High-pressure Phase Transformation Research Articles

Pressure-induced phase transition and metallization in zirconium disulfide under different hydrostatic environments up to 25.3 GPa

The pressurized behaviours of structure, vibration and electrical transport in zirconium disulfide have been systematically investigated in a diamond anvil cell (DAC) under different hydrostatic environments up to 25.3 GPa utilizing in-situ Raman spectroscopy, electrical conductivity measurement, high-resolution transmission electron microscopy (HRTEM) and first-principles theoretical calculations. Upon compression, our experimental results showed that ZrS2 underwent a phase transition accompanied by a semiconductor-to-metal transformation at 15.4 GPa under non-hydrostatic condition. Under hydrostatic condition, the corresponding phase transition occurred at a higher pressure of 17.5 GPa, indicating that the high-pressure phase transition is sensitive to the hydrostaticity in the sample chamber of DAC. Upon decompression, some absolutely new Raman peaks emerged at the pressures below 4.8 GPa and 2.9 GPa under non-hydrostatic and hydrostatic environments, respectively, which revealed an irreversible phase transition in ZrS2. Furthermore, our observations from selected-area electron diffraction patterns confirmed the irreversibility of the high-pressure phase transformation.

Electron Microscope Study of the Pressure-Induced Phase Transformation and Microstructure Change of TiO2 Nanocrystals.

Microstructure transformation of materials under compression is crucial to understanding their high-pressure phase transformation. However, direct observation of the microstructure of compressive materials is a considerable challenge, which impedes the understanding of the relations among phase transformation, microstructure, and material properties. In this study, we used transmission Kikuchi diffraction and transmission electron microscopy to intuitively characterize pressure-induced phase transformation and microstructure of TiO2. We observed the changes of twin boundaries with increasing pressure and intermediate phase TiO2-I of anatase transformed into TiO2-II (α-PbO2 phase) for the first time. The following changes occur during this transformation: anatase (diameter of ∼100 nm) → anatase twins 60° along the [110] zone axis → intermediate TiO2-I twins 60° along the [010] zone axis → TiO2-II twins 90° along the [010] zone axis. These results directly reveal the crystallographic relation among these structures, enhancing our understanding of the phase transformation in TiO2 nanocrystals.

Effect of dislocation slip and deformation twinning on the high-pressure phase transformation in Zirconium

Zirconium transforms from the α→ω phase under high hydrostatic pressures. The critical transformation pressure is expected to be affected by internal stresses associated with defects. This study focuses on the effects of dislocations and twins and their associated stress fields on the transformation. Samples are pre-loaded to seed dislocations or twins. In-situ high-hydrostatic-pressure X-ray synchrotron experiments are performed revealing that microstructures with pre-existing prismatic 〈a〉 dislocations and {101¯2} twins promote the transformation more effectively than pyramidal 〈c + a〉 dislocations or {112¯2} twins. Post-mortem electron backscatter diffraction further shows that pre-seeded defects stabilize the ω-phase at ambient conditions. In-situ stress-hold neutron diffraction experiments are also performed combining both hydrostatic and deviatoric stresses capturing the role of deviatoric stresses on phase transformation kinetics. These results support the findings of recent atomistic simulations indicating flow of prismatic 〈a〉 dislocations at an α-ω interface promotes the growth of the ω domain.

Exploring the high-temperature deformation behavior of monocrystalline silicon – An advanced nanoindentation study

