Pressure-driven Phase Transition Research Articles

A search for pressure-induced phase transition in the layered perovskite-like Pr2Ti2O7

In our study, we present comprehensive findings on the structural properties of Pr2Ti2O7 across a broad pressure range of 0-30GPa. Neutron diffraction experiments, conducted under ambient conditions, offer crucial structural insights into the initial monoclinic phase with the P21 space group. As pressure increased, a significant phase transition to the monoclinic P21/m phase occurred at 13.8GPa. Structural data analysis from X-ray diffraction highlights the essence of this transition, identifying it as the tilting of Ti-O6 octahedra. High-pressure Raman spectroscopy data unequivocally confirms the phase transition, detecting anomalies in the baric dependencies of some vibration modes of Pr2Ti2O7 and the emergence of new modes in the Raman spectra within the pressure region associated with the phase transition. The analysis of these novel vibration modes points to alterations in the Ti-O6 octahedra, emphasizing the pivotal role played by Ti4+ and O2- ions in the mechanism of the pressure-driven phase transition.

Self-Trapped Exciton Emission Enhancement in 3D Cationic Lead Halide Hybrids Via Phase Transition Engineering.

Three-dimensional (3D) cationic lead halide hybrids constructed by organic ions and inorganic networks via coordination bonds are a promising material for solid-state lighting due to their exceptional environmental stability and broad-spectrum emission. Nevertheless, their fluorescence properties are hindered by the limited lattice distortion from extensive connectivity within the inorganic network. Here, a dramatic 100-fold enhancement of self-trapped exciton (STE) emission is achieved in 3D hybrid material [Pb2Br2][O2C(CH2)4CO2] via pressure-triggered phase transition. Notably, pressure-treated material exhibits a 110 nm redshift with 1.5-fold enhancement compared to the initial state after pressure was completely released. The irreversible structural phase transition intensifies the [PbBr3O3] octahedral distortion, which is highly responsible for the optimization of quenched emission. These findings present a promising strategy for improving the optical properties of 3D halide hybrids with relatively high stability and thus facilitate their practical applications by pressure-driven phase transition engineering.

Imaging Ultrafast Dynamics of Pressure-Driven Phase Transitions in Black Phosphorus and Anomalous Coherent Phonon Softening.

Applying high pressure to effectively modulate the electronic and lattice structures of materials could unravel various physical properties associated with phase transitions. In this work, high-pressure-compatible femtosecond pump-probe microscopy was constructed to study the pressure-dependent ultrafast dynamics in black phosphorus (BP) thin films. We observed pressure-driven evolution of the electronic topological transition and three structural phases as the pressure reached ∼22 GPa, which could be clearly differentiated in the transient absorption images containing spatially resolved ultrafast carrier and coherent phonon dynamics. Surprisingly, an anomalous coherent acoustic phonon mode with pressure softening behavior was observed within the range of ∼3-8 GPa, showing distinct laser power and time dependences. Density functional theory calculations show that this mode, identified as the shear mode along the armchair orientation, gains significant electron-phonon coupling strength from out-of-plane compression that leads to decreased phonon frequency. Our results provide insights into the structure evolution of BP with pressure and hold potential for applications in microelectromechanical devices.

Numerical Simulation Method for Flash Evaporation with Circulating Water Based on a Modified Lee Model

Flash evaporation processes are widely adopted in the desalination, food processing, waste heat recovery and other industries for heat extraction or product separation. In this paper, a pressure-driven phase transition model is developed by improving the Lee model and combined with the VOF (Volume of Fluid) method to numerically simulate the flash evaporation process. In this modified Lee phase transition model, the driving force for the rates of the local phase transition is calculated using the local temperature and static pressure magnitude. Numerical simulations are carried out in a water-circulating flash chamber and compared with the experimental results to obtain the values of the time relaxation parameters. And the non-equilibrium fraction of the outlet water can be effectively obtained under different conditions of flow rate, inlet temperature and initial liquid level height. The time relaxation factor takes values from 0.195 to 0.43 (Pout,v = 19.9 kPa) and from 0.31 to 0.92 (Pout,v = 31.2 kPa) with increasing superheat. In addition, the model can effectively represent the evolution of the unstable flow flash evaporation from the initial rapid boiling state to dynamic equilibrium.

