Pressure-induced Structural Changes Research Articles

Pressure-induced structural modifications of imogolite nanotubes and of their methylated analogues

Structural modifications of single-walled aluminosilicate clay nanotubes have been studied under hydrostatic pressure by in situ synchrotron X-ray scattering. Imogolite nanotubes (INT) of nominal composition (OH)3Al2O3SiOH, and methyl-modified imogolite nanotubes (m-INT), (OH)3Al2O3SiCH3, have hydrophilic and hydrophobic internal cavities, respectively. Nanotube chiralities also differ, with zigzag (INT) and armchair (m-INT) chirality. In this work, pressure-induced changes in nanotube morphology and atomic structure are studied as a function of chirality, affinity of the inner cavity, and the pressure-transmitting medium used. Radial deformation and collapse of nanotubes are evidenced below 3 GPa, followed by the formation of a lamellar phase at higher pressures. In the case of INT, the collapse pressure value depends on the pressure transmitting medium chosen. Axial compressibility is measured, and a pseudo Young's modulus Y is determined to be equal to ∼265 GPa for INT and below 80 GPa for m-INT, underpinning the role of nanotube chirality in mechanical properties.

In situ high-pressure pair distribution function measurement of liquid and glass by using 100keV pink beam.

Understanding the pressure-induced structural changes in liquids and amorphous materials is fundamental in a wide range of scientific fields. However, experimental investigation of the structure of liquid and amorphous material under in situ high-pressure conditions is still limited due to the experimental difficulties. In particular, the range of the momentum transfer (Q) in the structure factor [S(Q)] measurement under high-pressure conditions has been limited at relatively low Q, which makes it difficult to conduct detailed structural analysis of liquid and amorphous material. Here, we show the in situ high-pressure pair distribution function measurement of liquid and glass by using the 100keV pink beam. Structures of liquids and glasses are measured under in situ high-pressure conditions in the Paris-Edinburgh press by high-energy x-ray diffraction measurement using a double-slit collimation setup with a point detector. The experiment enables us to measure S(Q) of GeO2 and SiO2 glasses and liquid Ge at a wide range of Q up to 20-29 Å-1 under in situ high-pressure and high-temperature conditions, which is almost two times larger than that of the conventional high-pressure angle-dispersive x-ray diffraction measurement. The high-pressure experimental S(Q) precisely determined at a wide range of Q opens the way to investigate detailed structural features of liquids and amorphous materials under in situ high-pressure and high-temperature conditions, as well as ambient pressure study.

Thickness dependence of optical and electronic properties of FeCl2 films under high pressure

In this study, we propose an unconventional approach to investigate the pressure-induced changes in crystal structure, electrical transport, and optical bandgap by examining bulk FeCl2 and FeCl2 films with a thickness of 600 nm. Our analysis involved Raman spectroscopy, electrical transport measurements, and Ultraviolet–visible (UV–vis) absorption spectroscopy. Analyzing the Raman spectroscopy data, we discovered subtle distinctions in the sequence of structural phase transitions between bulk FeCl2 and the 600 nm thick film of FeCl2 under varying pressures. Specifically, while bulk FeCl2 underwent an insulating-to-metal transition at 52 GPa, the film of FeCl2 maintained its semiconductor behavior even at pressures as high as 60 GPa. Furthermore, by studying the optical absorption spectra, we observed a consistent decrease in the direct band gap of bulk FeCl2 with increasing pressure. In contrast, the film of FeCl2 exhibited an increase in the optical band gap beyond 34.1 GPa. These remarkable findings offer valuable insights into the manipulation of the mechanical, electrical, and optical properties of layered nanomaterials.

Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO3 Nickelate

Rare-earth perovskite nickelates RNiO3 (R stands for Y, rare earths, Tl or Bi) have attracted wide attention in the past few decades because of their unique electronic properties that emerge from the insulator–metal transition (IMT). This transition can be tuned by chemical substitutions or by external variables, like pressure and temperature, but the mechanism of this transition still remains debated. While most previous studies focused on nickelates with partially filled 4f cations on the R site, here we studied the mechanism of the pressure-induced phase transition on a rare Tl3+ nickelate, which comprises fully filled 4f orbitals. We probed the pressure-induced structural and electronic changes taking place at the bulk, medium, and local atomic level at room temperature by using synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) as probing techniques, respectively. High-resolution XRD data under ambient conditions show evidence for the monoclinic phase of TlNiO3, as characterized by the charge ordering between Ni3−δ and Ni3+δ in the insulator regime. High-pressure XRD data elucidated an anomaly in the evolution of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P21/n → Pbnm. High-pressure EXAFS data at the Ni K-edge provide a solid proof for a short-order charge disproportionation under near ambient conditions. The Ni–O pair-bond compression curves unveiled an isotropic overlapping Ni t2g6eg1 → O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distance variance (Debye–Waller exponents) lodged a hardening at this pressure point, suggesting constrained atomic displacements in the average range of ±0.04(5) Å. We propose that this reduced motional freedom could be linked to an electron–phonon coupling, which is the driving force behind the mechanisms of the IMT. We demonstrated that the XANES technique can be applied to ascertain the pressure-induced transition, by tracking the anomalous behavior of the position and width (full width at half-maximum) of the XANES features.

