Structural Phase Diagram Research Articles

Evidence of correlated incommensurate structural and magnetic order in highly oxygen-doped layered nickelate Nd2NiO4.23

$\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ is a nonstoichiometric oxide, crystallizing in the Ruddlesden-Popper-type framework and exhibiting a wide range of oxygen nonstoichiometry $\ensuremath{\delta}$ with a complex structural phase diagram. Excess oxygen atoms insert holes, directly influencing the Ni valence state and associated electronic structure. We report here the existence of a correlated incommensurate structural and antiferromagnetic order below $T{}_{N}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 150 K in highly oxygen-doped $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ with $\ensuremath{\delta} \ensuremath{\approx}$ 0.23. The crystal and magnetic structures are investigated by polarized and unpolarized single-crystal neutron diffraction studies together with synchrotron x-ray powder diffraction and macroscopic magnetization measurements. Due to the long-range ordering of excess oxygen atoms, the real structure of the compound is incommensurately modulated, represented by $\mathbf{Q}{}_{n}=\ifmmode\pm\else\textpm\fi{}0.813{\mathbf{a}}^{*}\ifmmode\pm\else\textpm\fi{}0.519{\mathbf{b}}^{*}$ wave vectors. The antiferromagnetic order is characterized by the presence of incommensurate Bragg peaks of type ($h\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}\ensuremath{\varepsilon}$, 0, $l$/2), with $h$ and $l$ being odd-integer numbers and the magnetic incommensurability $\ensuremath{\varepsilon}\ensuremath{\approx}$ 0.36. The in-plane magnetic correlation length is found to be $\ensuremath{\xi}{}_{ab}$ = 184(1) \AA{} but is significantly reduced along the $c$ axis to $\ensuremath{\xi}{}_{c}$ = 39(2) \AA{}, indicating the quasi-two-dimensional nature of the antiferromagnetic correlations. Our study indicates that the excess oxygen atoms nevertheless strongly enhance the magnetic correlations between the ${\mathrm{NiO}}_{2}$ planes, resulting in a doubling of the magnetic unit cell along the $c$ axis. In contrast to $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4.1}$, in which the incommensurate periodicity of oxygen and magnetic ordering was found to be identical, the structural and electronic orderings in heavily oxygen-doped ${\mathrm{Nd}}_{2}{\mathrm{NiO}}_{4.23}$ show two independent modulation vectors. Our study thus unravels a strong correlation between structural and electronic orderings in the highly oxygen-doped $\mathrm{Ln}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ system.

Orbital structure and magnetic phase diagram of the four-sublattice ferromagnet PbMnBO4

A role of quadric and quartic single-ion anisotropy (SIA) of Mn3+-ions, dipole–dipole and Dzyaloshinsky–Moriya (DM) interactions for the magnetic ordering of the four-sublattice ferromagnet PbMnBO4 is investigated. A phase diagram of low-lying magnetic phases on the plane of the DM exchange components is obtained for different values of the SIA parameters. An effect of the isotropic interchain exchange on the spin orientation and, as a result, on the SIA energy in different magnetic phases is considered at the comparable values of the interactions. The determinative role of DM exchange for the magnetic anisotropy of PbMnBO4 is discussed.

