Homogeneous Strain Deformation Path for the Wurtzite to Rocksalt High-Pressure Phase Transition in GaN

Artificial neural network for deciphering the structural transformation of condensed ZnO by extended x-ray absorption fine structure spectroscopy

Pressure-induced structural phase transitions play a pivotal role in unlocking novel material functionalities and facilitating innovations in materials science. Nonetheless, unveiling the mechanisms of densification, which relies heavily on precise and comprehensive structural analysis, remains a challenge. Herein, we investigated the archetypalB4 →B1 phase transition pathway in ZnO by combining x-ray absorption fine structure (XAFS) spectroscopy with machine learning. Specifically, we developed an artificial neural network (NN) to decipher the extended-XAFS spectra by reconstructing the partial radial distribution functions of Zn-O/Zn pairs. This provided us with access to the evolution of the structural statistics for all the coordination shells in condensed ZnO, enabling us to accurately track the changes in the internal structural parameteruand the anharmonic effect. We observed a clear decrease inuand an increased anharmonicity near the onset of theB4 →B1 phase transition, indicating a preference for the iT phase as the intermediate state to initiate the phase transition that can arise from the softening of shear phonon modes. This study suggests that NN-based approach can facilitate a more comprehensive and efficient interpretation of XAFS under complexin-situconditions, which paves the way for highly automated data processing pipelines for high-throughput and real-time characterizations in next-generation synchrotron photon sources.

Size-Dependent Nucleation in Crystal Phase Transition from Machine Learning Metadynamics

In this Letter, we present a framework that combines machine learning potential (MLP) and metadynamics to investigate solid-solid phase transition. Based on the spectral descriptors and neural networks regression, we develop a scalable MLP model to warrant an accurate interpolation of the energy surface where two phases coexist. Applying it to the simulation of B4-B1 phase transition of GaN under 50 GPa with different model sizes, we observe sequential change of the phase transition mechanism from collective modes to nucleation and growths. When the size is at or below 128 000 atoms, the nucleation and growth appear to follow a preferred direction. At larger sizes, the nuclei occur at multiple sites simultaneously and grow to microstructures by passing the critical size. The observed change of the atomistic mechanism manifests the importance of statistical sampling with large system size in phase transition modeling.

Induced crystallographic changes in Cd1−xZnxO films grown on r-sapphire by AP-MOCVD: the effects of the Zn content when x ≤ 0.5

HRXRD, SEM and TEM techniques were used to investigate crystallographic characteristics of Cd_1−xZn_xO films grown by MOCVD on r-plane sapphire in the transition process from the rock-salt to the wurtzite structure.

Predictions on High-Power Trivalent Metal Pentazolate Salts

High-energy-density materials (HEDMs) have been intensively studied for their significance in fundamental sciences and practical applications. Here, using the molecular crystal structure search method based on first-principles calculations, we have predicted a series of metastable energetic trivalent metal pentazolate salts MN₁₅ (M= Al, Ga, Sc, and Y). These compounds have high energy densities, with the highest nitrogen content among the studied nitrides so far. Pentazolate N₅^- molecules stack up face-to-face and form wave-like patterns in the C222₁ and Cc symmetries. The strong covalent bonding and very weak noncovalent interactions with nonbonded overlaps coexist in these ionic-like structures. We find MN₁₅ molecular structures are mechanically stable up to high temperature (∼1000 K) and ambient pressure. More importantly, these trivalent metal pentazolate salts have high detonation pressure (∼80 GPa) and velocity (∼12 km/s). Their detonation pressures exceeding that of TNT and HMX make them good candidates for high-brisance green energetic materials.

Redistribution of native defects and photoconductivity in ZnO under pressure

Control and design of native defects in semiconductors are extremely important for industrial applications. Here, we investigated the effect of external hydrostatic pressure on the redistribution of native defects and their impact on structural phase transitions and photoconductivity in ZnO. We investigated morphologically distinct rod- (ZnO-R) and flower-like (ZnO-F) ZnO microstructures where the latter contains several native defects namely, oxygen vacancies, zinc interstitials and oxygen interstitials. Synchrotron X-ray diffraction reveals pressure-induced irreversible phase transformation of ZnO-F with the emergence of a hexagonal metallic Zn phase due to enhanced diffusion of interstitial Zn during decompression. In contrast, ZnO-R undergoes a reversible structural phase transition displaying a large hysteresis during decompression. We evidenced that the pressure-induced strain and inhomogeneous distribution of defects play crucial roles at structural phase transition. Raman spectroscopy and emission studies further confirm that the recovered ZnO-R appears less defective than ZnO-F. It resulted in lower photocurrent gain and slower photoresponse during time-dependent transient photoresponse with the synergistic application of pressure and illumination (ultra-violet). While successive pressure treatments improved the photoconductivity in ZnO-R, ZnO-F failed to recover even its ambient photoresponse. Pressure-induced redistribution of native defects and the optoelectronic response in ZnO might provide new opportunities in promising semiconductors.

