Phase Diagram Of Crystals Research Articles

Ab initio phase diagram of PbSe crystals calculated with the random phase approximation

Understanding the phase behavior of semiconductor materials is important for applications in solid state physics and nanoscience. Accurate experimental data is often difficult to obtain due to strong kinetic effects. In this work, we calculate the temperature-pressure phase diagram of lead selenide (PbSe) using the random phase approximation (RPA), an accurate wavefunction based many-body technique. We consider three crystalline phases, the low pressure B1 phase (NaCl-type), the intermediate B33 phase (CrB-type), and the high pressure B2 phase (CsCl-type). The electronic contributions to the free energy (at T=0K) are calculated in the Born-Oppenheimer approximation using the RPA, whereas phononic contributions are computed in the quasi-harmonic approximation using DFT and the PBEsol functional. At room temperature, we find transition pressures of 4.6 +/- 0.3 GPa for the B1-B33 transition and 18.7 +/- 0.3 GPa for the B33-B2 transition, in good agreement with experiments. In contrast to the interpretation of recent experiments, we observe a negative Clapeyron slope for both transitions. Gibbs free energy differences between competing structures have small gradients close to coexistence, consistent with pronounced hysteresis observed in experiments. The phase diagram presented in this work can serve as a reference for future studies of PbSe and should prove useful in the development of accurate and efficient force fields.

Structural and Superconducting Properties of Tungsten Hydrides Under High Pressure

Unveiling the relation between crystal structure and superconductivity of hydrides becomes a fascinating research area in chemistry and condensed-mater physics. Although much efforts have been made to study chemical reaction between tungsten and hydrogen, the crystal structures, superconductivity, and phase diagram of tungsten hydrides under high pressure have not been fully explored and built thus far. In this work, we carried out extensive structural search on W-H binary compounds through first-principles swarm-intelligence structural search calculations. Besides reproducing the known W-H compounds, a new stoichiometry WH5 with P6mm symmetry becomes stable above 230.2 GPa. Intriguingly, P6mm WH5 shows a critical temperature (Tc) value of 60.8 K, which is much higher than 31.6 K in WH6. This finding is different from the knowledge of compounds with higher H content exhibiting higher Tc, which might be attributed to the appearance of unique H network and tetrahedron H units in WH5. Electronic property and superconductivity of the other tungsten hydrides are also investigated. The built pressure-composition phase diagram provides some useful information for experimental synthesis.

Forced Convection Evaporation for Bulk Protein Crystallization

Forced convection evaporation was adopted for bulk protein crystallization. As a model system ovalbumin in aqueous solutions of ammonium sulfate was used. The crystallization process was performed in a drying chamber, where the protein solution was contacted with a warm air stream at a temperature varied from 35 to 52 °C. The slow evaporation of water induced gradual crystallization of the protein. Heat and mass transfer in the water–air system during water evaporation caused the temperature of the protein solution to remain distinctly below the air temperature. This allowed processing the protein under mild conditions. The process design was guided by crystallization phase diagram and kinetic measurements. The supersaturation degree, which determines the nucleation and crystal growth rates, was adjusted by the rate of water evaporation. The latter was precisely controlled by matching the temperature in the chamber to the air humidity. The process could be accomplished in a single crystallization stage in...

Experimental and theoretical aspects of thermodynamic properties of quasi-1D and quasi-2D organic conductors and superconductors

We deal with thermodynamic features of organic conductors and superconductors where itinerant [Formula: see text]-electrons/holes released from organic molecules are playing essential roles for electronic properties. Since they are low-dimensional electronic systems with relatively soft lattice framework, they show variety of phenomena related to electron correlations and electron–lattice coupling. The drastic changes of conductive and magnetic properties owing to quantum features of [Formula: see text]-electrons can be induced by external perturbations such as magnetic/electric field, pressure, etc. It is especially emphasized that the possible mechanism and relation with other phenomena of the superconductivity in [Formula: see text]-electrons system remains to be one of the interesting research areas in fundamental condensed matter science. In this review paper, we consider several topics of organic conductors and superconductors from the standpoints of thermodynamic experiments, data analyses and theories performed up to now. Starting from the overall picture of the electronic states in charge transfer complexes, thermodynamic properties of the quasi-one-dimensional systems, quasi-two-dimensional systems and [Formula: see text]–d interacting systems are reviewed. The thermodynamic parameters of the superconductive compounds in them are compared and discussed. The relations with crystal structures, electronic states, phase diagram and other experiments are also discussed in comparison with these thermodynamic properties. The possible pairing symmetries in organic superconductors and some models are mentioned in the last part. This review deals with a wide scope of theoretical and experimental topics in superconductivity in molecule-based conductive systems.