Effects of lattice distortion and amount of d electrons on the pressure-driven structural phase transition in ZnO: A comparative study of V-, Mn-, and Co-doped ZnO

The pressure-driven phase transition from fourfold würtzite (B4) to sixfold coordinated rock salt (B1) structure in ZnO, in particular in transition metal (TM) doped ones, exhibited very scattered results and remained as an outstanding issue. The in situ angle-dispersive x-ray diffraction revealed that TM doping can result in significant reduction on the onset pressure at which the pressure-driven B4-to-B1 phase transition starts to emerge, albeit the transition path appeared to be very much dependent on the doping transition metal element. Systematic comparisons on a series of V-, Mn-, and Co-doped ZnO materials indicated that, in addition to ionic sized difference-induced lattice distortion, the amount of the 3d electrons carried by the doped TM atom may also play a prominent role in the underlying mechanism of the pressure-driven phase transition. Specifically, the existence of 3d electrons may have resulted in increased screening of the Coulomb forces between the cations at increasing pressure, leading to significant softening of the c44 and c66 elastic constants, which, in turn, lowers the onset pressure in triggering the B4-to-B1 phase transition in ZnO. Our results also indicate that the initial lattice distortion induced by the ionic size difference may change the landscape of energy barrier and alter the phase transition path.

Pressure-driven structural transition in CoNi-based multi-principal element alloys

Pressure-driven phase transition in metals has been a hot topic because it is an effective means to induce fresh phase, benefit of tuning the properties of materials. Herein, CoNiFe, CoNiCr, and CoNiV multi-principal element alloys (MPEAs) were investigated by an in situ high-pressure x-ray diffraction technique. It is found that the pressure-induced phase transition from face-centered cubic to hexagonal close-packed phase occurs at 15.60, 13.84, and 8.20 GPa, respectively. The atomic size misfit of CoNiFe, CoNiCr, and CoNiV MPEAs is estimated to be 0.653%, 2.077%, and 3.013%, respectively, illustrating that the lattice distortion degree is increasing. The increase in lattice distortion can decrease the initial phase-transition-pressure because lattice distortion could reduce the strain to nucleate Shockley partial dislocation, which promotes the formation of a stacking fault (SF) stack of three atomic layers with hcp stacking. However, the quantitative calculation of stacking fault probability α as a function of pressure demonstrates that the probability of SF formation gradually increases in order of CoNiFe, CoNiCr, and CoNiV, which is in line with the critical pressure of phase transition decreasing orderly. Furthermore, the first peak in the pair distribution function curve after entirely decompression not fully reverts to its initial state, proving the densification of MPEAs under pressure. These findings provide an innovative light for understanding pressure-induced phase transitions in MPEAs.

Pressure engineering of van der Waals compound RhI3: bandgap narrowing, metallization, and remarkable enhancement of photoelectric activity

The realization of high tunability in layered van der Waals halides may create new interesting optical and optoelectronic properties. These soft materials are particularly sensitive to external stress or pressure, suggesting a feasible route to pinpoint their structure with extraordinary behavior. However, a very sensitive pressure response usually lead to a detrimental phase transition and/or lattice distortion, making the approach of materials manipulation in a continuous manner remain challenging. Here, the extremely weak interlayer coupling and high tunability of layered RhI3 crystals are observed. A pressure-driven phase transition occurs at a moderate pressure of 5 GPa, interlinking to a change of layer stack mode. Strikingly, such a phase transition does not affect the tendency of quasi-linear bandgap narrowing, and a metallization with an ultra-broad tunability of 1.3 eV redshift is observed at higher pressures. Moreover, the carrier concentration of RhI3 increases by 4 orders of magnitude at 30 GPa, and the photocurrent enhances by 5 orders of magnitude at 7.8 GPa. These findings create new opportunities for exploring, tuning, and understanding the van der Waals halides by harnessing their unusual feature of a layered structure, which is promising for future devices based on materials-by-design that are atomically thin.

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.

