Pressure-induced Structural Transformations Research Articles

Temperature and volumetric effects on structural and dielectric properties of hybrid perovskites

Three-dimensional organic-inorganic perovskites are rapidly evolving materials with diverse applications. This study focuses on their two representatives - acetamidinium manganese(II) formate (AceMn) and formamidinium manganese(II) formate (FMDMn) – subjected to varying temperature and pressure. We show that AceMn undergoes atypical pressure-induced structural transformations at room temperature, increasing the symmetry from ambient-pressure P21/n phase II to the high-pressure Pbca phase III. In turn, FMDMn in its C2/c phase II displays temperature- and pressure-induced ordering of cage cations that proceeds without changing the phase symmetry or energy barriers. The FMD+ cations do not order under constant volume across the pressure-temperature plane, despite similar pressure and temperature evolution of the unit-cell parameters. Temperature and pressure affect the cage cations differently, which is particularly pronounced in their relaxation dynamics seen by dielectric spectroscopy. Their motion require a rearrangement of the metal-formate framework, resulting in the energy and volumetric barriers defined by temperature-independent activation energy and activation volume parameters. As this process is phonon-assisted, the relaxation time is strongly temperature-dependent. Consequently, relaxation times do not scale with unit-cell volume nor H-bond lengths in formates, offering the possibility of tuning their electronic properties by external stimuli (like temperature or pressure) even without any structural changes.

Robustness of the pyrochlore structure in rare-earth A2Ir2O7 iridates and pressure-induced structural transformation in IrO2

A comprehensive study of the structural properties of the heavily investigated rare-earth A2Ir2O7 iridate series under extreme conditions is presented. From Pr2Ir2O7 to Lu2Ir2O7, the series is sufficiently covered by iridates with A = Pr, Sm, Dy, Ho, Er, Tm, Yb, and Lu; general trends and systematics within the series, including the understudied heavy-rare-earth members, are dependably followed. Temperature- and pressure-dependent synchrotron X-ray powder diffraction experiments reveal robustness of the pyrochlore structure throughout the series, down to 4 K and up to 20 GPa. The thermal expansivity of the pyrochlore lattice is determined, all falling in the Debye temperature range of θD = 360–420 K. The pressure compressibility shows a systematic increase of the bulk modulus with the rare-earth atomic number from K = 180–210 GPa. Combining the results of thermal measurements (Debye temperature) and pressure measurements (bulk modulus) enables us to determine the Grüneisen parameter of selected members and compare it to previous studies. Temperature and pressure evolution of the fractional coordinate of oxygen at 48f Wyckoff position, the sole free fractional coordinate in the crystal structure, is investigated and discussed regarding the antiferromagnetic ordering of the Ir magnetic moments. In addition to results on A2Ir2O7 iridates, the temperature and pressure evolution of the crystal structure of an IrO2 minority phase is followed. The tetragonal rutile-type structure is stable down to the lowest temperature. However, an application of pressure of approximately 15 GPa induces a structural transition: The tetragonal structure is orthorhombically distorted. The orthorhombic structure is still not fully stabilised at 20 GPa, and further distortion of the lattice (or subsequent structural transformations) is expected with increasing external pressure.

Systemic approach in the study of the properties and pressure-induced structural transformations in TaN: First-principles molecular dynamics simulations

First-principles molecular dynamics simulations were used to investigate pressure- and temperature-induced phase transitions in different TaN polymorphs. The driving forces and intermediate structures were established for the CoSn-like TaN (ε phase) to WC-type TaN (θ phase) and ε to NaCl type-TaN (δ phase) structural transformations that were observed in experiments and for the θ to hP4-194, θ to cP2-221, δ to cP2-221 and δ to tP4-129 phase transitions that were predicted in this investigation. Other possible TaN polytypes were systematically investigated and five new prospective phases were found: oP8-25, oP12-25, hP8-194, tP8-105 and tI8-109. All new structures are thermodynamically, dynamically and mechanically stable, and their formation energy is only slightly higher than that of the θ phase (by 0.014–0.032 eV/atom). The relative stability, electronic structure, chemical bonding, vibrational spectra, elastic moduli and their spatial anisotropy, Vickers hardness, fracture toughness, Debye temperature and optical properties (real and imaginary parts of the dielectric function, reflectivity spectrum) of the tantalum mononitrides mentioned above were systematically calculated and discussed. The oP12-25, tP8-105 and tI8-109 novel phases exhibit the Vickers hardness (33.1–34.2 GPa), fracture toughness (5.52–5.59 MPa m1/2) and Debye temperatures (609.7–616.1 K) that are higher compared to those of the known TaN structures. The TaN polytypes with the strong Ta-N bonds with minimal fraction of ionicity were found to possess the best mechanical properties. Further experimental investigations are highly desirable to confirm the results presented in this paper.