Knowing the mechanical behavior of silicon at elevated temperatures is crucial for many high-temperature applications and processes. This work aims to expand the knowledge of these properties, especially on length scales relevant to miniaturized silicon structures. Therefore, nanoindentation experiments were performed on two differently doped (1 0 0) silicon wafers with varying oxygen content, from room temperature up to 950 °C. Residual impressions were microscopically characterized via confocal laser scanning microscopy. From this dataset covering an exceptionally large temperature range the elastic modulus and hardness as well as strain rate sensitivity, activation volume, and energy were evaluated. Generally, a change in deformation behavior can be identified between 300 °C and 400 °C. Above this transition temperature, the hardness drops exponentially. Also, the material becomes strain rate sensitive. These observations support the assumption that the deformation mechanism shifts from high-pressure phase transformation to dislocation-controlled plasticity with increasing temperature. Furthermore, a change in strain rate sensitivity, activation energy, and activation volume above 800 °C implies a further shift in the rate-controlling mechanism of dislocation motion. Lastly, the more strongly doped sample with the higher oxygen content showing higher mechanical strength is discussed regarding solid-solution strengthening and dislocation locking by oxygen atoms.

Phase transformation behavior of aluminum under high hydrostatic pressure: A molecular dynamics study

The atomic mechanism of phase transformation under high pressure has been the focus of researches. In the present study, the roles of hydrostatic pressure (P) on phase transformation temperature and microstructural evolution in aluminum (Al) during cyclic thermal loading are studied from molecular dynamics simulations. The FCC → Liquid → FCC, FCC → liquid → FCC + BCC, FCC → BCC → Liquid → BCC and BCC → Liquid → BCC transformation pathways were observed under 0 ≤ P ≤ 14 GPa, 15 ≤ P ≤ 21 GPa, 22 ≤ P ≤ 25 GPa and 26 ≤ P ≤ 100 GPa, respectively, during cyclic thermal loading. It should be noted that the melting temperature predicted by the used potential at high P is obviously lower than experimental results. The FCC → BCC transformation involves nucleation and growth of BCC phase. The nucleation of BCC phase undergoes by the continuous deformation of FCC lattice, corresponding to the Bain transformation path. The growth of BCC phase is completed by the atomic diffusion. With increasing P, the 111<112̅> twin and {111} stacking fault in FCC phase evolve to five-fold twin and HCP phase, respectively. The microstructure of BCC phase is dominated by the {112}<110> twin. Insight obtained in this work paves the way for understanding the atomic mechanism of high pressure phase transformation.

Electron microscopic study on high-pressure induced deformation of nano-TiO&lt;sub&gt;2&lt;/sub&gt;

The high-pressure &lt;i&gt;α&lt;/i&gt;-PbO&lt;sub&gt;2&lt;/sub&gt; phase of TiO&lt;sub&gt;2&lt;/sub&gt; has suitable band gap and photocatalytic capability in the visible light range, which is an environmentally friendly and efficient photocatalytic material. In this work, &lt;i&gt;α&lt;/i&gt;-PbO&lt;sub&gt;2&lt;/sub&gt; phase of TiO&lt;sub&gt;2&lt;/sub&gt; is obtained by the pressure-relief treatment of anatase nanospheres through using diamond anvil cell, and transmission electron microscope (TEM) observation shows the obvious deformation of TiO&lt;sub&gt;2&lt;/sub&gt; nanospheres. High-esolution TEM shows that there are a large number of stacking faults along the [100] direction and deformation twins in the grain. Specifically, the deformation twin band with lens lamellar structure is formed in the submicron grain. The fan-shaped multiple deformation twins are formed in the nanocrystalline grains. This study shows that anatase TiO&lt;sub&gt;2&lt;/sub&gt; can be deformed under high pressure, and its micro mechanism of deformation is similar to metal’s, mainly including deformation twins and stacking fault slip. There is obvious size effect in the formation of deformation twins. These results provide a new breakthrough point for the study of the size effect of high-pressure phase transformation of TiO&lt;sub&gt;2&lt;/sub&gt;, and also point out an experimental direction for preparing the twin high-pressure &lt;i&gt;α&lt;/i&gt;-PbO&lt;sub&gt;2&lt;/sub&gt; phase.