First-principles study of thermoelasticity and structural phase diagram of CaO

We present pressure $(P)$ and temperature $(T)$ variations of elastic and anisotropic properties, solid-solid (rocksalt, B1 to cesium chloride, B2) and solid-liquid structural phase transitions of CaO. We employed first-principles density functional theory supplemented with an anharmonic contribution to phonon dynamics. Good agreement is obtained for all properties up to the pressure and temperature range relevant to the Earth's mantle and outer core. Born elastic criteria are generalized for arbitrary stress to elaborate the structural phase diagram from a thermoelastic viewpoint. We propose a self-consistent computational scheme to incorporate the effect of thermal hysteresis into Lindemann's melting law. This improvised melting law exhibits quantitative agreement with reported findings for the high-$P$ melting curve. We find the triple point (i.e., the coexistence of B1, B2, and liquid phases) at 23 GPa and 4600 K. By examining the pressure-term included elastic properties, we can show that the solid-solid transition is mechanical in a pressure range of 0--200 GPa and temperature up to 3000 K. The softening of shear elastic constant ${C}_{44}$ drives the B1-B2 phase transition. This assertion is corroborated by examining the phonon dispersion curve and mode Gr\"uneisen parameter at different pressures for both B1 and B2 phases. The solid-liquid phase boundary can be treated accurately through the temperature-dependent thermodynamic Gr\"uneisen parameter. Furthermore, in this paper, we predict a negative melting slope $>140\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ with a peak temperature of 7800 K, suggesting a smaller molar volume on the liquid side than that of the solid phase. This finding is supported by electronic band structure calculation. It is proposed that the hybridization of the empty $3d$ band of the cation with $s$ orbitals lowers the conduction band to cross the Fermi energy at the Brillouin zone center and leads the insulator-to-metal/semimetal phase transition $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Different electronic states on solid and liquid sides make the liquid phase more compressible than the solid phase, eventually reducing the melting temperature with pressure.

PETROGRAPHY AND THERMODYNAMIC REACTIONS OF SOME OF THE ROCKS IN THE BANKET SERIES OF THE TARKWAIAN SUPERGROUP

The Banket Series, which is a paleoplacer deposit, is the third formation in the stratigraphy of Tarkwaian Supergroup. This project seeks to study petrographically; the lithologies in the banket series to confirm the rocks identity as already known or state otherwise. Two different photomicrographs of each of the rock samples, coupled by their petrographic descriptions were obtained from the laboratory. The main areas of our arguments were based in Bowen’s reaction series, metamorphic facies of the rocks, the earth structure and thermodynamic phase diagrams. The sampled rocks can be confirmed with the help of thin sections and mineral facies to be conglomerate, dolerite and quartzite. The minerals in the banket series range in temperature between 200°C and 800°C. The abundance of silica (SiO2) in the rocks conglomerate, dolerite and quartzite are of crustal origin, whereas pyroxene and olivine originate from the upper mantle. The common stable minerals in the studied rocks, namely, amphibole, chlorite and plagioclase are less likely to change compared to the other minerals in the rocks. The three (3) different metamorphic facies of the rocks conglomerate, dolerite and quartzite are respectively greenschist, epidote-amphibolite and amphibolite facies.

Investigation of thermal control in phase-changing ABO3 perovskites via first-principles predictions: general mechanism of solar absorptivity.

The fundamental mechanism of solar absorbance during the phase-change process is investigated in ABO3 perovskites based on first-principles predictions. A Gaussian-like relationship between the solar absorbance and band gaps is established, which follows the Shockley-Queisser limiting efficiency. For ABO3 perovskites with bandgaps of Eg > 3.5 eV, a low solar absorbance is obtained, whereas a high solar absorbance is obtained for ABO3 perovskites, with band gaps ranging from 0.25 to 2.2 eV. The relationship between the orbital character of the density of states (DOS) and the absorption spectra reveals that ABO3 perovskites with magnetic (strongly interacting) and distorted crystal structures always exhibit a higher solar absorptivity. In contrast, non-magnetic and cubic ABO3 perovskites always exhibit a lower solar absorptivity. Moreover, the tunable solar absorptivity always undergoes a phase change from cubic to large distorted crystal structures in ABO3 perovskites with strong interactions. These results can be attributed to a rich structural, electronic, and magnetic phase diagram resulting from the strong interplay between the lattice, spin, and orbital degrees of freedom, which induce highly tunable optical characteristics in the phase-change process. The findings presented in this study are critical for the development of ABO3 perovskite-based smart thermal control materials in the spacecraft field.

Ab initio structural optimization at finite temperatures based on anharmonic phonon theory: Application to the structural phase transitions of BaTiO3

We formulate a first-principle scheme for structural optimization at finite temperature ($T$) based on the self-consistent phonon (SCP) theory, which accurately takes into account the effect of strong phonon anharmonicity. The $T$ dependence of the shape of the unit cell and internal atomic configuration is determined by minimizing the variational free energy in the SCP theory. At each optimization step, the interatomic force constants in the new structure are calculated without running additional electronic structure calculations, which makes the method dramatically efficient. We demonstrate that the thermal expansion of silicon and the three-step structural phase transitions in ${\mathrm{BaTiO}}_{3}$ and its pressure-temperature ($p\text{\ensuremath{-}}T$) phase diagram are successfully reproduced. The present formalism will open the way to the nonempirical prediction of physical properties at finite $T$ of materials having a complex structural phase diagram.