Layered Indium Selenide under High Pressure: A Review

This paper intends a short review of the research work done on the structural and electronic properties of layered Indium Selenide (InSe) and related III–VI semiconductors under high pressure conditions. The paper will mainly focus on the crucial role played by high pressure experimental and theoretical tools to investigate the electronic structure of InSe. This objective involves a previous revision of results on the pressure dependence of the InSe crystal structure and related topics such as the equation of state and the pressure-temperature crystal phase diagram. The main part of the paper will be devoted to reviewing the literature on the optical properties of InSe under high pressure, especially the absorption experiments that led to the identification of the main optical transitions, and their assignment to specific features of the electronic structure, with the help of modern first-principles band structure calculations. In connection with these achievements we will also review relevant results on the lattice dynamical, dielectric, and transport properties of InSe, as they provided very useful supplementary information on the electronic structure of the material.

Ultrasonic Irradiation and Seeding To Prevent Metastable Liquid–Liquid Phase Separation and Intensify Crystallization

During the crystallization of complex pharmaceutical molecules, a liquid–liquid phase (LLP) separation phenomenon may occur that could hinder crystallization processes and adversely affect the crystal quality and process robustness. In this study, the LLP separation behavior of a vanillin and water mixture was investigated using a hot-stage microscope and a cooling crystallization process. Thermodynamic stability of phases and the crystallization phase diagram including the metastable zone width, nucleation, and LLP separation were developed. The impact of ultrasound and seeding on LLP separation was investigated and used to optimize the crystallization process. Our results show that the LLP separation may exist in both the stable and metastable zones of the crystallization phase diagram. Ultrasound can effectively promote nucleation, narrow the metastable zone of LLP separation, and inhibit LLP separation within the concentration range of 3.8–4.8 g/100 g water. Moreover, ultrasonic crystallization was op...

Single crystal growth and characterization of the 112-type iron-pnictide EuFeAs2

The 112-type (Ca, RE)FeAs2 (RE=rare earth) superconductors are very special among the iron-based superconductors for their particular crystal structures with arsenic chain configuration and attractive electronic phase diagram with the coexistence of superconductivity and antiferromagnetism upon carrier doping, while the chemical phases are absent for the low doping level or undoped parent compound. Here we report the single crystal growth method and physical characterizations for the newly discovered Eu 112 type parent compound EuFeAs2. The single crystal of EuFeAs2 is grown by high temperature solution method through using CsCl as the flux under the constant temperature of 800℃ with the molar ratio of the starting materials Eu:Fe:As:CsCl=1:1:4:18. The as-grown crystal is shinyplatelike piece with a typical size of 1 mm1 mm0.2 mm, and quite stable in air. The chemical composition of EuFeAs2 crystal is confirmed by energy-dispersive X-ray spectroscopy. The single crystal X-ray diffraction analysis at room temperature indicates that EuFeAs2 crystallizes into an orthorhombic crystal structure with the space group Imm2 (No. 44), and the refined lattice parameters are a=21.285(9) , b=3.9082(10) , c=3.9752(9) , which are different from those of the Ca 112 compound, but similar to those of unique zigzag As-As chain configuration presented in the layered crystal structure. Electrical resistivity measurements show three anomalies near 110 K, 98 K, and 46 K. The former two anomalies with relatively high temperature imply that the structural and antiferromagnetic transitions are related to Fe2+ sublattice, which is similar to other iron-based parent compounds. The low temperature anomaly at 46 K is attributed to the antiferromagnetic transition of Eu2+ sublattice, which is also confirmed by the corresponding transition observed in the direct current magnetic susceptibility measurement. The magnetic susceptibility of EuFeAs2 exhibits obvious anisotropy blow 46 K when the magnetic field is parallel or perpendicular to the bc plane, while the exact orientation of the Eu2+ moment needs further studying. The discovery of EuFeAs2 provides a new platform for further studying the unique crystal structure and electronic state phase diagrams in the 112-type iron-based superconducting family, and may shed new light on the correlations between superconductivity and magnetism.