Intensified chipping during nanoindentation and the effect of friction on the interfacial fracture for thin films used in N/MEMS

Nanoindentation was performed on Ti-B-Si-C and SiCN films deposited on Si substrates at varying loads and depths of penetrations. The intensity of chipping got enhanced with an increasing load which was represented geometrically. The pop-in and pop-out events occurring in the load and unloading were explained as cracking and high-pressure phase transformation. The interfacial fracture stability and adhesion of the film-substrate system, crucial for N/MEMS devices were analyzed with respect to frictional coefficient with the help of Finite Element Modelling (FEM).

Hard nanocrystalline gold materials prepared via high-pressure phase transformation

Hard nanocrystalline gold materials prepared via high-pressure phase transformation

Octahedral tilting dominated phase transition in compressed double perovskite Ba2SmBiO6

The comprehension of structural behaviors in double perovskites is crucial for their functional optimization, especially when applying external regulations. Here, to inquire about potential structures with better magnetic performance, high-pressure phase transformation in double perovskite Ba2SmBiO6 was first investigated up to 50 GPa via in situ high-pressure x-ray diffraction and Raman spectroscopy. A pressure-induced phase transition from cubic Fm-3m to orthorhombic Pnma is discovered at 4.8 GPa, accompanied by the splitting of the diffraction peaks. Above 19.8 GPa, the new phase becomes distorted as shown by the peak recombination and broadening. The variation of Raman spectra also confirms the formation and distortion of the high-pressure phase during compression, through the evolution of Bi–O stretching, Bi–O bending, octahedral rotation, and Ba-sites translation modes. The analysis of tilt angles and distortion factor evinced that the octahedral BiO6 tilting is the key factor for the phase transition occurrence. Based on the Mulliken populations analyses, the Bi–O bonds undergo a covalent-ionic-antibonding transition across the phase transition under compression. Our exploration of the phase transition mechanism guides the modulation of the magnetic and electronic properties under extreme conditions.

In situ investigation of nanometric cutting of 3C-SiC using scanning electron microscope

Experimentally revealing the nanometric deformation behavior of 3C-SiC is challenging due to its ultra-small feature size for brittle-to-ductile transition. In the present work, we elucidated the nanometric cutting mechanisms of 3C-SiC by performing in situ nanometric cutting experiments under scanning electron microscope (SEM), as well as post-characterization by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). In particular, a new method based on the combination of image processing technology and SEM online observation was proposed to achieve in situ measurement of cutting force with an uncertainty less than 1 mN. Furthermore, the cutting cross-section was characterized by atomic force microscope (AFM) to access the specific cutting energy. The results revealed that the specific cutting energy increase non-linearly with the decrease of cutting depth due to the size effect of cutting tool in nanometric cutting. The high-pressure phase transformation (HPPT) may play the major role in 3C-SiC ductile machining under the parameters of this experiment.

Bactericidal surfaces: An emerging 21st-century ultra-precision manufacturing and materials puzzle

Progress made by materials scientists in recent years has greatly helped the field of ultra-precision manufacturing. Ranging from healthcare to electronics components, phenomena such as twinning, dislocation nucleation, and high-pressure phase transformation have helped to exploit plasticity across a wide range of metallic and semiconductor materials. One current problem at the forefront of the healthcare sector that can benefit from these advances is that of bacterial infections in implanted prosthetic devices. The treatment of implant infections is often complicated by the growth of bacterial biofilms on implant surfaces, which form a barrier that effectively protects the infecting organisms from host immune defenses and exogenous antibiotics. Further surgery is usually required to disrupt the biofilm, or to remove the implant altogether to permit antibiotics to clear the infection, incurring considerable cost and healthcare burdens. In this review, we focus on elucidating aspects of bactericidal surfaces inspired by the biological world to inform the design of implant surface treatments that will suppress bacterial colonization. Alongside manufacturing and materials related challenges, the review identifies the most promising natural bactericidal surfaces and provides representative models of their structure, highlighting the importance of the critical slope presented by these surfaces. The scalable production of these complex hierarchical structures on freeform metallic implant surfaces has remained a scientific challenge to date and, as identified by this review, is one of the many 21st-century puzzles to be addressed by the field of applied physics.

