High-pressure Metastable Phase Research Articles

Unusual structure and properties of germanium under pressure

Structural phase transitions in group 14 element germanium (Ge) under high pressure were investigated through first-principle calculations along with unbiased CALYPSO structure prediction method. We successfully reproduced experimental structures Fd-3m, I41/amd, Imma, P6/mmm, Cmca and P63/mmc were in our study on crystal structures. In addition, we found four new high-pressure metastable phases (Fmmm(I) and Cmcm at 75 GPa, Fmmm(II) and Fmm2 at 130 GPa). It was found in electronic density of states (DOS) and electronic band structure diagrams that all predicted new phases were metallic phases. The results of crystal structures, X-ray diffraction and phonon dispersion curve showed that our predicted phases (Cmcm and Fmmm(II)) could be incommensurate modulation structures. The findings of this study enriched the phases of Ge under high pressures and offered fundamental insight for further investigation of structural phase transitions in other group 14 elements under high pressures.

A15 Nb3Si: a ‘high’ T c superconductor synthesized at a pressure of one megabar and metastable at ambient conditions

A15 Nb3Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb3Si and its production from tetragonal Nb3Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb3Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c (P) data for A15 Nb3Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb3Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb3Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb3Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb3Si structural transformation. Our theoretical calculations show that A15 Nb3Si has an enthalpy vs the tetragonal structure that is 70 meV atom−1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom−1. The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom−1.

Evolution of high-pressure metastable phase Si-XIII during silicon nanoindentation: A molecular dynamics study

Berkovich indenters are widely employed in nanoindentation experiments of single-crystal silicon, in molecular dynamic simulations of silicon nanoindentation, however, spherical indenters are generally employed. To close this gap, in this paper, a series of molecular dynamic simulations are conducted on (001), (101), and (111) silicon surfaces to explore the evolution of high-pressure phases. Several possible silicon phases are tracked by coordination number (CN), radial distribution function (RDF), and angular distribution function (ADF). Results show that the metastable phases and the high-pressure phases present different symmetrical patterns along different crystallographic orientations. A large amount of high-pressure phase Si-XIII, which is seldomly observed in nanoindentation using a spherical indenter, is first discovered under the Berkovich indenter, and the evolution mechanism is characterized by tracking the position transition of labeled silicon atoms. As a transition phase for high-pressure phase Si-II and Si-XIII, bct5 phase is found in the {111} [110] slip systems, which is related with slipping flatten lattice structures. Different from the phase transformation criterion for Si-II, the transformation from Si-I to Si-XIII companies with the local elastic deformation of (001) surface. The atoms in the three neighboring unit cells are compressed into a uniform long-distance ordered crystal structure. Upon indenter extraction and rapid stress releasing, the elastic deformation of the crystal surface resumes incompletely. A part of Si-XIII recovers to pristine Si-I and the rest changes into amorphous phase. The distribution of local hydrostatic pressure and von Mises stress in the phase transformation regions indicates that the concentrated hydrostatic pressure and specific stress induce this high-pressure silicon phase. This research clarifies the evolutionary process of Si-XIII phase generation and enriches fundamental understanding on the mechanisms of silicon phase transformations in nanoindentation.

High-pressure phases of AlxIn1−xN compounds: First principles calculations

Novel high pressure metastable phases within Al-In-N system have been comprehensively explored via evolutionary algorithm USPEX, under both variable and fixed composition schemes. At 2.5 GPa, only Al7In2N9 (Cm) and Al2In2N4 (I4¯2d) are identified. Candidate structures at 5.0 GPa expand to orthorhombic Al4In2N6 (Cmc21), two tetragonal Al2In2N4 (I4¯2d) plus AlIN4N5 (I4¯) and trigonal AlIn5N6 (R3). Increasing the pressure beyond 5.0 GPa appears to upset the stability of AlxIn1-xN. Using ab initio calculations based on systematic density functional theory, we also outline the electronic, lattice, mechanical and thermodynamic properties of these phases. Band structure, PDOS and Bader analyses show that these alloys are direct band gap semiconductor with certain degree of covalent nature in an ionic bonding. Corrected energy gap ranges from 0.771 eV to 4.995 eV. Mechanical and dynamical stabilities of abovementioned alloys are further revealed by the computations of independent elastic parameters and phonon dispersion curves. All the studied species demonstrate considerable elastic anisotropy and moderate hardness. Their simulated Vickers hardness ranges between 13.3 GPa and 21.6 GPa. Through quasi harmonic Debye model, composition dependence of thermal influenced macroscopic properties is quantitatively analyzed as well.

Nanosize effects assisted synthesis of the high pressure metastable phase in ZrO2.