Ab initio prediction of the polymorph phase diagram for crystalline methanol.

Organic crystals frequently adopt multiple distinct polymorphs exhibiting different properties. The ability to predict not only what crystal forms might occur, but under what experimental thermodynamic conditions those polymorphs are stable would be immensely valuable to the pharmaceutical industry and others. Starting only from knowledge of the experimental crystal structures, this study successfully predicts the methanol crystal polymorph phase diagram from first-principles quantum chemistry, mapping out the thermodynamic regions of stability for three polymorphs over the range 0-400 K and 0-6 GPa. The agreement between the predicted and experimental phase diagrams corresponds to predicting the relative polymorph free energies to within ∼0.5 kJ mol-1 accuracy, which is achieved by employing fragment-based second-order Møller-Plesset perturbation theory and coupled cluster theory plus a quasi-harmonic treatment of the phonons.

Magnetic Properties and Magnetic Phase Diagrams of Trigonal DyNi3Ga9

We report the crystal structure, magnetic properties, and magnetic phase diagrams of single crystalline DyNi3Ga9 studied using X-ray diffraction, electrical resistivity, specific heat, and magnetization measurements. DyNi3Ga9 crystallizes in the chiral structure with space group R32. The dysprosium ions, which are responsible for the magnetism in this compound, form a two-dimensional honeycomb structure on a (0001) plane. We show that DyNi3Ga9 exhibits successive phase transitions at TN = 10 K and \(T'_{\text{N}} = 9\) K. The former suggests quadrupolar ordering, and the latter is attributed to the antiferromagnetic order. It is considered that DyNi3Ga9 forms the canted-antiferromagnetic structure below \(T'_{\text{N}}\) owing to a small hysteresis loop of the low-field magnetization curve. We observe the strong easy-plane anisotropy, and the multiple-metamagnetic transitions with magnetization-plateaus under the field applied along the honeycomb plane. For \(H\parallel [2\bar{1}\bar{1}0]\), the plateau-r...

The random field Blume-Capel model revisited

We have revisited the mean-field treatment for the Blume-Capel model under the presence of a discrete random magnetic field as introduced by Kaufman and Kanner (1990). The magnetic field (H) versus temperature (T) phase diagrams for given values of the crystal field D were recovered in accordance to Kaufman and Kanner original work. However, our main goal in the present work was to investigate the distinct structures of the crystal field versus temperature phase diagrams as the random magnetic field is varied because similar models have presented reentrant phenomenon due to randomness. Following previous works we have classified the distinct phase diagrams according to five different topologies. The topological structure of the phase diagrams is maintained for both H-T and D-T cases. Although the phase diagrams exhibit a richness of multicritical phenomena we did not found any reentrant effect as have been seen in similar models.

X-ray diffraction, Mössbauer spectrometry and thermal studies of the mechanically alloyed (Fe1−xMnx)2P powders

(Fe1−xMnx)2P phosphide powders in the composition range 0.15 ≤ x ≤ 0.75 have been mechanically alloyed and their structural, magnetic and thermal changes with composition have been investigated by means of X-ray diffraction, 57Fe Mössbauer spectrometry, magnetization measurements and differential scanning calorimetry. The milling process induces changes in the crystal phase diagram of the (Fe1−xMnx)2P system. The XRD results reveal the coexistence of a bcc Fe(Mn)-type, hexagonal (Fe2P and Mn2P-type), orthorhombic (MnP-type) and tetragonal Fe3P-type structures for all compositions. The room temperature Mössbauer spectra confirm the formation of the Fe(Mn)-type, non-stoichiometric Fe2P-type, FeP-type and Fe3P-type structures. Saturation magnetization exhibits a comparable behavior to that of the average hyperfine magnetic field. The DSC scans show the existence of several endothermic and exothermic peaks in the temperature range (100–700) °C related to different phase transitions. The endothermic peak at about 582–589 °C can be related to the ferromagnetic/paramagnetic transition temperature (Curie temperature, TC) of the Fe(Mn)-type structure.

Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation.