“Volume collapse” instabilities in deep-focus earthquakes: A shear source nucleated and driven by pressure

Symmetry-breaking instabilities in high-pressure phase transformation produce the counterintuitive phenomenon of “volume collapse” producing only shear radiation, with little, or no, volumetric component, even under conditions of full isotropy, and explain the mystery of the long-standing observations in deep-focus earthquakes (400–700 km). Due to instability, at a critical “nucleation pressure”, an arbitrarily small densified region in the shape of a “pancake-like” flattened ellipsoidal Eshelby inclusion, grows self-similarly as a “lacuna” (zero particle velocity) with the phase transformation occurring under conditions of equilibrium in uniform strain/stress and at constant potential energy (at the vanishing of the M integral, when the radius-expanding driving force pεkk∗ overcomes the radius-shrinking self-force). The symmetry-breaking flattened shape favors minimization of the energy needed for the boundary to grow large, while for the accommodation of the large collapsing volume in the very thin inclusion deviatoric stresses are developed to avoid openings and overlaps. It is shown that, if an arbitrarily small flattened densified region is generated planarly, and the pressure exceeds the critical nucleation value, then it will necessarily produce a shear seismic source, with little or no, volumetric component, nucleated and driven to propagate by the pressure. The ellipsoid of phase change forms in the direction that minimizes the interaction energy with the pre-stress field and will be close to the direction of max shear pre-stress in the mantle. The obtained stress/deformation fields of a densified 2D flattened elliptical inclusion constitute a new defect that models the “anticrack” in geophysics and densified shear bands. The instability analysis can be extended to the nucleation and growth of the phase transition from water to a solid ice phase under high pressure, with the discovered instabilities providing insight to other phenomena of dynamic phase transformations, such as failure waves, amorphization, planetary impacts, etc.

High-Pressure Structural Phase Transformation of Ferroelectric Bis-benzylammonium Lead Tetrachloride Studied by Raman Spectroscopy and X-ray Diffraction

Hybrid organic-inorganic 2-D perovskite bis-benzylammonium lead tetrachloride (BALC) is a room-temperature ferroelectric semiconductor. A structural phase transformation from the ambient Cmc21 structure is evident at 1.8 GPa from the Raman spectra, and this is confirmed by our high-pressure X-ray diffraction studies that point to a centrosymmetric structure Cmcm at 1.7 GPa. The ambient phase is recoverable on decompression. Using density functional theory calculations, we have studied the intermolecular and intramolecular vibrations to get an idea of the structural changes as a function of pressure. The high-pressure transition is identified to be due to a distortion in the PbCl6 octahedra and a conformation change in the molecule. There are several discontinuities, broadening, and splitting of the Raman bands, corresponding to NH3 units above 1.8 GPa that point to rearrangements in the hydrogen bond network in the new phase. The ambient structure shows anisotropic compressibility, with a bulk modulus of 14.5 ± 0.33 GPa. As the new phase is a centrosymmetric structure, BALC is expected to lose its ferroelectricity above ∼1.8 GPa.

High-pressure phase transformation of carbonate malachite Cu2(CO3)(OH)2 driven by [CuO6] regularization and [CO3] rotation

High-pressure synchrotron X-ray diffraction and infrared absorption spectroscopy have been employed to study the crystal chemistry and phase transitions in an [OH]-bearing carbonate, malachite Cu2(CO3)(OH)2, to determine the effect of [OH] on the stability of carbonate. We found that the crystal structure of malachite is stabilized by a high degree of [CuO6]-octahedron distortion, as is manifested by large variations in Cu–O bond lengths resulting from oxygen atoms that connect to hydrogen at crystallographically different sites. External pressure offsets the effect of hydrogen bond, promotes [CuO6] compression and regularization and accordingly [CO3] rotation. Rotation of [CO3]-triangles, in turn, assists in a conversion in the crystal orientation of the [CuO6] structural unit. During compression to above ~6 ​GPa, malachite begins to turn into the rosasite lattice, accompanied with a jump in density of 3.3%. Rosasite is characterized with a hardened lattice and preserves to the maximum pressure (18.2 ​GPa) of the present study. Phase transformation mechanism of malachite to rosasite is different from that of carbonates, with the latter being driven by an almost uniform compression of [MO6]-octahedron (M ​= ​Ca, Cd, Mn, Fe, Zn, Mg, etc.) and rotation/translation of [CO3]-triangle under pressure.

