Reconstructive Phase Transition Research Articles

Phase Transition in Silicon from Machine Learning Informed Metadynamics.

Investigating reconstructive phase transitions in large-sized systems requires a highly efficient computational framework with computational cost proportional to the system size. Traditionally, widely used frameworks such as density functional theory (DFT) have been prohibitively expensive for extensive simulations on large systems that require long-time scales. To address this challenge, this study employed well-trained machine learning potential to simulate phase transitions in a large-size system. This work integrates the metadynamics simulation approach with machine learning potential, specifically deep potential, to enhance computational efficiency and accelerate the study of phase transition and consequent development of grains and dislocation defects in a system. The new method is demonstrated using the phase transitions of bulk silicon under high pressure. This approach has revealed the transition path and formation of polycrystalline silicon systems under specific stress conditions, demonstrating the effectiveness of deep potential-driven metadynamics simulations in gaining insights into complex material behaviors in large-sized systems.

Layer-by-layer phase transformation in Ti3O5 revealed by machine-learning molecular dynamics simulations

Reconstructive phase transitions involving breaking and reconstruction of primary chemical bonds are ubiquitous and important for many technological applications. In contrast to displacive phase transitions, the dynamics of reconstructive phase transitions are usually slow due to the large energy barrier. Nevertheless, the reconstructive phase transformation from β- to λ-Ti3O5 exhibits an ultrafast and reversible behavior. Despite extensive studies, the underlying microscopic mechanism remains unclear. Here, we discover a kinetically favorable in-plane nucleated layer-by-layer transformation mechanism through metadynamics and large-scale molecular dynamics simulations. This is enabled by developing an efficient machine learning potential with near first-principles accuracy through an on-the-fly active learning method and an advanced sampling technique. Our results reveal that the β−λ phase transformation initiates with the formation of two-dimensional nuclei in the ab-plane and then proceeds layer-by-layer through a multistep barrier-lowering kinetic process via intermediate metastable phases. Our work not only provides important insight into the ultrafast and reversible nature of the β−λ transition, but also presents useful strategies and methods for tackling other complex structural phase transitions.

Manipulating the phase stability of a halide perovskite, CH3NH3PbI3 by high-pressure cycling

Methylammonium lead iodide perovskite, CH3NH3PbI3 (MAPbI3) is a highly promising photovoltaic material, whose structural and electronic properties are very sensitive to applied stresses. Here, we show that the phase stability of a MAPbI3 crystal can be controlled by structural defects and strains. We synthesized a single crystal of MAPbI3 and measured its electrical properties across two successive structural phase transitions under applied pressure up to 4–9 GPa under multiple high-pressure cycling. We revealed three types of effects of this high-pressure treatment. (i) An increasing the level of defects and strains in the crystal greatly delays the first reconstructive phase transition, conserving the ambient-pressure tetragonal phase up to a record pressure of ∼1.4 GPa, much higher compared to its typical stability range of up to 0.3–0.5 GPa only. (ii) A time exposure of the crystal at a fixed pressure corresponding to the beginning of the second phase transition to a “destabilized” structure eliminated some of defects and strains; it down-shifted the phase boundaries back. (iii) A recrystallization of the sample after pressure-induced amorphization enhanced its structural stability against amorphization upon subsequent pressurization cycle. We proposed that the most abundant defects and strains, which could appear in the pressure-cycled MAPbI3 crystals, were related to migration of iodine atoms from their regular sites to the MA interstices. Temperature-dependent photoluminescence measurements indicated that the sample recovered after the high-pressure cycling crystallized back into the initial tetragonal crystal structure. Whereas, its stability range expanded toward lower temperatures by 20 K and a band gap slightly increased, from 1.59 to 1.625 eV (at 290 K). Using this organic-inorganic perovskite as an example, we show how the phase stability of such materials can be manipulated by creation and elimination of structural defects and local strains.

Influence of aluminium doping on high purity quartz glass properties.