Surface functionalization of nanoscale materials has a significant impact on their properties. We have demonstrated the effect of different passivating ligands on the crystal phase of organometal halide perovskite quantum dots (PQDs). Using static and dynamic spectroscopy, we studied phase transitions in CH3NH3PbBr3 PQDs ligated with either octylaminebromide (P-OABr) or 3-aminopropyl triethoxysilane (P-APTES). Around 140 K, P-OABr underwent a structural phase transition from tetragonal to orthorhombic, established by the emergence of a higher energy band in the photoluminescence (PL) spectrum. This was not observed in P-APTES, despite cooling down to 20 K. Additionally, time-resolved and excitation power-dependent PL, as well as Raman spectroscopy over a range of 300-20 K, revealed that recombination rates and types of charge carriers involved are significantly different in P-APTES and P-OABr. Our findings highlight how aspects of PQD phase stabilization are linked to nanoscale morphology and the crystal phase diagram.

Automated Phase Segmentation for Large-Scale X-ray Diffraction Data Using a Graph-Based Phase Segmentation (GPhase) Algorithm.

The creation of composition-processing-structure relationships currently represents a key bottleneck for data analysis for high-throughput experimental (HTE) material studies. Here we propose an automated phase diagram attribution algorithm for HTE data analysis that uses a graph-based segmentation algorithm and Delaunay tessellation to create a crystal phase diagram from high throughput libraries of X-ray diffraction (XRD) patterns. We also propose the sample-pair based objective evaluation measures for the phase diagram prediction problem. Our approach was validated using 278 diffraction patterns from a Fe-Ga-Pd composition spread sample with a prediction precision of 0.934 and a Matthews Correlation Coefficient score of 0.823. The algorithm was then applied to the open Ni-Mn-Al thin-film composition spread sample to obtain the first predicted phase diagram mapping for that sample.

Pressure-induced superconductivity in Bi single crystals

Measurements on resistivity and magnetic susceptibility have been carried out for Bi single crystals under pressures up to 10.5GPa. The temperature dependent resistivity shows a semimetallic behavior at ambient and low pressures (below about 1.6GPa). This is followed by an upturn of resistivity in low temperature region when the pressure is increased, which is explained as a semiconductor behavior. This feature gradually gets enhanced up to a pressure of about 2.52GPa. Then a non-monotonic temperature dependent resistivity appears upon further increasing pressure, which is accompanied by a strong suppression to the low temperature resistivity upturn. Simultaneously, a superconducting transition occurs at about 3.92K under a pressure of about 2.63GPa. With further increasing pressure, a second superconducting transition emerges at about 7K under about 2.8GPa. For these two superconducting states, the superconductivity induced magnetic screening volumes are quite large. As the pressure further increased to 8.1GPa, we observe the third superconducting transition at about 8.2K. The resistivity measurements under magnetic field allow us to determine the upper critical fields $\mu_0 H_{c2}$ of the superconducting phases. The upper critical field for the phase with $T_c=3.92$K is extremely low. Based on the Werthamer-Helfand-Hohenberg (WHH) theory, the estimated value of $\mu_0 H_{c2}$ for this phase is about 0.103T. While the upper critical field for the phase with $T_c$=7K is very high with a value of about 4.56T. Finally, we present a pressure dependent phase diagram of Bi single crystals. Our results reveal the interesting and rich physics in bismuth single crystals under high pressure.

The global liquid crystal phase diagram of the nematogenic square-well line model

ABSTRACTMonte–Carlo simulations in the isothermal–isobaric ensemble are employed to characterise the boundaries of isotropic, nematic, smectic and hexatic phases of square-well prolate spherocylinders with a continuous line of interaction sites. Our results show clear first-order phase transitions between the different mesogenic phases. This work considers different issues not previously published for this model that largely comprised the features of real substances without explicit energetical anisotropy parameters. Particular emphasis is given to the enhancement of the stability of the nematic phase in comparison with other prolate fluids of ellipsoidal and spherocylindrical geometry.