Pressure-induced evolution of the lattice dynamics for selected UO3 polymorphs

Uranium trioxide (UO3) is a stable chemical form of uranium oxide with multiple polymorphs found throughout the nuclear fuel cycle. The pressure-induced changes in the structure and lattice dynamics of four of these polymorphs are simulated with density functional perturbation theory and analyzed. Two phases, α- and δ-UO3 are found to exhibit an isotropic response to pressure and do not undergo any changes in coordination geometry up to ∼40 GPa. In contrast, the other two phases investigated, β- and γ-UO3, exhibit an anisotropic response to pressure. Decomposition of the phonon eigenvectors allows us to assign specific pressure-induced structural changes to individual phonon modes. This analysis has been performed on a per atom basis for the relatively simple α- and δ-UO3 structures, which have one symmetrically unique uranium site, and on a per coordination environment basis for β- and γ-UO3, which have multiple U sites.

Structure and equation of state of Bi2Sr2Can−1CunO2n+4+δ from x-ray diffraction to megabar pressures

Pressure is a unique tuning parameter for probing the properties of materials, and it has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasihydrostatic compression produced by a neon pressure medium on the structures of bismuth-based high-Tc cuprate superconductors with the nominal composition Bi2Sr2Can−1CunO2n+4+δ (n=1,2,3) up to 155 GPa. The structures of all three compositions obtained by synchrotron x-ray diffraction can be described as pseudotetragonal over the entire pressure range studied. We show that previously reported pressure-induced distortions and structural changes arise from the large strains that can be induced in these layered materials by nonhydrostatic stresses. The pressure-volume equations of state (EOS) measured under these quasihydrostatic conditions cannot be fit to single phenomenological formulation over the pressure ranges studied, starting below 20 GPa. This intrinsic anomalous compression as well as the sensitivity of Bi2Sr2Can−1CunO2n+4+δ to deviatoric stresses provide explanations for the numerous inconsistencies in reported EOS parameters for these materials. We conclude that the anomalous compressional behavior of all three compositions is a manifestation of the changes in electronic properties that are also responsible for the remarkable nonmonotonic dependence of Tc with pressure, including the increase in Tc at the highest pressures studied so far for each. Transport and spectroscopic measurements up to megabar pressures are needed to fully characterize these cuprates and explore higher possible critical temperatures in these materials.1 MoreReceived 15 December 2022Accepted 5 May 2023DOI:https://doi.org/10.1103/PhysRevMaterials.7.064803©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEquations of stateHigh-pressure studiesPhysical SystemsCupratesTechniquesX-ray diffractionCondensed Matter, Materials & Applied Physics

Origin and Mechanism of Anisotropic Piezoelectric Generation on the ZnO(101̅0) Crystal Plane

The experimental observation of pressure-induced structural changes is crucial for establishing the correct piezoelectric mechanism. Here, we show the in-plane anisotropic piezoelectric generation behavior on a ZnO non-polar (101̅0) crystal plane. The piezoelectric and rectification effects are found in the [0001] polar direction on the (101̅0) crystal facet. Neither effect was observed in the [12̅10] non-polar direction perpendicular to the [0001] direction. The piezoelectric generation and rectification properties can be reduced and vanished under pressure in the [0001] direction. The spontaneous polarization in the [0001] direction is evidenced to be the origin of piezoelectric effect. Pressure-induced structural change of ZnO is observed for the first time. In the piezoelectric effect, the applied stress causes the polar structure of ZnO to change into a non-polar structure, and the piezoelectric current is generated. This novel piezoelectric model will overturn the existing piezoelectric theory and piezoelectric generation mechanism and contribute to comprehending the crystal orientation-dependent piezoelectric properties and the development of new high-performance piezoelectric materials and devices.