High purity natural quartz is used as raw material for the manufacture of quartz glass crucibles for solar-grade silicon ingots production. One key challenge for cost-effective ingot pulling is to maximise the ability of the crucible to withstand the process conditions (i.e., silicon load and temperature about 1500 °C) without deformation. In order to improve this glass property, aluminium was coated into the raw quartz materials. Our results showed that an addition of up to 1000 wt ppm Al substantially reduces deformation of glass and improves viscosity at high temperatures. This is likely due to the reduction of stability of OH groups in the quartz glass as well as a trapping effect of aluminium on oxygen vacancies. This hypothesis is also supported by atomistic models. In the presence of Al, formation energies of silanol groups (Si-O-H) were much higher than without. Furthermore, the presence of Al in the structure significantly reduces mobility of the oxygen vacancies. It was also found that formation of oxygen vacancies hinders cristobalite crystallisation, on the other hand, Al atoms themselves induce local weakening of the Si-O bond which accelerates the kinetics of the reconstructive phase transition from glassy state to crystalline phase. This was also confirmed experimentally in our study.

Pressure-induced phase transition in cubic Yb2O3 and phase transition enthalpies

The high pressure structural evolution of cubic Yb2O3 has been studied using in situ synchrotron angle dispersive x-ray diffraction in combination with diamond anvil cell techniques up to 44.1 GPa. The XRD measurements revealed an irreversible reconstructive phase transition from cubic to monoclinic Yb2O3 at 11.2 GPa and extending up to 28.1 GPa with ∼8.1% volume collapse and a subsequent reversible displacive transition from monoclinic to hexagonal phase starting at 22.7 GPa. The monoclinic phase coexists with the hexagonal phase up to 44.1 GPa. After pressure releases, the hexagonal Yb2O3 reverts to the monoclinic structure. The second-order Birch–Murnaghan equation of state fit to the pressure–volume data yields a bulk modulus of 201 (4), 187 (6), and 200 (4) GPa for the cubic, monoclinic, and hexagonal phases, respectively. Furthermore, the effects of the hydrostatic pressure state on the diffraction patterns, bulk modulus, and onset transition pressure of Yb2O3 under high pressure have been discussed. It is concluded that the bulk modulus of the cubic Ln2O3 phase increases with decreasing cation radius due to lanthanide contraction. Another important work in this study is the determination of the enthalpies of the cubic to monoclinic and monoclinic to hexagonal phase transitions of Yb2O3 of 37.0 and 17.4 kJ/mol, respectively, based on the basic thermodynamic equations and using the onset transition pressures and corresponding volume changes obtained from high pressure XRD experiments.

Synthesis of Hexagonal Nanophases in the La2O3–MO3 (M = Mo, W) Systems

We report a study of nanophases in the La2O3–MO3 (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La2O3 + MO3 (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La15M8.5O48 (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La15Mo8.5O48 and ~685–760 °C for La15W8.5O48, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La15Mo8.5O48 to tetragonal γ-La2MoO6 and of hexagonal La15W8.5O48 to orthorhombic β-La2WO6. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La18W10O57 (phase I, 6H polytype) ceramic. Low-density La15W8.5O48 and La15Mo8.5O48 (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.

Phase transition of titanium dioxide based on quantum dynamics.

Polymorphic materials are ubiquitous in nature. Their structural features play a critical role in determining the characteristics. The investigation of phase transition mechanisms between different phases of polymorphs is of great significance for designing material structures, tuning phase components and creating novel materials with specific properties. In the current work, the phase transition of titanium dioxide (TiO2) crystals from brookite to columbite was successfully revealed by quantum dynamics simulations. The reconstructive phase transition mechanism of brookite-to-columbite TiO2 was proposed by systematically simulating the XRD spectra of various states, tracking the breakage and formation of Ti-O bonds, counting the connections of the [TiO6] octahedra, and calculating the Lindemann parameter. This work is expected to shed light on the theoretical basis for regulating the crystal phase composition of TiO2 materials and the phase transition mechanisms between polymorphs.