High-pressure polymorphism in amoxicillin

This work is devoted to the pressure-induced structural transformations of amoxicillin by Raman and infrared spectroscopy at pressures up to 10 GPa. The Raman and infrared results indicate that amoxicillin underwent two phase transitions upon compression, specifically phase α → β and β → γ phase transitions at 2.71 GPa and 7.45 GPa. The significant changes of hydrogen bonding are indicated by the softened Raman modes, suggesting that the α → β phase transition may be a molecular layering reorganization process. The retrieved Raman and FT-IR spectra demonstrate that pressure-induced phase transitions are reversible upon decompression from 10 GPa to ambient pressure.

Breathing porous liquids based on responsive metal-organic framework particles

Responsive metal-organic frameworks (MOFs) that display sigmoidal gas sorption isotherms triggered by discrete gas pressure-induced structural transformations are highly promising materials for energy related applications. However, their lack of transportability via continuous flow hinders their application in systems and designs that rely on liquid agents. We herein present examples of responsive liquid systems which exhibit a breathing behaviour and show step-shaped gas sorption isotherms, akin to the distinct oxygen saturation curve of haemoglobin in blood. Dispersions of flexible MOF nanocrystals in a size-excluded silicone oil form stable porous liquids exhibiting gated uptake for CO2, propane and propylene, as characterized by sigmoidal gas sorption isotherms with distinct transition steps. In situ X-ray diffraction studies show that the sigmoidal gas sorption curve is caused by a narrow pore to large pore phase transformation of the flexible MOF nanocrystals, which respond to gas pressure despite being dispersed in silicone oil. Given the established flexible nature and tunability of a range of MOFs, these results herald the advent of breathing porous liquids whose sorption properties can be tuned rationally for a variety of technological applications.

Temperature- and pressure-induced structural transformations in NbN: A first-principles study

Structural transformations at high temperatures and pressures, elastic moduli, hardness, fracture toughness, Debye temperature, stress-shear strain relations, electronic structure, and lattice dynamics in the experimentaly observed NaCl–NbN (δ, Fm-3m), anti-TiP-NbN (ε, P63/mmc), anti-NiAs-NbN (δ′, P63/mmc), WC-NbN (η, P-6m2), and TiP–NbN (ε′, P63/mmc) phases and hypothetical new tP4-129 (P4/nmm), hP6-189 (P-62m), oP8-25 (Pmm2) and cP2-221 (Pm-3m) structures are studied by using first-principles calculations and molecular dynamics simulations. The possible mechanisms of the phase transitions between these structures based on the condensation of a certain phonon mode with subsequent spontaneous strains are suggested. The ε, η and δ′, and cP2-221 structures are brittle materials and exhibit highest shear moduli (200.5–216.8 GPa), Young moduli (492.4–528.8 GPa), Vickers hardness (23.9–27.1 GPa), fracture toughness (4.54–4.72 MPa m1/2), and Debye temperatures (730.5–767.0 K). It is found that the main slip systems should be (0001)<10-10> for ε and η, and both (0001)<10-10> and (0001)<-12-10> for δ’.

Compressive Symbol: A New Way to Evaluate the High-Pressure Behaviors of Energetic Tetrazole Materials.

Energetic materials may transit to different phases or decompose directly under compression. Their reactivity in the explosions can be evaluated by their high-pressure induced behaviors, including polymorphism or phase transition. Here, we applied DFT methods to understand high-pressure behaviors of four typical tetrazole derivate crystals, including 5-aminotetrazole (ATZ), 1,5-aminotetrazole (DAT), 5-hydrazinotetrazole (HTZ), and 5-azidotetrazole (ADT), under the gradually increased pressure from ambient pressure to 200 GPa. In response to the extreme-high pressures, the performances are dominated by compressibility of crystals, reflected by compressive symbols on the basis of the molecular orientation in crystals. The crystal with weak compressibility (large symbol) generally dissociates, triggered by cleavage of weak bonds. However, the crystal with low compressive symbol is generally corresponding to a pressure-induced structural transformation or phase transition.

