmetric phase transition, reconstructive in character, between 6.44 and 7.13 GPa (to the ulexite-II polymorph); the crystal structure of ulexite-II was successfully solved and refined. For the low-P polymorph of ulexite (stable in all the working conditions as aggregate in concretes), the isothermal bulk modulus here obtained is KV0 = 36.9(5) GPa, with a strongly anisotropic compressional pattern (finite-Eulerian principal components of the unit-strain ellipsoid: ε1:ε2:ε3 ∼7:3:1). Ulexite-II also shows a marked anisotropic compressional pattern and with a resulting bulk softening (KV0 = 26(4) GPa).