Effects of Anisotropy on Single Crystal Silicon in Polishing Non-Continuous Surface.

A molecular dynamics model of the diamond abrasive polishing the single crystal silicon is established. Crystal surfaces of the single crystal silicon in the Y-direction are (010), (011), and (111) surfaces, respectively. The effects of crystallographic orientations on polishing the non-continuous single crystal silicon surfaces are discussed from the aspects of surface morphology, displacement, polishing force, and phase transformation. The simulation results show that the Si(010) surface accumulates chips more easily than Si(011) and Si(111) surfaces. Si(010) and Si(011) workpieces are deformed in the entire pore walls on the entry areas of pores, while the Si(111) workpiece is a local large deformation on entry areas of the pores. Comparing the recovery value of the displacement in different workpieces, it can be seen that the elastic deformation of the A side in the Si(011) workpiece is larger than that of the A side in other workpieces. Pores cause the tangential force and normal force to fluctuate. The fluctuation range of the tangential force is small, and the fluctuation range of the normal force is large. Crystallographic orientations mainly affect the position where the tangential force reaches the maximum and minimum values and the magnitude of the decrease in the tangential force near the pores. The position of the normal force reaching the maximum and minimum values near the pores is basically the same, and different crystallographic orientations have no obvious effect on the drop of the normal force, except for a slight fluctuation in the value. The high-pressure phase transformation is the main way to change the crystal structure. The Si(111) surface is the cleavage surface of single crystal silicon, and the total number of main phase transformation atoms on the Si(111) surface is the largest among the three types of workpieces. In addition, the phase transformation in Si(010) and Si(011) workpieces extends to the bottom of pores, and the Si(111) workpiece does not extend to the bottom of pores.

Molecular dynamics simulation of nano-polishing of single crystal silicon on non-continuous surface

A molecular dynamic model of diamond abrasive polishing single crystal silicon is established. The effects of pores on the material removal process and the mechanism of pore wall material removal are studied from the aspects of displacement, polishing force, and phase transformation. The simulation results show that the removal method of material at pore walls is mainly shear and extrusion. The deformation of the pore walls in the entry areas of pores is an elastoplastic mixed deformation, and the deformation of the pore walls in the exit areas of pores is plastic deformation. Increasing the polishing depth increases plastic removal of material at pores. The existence of pores makes the tangential and normal forces decrease to a certain extent during the entire polishing process. The fluctuation of the normal polishing force is greater than that of the tangential polishing force. The lack of material at the pores is the main reason for the decrease of the normal polishing force. As the polishing depth increases, the tangential force increases significantly, and the normal force increases less. During the polishing process, phase transformation atoms with coordination number of 5 and phase transformation atoms with coordination number of 6 are main types of phase transformation atoms. High-pressure phase transformation is the main form of phase transformation. Due to the lack of atoms at the pores, the phase transformation of the pore walls in the entry areas of pores will be extended longitudinally. In the process of increasing the polishing depth, the number of main phase transformation atoms increased, and the degree of phase transformation in the pore area deepened.