The pressure-induced structural transformation of La2S3

As a fundamental thermodynamic parameter, pressure can cause matters to produce new phases with unique physical properties. In this paper, the pressure-induced behavior of La2S3 has been investigated in depth by in-situ high pressure synchrotron angle dispersive X-ray diffraction (ADXRD), and the pressure is up to 26 GPa. We find the conventional La2S3 which was stable at ambient pressure, was destabilized with the application of pressure and decomposed into La3S4 and LaS2 at 13.4 GPa. This is the first time that the high pressure decomposition of La2S3 has been found in the experiments. The La2S3 possesses the lowest formation enthalpy at atmospheric pressure, enthalpy change increases with the pressure increasing, leading to the decomposition of La2S3. The high pressure inhibits the ability of S atoms to accept electrons and promotes the polymerization of S atoms to form shared electron pairs, brings in the change of S–S bonding modes, and also further leads to the change of structural. The study of pressure relief experiment shows that the pressure induced decomposition in La2S3 is an irreversible phase transition. The different radial direction has the different compression effects under pressure, the initial structure decays rapidly driven by pressure. With the pressure rising, the decay rate becomes gentle. The volume expansion of La3S4 and LaS2 may be due to some defects, and the smaller the volume, the greater the bulk modulus. This work provides a strong support for the theoretical research results and is expected to provide guidance for the experiment study of the physical properties and structural characteristics of La–S system and other rare earth sulfur compounds.

Structural transformation and micro-phase separation of CaO-P2O5-SiO2 system under compression

This study investigated the pressure-induced structural transformation and micro-phase separation of the CaO-P2O5-SiO2 system at 3000 K and in the pressure range of 0–100 GPa based on molecular dynamics simulations. The structural characteristics were investigated based on the short range order, intermediate range order, and glassy network structure. The evolution of the local environment of network-forming ions (P5+, Si4+) and clustering behavior of network modifiers (Ca2+ ions) were clarified based on the radial distribution function, coordination number, and O-(P, Si, Ca) linkage distribution. The topologies of TOx (T = Si, P; x = 4, 5, 6) polyhedra were determined using the bond angle and bond length distribution. The linkages among TOx polyhedra, Ca2+ ion distribution, compositional and structural heterogeneities, micro-phase separation, and mechanism of Ca2+ incorporation into glassy networks were also visually analyzed and investigated in detail.

Pressure-Induced Structural Transformations and Electronic Transitions in TeO2 Glass by Raman Spectroscopy.

TeO2 glass has been studied by Raman spectroscopy up to the record pressure of 70 GPa. The boson peak frequency ωb exhibits a decrease of the ∂ωb/∂P slope at 5-6 GPa and saturates above 30 GPa with a practically constant value up to 70 GPa. Experiment and theory indicate that pressures up to 20 GPa induce the transformation of single Te-O-Te bridges to double Te-O2-Te bridges, leading to a more compact structure, while Raman activity developing at higher pressures around 580 cm-1 signals the increase of Te coordination from 4- to 6-fold. Natural bond orbital analysis shows that double Te-O2-Te bridges favor the s → d transition and promote the increase of Te coordination through d2sp3 hybridization. This transition leads to the formation of TeO6 octahedra, in strict difference with crystalline TeO2 at the same pressure range, and to the development of a 3D network that freezes the medium range order.

Pressure-induced structural phase transition, irreversible amorphization and upconversion luminescence enhancement in Ln3+-codoped LiYF4 and LiLuF4

The pressure-induced structural transformation and irreversible amorphization contribute to 1.2–2.6 times upconversion luminescence enhancement in Ln3+ codoped LiYF4 and LiLuF4.

Giant power density from BiFeO3-based ferroelectric ceramics by shock compression

Ferroelectric pulsed-power sources with rapid response time and high output energy are widely applied in the defense industry and mining areas. As the core materials, ferroelectric materials with large remnant polarization and high electrical breakdown field should generate high power under compression. Currently, lead zirconate titanate 95/5 ferroelectric ceramics dominated in this area. Due to environmental damage and limited output power of lead-based materials, lead-free ferroelectrics are highly desirable. Here, the electrical response of 0.9BiFeO3-0.1BaTiO3 (BFO-BT) ferroelectric ceramics under shock-wave compression was reported, and a record-high power density of 4.21 × 108 W/kg was obtained, which was much higher than any existing lead-based ceramics and other available energy storage materials. By in situ high-pressure neutron diffraction, the mechanism of shock-induced depolarization of the BFO-BT ceramics was attributed to pressure-induced structural transformation, and the excellent performance was further elaborated by analyzing magnetic structure parameters under high pressures. This work provides a high-performance alternative to lead-based ferroelectrics and guidance for the further development of new materials.