A novel technique for probing phase transitions in ferroelectric functional materials: Condensed matter spectroscopy

As relaxor ferroelectric functional materials, their crystal structures depend on temperature, component, electric filed, pressure, and so on, which are important for the applications of sensors, transducers, and actuators. For the case of PbTiO3-based ferroelectrics Pb(Zn1/3Nb2/3)O3-PbTiO3, Pb(Mg1/3Nb2/3)O3-PbTiO3, Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 and some other crystals, they have been extensively investigated due to the excellent electromechanical and piezoelectric properties. Generally, ferroelectric crystal structure and corresponding phase diagram are detected by temperature-dependent high resolution X-ray diffraction, low frequency dielectric permittivity, and domain structures. In this review, we focus on the novel condensed matter spectroscopy (i.e., spectroscopic ellipsometry, transmittance, photoluminescence spectra as well as Raman spectra), which is nondestructive, noncontact, and sensitive optical techniques for probing symmetries, phase transitions and phase diagrams of ferroelectric crystals. Besides, it can supply some other physical and chemical information for ferroelectric and semiconductor functional materials such as optical band gap, electronic transitions, dielectric functions, optical conductivity, absorption, phonon modes, lattice dynamics as functions of temperature and PT composition.

Enhancement of superconductivity under pressure and the magnetic phase diagram of tantalum disulfide single crystals.

In low-dimensional electron systems, charge density waves (CDW) and superconductivity are two of the most fundamental collective quantum phenomena. For all known quasi-two-dimensional superconductors, the origin and exact boundary of the electronic orderings and superconductivity are still attractive problems. Through transport and thermodynamic measurements, we report on the field-temperature phase diagram in 2H-TaS2 single crystals. We show that the superconducting transition temperature (Tc) increases by one order of magnitude from temperatures at 0.98 K up to 9.15 K at 8.7 GPa when the Tc becomes very sharp. Additionally, the effects of 8.7 GPa illustrate a suppression of the CDW ground state, with critically small Fermi surfaces. Below the Tc the lattice of magnetic flux lines melts from a solid-like state to a broad vortex liquid phase region. Our measurements indicate an unconventional s-wave-like picture with two energy gaps evidencing its multi-band nature.

Exploration of Vanadate Selenites Solid Phase Space, Crystal Structures, and Polymorphism

Our recent exploration of the Pb–Ni–Se–O “crystallization phase diagram” under hydrothermal conditions has led to the discovery of three new selenites. Here we have applied a similar systematic method to the Pb–V–Se–O system using both hydrothermal and chemical vapor transport crystal growth using either PbO or PbCl2 as the source of lead. In our approach, phases in the presence are meticulously characterized by single crystal X-ray diffraction tests, on the basis of their morphology. Besides the determination of stable predominant phases in the diagram such as Pb2(VO3)-(SeO3)2Cl, it enables the crystallization of three novel vanadate selenites, β-(V5+2O3)(SeO3)2 (I), Pb2(V4+O)(SeO3)3 (II), and β-Pb4(V5+3O8)2(SeO3)3(H2O) (III), depending on the method, pH, and potential of the solution. It is worth noting that in our conditions the solid/liquid crystal growth could be driven by kinetic or thermodynamic control which does not lead to determination of a formal phase diagram, but rather to a “solid-phase spa...

Phase transitional behaviors of bent-cored liquid crystal in electric field

Monte Carlo (MC) simulations based on lattice model were performed to study the phase diagram (anisotropy, uniaxiality and biaxiality) of liquid crystals formed by bent-cored molecules with a strong transverse dipole moment deviating from their angular bisector. It is shown that the asymmetric strong dipolar interaction enhances biaxiality slightly but encourages uniaxiality greatly and as a result suppresses the system’s isotropic order, which is different from a system free from external field in that dipole moment increases biaxiality by suppressing the uniaxial and the isotropic orders simultaneously. It is also found that an external electric field encourages the biaxiality slightly but considerably enhances the uniaxiality of bent-cored liquid crystal.

Modeling Polymorphic Molecular Crystals with Electronic Structure Theory.

Interest in molecular crystals has grown thanks to their relevance to pharmaceuticals, organic semiconductor materials, foods, and many other applications. Electronic structure methods have become an increasingly important tool for modeling molecular crystals and polymorphism. This article reviews electronic structure techniques used to model molecular crystals, including periodic density functional theory, periodic second-order Møller-Plesset perturbation theory, fragment-based electronic structure methods, and diffusion Monte Carlo. It also discusses the use of these models for predicting a variety of crystal properties that are relevant to the study of polymorphism, including lattice energies, structures, crystal structure prediction, polymorphism, phase diagrams, vibrational spectroscopies, and nuclear magnetic resonance spectroscopy. Finally, tools for analyzing crystal structures and intermolecular interactions are briefly discussed.