First-principles calculations to investigate pressure-driven electronic phase transition of lead-free halide perovskites KMCl3 (M = Ge, Sn) for superior optoelectronic performance

This article investigates the physical properties of lead-free tin- and germanium-based halide perovskites under pressure via the density functional theory to use as potential photovoltaic materials. Specifically, the structural, electronic, optical, and mechanical properties of KMCl3 (M = Ge, Sn) under diverse hydrostatic pressures ranging from 0 to 8GPa are examined to vindicate the compounds' superiority for useful applications. The structures show high precision in terms of lattice constants, which approves the formerly published data. The calculated lattice constant (5.261 and 5.58 Å for KGeCl3 and KGeCl3, respectively, at 0 GPa) and unit cell volume (145.67 and 173.80 Å3 for KGeCl3 and KGeCl3, respectively, at 0 GPa) are significantly reduced ((lattice constant 4.924 Å (5.183 Å) and unit cell volume 119.41 Å3 (139.39 Å3) for KGeCl3 (KSnCl3) at 8 GPa) due to the pressure effect, while the cubic phase stability is maintained. Under ambient pressure, the calculated band gap reveals the compounds' semiconducting nature. Nevertheless, when pressure is increased, the band gap narrows, enhancing its conductivity and igniting its route towards semiconductor to metallic transition. The ionic and covalent bonding nature of K-Cl and Ge(Sn)-Cl, respectively; as well as the decrement of bond length due to external pressure are marked by charge density mapping. The optical functions are also enhanced when pressure is devoted, vindicating the chosen perovskites' suitability in various optoelectronic devices in the visible and ultraviolet ranges. Likewise, while maintaining mechanical stability, hydrostatic pressure significantly impacts mechanical properties. The ductility and anisotropic behavior of both perovskites are intensified under applied pressure.

Pressure-driven structural phase transitions and metallization in the two-dimensional ferromagnetic semiconductor CrBr3.

High-pressure structural, magnetic and electrical transport characteristics of CrBr3 were synthetically investigated using Raman scattering, electrical conductivity, high-resolution transmission electron microscopy (HRTEM) and first-principles theoretical calculations during compression and decompression under different hydrostatic conditions. Upon pressurization, CrBr3 underwent a second-order structural transition at 9.5 GPa, followed by the semiconductor-to-metal and magnetic switching at 25.9 GPa under non-hydrostatic conditions, whereas, an obvious pressure hysteresis of ∼3.0 GPa was detected in the occurrence of structural transitions and metallization under hydrostatic conditions due to the deviatoric stress. Upon decompression, the structural and electronic transitions of CrBr3 under different hydrostatic conditions were of good reversibility with a considerable pressure sluggishness of ∼5.0 GPa, which was corroborated well by the microstructural observation with HRTEM. Our systematic high-pressure investigation on CrBr3 not only reveals its underlying application in spintronic, magnetic and electronic devices but also advances the understanding of the physicochemical behaviors for 2D magnetic materials.

Phase Transitions and Electric Properties of PbBr2 under High Pressure: A First-Principles Study.

PbBr2 has recently attracted considerable attention as a precursor for lead halide perovskite-based devices because of its attractive properties. It is known that pressure can modify the chemical and physical properties of materials by altering the distance between atoms in the lattice. Here, a global structure-searching scheme was used to explore the high-pressure structures of PbBr2, whose structures and properties at high pressure are still far from clear. Three new phases of PbBr2 were predicted in the pressure range of 0-200 GPa, and the pressure-driven phase transition sequence of orthorhombic Pnma (0-52 GPa) → tetragonal I4/mmm (52-80 GPa) → orthorhombic Cmca (80-153.5 GPa) → orthorhombic Immm (153.5-200 GPa) is proposed. Electronic calculations indicate a semiconductor-to-metallic transition of PbBr2 in the Cmca phase at ~120 GPa. Our present results could be helpful in improving the understanding of fundamental physical properties and provide insights to modulate the structural and related photoelectric properties of PbBr2.

Pressure-driven ferroelectric phase transition for the Pnma-CsPbBr3: Mechanical and dynamical stability study

Using the first-principles method, a new ferroelectric phase transition from the Pnma structure to the P212121 polar structure is predicted under a hydrostatic pressure above 3.6 ​GPa for CsPbBr3 compound. This phase transition results from the condensation of the polar atomic displacements of the silent soft mode Au at the center of the Brillouin zone. The off-center displacement of Pb is dominated relative to that of Cs and Br atoms in this soft mode. Besides, the structural, elastic, dynamic, and electronic properties of both structures are reported in the present work. The Pnma structure shows mechanical stability under pressure and a large resistance to unidirectional compression along y-axis comparing to the shear deformation. The application of pressure decreases the ductility and enhances the stiffness of the compound.