Different evolutionary pathways from B4 to B1 phase in AlN and InN: metadynamics investigations

Pressure-induced B4-B1 phase transitions of AlN and InN at ambient temperature are systematically investigated using density functional-based metadynamics simulations. A homogeneous deformation path, which is energetically favorable, is through a hexagonal structure for AlN, and through a tetragonal structure for InN. Furthermore, the dynamical stability, instead of the mechanical stability, is crucial to determining the phase-transition paths: the intermediate hexagonal structure can remain stable, whereas the tetragonal structure is always unstable. The B4 phase always shows the direct band gap before the occurrence of structure transition, while the band gap of stable intermediate hexagonal phase is indirect for AlN. Finally, the band gap of the ultimate cubic phase is direct for AlN and indirect for InN, due to the strong p-d repulsion at the R point.

Phase transition induced strain in ZnO under high pressure

Under high pressure, the phase transition mechanism and mechanical property of material are supposed to be largely associated with the transformation induced elastic strain. However, the experimental evidences for such strain are scanty. The elastic and plastic properties of ZnO, a leading material for applications in chemical sensor, catalyst, and optical thin coatings, were determined using in situ high pressure synchrotron axial and radial x-ray diffraction. The abnormal elastic behaviors of selected lattice planes of ZnO during phase transition revealed the existence of internal elastic strain, which arise from the lattice misfit between wurtzite and rocksalt phase. Furthermore, the strength decrease of ZnO during phase transition under non-hydrostatic pressure was observed and could be attributed to such internal elastic strain, unveiling the relationship between pressure induced internal strain and mechanical property of material. These findings are of fundamental importance to understanding the mechanism of phase transition and the properties of materials under pressure.

Phase transition and band-structure tuning in InN through uniaxial and biaxial strains

The phase transitions and band structure of InN under uniaxial and biaxial strains are systematically investigated using first-principles calculations. The main findings are summarized as follows: (I) although graphite-like phases are observed for both types of strain, the phase transitions are drastically different: second order for uniaxial strain and first order for biaxial strain. Furthermore, the second-order transition is driven by elastic and dynamical instabilities, whereas the first-order transition is driven only by elastic instability. (II) The wurtzite bandgap is always direct and that of the graphite-like phase is always indirect. Furthermore, the wurtzite bandgap is drastically enhanced by compressive uniaxial strain but reduced by tensile uniaxial strain. However, both biaxial strains greatly reduce the bandgap and eventually the semi-metallic phases are achieved.

Hybrid density functional theory studies of AlN and GaN under uniaxial strain

The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).

Ideal strengths, structure transitions, and bonding properties of a ZnO single crystal under tension

We perform a first-principles computational tensile test (FPCTT) on a ZnO single crystal based on density functional theory to systematically investigate structural transitions, mechanical, and intrinsic bonding properties in the three representative directions, [Formula: see text], [0001], and [Formula: see text]. Stress as a function of tensile strain shows that the ideal tensile strengths in the three directions are 16.2 GPa, 22.4 GPa, and 19.0 GPa, corresponding to strains of 0.20, 0.16, and 0.16, respectively. The [0001] is the strongest direction due to the strongest bonding between the most closely packed Zn and O(0001) layers. We demonstrate that different structures in these three directions lead to different structural transitions, i.e. from a wurtzite (WZ) to a body-centered tetragonal (BCT) structure for [Formula: see text], to a graphite-like (GP-like) structure for [0001], and to a quasi-hexagonal (quasi-HX) structure for [Formula: see text], respectively. Bond length and charge density evolution under tension indicate the occurrence of bond formation and disassociation during these structure transitions. New O-Zn bonds form in the WZ [Formula: see text] BCT and WZ [Formula: see text] quasi-HX transitions, and the original O-Zn bonds break in the WZ [Formula: see text] GP-like transition.

Wurtzite-type CoO nanocrystals in ultrathin ZnCoO films

Surface x-ray diffraction experiments reveal that, in cobalt-doped ZnO films two to five monolayers thick, Wurtzite-type CoO nanocrystals are coherently embedded within a hexagonal boron-nitride- (h-BN)-type ZnO matrix, supporting the model of a phase separation. First-principles calculations confirm that, in contrast with ZnO, the formation of h-BN-type CoO is unfavorable in the ultrathin film limit. Our results are important for understanding magnetic properties of transition metal-doped semiconductors in general.

Walking the path from B4- to B1-type structures in GaN

Molecular dynamics simulations are performed on the wurtzite-type structure (B4) to the rocksalt-type structure (B1) pressure-induced phase transition in GaN. From this, a nucleation and growth mechanism through a tetragonal metastable configuration is found. An intermediate of h-MgO type structure suggested from static calculations is ruled out. However, the pathway through the tetragonal intermediate may be altered by defect incorporation. While the overall transformation mechanism is preserved for both vacancies and Ga substitution by indium, already a 5% aluminum substitution establishes a transition route which avoids the tetragonal structure. Changes in the transformation mechanism and the resulting stabilization of the previously metastable high-pressure modification is elaborated by tracing the interplay of phase nucleation and growth and defect incorporation.