Thermal and Magnetic Field Switching in a Two‐Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings

AbstractA MnIII spin crossover complex with atypical two‐step hysteretic thermal switching at 74 K and 84 K shows rich structural–magnetic interplay and magnetic‐field‐induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X‐ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry‐breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low‐temperature phase with two inequivalent low‐spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Thermal and Magnetic Field Switching in a Two-Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings.

A MnIII spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Phase field crystal simulation of crack propagation based on a symmetry mode

Phase field crystal models have been extensively employed to investigate solidification process of melt, solid-state precipitation, crack propagation, reconstructive phase transitions and thermomechanical process. However, the models suggested in literatures have little consideration on the ordering structures of the simulation system. This paper proposes a universal framework to correlate the ordering structures, i.e. the crystal symmetry mode to a phase field crystal model. Demonstrations are included so as to show how to deal with the coexisting equilibrium density function of a three-dimensional ordering structure. As a validation, the phase field crystal simulation of crack propagation has been implemented based on the constructed symmetry mode. Furthermore, the three-dimensional atomic density functions for the bcc, TET, ORC, HEX and fcc lattice system have also been given.

Polymorphism and Fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi2 P3.

The all‐solid‐state battery (ASSB) is a promising candidate for electrochemical energy storage. In view of the limited availability of lithium, however, alternative systems based on earth‐abundant and inexpensive elements are urgently sought. Besides well‐studied sodium compounds, potassium‐based systems offer the advantage of low cost and a large electrochemical window, but are hardly explored. Here we report the synthesis and crystal structure of K‐ion conducting T5 KSi2P3 inspired by recent discoveries of fast ion conductors in alkaline phosphidosilicates. KSi2P3 is composed of SiP4 tetrahedra forming interpenetrating networks of large T5 supertetrahedra. The compound passes through a reconstructive phase transition from the known T3 to the new tetragonal T5 polymorph at 1020 °C with enantiotropic displacive phase transitions upon cooling at about 155 °C and 80 °C. The potassium ions are located in large channels between the T5 supertetrahedral networks and show facile movement through the structure. The bulk ionic conductivity is up to 2.6×10−4 S cm−1 at 25 °C with an average activation energy of 0.20 eV. This is remarkably high for a potassium ion conductor at room temperature, and marks KSi2P3 as the first non‐oxide solid potassium ion conductor.

Machine learning metadynamics simulation of reconstructive phase transition

Simulating reconstructive phase transition requires an accurate description of potential energy surface (PES). Density-functional-theory (DFT) based molecular dynamics can achieve the desired accuracy but it is computationally unfeasible for large systems and/or long simulation times. Here we introduce an approach that combines the metadynamics simulation and machine learning representation of PES at the accuracy close to the DFT calculations, but with the computational cost several orders of magnitude less, and scaling with system size approximately linear. The high accuracy of the method is demonstrated in the simulation of pressure-induced $B4\text{\ensuremath{-}}B1$ phase transition in gallium nitride (GaN). The large-scale simulation using a 4096-atom simulation box reveals the phase transition with excellent detail, revealing different simulated transition paths under particular stress conditions. With well-trained machine learning potentials, this method can be easily applied to all types of systems for accurate scalable simulations of solid-solid reconstructive phase transition.

Revisiting metal fluorides as lithium-ion battery cathodes.

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF3 using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF2 and CuF2. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F- sublattices and that of LiF is established. Initial lithiation of FeF3 forms FeF2 on the particle's surface, along with a cation-ordered and stacking-disordered phase, A-LixFeyF3, which is structurally related to α-/β-LiMn2+Fe3+F6 and which topotactically transforms to B- and then C-LixFeyF3, before forming LiF and Fe. Lithiation of FeF2 and CuF2 results in a buffer phase between FeF2/CuF2 and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.

