Pressure-induced Structural Evolution Research Articles

Macroscale structural superlubricity: Dynamic evolution of tribolayers in two-dimensional materials under extreme pressure

Achieving macroscale structural superlubricity with two-dimensional (2D) materials under ultrahigh contact pressure in ambient condition is particularly challenging. Furthermore, the mechanisms underlying the disparate trans-scale tribological behaviors of 2D materials continue to be a subject of debate. Here, we propose a novel principle concerning pressure-induced dynamic structural evolution and tribochemical behaviors of tribolayers to broaden the macroscale structural superlubricity. For the first time, robust macroscale structural superlubricity with ultralow wear rate is realized by 2D material coating in ambient condition by sliding steel counterparts under ultrahigh contact pressure. The results reveal that macroscale structural superlubricity of 2D materials is highly dependent on the dynamic evolution of tribolayers nanostructures, as well as the adsorption and tribochemical behaviors governed by extreme pressure. These findings shed light on achieving robust macroscale structural superlubricity with 2D materials for harsh engineering conditions.

Compressibility and pressure-induced structural evolution of kokchetavite, hexagonal polymorph of KAlSi3O8, by single-crystal X-ray diffraction

Abstract Compressibility and pressure-induced structural evolution of kokchetavite, the hexagonal polymorph of KAlSi3O8, has been studied up to 11.8 GPa using synchrotron single-crystal X-ray diffraction. Two phase transitions were observed at pressures of ~0.3 and 10.4 GPa. Kokchetavite-I (as-synthesized, P6/mcc) transforms into kokchetavite-II with the P6c2 space group. Kokchetavite-II → kokchetavite-III phase transition at ~10.4 GPa is accompanied by a change of symmetry to probably orthorhombic. After pressure release, kokchetavite reverts to the initial single-crystal state with P6/mcc space group. A second-order Birch-Murnaghan equation of state was calculated for phase kokchetavite-II with coefficients V0 = 1486(3) Å3, K0 = 59(2) GPa.

Realizing Persistent Zero Area Compressibility over a Wide Pressure Range in Cu2 GeO4 by Microscopic Orthogonal-Braiding Strategy.

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO4 ] unit, Cu2 GeO4 , that possesses an area compressibility as low as 1.58(26) TPa-1 over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO4 ] and the expansion effect from the increase in the [CuO4 ]-[CuO4 ] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding dx 2 -y 2 to out-of-plane nonbonding dz 2 orbitals within copper atoms causes the counterintuitive extension of the [CuO4 ]-[CuO4 ] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.

Pressure-induced structural evolution to a nearly perfect kagome lattice in β-Mn2(OH)3Cl nanosheets

The frustrated magnets exhibiting kagome geometries and the investigation of their structure-property relationships have garnered substantial attention as potential candidates for quantum spin liquid (QSL). In this study, β-Mn2(OH)3Cl nanosheets with distorted kagome geometries were synthesized via a solid-state reaction, and low-temperature magnetic measurements revealed a spin-glass-like magnetic phase transition around 40 K in the frustrated β-Mn2(OH)3Cl. A pressure-induced structural phase transition was further realized by employing in situ synchrotron X-ray diffraction, leading to a hexagonal crystal structure with a nearly perfect kagome lattice of magnetic Mn2+ ions. High-pressure Raman and UV–vis absorption measurements indicates that the topological evolution can be attributed to the synergistic effects of pressure driven internal hydrogen bonds and the intrinsic Jahn-Teller effects. This study provides a strategy for synthesizing candidate compounds of QSL and sheds light on the mechanism underlying the topological evolution of kagome lattices under high pressure.

Pressure-Induced Structural Evolution with a Turnover Point in the Honeycomb Iridate Na2IrO3

Pressure-Induced Structural Evolution with a Turnover Point in the Honeycomb Iridate Na₂IrO₃