High-pressure investigations in CH3NH3PbX3 ( X = I, Br, and Cl): Suppression of ion migration and stabilization of low-temperature structure

Hybrid organic-inorganic halide perovskites represent a promising next-generation photovoltaic material with drawbacks for structure stability and composition concerns. Demonstrations of ion migration and molecular dynamics suggest room for structural contraction and subsequent property adjustments. Here, we deploy dielectric and infrared spectroscopy under external pressure to probe the full structural phase diagram and dielectric response of methylammonium lead halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{X}_{3}$ ($X\phantom{\rule{4pt}{0ex}}=$ I, Br, or Cl). Ion migration can be fully suppressed by pressure beyond 4 GPa. The low-temperature orthorhombic phase transition can be gradually enhanced and stabilized at ambient conditions with increasing pressure. A slow relaxation mode, presumably the motion of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cation, is observed at lower pressure and is absent in the orthorhombic phase for every halide.

Stable nitrogen-rich yttrium nitrides under high pressure

Polynitrides have excellent physical and chemical properties as well as have drawn wide attention. Through the first-principles calculations and CALYPSO structure searching approach, we calculated the overall structure of YN components (from 0.5 to 10) and established the pressure versus composition diagram and pressure versus structure phase diagram of the YN system. This paper focuses on the YN2 structure (space group P21) which can be stable at 0 GPa and the YN10 structure with the highest nitrogen content (space group Imm2) with the intriguing chain-like N10 cage structure appears. Under ambient conditions, the YN10 structure has a high energy density (9.33 kJ g−1). By calculating the phonon dispersion curve, the structure has better dynamic stability at 250 GPa. Seven structures (Y2N, YN, YN2, YN3, YN5, YN8, YN10) that appear on convex hull graphs in all YN systems are also calculated. Their dynamics, mechanics and related electronic properties are described in the appendix. We hope that our theoretical research can facilitate future experimental realization and help to advance the research on nitrogen chains.

Mixology of MA1-x EA x PbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics, and Photoluminescence.

Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)-ethylammonium (EA) MA1-x EA x PbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.

Strain effect on the ground-state structure of Sr2SnO4 Ruddlesden-Popper oxides

Ruddlesden-Popper (RP) oxides (${A}_{n+1}{B}_{n}{\mathrm{O}}_{3n+1}$) comprised of perovskite ${(AB{\mathrm{O}}_{3})}_{n}$ slabs can host a wider variety of structural distortions than their perovskite counterparts. This makes an accurate structural determination of RP oxides more challenging. In this study, we investigate the structural phase diagram of $n=1$ RP ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$, one of alkaline earth stannates that are promising for opto-electronic applications by using group theory based symmetry analysis and first-principles calculations. We explore the symmetry breaking effects of different dynamical instabilities, predict the energies of phases they lead to, and take into account different (biaxial strain and hydrostatic pressure) boundary conditions. We also address the effect of structural changes on the electronic structure and find that compressive biaxial strain drives ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$ into a regime with wider band gap and lower electron effective mass.

Formation of One-Dimensional van der Waals Heterostructures via Self-Assembly of Blue Phosphorene Nanoribbons to Carbon Nanotubes

Van der Waals heterostructures composed of low-dimensional atomic layers host rich physics for new device applications, such as magic-angle twisted bilayer graphene and coaxial multi-walled hetero-nanotubes. Aside from exploring their abnormal physical behavior, fabrication of such structures also presents a great challenge to this area, owing to the subtle and sensitive interactions among neighboring layers. Here we show by molecular dynamics simulations that narrow blue phosphorene nanoribbons can be encapsulated into carbon nanotubes driven by van der Waals interactions and form one-dimensional heterostructures. It shows that by varying carbon nanotube diameters and nanoribbon width, the nanoribbons can either retain their original straight structures or twist into tubular structures. Wrapping phases are also observed for large-sized blue phosphorus. It is found that the underlying mechanism originates from the competition between van der Waals energy and bending energy induced by tube curvature. A phase diagram of the resultant 1D structure is thus obtained based on a simple analysis of energetics. The results are expected to stimulate further experimental efforts in fabricating one-dimensional van der Waals heterostructues with desired functionality.