Pressure evolution of electronic and structural properties in transition metal dichalcogenide 1T-Co1.06Te2

Two-dimensional transition metal dichalcogenides (TMDs) are important materials for promising electronic devices because they usually exhibit excellent and highly tunable electronic properties. Here, we report on the pressure-driven electronic phase transition in a TMD 1T-Co1.06Te2. High-pressure transport measurements reveal a sign reversal of the Hall coefficients at a critical point of P C ∼ 32 GPa, evidencing a transition from hole band(s) dominated transport into one that is dominated by electron band(s). Synchrotron x-ray diffraction experiments demonstrate that no structural phase transition occurs below 46.3 GPa, indicating an electronic origin of the transition. Moreover, a kink anomaly of the lattice constant ratio c/a is also observed at P = P C. These results might indicate a Lifshitz transition which refers to a change of Fermi surface topology in absence of structural transition.

Pressure-induced phase transitions in weak interlayer coupling CdPS3

Metal phosphorus trichalcogenides (MPX3: M = Fe, Co, Ni, Cd, Mn; X = S or Se) represent a family of two-dimensional (2D) layered materials with an exceptional response to high pressure and a remarkable structural flexibility originating from the weak interlayer coupling. Despite their interest for applications, the knowledge about pressure-driven phase transitions of cadmium compounds is still limited. In this paper, we fill this gap and provide an accurate description of the structural evolution of CdPS3 by combining high-pressure experiments and first-principle calculations. We have performed high-throughput screening of the low-energy stacking configurations and found a phase evolution starting with C2/m space group at 12 GPa using the generalized evolutionary metadynamics method. Then, high-pressure experiments have been used to reveal a structural transition from phase-I (C2/m) to phase-II (R3¯) to phase-III (R3¯), which is marked by the appearance and vanishing of the Raman band at approximately 30–250 cm−1 in good agreement with our theoretical predictions. Our study paves the way to the understanding of pressure-induced phase transitions in weak interlayer coupling 2D CdPS3 materials.

Pressure driven phase transitions in honeycomb Fe4Nb2O9: A possible re-entrant multiferroic behavior

A detailed high-pressure investigation is carried out on Fe4Nb2O9 using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of Nb–O6 octahedra as seen from x-ray diffraction analysis and a large pressure dependence of an Nb–O6 octahedra breathing Raman mode. Anomalous behavior of Raman modes and an increase in the phonon lifetime at the phase transition pressure indicate a possible change in the magnetic property of the sample above 8.8 GPa. A decrease in the diffusive scattering rate of a low-frequency electronic contribution contradicts the results of a decrease in the intensity of a high-frequency electronic response and excludes the phenomenon of an insulator to metal transition. On the contrary, the enhancement of the intensity of the Raman modes up to about 8.8 GPa indicates a large change in ferroelectric polarization of the sample, indicating a possible pressure induced re-entrant multiferroic behavior in Fe4Nb2O9.

Pressure-induced two-dimensional to three-dimensional structural phase transition in 2H-type layered lead iodide PbI2

2H-type lead iodide (PbI2) is a two-dimensional (2D) layered semiconductor and has recently received considerable attention as a precursor for lead halide perovskite solar cells. Although the pressure-induced structural phase transitions of PbI2 have been investigated for several decades, the detailed crystalline structure of PbI2 at high pressure is still elusive because of lacking in situ x-ray diffraction (XRD) studies. In this report, in situ Raman spectra and XRD patterns of 2H-PbI2 under pressure have been investigated in a diamond-anvil cell. Both Raman spectra and XRD patterns provide consistent evidence for two structural phase transitions upon compression at approximately 0.58 and 2.6 GPa. The XRD patterns suggest that the complete pressure-driven phase transition sequence of PbI2 up to 20 GPa is two-dimensional hexagonal 2H-P3¯m1→polytype 4H-P3m1→three-dimensional (3D) orthorhombic Pnma. The uncovered 2D to 3D structural transition in 2H-PbI2 advances the understanding of structural stability of 2H-type layered semiconductors and lead halide perovskite.