High-pressure structural transition and magnetic phase diagram of CuB2O4

Noncentrosymmetric $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ is a prototypical system with interacting spin sublattices of different dimensionality, exhibiting diverse magnetic structures and magnetic phase transitions. Here we report pressure-induced structural evolution of $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ and its effect on the magnetic phase diagram. Powder x-ray diffraction (XRD), Raman scattering, and infrared phonon measurements have been performed under high pressure at room temperature. Although XRD shows lattice stability up to $\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, Raman and IR phonon measurements reveal structural anomaly near 3 GPa, followed by a structural phase transition at $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Pressure evolution of the zero-phonon lines in the low-temperature optical absorption studies helps identifying the symmetry change of the Cu sites modifying the crystal-field energy levels at the structural transition. High-pressure magnetization measurements have been performed with magnetic field applied perpendicular to the tetragonal $c$ axis. With increasing pressure the saturation magnetization of the commensurate weak ferromagnetic phase increases with systematic reduction of its turnup critical field. Pressure-induced systematic suppression of the dc susceptibility in this phase is attributed to the reduced critical field, indicating pressure-driven enhanced stability of the weak ferromagnetic phase at high pressure. Whereas, below 10 K the critical field of the incommensurate-commensurate transition increases with pressure, indicating enhanced stability of the incommensurate helical order at high pressure.

Pressure tuning of structure, magnetic frustration, and carrier conduction in the Kitaev spin liquid candidate Cu2IrO3

The layered honeycomb lattice iridate Cu$_2$IrO$_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of Cu$_2$IrO$_3$ by powder x-ray diffraction (PXRD) up to $\sim$17 GPa and Raman scattering measurements up to $\sim$25 GPa. A structural phase transition (monoclinic $C2/c \: \rightarrow$ triclinic $P\bar{1}$) is observed with a broad mixed phase pressure range ($\sim$4 to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4--300 K show resilient non-metallic $R$($T$) behaviour with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. Using first-principles density functional theoretical (DFT) calculations, we find that at ambient pressure $\rm Cu_2IrO_3$ exists in monoclinic $P2_1/c$ phase which is energetically lower than $C2/c$ phase (both the structures are consistent with experimental XRD pattern). DFT reveals structural transition from $P2_1/c$ to $P\bar{1}$ structure at 7 GPa (involving dimerization of Ir-Ir bonds) in agreement with experimentally observed transition pressure.

Fluorescence-based monitoring of the pressure-induced aggregation microenvironment evolution for an AIEgen under multiple excitation channels

The development of organic solid-state luminescent materials, especially those sensitive to aggregation microenvironment, is critical for their applications in devices such as pressure-sensitive elements, sensors, and photoelectric devices. However, it still faces certain challenges and a deep understanding of the corresponding internal mechanisms is required. Here, we put forward an unconventional strategy to explore the pressure-induced evolution of the aggregation microenvironment, involving changes in molecular conformation, stacking mode, and intermolecular interaction, by monitoring the emission under multiple excitation channels based on a luminogen with aggregation-induced emission characteristics of di(p-methoxylphenyl)dibenzofulvene. Under three excitation wavelengths, the distinct emission behaviors have been interestingly observed to reveal the pressure-induced structural evolution, well consistent with the results from ultraviolet-visible absorption, high-pressure angle-dispersive X-ray diffraction, and infrared studies, which have rarely been reported before. This finding provides important insights into the design of organic solid luminescent materials and greatly promotes the development of stimulus-responsive luminescent materials.

Pressure-induced structural evolution in boron-bearing model rhyolitic glasses under compression: Implications for boron isotope compositions and properties of deep melts in Earth’s interior

The pressure-induced structural evolutions of boron-bearing model rhyolitic melts under high pressures enable to infer the detailed geochemical processes (melting and fluid-rock-melt interactions) occurring in Earth interiors and to control the melt properties (viscosity and the boron isotope composition, δ11B) of complex magmatic melts, providing insights into the boron cycle toward the deeper part of the upper mantle (∼10 GPa). Despite the importance, the structures of multicomponent boron-bearing silicate melts above 3 GPa are currently unavailable. Here, we explore the structures, particularly, coordination transformation of constituent elements in boron-bearing nepheline and albite glasses – a model rhyolitic melts - upon compression to a depth of ∼270 km (∼9.2 GPa) in the mantle using multi-nuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. The results showed that the conversion of [3]B into [4]B is prominent upon compression up to 6 GPa. In contrast, the formation of [5,6]Al is accompanied by the formation of oxygen tricluster above 6 GPa, where all the nonbridging oxygens are consumed. We quantify how the melt composition affects tendency to form highly coordinated B, Al, and Si upon compression. Particularly, the increase in the [4]B population tends to be larger for the glasses with low Si content as pressure increases to 9.2 GPa. We reveal the relationship between such structural adaptations of the compressed melts at high pressure and the melt properties, including viscosity and element partition coefficient in boron-bearing melts. The current NMR results also unravel the structural origins of 11B/10B ratios in rhyolitic melts at high pressure. Considering a preferential partitioning of 10B to [4]B, an increase in [4]B population in the melts leads to an pressure-induced enrichment of 10B. As the increase in Si/B ratio in the melts tends to decrease the pressure-induced increase in [4]B fraction, the contribution of boron coordination transformation on the 11B/10B ratios in silicate melt would be somewhat minor in deep mantle melts with increasing Si content. The detailed boron environments in rhyolitic melts at high pressure yield useful constraints for the isotope composition (11B/10B) of dense mantle melts, thereby enabling quantification of deep boron cycle.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO 3

Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO ₃

Shear band formation during nanoindentation of EuB6 rare-earth hexaboride

Research on rare-earth hexaborides mainly focuses on tuning their electronic structure from insulating-to-metallic states during high pressure experiments. However, the structural evolution that contributes to their mechanical failure is not well understood. Here, we examine the pressure-induced structural evolution of a model rare-earth hexaboride, EuB6, during nanoindentation. Transmission electron microscopy reveals that nanoscale amorphous shear bands, mediated by dislocations, play a decisive role in deformation failure. Density functional theory calculations confirm that amorphous bands evolve by breaking boron-boron bonds within B6 octahedra during shear deformation. Our results underscore an important damage mechanism in hard and fragile hexaborides at high shear pressures.

Pressure-induced structural, electronic, and magnetic evolution in cubic inverse spinel NiFe2O4, an ab-initio study

Pressure-induced structural, electronic, and magnetic evolution in cubic inverse spinel NiFe2O4, an ab-initio study

Pressure-induced 1T to 3R structural phase transition in metallic VSe2 : X-ray diffraction and first-principles theory

We study pressure-induced structural evolution of vanadium diselenide (VSe2), a 1T polymorphic member of the transition metal di-chalcogenide (TMD) family using synchrotron-based powder X-ray diffraction (PXRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at P ~4 GPa in c/a ratio, V-Se bond length and Se-V-Se bond angle signalling an isostructural transition. This is followed by a first order structural transition from 1T (space group P-3m1) phase to a 3R (space group R-3m) phase at P ~11 GPa due to sliding of adjacent Se-V-Se layers. We present various scenarios to understand the experimental results within DFT and find that the 1T to 3R transition can be captured only after inclusion of enthalpic correction associated with errors in cell volume with underestimated transition pressure. The abrupt increase in the Debye-Waller factors of Se atoms by a factor of ~4 and hence the anharmonic effects across the structural transition pressure are hitherto not reported so far and hint a possible way to understand the mismatch between the experimental and theoretical transition pressure values.

Atomistic insights on the pressure-induced multi-layer graphene to diamond-like structure transformation

Recent results suggest a pressure-induced transformation of bilayer graphene to a diamond-like structure. Intriguingly, the reported transformation is absent or incomplete in multi-layer graphene beyond two layers, implying the suppression of the transformation by an unclear mechanism. In this work, we use reactive molecular dynamics simulations to describe the pressure-induced structural evolution of multi-layer graphene with two to six layers to pressures up to 200 GPa. The results show that bilayer graphene transforms into a diamond-like film, i.e., more than 75% sp3 hybridized atoms. In contrast, three- to six-layer graphene with AB layer stacking fail to form a diamond-like film up to 200 GPa. Bond formation analysis indicates that the transformation or its suppression is related to the generation of a bond chain structure across layers, connecting sp3 hybridized atoms. The result suggests that the ABA layer stacking is directly related to the hindering of the chain structure formation and the suppression of the transformation. Surprisingly, the bond-chain-structure transformation mechanism is effective in three-layer graphene with ABC layer stacking, as well as five-layer graphene with ABCAB layer stacking. That suggests that stacking faults are effective catalysts for the high-pressure transformation of multi-layer graphene to diamond.

Structural Phase Transitions, Electronic Properties, and Hardness of RuB4 under High Pressure in Comparison with FeB4 and OsB4

We have employed an evolutionary algorithm with first-principles calculations to investigate the pressure-induced structural evolution of RuB4 up to 500 GPa. The ambient phase is predicted to be a ...