First-principles study of the wurtzite-to-rocksalt phase transition in zinc oxide

Wurtzite (B4) compounds commonly undergo a structural transition to the rocksalt (B1) phase under high pressure. The underlying transition mechanism, or the so-called transition path, has been extensively investigated in recent years. Two different transition paths have been proposed for the B4-B1 phase transition, that is the 'hexagonal' path and the 'tetragonal' path. In this work, taking zinc oxide (ZnO) as an example, we have made a comparative study of these two paths from first-principles. The calculated results lead to the conclusion that the tetragonal path is more favourable under lower pressure but the hexagonal path is more favourable under higher pressure, which indicates a competition between these two paths in the ZnO case. We have also investigated the evolution of structural and electronic properties along the two different paths; the axial ratio c/a is suggested as a good indicator in experiments to identify the transition path.

Novel phase transformation in ZnO nanowires under tensile loading

We predict a previously unknown phase transformation from wurtzite to a graphitelike (P6(3)/mmc) hexagonal structure in [0110]-oriented ZnO nanowires under uniaxial tensile loading. Molecular dynamics simulations and first principles calculations show that this structure corresponds to a distinct minimum on the enthalpy surfaces of ZnO for such loading conditions. This transformation is reversible with a low level of hysteretic dissipation of 0.16 J/m3 and, along with elastic stretching, endows the nanowires with the ability to recover pseudoelastic strains up to 15%.

Mechanisms of the wurtzite to rocksalt transformation in CdSe nanocrystals

We study the pressure-driven phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure in CdSe nanocrystals with molecular dynamics computer simulations. With an ideal gas as the pressure medium, we apply hydrostatic pressure to spherical and faceted nanocrystals ranging in diameter from 25 to 62 A. In spherical crystals, the main mechanism of the transformation involves the sliding of (100) planes, but depending on the specific surface structure we also observe a second mechanism proceeding through the flattening of (100) planes. In faceted crystals, the transition proceeds via a five-coordinated hexagonal structure, which is stabilized at intermediate pressures due to dominant surface energetics.

Simulation of the pressure-driven wurtzite to rock salt phase transition in nanocrystals

Nanocrystals in the size range 12-21 nm of a model binary ionic material in the wurtzite (B4) structure were constructed with morphologies which minimize the surface energy. These were then embedded in a pressurization medium, consisting of a binary Lennard-Jones-type fluid and progressively pressurized in "constant pressure" molecular dynamics simulation runs. Phase transitions to the rocksalt (B1) phase were confirmed by examining calculated powder diffraction patterns, which show the same changes in features as seen for experimental systems. By directly observing the atomic trajectories throughout the duration of the transition the local mechanism has been identified. The transition proceeds via a trigonal bipyramidal intermediate, denoted as the h-MgO structure. It is initiated by a single nucleation event at a [1120]B4 surface with subsequent growth of the B1 region throughout the remainder of the nanocrystal. The consequences of this mechanism for the particle shape of the product phase are detailed and contrasted with those previously found for initially zincblende (B3) structured nanoparticles, using the same interaction potential. The observed transition pressures are elevated relative to the thermodynamically predicted pressure for the bulk, but there is no observable system size effect on the transition pressure across the size range of nanocrystals investigated.

Phase Equilibria and Transition Mechanisms in High-Pressure AgCl by Ab Initio Methods

The theoretical study of pressure-driven phase transformations by means of ab initio quantum mechanical methods, in the frame of the extended Landau approach, is considered. A specific application to AgCl is presented: the system shows, on increasing pressure, four polymorphs with rock salt- (Fmm), KOH- (P2(1)/m), TlI- (Cmcm), and CsCl- (Pmm) type structures. The method of constant-pressure enthalpy minimization was used for all phases, by fully relaxing the corresponding crystal structures. Periodic ab initio energy calculations were performed by the CRYSTAL03 code, employing a DFT-GGA-PBE functional with a localized basis set of Gaussian-type functions. The three phase transitions were predicted to occur at 3.5, 6.0, and 17.7 GPa, respectively, against pressures of 6.6, 10.8, and 17 GPa from literature experimental results. The rock salt- to KOH-type and KOH- to TlI-type displacive transformations show a weak first-order character. The TlI- to CsCl-type reconstructive transition is sharply first-order, and its kinetic mechanism was studied in detail on the basis of a P2(1)/m pathway, similar to that previously found for the rock salt- to CsCl-type transformation of NaCl. An activation enthalpy of 0.011 eV was found at the equilibrium pressure of 17.7 GPa.

Orthorhombic intermediate state in the zinc blende to rocksalt transformation path of SiC at high pressure

The mechanism of the B3/B1 phase transition of SiC has been investigated by periodic LCAO-DFT least-enthalpy calculations. A new transformation pathway, based on a Pmm2 orthorhombic intermediate state with two SiC units per cell, is found to be energetically favored over the traditional R3m mechanism. The computed activation enthalpy is 0.75 eV/SiC unit at the predicted transition pressure of 92 GPa (B3LYP functional). Activation enthalpy and activation volume vs pressure are analyzed to characterize the kinetic aspects of the transformation.