Crystal Prediction via Genetic Algorithms in a Model Chiral System.

Chiral crystals and their constituent molecules play a prominent role in theories about the origin of biological homochirality and in drug discovery, design, and stability. Although the prediction and identification of stable chiral crystal structures is crucial for numerous technologies, including separation processes and polymorph selection and control, predictive ability is often complicated by a combination of many-body interactions and molecular complexity and handedness. In this work, we address these challenges by applying genetic algorithms to predict the ground-state crystal lattices formed by a chiral tetramer molecular model, which we have previously shown to exhibit complex fluid-phase behavior. Using this approach, we explore the relative stability and structures of the model's conglomerate and racemic crystals, and present a structural phase diagram for the stable Bravais crystal types in the zero-temperature limit.

Influence of depolarization field on temperature-misfit strain phase diagram of PbZr0.4Ti0.6O3 ultrathin films

Phase-field simulations are performed to study domain structures in (001)-oriented PbZr0.4Ti0.6O3 epitaxial thin films. Temperature-misfit strain phase diagrams of the domain structures are constructed under various depolarization fields by adjusting surface screening conditions and film thickness. We find that the vortex-array state area in the phase diagram enlarges with the decrement of the depolarization field, which is contrary to the general view of depolarization field conductive to vortex formation. Moreover, a strain-mediated ferroelectric failure mechanism is found, wherein the ferroelectric characteristics of an ultrathin film with a thickness of 4 nm completely disappear (Curie temperature drops to 0 K) under the open-circuit condition when the compressive misfit strain drops to −2.69%. The results guide further understanding and manipulation of domain structures in ferroelectric nanomaterials.

Metamagnetism and tricritical behavior in the Kondo lattice model

The phase diagram of the Kondo lattice model is explored around the ferromagnetic region, including an external magnetic field, and allowing the coexistence of ferromagnetism and Kondo effect ($\mathrm{FM}+\mathrm{K}$). Considering the combined effect of temperature and pressure, the evolution of the $\mathrm{FM}+\mathrm{K}$ phase yields a wing structure phase diagram with a tricritical point and a ferromagnetic quantum critical end point. The evolution of the spin-selective Kondo insulator phase, inside the coexistence region, introduces another critical end point. The results show a correspondence with the experimental phase diagram of heavy-fermion compounds, in particular, the ferromagnetic superconductor ${\mathrm{UGe}}_{2}$.

Comprehensive Screening of Halogen-Containing Oxide Double Perovskites A2BXO6 (X = Cl, Br, and I) for Photovoltaic Applications.

The new halogen-containing oxide double perovskites A2BXO6 (X = Cl, Br, and I) have attracted much attention because of their superb electronic properties in halide double perovskites and their high stability in oxide double perovskites. Herein, 408 A2BXO6 double perovskites have been systematically screened by high-throughput computation. Refer to the empirical structural factors phase diagram (t-u), which uses large-scale first-principles calculations. Fourteen stable perovskites are finally confirmed; moreover, 11 of them have never been reported before. Our results show that Ba2AgIO6 and Sr2AgIO6 are the most preferable candidates for photovoltaic applications, of which Sr2AgIO6 has balanceable electron and hole effective masses, a quasi-direct band gap, and strong optical absorption. Importantly, Sr2AgIO6 was successfully synthesized by the solution method. Our work enriches the family of double perovskites, and the tentative experimental evidence undoubtedly hints at their great potential applications in the near future.

Structural phase-field crystal model for Lennard–Jones pair interaction potential

A modification of the structural phase-field crystal model for a Lennard–Jones (LJ) pair interaction potential is presented. Formation of 1D and 2D structures for the LJ-potential was studied numerically. The equilibrium lattice parameters for the obtained structures were found consistent to the correspondent LJ-distance parameters. The lattice parameter of 2D triangle’s structure matches the periodical in 1D, which is consistent to the theory of freezing from the isotropic liquids. Numerically obtained phase diagram of two-dimensional structures qualitatively reproduces classical PFC diagram and coincides with the melting region of high-temperature part of LJ diagram.