Pressure-induced hydrogen bonds reconstruction of energetic material 3,6-dihydrazino-1,2,4,5-tetrazine: A DFT study

Pressure-induced changes in molecular structure can result in the reconstruction of hydrogen bonds. The structure and vibrational properties of hydrogen-bonded material 3,6-dihydrazino-1,2,4,5-tetrazine were calculated to 50 GPa. When compression, the compressibility displays anisotropy. At 37 GPa, the N-H length increases abruptly to 1.51 Å, suggesting that N-H has broken. The discontinuous changes indicate that 37 GPa is a critical point for structural changes. The hydrogen bond network becomes strengthened, resulting in a decrease in the vibrational activity of NH/NH2. At 37 GPa, the direction of NH correlated vibration modes change. In Hirshfeld analysis, the strengthening of N…H interactions was confirmed.

Study on pressure-induced isostructural phase transition and electrical transport properties of BiOBr.

In this work, we prepared a BiOBr powder sample by the coprecipitation method for in situ high-pressure AC impedance spectroscopy tests, in situ high-pressure Raman measurements and in situ high-pressure X-ray diffraction experiments to explore its structural properties and electrical transport processes under compression. Two pressure-driven isostructural phase transitions, T-T' and T'-T'' (T - tetragonal, T' - tetragonal 1 and T'' - tetragonal 2), were discovered at around 10.0 and 15.0 GPa, respectively. The pressure-induced changes in the crystal structure and electrical transport of BiOBr can provide a reference for explaining the mechanism of the isostructural phase transition of other similar compounds after compression.

Pressure-Induced Abnormal Electrical Transport Transition from Pure Electronic to Mixed Ionic–Electronic in Multiferroic BiFeO3 Ceramics

Pressure-induced structural changes could induce changes in transport properties and lead to a better understanding of the structure–property relationship. The evolution of the carrier transport properties of BiFeO3 (BFO) ceramics under a high pressure was investigated through impedance spectroscopy measurements at room temperature combined with first-principles calculations. A pressure-induced abnormal transition from pure electronic to mixed ionic–electronic was found in the BFO ceramics Cmmm and Pnma phases at 7.67 and 11.01 GPa, respectively. The pressure-induced structural phase transition from the R3c phase to Cmmm and then to Pnma was responsible for the change in electrical transport behavior from pure electronic to mixed ionic–electronic conduction, accompanied by a decrease in ionic resistance. The calculations of electronic structures and electron localization function from 1 atm to 40 GPa indicated that the ionic conduction in the Pnma phase resulted from the weakened Coulomb screen of the localized electron background to O2– between Bi3+ and O2–. This work provides a critical insight into the understanding of the relationship between structure and conduction and facilitates the application of ferroelectric materials in photoelectric fields.

Structuration of Water in Microporous CAU-10-H under Gigapascal Pressure.

Employing metal-organic frameworks (MOFs) as water adsorbents has become a flourishing field in recent years. Aluminum isophthalate [Al(OH)(bdc)]·nH2O, referred to as CAU-10-H, is renowned for its high physical and chemical stability under external mechanical stimuli, which is crucial for use in sorption-based heat exchange. We report pressure-induced structural changes and water uptake/release of CAU-10-H in the gigapascal regime. CAU-10-H displays four different phases during pressurization in water up to 4.08(1) GPa. Upon immersion in water at ambient conditions, the unit cell volume expands by 1.3(1)%, by increasing water content from 78.7(1) to 85.6(1) H2O per unit cell. Upon pressurization to 1.08(1) GPa, the water content increases gradually to 112.5(1) H2O per unit cell and reduces the symmetry to P1 phase. At 1.72(1) GPa, a correlated "gate-opening/channel-closing" transition to a centrosymmetric Pnma phase occurs to increase the unit cell volume by 6.9(1)% and water content to 138.8(1) H2O per unit cell. Further increase in pressure to 4.08(1) GPa then leads to a gradual decrease in water content to 119.2(1) H2O per unit cell and redistribution of the inserted water molecules during a "gate-closing/channel-opening" transition to an I41/amd phase. After pressure removal, CAU-10-H reverts to its initial state in terms of crystallinity and water content. This pressure-induced water adsorption/desorption and concerted movements of gate-constraining ligands provides a detailed understanding of the water-MOF interplay, which can be used as a basis for designing new materials, possibly utilizing pressure as a tool for chemical modifications.

Pressure-Induced Structural Effects in the Square Lattice (sql) Topology Coordination Network Sql-1-Co-NCS·4OX.