Phase transitions in CaCO3 under hydrous and anhydrous conditions: Implications for the structural transformations of CaCO3 during subduction processes

AbstractThe transport of calcium carbonate (CaCO3) into the Earth’s interior through subduction is one of the key processes in the global cycling of carbon. To develop a better understanding of the CaCO3 structural stability during subduction processes, the phase transitions among CaCO3-I (calcite), CaCO3-II, -III/IIIb, and aragonite under pressure-temperature (P-T) conditions up to 2.5 GPa and 600 °C, in hydrous and anhydrous environments, were investigated using a hydrothermal diamond-anvil cell. One displacive and two reconstructive processes during the phase transitions among CaCO3 polymorphs were confirmed from the results obtained from in situ observations and Raman spectroscopic measurements. Meanwhile, the effect of Ca-substitutional metal cations (e.g., Mg2+) in CaCO3 and the presence of an aqueous fluid on the phase transition processes have been determined. Specifically, the CaCO3-I ↔ -II phase transition is a displacive process, occurring instantly at pressures varying from 1.6 GPa at room temperature to 1.5 GPa at 500 °C with the phase equilibrium boundary having a minimum P-T point at ~1.4 GPa at 300 °C, and is completely reversible upon cooling and decompression. The CaCO3-II → -III phase transition is a reconstructive process, observed at P-T conditions from 2.0 GPa at room temperature to 2.5 GPa at 150 °C, and is accomplished by solid recrystallization starting from CaCO3-II, transitioning through an intermediate CaCO3-IIIb, and ending at the CaCO3-III structure. The phase transition between CaCO3-I or -II and aragonite, which is also a reconstructive process, was found to occur by progressive solid recrystallization under high P-T hydrous and anhydrous conditions, or alternatively, via dissolution-precipitation under low-P-T hydrous conditions, depending on the presence of aqueous fluids and the heating rate of the system. The substitution for Ca2+ by other metal cations (e.g., Mg2+, Mn2+, Fe2+) in CaCO3 results in a significant increase in the pressures for the displacive and solid recrystallization reconstructive phase transitions, but has no detectable influence on the CaCO3-I/II ↔ aragonite transformation via a dissolution-precipitation process under hydrous conditions. Our results show that the presence of Ca-substitutional metal cations in CaCO3 is a key factor controlling the phase stability of CaCO3 under high P-T conditions, and suggest that aragonite should be the predominant phase in the upper mantle in subduction zones where the heating rate is very low and slab dehydration is prevalent.

Pressure-induced reconstructive phase transitions, polarization with metallicity, and enhanced hardness in antiperovskite MgCNi3.

In general, hydrostatic pressure can suppress electrical polarization, instead of creating and/or enhancing polarization like strain engineering. Here, a combination of first-principles calculations and CALYPSO crystal structures prediction is used to point out that hydrostatic pressure applied on antiperovskite MgCNi3 can stabilize polarization with metallicity, and thus a polar metal can exist under high pressure. Strikingly, the metallic polar phase of MgCNi3 exhibits an original linear-cubic coupling between polar and nonpolar modes, resulting in an asymmetrical double-well when the polarization is switched. Moreover, another novel phase of MgCNi3 under high pressure possesses an enhanced hardness stemming from a robust s-s electrons interaction of an unexpected C-C bond, rather than typical sp3 orbital hybridization. These discoveries open new routes to design superhard materials and polar metals.

Pressure-Induced High-κ Dielectric Properties and Multiple Phase Transitions between Novel Nonperovskite and Perovskite Phases in LiSbO3: A First-Principles Study

The pressure effects on LiSbO3 have been investigated via first-principles simulation. Multiple pressure-induced reconstructive phase transitions between nonperovskite and perovskite states are rev...

Enhancing the Stability of Orthorhombic CsSnI3 Perovskite via Oriented π-Conjugated Ligand Passivation.