Application of machine learning to reflection high-energy electron diffraction images for automated structural phase mapping

We have developed a phase mapping method based on machine learning analysis of reflection high-energy electron diffraction (RHEED) images. RHEED produces diffraction patterns containing a wealth of static and dynamic information and is commonly used to determine the growth rate, the growth mode, and the surface morphology of epitaxial thin films. However, the ability to extract quantitative structural information from the RHEED patterns that appear during film growth is limited by the lack of versatile and automated analysis techniques. We have created a deep learning-based analysis method for automating the identification of different RHEED pattern types that occur during the growth of a material. Our approach combines several supervised and unsupervised machine learning techniques and permits the extraction of quantitative phase composition information. We applied this method to the mapping of the structural phase diagram of ${\mathrm{Fe}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}$ thin films grown by pulsed laser deposition as a function of growth temperature and oxygen pressure close to the hematite-magnetite phase boundary. The in situ RHEED-based mapping method produces results that are qualitatively similar to postsynthesis x-ray diffraction analysis.

Magnetic, structural and magnetocaloric properties of Y0.9Gd0.1Fe2Hx hydrides

At 300 K, Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$H$_{x}$ hydrides crystallize sequentially with increasing H concentration in various structures related to a lowering of the cubic MgCu$_{2}$ type structure of the parent alloy: cubic C1, monoclinic M1, cubic C2, monoclinic M2, cubic C3, orthorhombic O. Above 300 K, they undergo a first-order transition at a T$_{O-D}$ temperature driven by order-disorder of hydrogen atoms into interstitial sites. Their magnetic, structural and magnetocaloric properties have been investigated through magnetic measurements, and high-resolution synchrotron diffraction experiments. The magnetization at 5 K decreases slightly from 4 to 3.8 ${\mu}_{B}$ for x = 3 to 3.9 H f.u., then with a larger slope for higher H content. A discontinuous decrease of the magnetic transition temperature is observed: M1 and C2 hydrides are ferrimagnetic with T$_{C}$ near 300 K, M2 hydride displays a sharp ferromagnetic-antiferromagnetic transition at T$_{FM-AFM}$ =144 K, whereas C3 and O hydrides present only a sharp increase of the magnetization below 15 K and a weak magnetization up to RT. Negative magnetic entropy variations (${\Delta}$S$_{M}$) are measured near T$_{C}$ for the M1 and C2 phases, near T$_{FM-AFM}$ for the M2 phase, whereas positive ${\Delta}$S$_{M}$ peaks due to inverse MCE effect are found near T$_{O-D}$. A structural and magnetic phase diagram is proposed.

High Chern number phase in topological insulator multilayer structures: A Dirac cone model study

We employ the Dirac cone model to explore the high Chern number (C) phases that are realized in the magnetic-doped topological insulator (TI) multilayer structures by Zhao et al. [Nature 588 419 (2020)]. The Chern number is calculated by capturing the evolution of the phase boundaries with the parameters, then the Chern number phase diagrams of the TI multilayer structures are obtained. The high-C behavior is attributed to the band inversion of the renormalized Dirac cones, along with which the spin polarization at the Γ point will get increased. Moreover, another two TI multilayer structures as well as the TI superlattice structures are studied.

Ferromagnetic-antiferromagnetic Competition in Diluted Magnetic Semiconductors

The signature of the magnetic competition in a nearly filled impurity band diluted magnetic semiconductors is addressed. In the formalism of the Kondo lattice model, self-consistent equations determining the Green function of the itinerant carrier in the system are evaluated by use of the dynamical mean-field theory. An analytical solution of the static spin susceptibility of the itinerant carrier to specify the magnetic instability is then established. Once the impurity band is nearly filled, one finds the antiferromagnetic instability against the ferromagnetic state. Phase diagrams of the magnetic structures in the diluted magnetic semiconductor were also discussed.&#x0D;