A high-pressure study of a switching coordination network of square lattice topology (sql) loaded with o-xylene (OX), [Co(4,4'-bipyridine)2(NCS)2] n ·4nC8H10 (sql-1-Co-NCS·4OX), was conducted up to approximately 1 GPa to investigate pressure-induced structural changes. Previous reports revealed that sql-1-Co-NCS exhibits multiple phases thanks to its ability to switch between closed (nonporous) and several open (porous) phases in the presence of various gases, vapors, and liquids. Networks of such properties are of topical interest because they can offer high working capacity and improved recyclability for gas adsorption. The monoclinic crystal structure of sql-1-Co-NCS·4OX at 100 K was previously reported to show an increase in interlayer separation of more than 100% compared to the corresponding closed phase, sql-1-Co-NCS, when exposed to gases or vapors under ambient conditions. Herein, a tetragonal crystal form of sql-1-Co-NCS·4OX (space group I4/mmm, Phase I) that exists at 0.1 MPa/303 K is reported. Exposure of Phase I to high pressure using penetrable pressure transmitting media (OX and 1:1 vol MeOH/EtOH) did not result in further separation of the sql networks. Rather, compression of the crystals and release of adsorbed OX molecules occurred. These pressure-induced changes are discussed in terms of structural voids, framework conformation, and molecular packing of the sql layers. Although Phase I retained tetragonal symmetry throughout the investigated pressure range, the interlayer voids occupied by OX molecules were significantly reduced between 0.3 and 0.5 GPa; further compression above 0.5 GPa induced structural disorder. Additionally, analysis of the electron count present in the pores of sql-1-Co-NCS confirmed the multistep evacuation of OX molecules from the crystal, and two intermediate phases, Ia and Ib, differing in the OX loading level, are postulated.

High-pressure induced structural changes of energetic ionic salts: Dihydroxylammonium 3,3′-dinitro-5,5′-bis-1,2,4-triazole-1,1′-diolate (MAD-X1)

Pressure-induced structural changes of dihydroxylammonium 3,3′-dinitro-5,5′-bis-1,2,4-triazole-1,1′-diolate (MAD-X1), an insensitive energetic ionic salt with high detonation performance, was investigated under simulated conditions (0 to 10 GPa). Using the first-principles density-functional theory, the geometrical structure, intra/intermolecular interactions, electronic properties and Raman spectrum were calculated in detail to elucidate material stability. The anisotropic compressibility in MAD-X1 was revealed by variations of lattice parameters. Furthermore, combining internal hydrogen bonds and geometric parameters of each moiety, we could explain the abrupt changes on lattice constants from two aspects, which demonstrated the contribution of hydrogen bonding to decreasing the material sensitivity. Studies on band gap and partial density of state showed that excessive pressure probably inhibit the electron transfer. Additionally, the simulation of vibrational properties suggested a possible phase transition at 7–8 GPa, implying that MAD-X1 could maintain its stability up to 7 GPa.

Thermodynamic Stability, Structure, and Optical Properties of Perovskite-Related CsPb2Br5 Single Crystals under Pressure

CsPb2Br5 belongs to all inorganic perovskite-related quasi-two-dimensional materials that have attracted considerable attention due to their potential for optoelectronic applications. In this study, we solve numerous controversies on the physical properties of this material. We show that optical absorption in the visible spectrum and green photoluminescence are due to microcrystallites of the three-dimensional CsPbBr3 perovskite settled on the CsPb2Br5 plates and that carefully cleaned crystal plates are devoid of these features. The high-pressure structural and spectroscopic experiments, performed on the single crystals free of CsPbBr3 impurities, evidenced that the layered tetragonal structure of CsPb2Br5 is stable at least up to 6 GPa. The absorption edge is located in the ultraviolet at around 350 nm and continuously red shifts under pressure. Moderate band gap narrowing is well correlated to the pressure-induced changes in the crystal structure. Although the compressibility of CsPb2Br5 is much higher than for CsPbBr3, the response in optical properties is weaker because the Pb–Br layers responsible for the optical absorption are much less affected by hydrostatic pressure than those built of Cs+ cations. Our study clarifies the confusing data in the literature on the optical properties and thermodynamic stability of CsPb2Br5.