The size effects on the high pressure behaviors of monoclinic (MI) ZrO2 nanoparticles were studied using in situ high pressure synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). A size-dependent phase transition behavior under high pressure was found in nanoscale ZrO2. The normal phase transition sequence of MI-orthorhombic I (OI)-orthorhombic II (OII) occurs in 100-300 nm ZrO2 nanoparticles, while only the transition of MI-OI exists in ultrafine ∼5 nm ZrO2 nanoparticles up to the highest experimental pressure of ∼52 GPa. This indicates that the size effects preclude the transition from the OI to the OII phase in ∼5 nm nanoparticles. Upon decompression, the OII and OI phases are retained down to ambient pressure, respectively. This is the first observation of the pure OI phase ZrO2 under ambient conditions. The bulk moduli of the MI ZrO2 nanoparticles were determined to be B0 = 192 (7) GPa for the 100-300 nm nanoparticles and B0 = 218 (12) GPa for the ∼5 nm nanoparticles. We suggest that the significant high surface energy precludes the transition from the OI to the OII phase and the nanosize effects enhance the incompressibility in the ultrafine ZrO2 nanoparticles (∼5 nm). Our study indicates that this is a potential way of preparing novel nanomaterials with high pressure structures using nanosize effects.

Prediction of novel hard phases of Si3N4: First-principles calculations

Exploration of novel hard metastable phases of silicon nitride was performed using a recently developed particle-swarm optimization method within the CALYPSO software package. Three potential hard metastable phases of t-Si3N4, m-Si3N4, and o-Si3N4 were predicted. These phases are mechanically and dynamically stable at ambient pressure based on their elastic constants and phonon dispersions. t-Si3N4 and m-Si3N4 exhibit lower energies than γ-Si3N4 at pressures below 2.5GPa and 2.9GPa, respectively, which promise that the formers could be obtained by quenching from γ-Si3N4. o-Si3N4 is a better high-pressure metastable phase than CaTi2O4-type Si3N4 proposed by Tatsumi et al. and it can come from the transition of γ-Si3N4 under 198GPa. The theoretical band gaps of t-Si3N4, m-Si3N4, and o-Si3N4 at ambient pressure were 3.15eV, 3.90eV, and 3.36eV, respectively. At ambient pressure, the Vickers hardness values of t-Si3N4 (32.6GPa), m-Si3N4 (31.5GPa), and o-Si3N4 (36.1GPa) are comparable to β-Si3N4 and γ-Si3N4. With the pressure increasing, t-Si3N4, m-Si3N4, and o-Si3N4 will change from the brittle to ductile state at about 15.7GPa, 7.3GPa and 28.9GPa, respectively.

Pressure-Induced Transformations in PrVO4 and SmVO4 and Isolation of High-Pressure Metastable Phases

Zircon-type PrVO4 and SmVO4 have been studied by high-pressure Raman spectroscopy up to 17 GPa. The occurrence of phase transitions has been detected when compression exceeds 6 GPa. The transformations are not reversible. Raman spectra of the high-pressure phases show similarities with those expected for a monazite-type phase in PrVO4 and a scheelite-type phase in SmVO4.The high-pressure phases have been also synthesized using a large-volume press and recovered at ambient conditions. X-ray diffraction measurements of the metastable products recovered after decompression confirms the monazite (PrVO4) and scheelite (SmVO4) structures of the high-pressure phases. Based upon optical properties of the reported new polymorphs, novel applications for rare-earth vanadates are proposed, including photocatalytic hydrogen production.

Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

We have combined first-principles calculations and high-pressure experiments to study pressure-induced phase transitions in silicon nitride (Si${}_{3}$N${}_{4}$). Within the quasi-harmonic approximation, we predict that the $\ensuremath{\alpha}$ phase is always metastable relative to the $\ensuremath{\beta}$ phase over a wide pressure-temperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the $\ensuremath{\beta}$-Si${}_{3}$N${}_{4}$, while phonon softening in the $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$ is rather moderate. When the previously observed equilibrium high-pressure and high-temperature $\ensuremath{\beta}$ $\ensuremath{\rightarrow}$ $\ensuremath{\gamma}$ transition is bypassed at room temperature (RT) due to kinetic reasons, the $\ensuremath{\beta}$ phase is predicted to undergo a first-order structural transformation to a denser $P\overline{6}$ phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV$/$atom, which suggests that the transition is kinetically possible around RT. This predicted new high-pressure metastable phase should be classified as a ``postphenacite'' phase. Our high-pressure x-ray diffraction experiment confirms this predicted RT phase transition around 34 GPa. No similar RT phase transition is predicted for $\ensuremath{\alpha}$-Si${}_{3}$N${}_{4}$. Furthermore, we discuss the differences in the pressure dependencies of phonon modes among the $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Gr\"uneisen ratios in the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases as the cause of the predicted negative thermal expansion coefficients (TECs) at low temperatures in these two phases, and predict no negative TECs in the $\ensuremath{\gamma}$ phase.