High-pressure study of hydrogen storage material ethylenediamine bisborane from first-principle calculations

Ethylenediamine bisborane (EDAB), as a hydrogen storage material, is one of the most promising possible candidates. This work reports the effects of pressure on the structural, mechanical, electronic, and vibrational behaviors of EDAB using density functional theory calculations. The predicted lattice parameters, bulk modulus B0 and its first pressure derivative B′ are in accord with available experimental data. From the unit-cell constants as a function of pressure, crystal EDAB is found to present an anisotropic compressibility. The calculated elastic properties show that the ambient phase EDAB is mechanical unstable at 3 GPa. Furthermore, the discontinuous behavior was found in the pressure dependence of crystal and electronic structure, suggesting a structural transformation occurred around 3 GPa. Finally, vibrational spectra are reported together with their infrared active modes assignment. The NH and BH stretching modes soften under compression, indicating the strengthening of hydrogen bonding, which leads to the occurrence of pressure-induced structural transformation.

Chemical pressure-induced structural, optical, and magnetic property transformations of PrAlO3

Progressive substitution of non-magnetic Al3+-ion with magnetic Cr3+-ion in PrAlO3 has been carried out in the present study by preparing the samples using the epoxide mediated sol-gel method. Extensive characterization of these samples was performed to comprehend the effect of chemical pressure on the structure and optical and magnetic properties. The hexagonal system of PrAlO3 with rhombohedral symmetry (R-3c space group) was preserved up to 60 mol % inclusion of Cr3+ in place of Al3+, beyond which the samples exhibited orthorhombic symmetry (Pnma space group). FTIR and Raman's spectra also confirmed the symmetry reduction in these samples. The transitions associated with Pr3+ and Cr3+ ions were observed in the optical spectra. The optical bandgap estimated using Tauc plot decreased from 3.2 (x = 0.10) to 1.75 eV (x = 0.60) in PrAl1-xCrxO3. With higher Cr3+-concentrations, the optical bandgap increased from 1.75 (x = 0.70) to 3.2 eV (PrCrO3). The field-dependent magnetic measurements indicated that all the samples were paramagnetic at room temperature and ferromagnetic at 2 K. Inclusion of Cr3+ -ion in place of Al3+ -ion in PrAlO3 shifts the transition from 109 to 231 K. They resulted in the increased effective magnetic moment.

Pressure-induced structural transformation of clathrate Ge136 via ultrafast recrystallization of an amorphous intermediate

We study the pressure-induced structural transformation of Ge$_{136}$ clathrate by ab initio molecular dynamics and metadynamics. The system under pressure first undergoes amorphization followed by an ultrafast recrystallization to the $\beta$-tin structure on the time scale of 30 ps. The initial pressure-induced amorphization of clathrate is triggered by high pressure while the subsequent fast recrystallization to $\beta$-tin is driven by low temperature. Interestingly, the amorphous intermediate is still diffusive even at room temperature, in spite of very strong undercooling, making the ultrafast recrystallization possible. The system provides an explicit example of structural transformation between two crystalline phases proceeding via non-crystalline intermediate. Upon fast decompression of the amorphous structure with incipient crystalline order the recrystallization is blocked and the system instead proceeds to the tetrahedral LDA amorphous phase.

Semiconducting BaS3 phase featuring v-shape S3 unit at high pressure

Pressure-induced structural transformation of known compounds has become an effective routine to obtain new materials. The sulfur motifs that are present in compounds have a considerable effect on their crystal structures and electronic properties. Here, we focus on exploring the evolution of S motifs in the $\mathrm{Ba}{\mathrm{S}}_{3}$ compound under high pressure. A novel $P{3}_{1}21\text{\ensuremath{-}}\mathrm{Ba}{\mathrm{S}}_{3}$, consisting of an equivalent V-shape ${\mathrm{S}}_{3}$ unit and quasi-hexagon Ba ring, is identified through the combination of first-principles swarm-intelligence search and phonon softening calculations. The bond angle in the ${\mathrm{S}}_{3}$ unit decreases, accompanied by pressure-induced structural phase transition. $P{3}_{1}21\text{\ensuremath{-}}\mathrm{Ba}{\mathrm{S}}_{3}$ shows a semiconducting character, mainly originating from the contribution of S $3p\text{\ensuremath{-}}\mathrm{electron}$ states. Along with the newly proposed $\mathrm{Ba}{\mathrm{S}}_{3}$ structure, the temperature effect on stability is estimated using the temperature-dependent effective potential method. For another S-rich $\mathrm{Ba}{\mathrm{S}}_{2}$, the disappearance of the ${\mathrm{S}}_{2}$ unit induced by pressure ($&gt;150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$) is mainly responsible for structural instability.