Effect of defects on the phase transition of Al0.1CoCrFeNi high-entropy alloy under high pressure

Knowledge pressure-driven phase transition in condensed physics requires an understanding of how phase-transition-pressure in a substance is affected by defect concentration and type. The effect of defects on phase transition was investigated in this paper by comparing structure evolutions in Al0.1CoCrFeNi high-entropy alloy (HEA) prepared by mechanical alloying with as-cast one using in situ synchrotron radiation X-ray diffraction technique and high-resolution transmission electron microscope (HRTEM), as well as the calculated dislocation density. Pressure-induced partial irreversible phase transitions from face-centered cubic (fcc) to hexagonal close-packed (hcp) phase has been observed in both ball-milled and as-cast Al0.1CoCrFeNi HEAs. In comparison to the as-cast Al0.1CoCrFeNi HEA, the ball-milled HEA has a large number of defects, especially higher density dislocations, and a low nucleation energy barrier, resulting in a lower phase-transition-pressure of 7.0 GPa and a higher fraction of hcp phase of 32.12% at pressure ∼30 GPa. The HRTEM result confirms that hcp and fcc phases coexist after pressure releasing. The investigation of the link between defects and phase transition process might lead to a potential approach for customizing the percentage of fcc and hcp phases in HEAs, allowing them to improve their structure and characteristics as a novel engineering material.

A large enhancement of ionic conductivity in SrCoO2.5 controlled by isostructural phase transition and negative linear compressibility

The exploration of next-generation materials and the underlying mechanisms for high ionic conductivity have been the mainstay of the study in the solid state ionic field, but with limited success. Here, we performed closely integrated in situ high-pressure structural and electronic characterizations on SrCoO2.5, a typical material for solid fuel cell application. We discovered that the activation energy for ionic transport decreased by approximately 47% at 13.6 GPa upon compression, demonstrating a large enhancement of conductivity. Such a desired functional behavior is strongly interlinked with the pressure-driven isostructural phase transition at which negative linear compressibility occurs along c-lattice, weakening the interaction between Co and O atoms and then enhancing the ionic diffusion. Our results provide fundamental insight of the solid ionic mechanism and materials-by-design for future high-performance oxides.

Pressure-induced structural phase transitions in bismuth tungstate Bi2WO6

The pressure-induced structural phase transitions in bismuth tungstate Bi2WO6 have been studied using neutron diffraction and Raman spectroscopy at high pressures up to 7 and 30 GPa, respectively. A rich structural polymorphism was revealed. At P ≃ 3.5 GPa a phase transition from the initial orthorhombic phase of P21 ab symmetry to an orthorhombic phase of B2cb symmetry was observed. This transition is caused by the complex spatial rotation of the WO6 octahedra. A subsequent isostructural phase transition to another orthorhombic phase of B2cb symmetry was detected at P ≃ 5.9 GPa, accompanied by changes in both the mutual rotation and tilting of the oxygen octahedra with respect to the crystal b axis. Two more pressure-induced phase transitions in Bi2WO6 at high pressures of 11.5 and 20 GPa were observed in the Raman spectra. These pressure-driven phase transitions in bismuth tungstate are accompanied by anomalies in the pressure dependences of the unit-cell parameters, bond lengths and angles, and in the vibrational modes.

Ultrafast crystallization mechanism of amorphous Ge15Sb85 unraveled by pressure-driven simulations

Crystallization rate determines the switching speed of phase-change memory for next-generation cache-type applications. Using ab initio molecular dynamics simulations, we studied the pressure-driven rapid crystallization behavior of amorphous Ge15Sb85 (a-GS), a technologically important Te-free phase-change material (PCM) that exhibits rapid growth-controlled phase transition. We concentrated on the fluctuations and competitions between the short-range order (SRO) and medium-range order (MRO) in a-GS under different pressures. The results reveal that the a-GS at zero pressure is composed of Ge-centered tetrahedrons and defective Sb-centered octahedrons, and these special SRO motifs are connected by 5-fold rings. The pressure-driven rapid phase transition of a-GS involve two stages: the incubation stage and the fast growth stage. In the incubation stage, Ge-centered tetrahedrons transform into octahedrons and meanwhile Sb-centered octahedrons increase as well. The disordered 5-fold rings evolve into a specific ordered spatial conformation, in which four atoms lie on one plane and one atom is pushed out with a dihedral angle of 90°. During the fast growth stage, the formation of 4-fold rings is linked with the existing defective octahedrons, requiring only minimum atomic movements with disappearance of 5-fold rings. Using pressure, we reveal this transient process which bridges the missing gap in the formation of new MRO clusters that are yet unknown in experiments. The unique crystallization mechanism of a-GS under pressure is of great significance for the design of high-speed phase-change memory.