High-pressure meta-stable phase of BeO: A first principle study

High-pressure phase transitions and phonon-dispersion relations of BeO have been studied by an ab initio method. A phase transition is found from a wurtzite structure (B4) to a cubic sodium chloride structure (B1) at about 122 GPa, but without a similar phase transition from B4 to a cubic zinc blende structure (B3). At ambient conditions, the B1 phase is not stable because in the calculated phonon-dispersion relations several modes with negative frequencies in this phase have been detected. However, they change to positive quickly with increasing pressures. The B3 and B4 phases are both stable at various pressures although the B3 phase is energetic meta-stable. When considering the temperature effect, a B4 → B3 phase transition can happen, but the transition pressure is higher than that of the B4 → B1 phase transition at the same temperature. Therefore, the B3 phase cannot be obtained by simply increasing pressures at high temperatures. Although there is the same atomic coordination at the plane (1 1 1) in the B3 phase and the plane (0 0 0 1) in the B4 phase, their sound velocities are different. The sound velocities of the longitudinal wave along the plane (0 0 0 1) in the B4 structure are always greater than that of the plane (1 1 1) in the B3 structure, and this difference almost keeps a constant with pressures. We think this could be a universal result for all A N B 8− N compounds.

Stable Oxoborate with Edge‐Sharing BO4 Tetrahedra Synthesized under Ambient Pressure

Back from the extreme: The formation of edge-sharing BO4 tetrahedra is excluded from borate structure chemistry and only rarely presents itself under extremely high pressure. Now this structure has been observed in an ambient pressure borate. Unlike high-pressure metastable phases, the title compound is stable up to its melting point. The tunnel structure of the compound is also accessible to mobile ions.

Ab initiosurvey of the electronic structure of tetrahedrally bonded phases of silicon

We present an ab initio study of the electronic structure of a number of high-pressure metastable phases of silicon with tetrahedral bonding. The phases studied include all experimentally determined phases that result from decompression from the metallic $\ensuremath{\beta}$-Sn phase, namely, the BC8 (Si III), hexagonal diamond (Si IV), and R8 (Si XII) phases. In addition to these we have also studied the hypothetical ST12 structure that is found upon decompression from $\ensuremath{\beta}$-Sn in germanium. We find that the local-density approximation incorrectly predicts the R8 phase to be semimetallic and that the quasiparticle spectrum exhibits a band gap. The effective masses found in R8 suggest that R8 may be useful for high-mobility applications. In addition, the ST12 phase is found to have a large density of electronic states at the band edge which could lead to interesting superconducting behavior.

Solidification characteristics of Al–9·6Mg alloy solidified under high pressure (4 GPa)

The present paper studies influences of high pressure up to 4 GPa at 1273 K on the microstructures and the phases of Al–9·6Mg alloy by means of optical microscopy (OM), X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results showed that fine dendrite with long primary arm formed at 4 GPa in comparison with the coarse discontinuous dendrite under normal pressure. Specially, a new high pressure metastable phase, Al(63·7)Mg(36·3), was produced besides a small amount of β-Al3Mg2 phases at 4 GPa, which was not found in other solidification conditions. Al(63·7)Mg(36·3) phase has bcc structure with the lattice constant a=8·495 Å. Evolution mechanism of microstructures and phases was studied in the present paper. Furthermore, Mg concentration in α-Al phase has increased by 42%. Correspondingly, the lattice constant of α-Al phase has increased by 0·44%. The nanohardness of α-Al phase in the alloy increased under high pressure, whereas that of the elastic modulus decreased.

Calculation of phonon spectra to predict the high-pressure metastable phase in an equilibrium immiscible Cu–Ta system

Under the framework of lattice dynamics, the phonon spectra of the possible metastable Cu3Ta compounds at zero pressure are computed based on an n-body potential by fitting to some physical properties acquired from ab initio calculations. For the three ideal lattices, i.e., L12, D03, and D019, imaginary phonons appear along all high symmetry directions in the Brillouin zone and disappear while subjected to high pressure at about 2.7, 11.8, and 14.7GPa, respectively. Interestingly, one of the predicted compounds in an L12 structure has indeed been obtained by ion-beam mixing under an appropriate condition.

The R8–BC8 phases and crystal growth in monocrystalline silicon under microindentation with a spherical indenter

The morphology and distribution of high-pressure metastable phases BC8 and R8, formed in monocrystalline silicon under microindentation, were identified and assessed using transmission electron microscopy nanodiffraction analysis. It was discovered that the crystal growth inside the transformation zone was stress-dependent with large crystals in its central region. The crystal size could also be increased using higher maximum indentation loads. The BC8 and R8 phases distributed unevenly across the transformation zone, with BC8 crystals mainly in the center of the zone and smaller R8 fragments in the peripheral regions. Such phase distribution was in agreement with the theoretical residual stress analysis.

Thermobaric quenching—A new method for producing metastable states

An alternative method of obtaining amorphous state by spontaneous amorphization of high-pressure metastable phases (HPP) after thermobaric quenching is discussed. Amorphous states in the ZnSb, CdSb, ZnCdSb, GaSb and AlGe systems have been obtained. All the initial crystalline HPPs are superconducting metal phases, and the resulting amorphous phases are semiconductors. The amorphization proceeds at temperatures −70 to +70 °C, doess not violate the compactness of the samples, is accompanied by a considerable volume increase and by large exothermal heat evolution.