Pressure-induced tuning of quantum spin liquid state in ZnCu3(OH)6Cl2

Mineral Herbertsmithite, ${\mathrm{ZnCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$, is an excellent candidate for S = 1/2 Heisenberg kagome antiferromagnet supporting quantum spin liquid (QSL) ground state at low temperature and ambient pressure. The pressure-induced structural evolution of the kagome lattice in Herbertsmithite has been investigated up to 16 GPa using x-ray diffraction and Raman scattering measurements. The system develops a distorted kagome lattice structure (clinoatacamite-like) beyond 8 GPa. This is accompanied by gradual suppression of the quasi-elastic Raman scattering intensity. Our high pressure magnetic susceptibility measurements indicate the emergence of a weak magnetic ordering in the distorted kagome lattice. Pressure-induced enhancement of the magnetic ordering temperature supports a gapped topological QSL rather than gapless ground state close to a quantum critical point. High-pressure optical absorption measurements support the structural evolution.

Tantalum doping in HfO2: orthorhombic phase formation at ambient conditions and change in path of pressure-induced structural evolution

ABSTRACT A systematic study of orthorhombic phase formation at ambient conditions beyond a critical concentration of dopant Tantalum (Ta) in HfO is reported. The effect on the path of structural evolution due to Ta doping in HfO was also investigated. Studies on Ta-doped HfO samples and their path of structural evolution under compression have significant implications for materials undergoing β-decay. The effect of continual doping due to β-decay from geophysical point of view is discussed.

Ab initio investigation of pressure-induced structural transitions and electronic evolution of Th3N4

ABSTRACTThe crystal structures, lattice dynamics, mechanical, electronic properties, and electron–phonon coupling of under environmental conditions and high pressures have been studied by merging first-principles calculations and particle-swarm optimization algorithm. Four structures are identified for , including the , , , and C2/m phases, in which the , , and C2/m phases are newly predicted. Their mechanical properties, including the Poisson's ratio σ, the elastic anisotropy factor , and the Pugh's ratio have been calculated and discussed. The results show that the , , and phases of behave ductile nature, while the C2/m phase behaves brittle nature. Among them, the phase of almost exhibits completely anisotropic nature. Besides, our electronic band structure calculations show that the pressure-induced semiconductor-metal transition occurs following the to phase transition. Further, the electron-phonon coupling of the phase has been analyzed. The results we obtained are of significance to further understand the physical essence of and its practical engineering applications.

Pressure-induced phase transitions and structural evolution across the insulator–metal transition in bulk and nanoscale BiFeO3

The pressure-induced phase-transition sequences and structural evolution across the insulator–metal transition (IMT) in multiferroic BiFeO3 still remain unclear. Here we use a combination of high-pressure XRD, XAFS experiment and first principle calculation to investigate the pressure-derived structural transformations and structure-related properties in bulk and nanoscale BiFeO3 up to 55 GPa. A new Imma structure of BiFeO3 has been discovered in the pressure range of 48–52 GPa, which presents ferromagnetic (FM) metallic properties and therefore plays a key role in the IMT. Local structure study reveals that the Bi3+ cation gradually shifts toward the centrosymmetric position in BiO12 cluster during IMT. Besides, the detailed structural information of post-perovskite Cmcm phase has also been determined and thus the complete phase sequence up to 60 GPa is obtained. Our research provides a structural origin of the IMT and a new way to understand the FM release in BiFeO3 system.

Unusual pressure-induced electronic structure evolution in organometal halide perovskite predicted from first-principles

Abstract Pressure has been demonstrated to be an effective parameter to alter the atomic and electronic structures of materials. By using the first-principles calculations based on density functional theory (DFT), we systematically investigated the changes in the atomic and electronic structures of the cubic MAPbI 3 phase under pressures. It is found that the band gap of the compressed cubic MAPbI 3 structure exhibits a remarkable redshift to 1.114/1.380 eV in DFT/HSE-SOC calculation under a mild pressure of 2.772 GPa, and subsequently shows a widening at higher pressures until ∼20 GPa. As the pressure further increases, the band gap closes at ∼80 GPa. Detailed structural and electronic characteristic analyses indicate that the band gap of the cubic MAPbI 3 structure is determined by two competing effects: the lattice contraction decreases its band gap while the PbI 6 octahedral tilting increases it. Given that, pressure can be a powerful tool to help understanding the optoelectronic properties of perovskite materials.