Oxide glasses under pressure: Recent insights from experiments and simulations

Deciphering the structure–property relations of densified oxide glasses is a problem of longstanding interest. For example, it is important for understanding the fracture mechanism under sharp contact loading as well as fabricating glasses with tunable physical characteristics. Recent advances in both experimental and simulation techniques have prompted research breakthroughs in understanding the response of glasses to high pressure. In this Perspective, we first briefly discuss the facilities for the high-pressure treatment of glasses, including in situ and ex situ investigations. The recent work on pressure-induced structural changes of archetypical oxide glass families (silicates, germanates, borates, aluminates, phosphates) is discussed and compared to the changes in macroscopic properties induced by densification, as densification treatment can be used to produce oxide glasses with improved hardness, stiffness, and toughness. We also discuss the new insights from atomistic simulations combined with topological analysis tools to unravel the densification mechanism of oxide glasses on the medium-range order length scale. Drawing on these recent studies, we clarify how densification treatment has proved to be an important tool to both understand the disordered nature of glasses and tune their physical properties, although many open questions and challenges remain that require further investigations.

Pressure-induced transition from pure electronic to mixed ionic-electronic conduction in strontium hydride

The heavier alkaline-earth hydrides AeH2 (Ae = Ca, Sr, and Ba) are considered as promising materials for hydrogen energy storage. Pressure-induced structural changes in AeH2 materials could improve hydrogen transport properties and result in a better understanding of the structure-property relationship. In this work, pressure evolution of carrier transport properties of SrH2 was investigated using impedance spectroscopy measurements at room temperature and first-principles calculations. The pressure-induced structure phase transition from a Pnma phase to a P63/mmc phase was accompanied by a transition from pure electronic conduction to mixed ionic-electronic conduction, which was related to the ionic migration barrier energy. In the P63/mmc phase, the H− ionic and electronic resistances of bulk and grain boundaries were distinguished, respectively. The total resistance of SrH2 decreased by about four orders of magnitude after the phase transition. This work provides critical insight into the structure-conduction relationship and the role of grain boundaries in the transport process of alkaline-earth hydrides under high pressure.

Structural phase transition of BiSb and formation of Weyl semimetallic phase under pressure: calculations and experiments

BiSb was found to transform into a Weyl semimetal at ∼4 GPa. Detailed pressure-induced structural phase transitions and changes in electrical transport properties are explored.

Superatomic Au25(SC2H5)18 Nanocluster under Pressure.

The past decade has witnessed significant advances in the synthesis and structure determination of atomically precise metal nanoclusters. However, little is known about the condensed matter properties of these nanosized metal nanoclusters packed in a crystal lattice under high pressure. Here using density functional theory calculations, we simulate the crystal of a representative superatomic gold cluster, Au25(SR)18 0 (R = C2H5), under various pressures. At ambient conditions, Au25(SC2H5)18 0 clusters are packed in a crystal via dispersion interactions; being a 7e superatom, each cluster carries a magnetic moment of 1 μB or one unpaired electron. Upon increasing compression (from 10 to 110 GPa), we observe the formation of intercluster Au-Au, Au-S, and S-S covalent bonds between staple motifs, thereby linking the clusters into a network. The pressure-induced structural change is accompanied by the vanishment of the magnetic moment and the semiconductor-to-metal transition. Our work shows that subjecting crystals of atomically precise metal nanoclusters to high pressures could lead to new crystalline states and physical properties.

Pressure induced superconductivity in MnSe

The rich phenomena in the FeSe and related compounds have attracted great interests as it provides fertile material to gain further insight into the mechanism of high temperature superconductivity. A natural follow-up work was to look into the possibility of superconductivity in MnSe. We demonstrated in this work that high pressure can effectively suppress the complex magnetic characters of MnSe, and induce superconductivity with Tc ~ 5 K at pressure ~12 GPa confirmed by both magnetic and resistive measurements. The highest Tc is ~ 9 K (magnetic result) at ~35 GPa. Our observations suggest the observed superconductivity may closely relate to the pressure-induced structural change. However, the interface between the metallic and insulating boundaries may also play an important role to the pressure induced superconductivity in MnSe.

Ab initio study of pressure-induced structural and electronic phase transitions in Ca2RuO4

${\mathrm{Ca}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ is a compound with a Mott insulating ground state, which responds to external pressure in a variety of ways, some of which are expected (an insulator-metal transition) while others are more surprising (an expansion of the $c$ lattice constant). We provide here a comprehensive study of these pressure-induced structural and electronic changes using DFT$+U$ calculations, and demonstrate generally good agreement with experiment. The insulator-metal transition is reproduced and coincides with an isostructural transition, the $c$ lattice expansion, $\mathrm{Ru}{\mathrm{O}}_{6}$ octahedral distortions, and associated changes to the Ru $4d$ orbital order. The metallic part of the phase diagram features several competing phases. The high-pressure $Bbcm$ phase is found to be unstable in the ground state over a broad pressure range and suggested to be thermally stabilized instead.