Irreversible high pressure phase transformation of onion-like carbon due to shell confinement

Onion-like carbon (OLC) was investigated at high pressure (up to 48 GPa) in a shear diamond anvil cell (SDAC) equipped with in-situ Raman spectroscopy control. The OLC was obtained from natural gas by a highly productive and efficient partial oxidation method. Vast bulk modulus B0 = 486 ± 15 GPa and outstanding stability under high pressure were registered in the experiment. A phase transition was observed at 20 GPa under conditions of shear deformation. The transition is characterized by an appearance of a new Raman band at 1560 cm−1 accompanied with disappearance of G band of OLC. We attribute these features to sp2-sp3 transition in the carbon nanoclusters. It was found however that the concentric multi-shell onion-like structure remains intact after pressure release, which may witness that the outer shell of the OLC serves as a confinement and prevents structural collapse. X-ray photoelectron spectroscopy revealed however that the phase transition was not completely reversible because the OLC after pressure release showed the presence of sp3-hybridized carbon in significant amounts.

Designing Novel Multiferroic Perovskite Oxide for Prospective Photovoltaic Applications

Multiferroics, with two or more coexisting ferroic orders (ferroelectric, ferro (antiferro)-magnetic) in a single phase, display promising photovoltaic characteristics which can be utilised in solar energy harvesting. However, the efficacy is seriously challenged due to their wide band gap, far from the ideal value of ~1.52 eV for photovoltaic applications, resulting in overall unimpressive performance. In the present work, an approach towards imparting multiferroism in an otherwise non-ferroic system was adopted through strain engineering. Bulk SrMnO3 (SMO) is antiferromagnetic-paraelectric. However, our previous first-principles studies predicted high-pressure phase transformation from bulk non-polar phase to a tetragonal polar phase. In light of the above, SMO was synthesised hydrothermally at 200°C for 96 h using water-soluble nitrate salts of strontium and manganese. FESEM study reveals the formation of hexagonal bipyramid shaped SMO crystals with elongated 1-D features. Powder x-ray diffraction studies and subsequent Rietveld refinement confirm the presence of hexagonal (P63/mmc) as well as tetragonal (P4mm) phases. Energy dispersive x-ray analysis (EDAX) confirms Sr/Mn ≈ 1, the stoichiometric ratio. UV-VIS spectroscopy was utilised to estimate the optical bandgap of the as-grown sample which was found to be in the range of 1.4-1.5 eV. Temperature-dependent magnetisation plot indicates the magnetic transition temperature, ~275K.

High temperature–high pressure phase transformation of Cu

DFT calculations (typically done at 0 K), Helmholtz free energy and Gibbs free energy calculations (for high temperature) within the quasi-harmonic approximation have been done in this manuscript to probe a metastable phase of Cu observed at high temperature and high pressure. The high electronic thermal conductivity observed for this new metastable phase of Cu may be beneficial for high temperature engineering applications in electronic and spacecraft device designs.

Investigation on the phase transformation of monocrystalline silicon during nanoindentation at cryogenic temperature by molecular dynamics simulation

High-pressure phase transformation of silicon is an important phenomenon as it is of scientific and technological importance for semiconductor industry and micro/nano electromechanical system. However, there are limited studies on the phase transformation at cryogenic temperature. In this study, molecular dynamics simulation was conducted to investigate the nanoindentation of monocrystalline silicon at 1 K. The force-displacement curve and the corresponding phase transformation were studied in details. During the loading process, the contact zone was affected by the applied stress and the original diamond cubic structure of silicon was distorted. With the increase of the depth, the distorted diamond cubic structure (DDS) was transformed into body-centered-tetragonal structure (bct-5) and Si-II. With the release of the force, the reverse transformations of Si-II to Si-III and Si-XII in junction with the pop-out events in the unloading curves can be clearly seen. It was also found that the phase distribution during the reverse transformation was strongly influenced by the indenter radius and indentation depth due to the different stress fields. At the higher depth, the unloading curves showed the unusual absence of pop-out and the appearance of surface extrusion. This can be ascribed by that the retained DDS outside the indentation after unloading not only restricts the transformation of Si-II but also brings the material extrusion on the surface due to the residual internal stress.