Pressure-induced structural transformations on linear carbon chains encapsulated in carbon nanotubes: A potential route for obtaining longer chains and ultra-hard composites

In this paper, we report high-pressure Raman experiments (0 − 28 GPa) in two different systems, i.e., linear carbon chains encapsulated by multi-walled (Cn@MWCNTs) and double-walled (Cn@DWCNTs) carbon nanotubes. By running high-pressure cycles, it is observed basically two changes in the Raman spectra of chains that are the softening and disappearance of Cn modes. We attributed the irreversible redshift of the Cn band to the coalescence between adjacent chains. On the other hand, the disappearance of the Cn band at the onset of the CNT collapse pressure is assigned to the tube-chain cross-linking. This effect appears to be independent of the Cn length and the number of walls of the tubes, depending only on the innermost CNT diameter. We show that the pressure to coalesce longer chains is higher than 10 GPa. Density functional theory and molecular dynamics calculations were performed in order to support the interpretation of experimental data. The calculations show an irreversible pressure-induced softening of frequency of the confined linear carbon chain. Furthermore, molecular dynamics calculations showed that coalescence between the shorter chains occurs in a region of lower pressure than that of the longer chains, thus supporting the experimental observations. Our findings shed new light on the understanding of the Cn@CNT system stability and pave the way for using high-pressure to obtain ultra-long chains (at lower pressures) and ultra-hard composites (at higher pressures).

A comparative study on pressure-induced structural transformations in a basaltic glass and melt from Ab initio molecular dynamics calculations

A comparative study on pressure-induced structural transformations in a basaltic glass and melt from Ab initio molecular dynamics calculations

First-Principles investigation of Pressure-Induced structural transformations of barium borates in the BaO-B2O3-BaF2 system in the range of 0–10 GPa

First-principles calculations within the density functional theory of the stability of barium borates of the BaO-B2O3-BaF2 ternary system have been performed at the pressure of up to 10 GPa. A brief summary on the known structures of ambient and high-pressure phases in the BaO–B2O3 and BaF2–Ba3B2O6 subsystems has been provided. In the BaO–B2O3 subsystem the Ba3B2O6, BaB2O4, and BaB4O7 phases tentatively are stable at up to 10 GPa, while the other known ambient-pressure borates Ba5B4O11, Ba2B6O11, and BaB8O13 decompose under the pressure of above 7.1, 0.6, and 2 GPa, respectively. Two new high-pressure polymorphic modifications of BaB2O4 compound, BaB2O4-Pna21 and BaB2O4-Pa3-, stable above 1.0 and 6.1 GPa, respectively, have been predicted. In the BaF2–Ba3B2O6 subsystem Ba7(BO3)4-xF2+3x solid solution is suggested to be stable in the considered pressure range, and Ba5(BO3)3F is suggested to decompose into Ba3B2O6 and Ba7(BO3)4-xF2+3x at pressures above 3–5 GPa. It has been shown that the enthalpy of Ba7(BO3)4-xF2+3x strongly depends on the distribution of the [(BO3)F]4− and [F4]4− groups in the structure. We consider the results obtained as a necessary basis for an experimental study aimed at obtaining barium borates under pressures of up to 10 GPa and studying their structure and properties.

From Semiconducting to Metallic: Jahn–Teller-Induced Phase Transformation in Skyrmion Host GaV4S8

Lacunar spinels, GaM4X8 (M = V, Nb, Mo, Ta, W; X = S, Se, Te), constitute a rare class of compounds with multiferroic properties. Recently, one member of this family, GaV4S8, received global attention due to the Néel-type skyrmions discovered in this material. Previous investigations strongly indicate the important role of the structure behind the multiferroicity, for example, the strong impact of ferroelectric transition on the exchange interactions at ∼40 K. Inspired by the delicate entanglement of lattice, spin, and charge degrees of freedom, in the present work, we aimed to use pressure to alter the structure and thus change the material’s properties to establish the inter-relation between the structural, optical, and electrical properties for a better understanding of this skyrmion host material. Upon this objective, in situ high-pressure measurements of single crystal/powder X-ray diffraction, electrical conductivity, and Raman spectroscopy were carried out by using a diamond anvil cell. These studies revealed the pressure-induced structural transformation from cubic to orthorhombic, along with a transition from semiconductor to metallic state in GaV4S8. The phase changes coincide with the variation in the optical property in this material explored by Raman spectra. We also determined the bulk modulus of the two phases of GaV4S8 by fitting the data set of unit cell volumes against pressure with the second-order birth-Murnaghan equation of state, and explained the mechanisms of phase transitions by means of the Jahn–Teller effect and the anisotropic changes in bonding lengths during compression.