Lead-free orthorhombic CsSnI3 (Bγ-CsSnI3) perovskite has been emerging as one of the potential candidates of photovoltaic materials with superior performance. However, the instability induced by rapid reconstructive phase transition and the oxidation of Sn2+ greatly limits their future application. We thus reported a strategy, oriented π-conjugated ligand passivation, for enhancing the stability of Bγ-CsSnI3, simulated using a Bγ-CsSnI3 slab model based on the first-principles computation. The phase stability was found to be strongly dependent on the orientations of phenylethylammonium (PEA+) ligands. The passivated Bγ-CsSnI3 slab with the ligand molecule axis along [414] was demonstrated as the most stable with the lowest adsorption energy (Eads). Based on this configuration, the calculated formation energies (Eform) of half- and full-monolayer coverage were even more negative than that of yellow phase (Y-) CsSnI3 passivated by PEA+ ligands, verifying the enhanced phase stability. Furthermore, the surface states could be effectively suppressed and the downshifted conduction band minimum (CBM) resulted in a reduced band gap for the completely capped Bγ-CsSnI3. Moreover, the CBM and the valence band maximum (VBM) of the system with complete coverage were respectively donated by the surface and bulky components of the slab, which might benefit the separation and transfer of photogenerated carriers.

Clathrate III (Bromine Hydrate): Structural Relationship with Clathrate I.

Tetragonal structure of clathrate III (TS-I) is found in bromine hydrate, ionic hydrates, in particular, in hydrates of tetraalkylammonium salts, as well as in clathrate compounds of the group IV elements ('Zintl clathrates'). In this paper new description of this clathrate structure is suggested. Main structural unit in this description is '4-column', the set of four nearest columns of 14-hedral T-cages together with its internal filling by the pillar of mutually orthogonal pairs of T-cages. Comparison based on this new description of clathrate III and clathrate I (CS-I) shows that 4-column is maximal common structural element of these two structures. On this ground it is argued that clathrate III can be considered as a modification of the clathrate I, formed as a result of multiple interruption of clathrate I crystal growth. At the same time, it is shown that due to crucial differences between these two structures there does not exist structural or reconstructive phase transition between them. Integral character of 4-columns as the structural units of clathrate III is confirmed by the analysis of the most studied case of ionic hydrates of tetraalkylammonium salts in which guest molecules occupy simultaneously T-cages in outer columns and inner T-cages. This analysis allows also to state important hypothesis that in general case for formation of clathrate III crystals presence of special guest molecules occupying several cages (or special ions in the host lattice) is necessary.

Microscopic Insights into the Reconstructive Phase Transition of KNaNbOF5 with 19F NMR Spectroscopy

The centrosymmetric polymorph of KNaNbOF5 undergoes a reconstructive transition from a rare A-site vacancy ordered perovskite to a unique nonperovskite high-temperature phase. Previously, the high ...

Thermal behavior and dielectric properties of Fe-doped Bi2CaNb2O9

Solid solutions of Bi2CaNb2–xFexO9-δ were obtained by the method of solid phase synthesis. The thermal analysis of the samples revealed diffuse low- and high-temperature exo-effects in the DSC curves in the temperature ranges of 493–673 K and 1123–1223 K. According to the high-temperature X-ray diffraction results, the exothermic effect in the range of 1123–1223 K is related to ferroelectric phase transition from polar (A21am) to nonpolar phase (I4/mmm). The high-temperature X-ray diffraction detected no reconstructive phase transitions in calcium bismuth niobate or its solid solution in the low temperature range of 493–673 K. The study of the electrical properties of Bi2CaNb2–xFexO9-δ samples (x = 0.06 and 0.10) showed that the substitution of niobium with iron led to the increase in the direct current electrical conductivity of the sample. In this case, the dielectric loss decreased and the dielectric permittivity increased. The studied solid solutions were characterized by the low-frequency ion-migration polarization. The radical change in the electrical characteristics including the almost double activation energy growth, the maxima of the LF and HF relaxation times and the maximum of polarization inhomogeneity were observed at the temperatures of 573–598 K. Which was probably caused by the phase transition accompanied by activation of oxygen ions. Thus, the impedance spectroscopy and thermal analysis data correlate well pointing at reproducibility of the low-temperature effect recorded by the thermal analysis methods.