Multinary Phases Research Articles

First-principles search of hot superconductivity in La-X-H ternary hydrides

Motivated by the recent claim of hot superconductivity with critical temperatures up to 550 K in La + x hydrides, we investigate the high-pressure phase diagram of compounds that may have formed in the experiment, using first-principles calculations for evolutionary crystal structure prediction and superconductivity. Starting from the hypothesis that the observed Tc may be realized by successive heating upon a pre-formed LaH10 phase, we examine plausible ternaries of lanthanum, hydrogen and other elements present in the diamond anvil cell: boron, nitrogen, carbon, platinum, gallium, gold. We find that only boron and, to a lesser extent, gallium form metastable superhydride-like structures that can host high-Tc superconductivity, but the predicted Tc’s are incompatible with the experimental reports. Our results indicate that, while the claims of hot superconductivity should be reconsidered, it is very likely that unknown H-rich ternary or multinary phases containing lanthanum, hydrogen, and possibly boron or gallium may have formed under the reported experimental conditions, and that these may exhibit superconducting properties comparable, or even superior, to those of currently known hydrides.

Training machine-learning potentials for crystal structure prediction using disordered structures

Prediction of the stable crystal structure for multinary (ternary or higher) compounds with unexplored compositions demands fast and accurate evaluation of free energies in exploring the vast configurational space. The machine-learning potential such as the neural network potential (NNP) is poised to meet this requirement but a dearth of information on the crystal structure poses a challenge in choosing training sets. Herein we propose constructing the training set from densityfunctional-theory (DFT) based dynamical trajectories of liquid and quenched amorphous phases, which does not require any preceding information on material structures except for the chemical composition. To demonstrate suitability of the trained NNP in the crystal structure prediction, we compare NNP and DFT energies for Ba2AgSi3, Mg2SiO4, LiAlCl4, and InTe2O5F over experimental phases as well as low-energy crystal structures that are generated theoretically. For every material, we find strong correlations between DFT and NNP energies, ensuring that the NNPs can properly rank energies among low-energy crystalline structures. We also find that the evolutionary search using the NNPs can identify low-energy metastable phases more efficiently than the DFTbased approach. By proposing a way to developing reliable machine-learning potentials for the crystal structure prediction, this work will pave the way to identifying unexplored multinary phases efficiently.

Intermetallic Reactivity: Ca3Cu7.8Al26.2 and the Role of Electronegativity in the Stabilization of Modular Structures

The structural chemistry of intermetallic phases is generally viewed in terms of what crystal structure will be most stable for a given combination of metallic atoms. Yet, individual atoms do not always make the best reference points. As the number of elements involved in compounds increases, their structures can often appear to be assembled from structural motifs derived from simpler compounds nearby in the phase diagram rather than fundamentally new arrangements of atoms. In this Article, we explore the notion that complex multinary phases can be viewed productively in terms of motif-preserving reactions between binary compounds, as opposed to direct reactions of the component elements. We present the targeted synthesis and structure solution of Ca3Cu7.8Al26.2, an intermetallic phase whose placement in the phase diagram is suggestive of a reaction between CaAl4 and CuAl2. Single-crystal X-ray diffraction analysis reveals that this compound crystallizes in the Y3TaNi6+xAl26 (or stuffed BaHg11) structure type and is constructed from three modules: Ca@(Cu/Al)16 polyhedra derived from the BaAl4 type, Cu@Al8 cubes, and Al13 cuboctahedra. To help understand this arrangement, we identify forces driving the reactivity of one of the supposed starting materials, CaAl4, through visualization of its atomic charge distribution and chemical pressure (CP) scheme, which suggest that the Al sites closest to the Ca atoms should show a high affinity for substitution by Cu atoms. Such a process on its own, however, would lead to overly long Ca-Cu distances and electron deficiency. When Cu is made available to CaAl4 in the Ca-Cu-Al ternary system, its incorporation in the Ca coordination environments instead nucleates domains of a fluorite-like CuAl2 phase, which act as nodes in the primitive cubic framework of CaAl4- and fluorite-like units. The cubic holes created by this framework are occupied by Al13 face-centered-cubic fragments that donate electrons while also resolving negative CPs in the Ca environments. This structural chemistry illustrates how new elements added to a binary compound at sites with conflicting electronic and atomic size preferences can serve as anchor points for the growth of domains of different bonding types, a notion that can be applied as a more general design strategy for new intermetallic intergrowth structures.

CRYSTAL CHEMISTRY OF THREE-COMPONENT WHITE DWARFS AND NEUTRON STAR CRUSTS: PHASE STABILITY, PHASE STRATIFICATION, AND PHYSICAL PROPERTIES

ABSTRACT A systematic search for multicomponent crystal structures is carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are “bred” by a genetic algorithm and optimized at constant pressure under the assumption of linear response (Thomas–Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not previously predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low-density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase-layering diagram and mass–radius-composition dependence, both of which are reported for He–C–O and C–O–Ne white dwarfs. Finite thickness interfacial phases (“interphases”) show up at the boundaries between single-component body-centered cubic (bcc) crystalline regions, some of which have lower lattice symmetry than cubic. A second application—quasi-static settling of heavy nuclei in white dwarfs—builds on our equilibrium phase-layering method. Tests of this nonequilibrium method reveal extra phases that play the role of transient host phases for the settling species.

Crystallographic data mining - A path to structure prediction in intermetallics?

"Many metals adapt very simple structure types, mainly sphere packings such as cubic and hexagonal close-packed, as well as simple body-centered cubic structures. In contrast, around 2 % of all intermetallic crystal structures exhibit unit cells containing 100 or more atoms [1]. These compounds are also termed ""complex intermetallics"" and, while the simplest packings approximate metal atoms as spheres and in doing so explain a number of frequent structure types, the existence of such intricate geometries is not immediately evident. From the analysis of a large number of intermetallic structures, we can recognize geometrical patterns (e.g., recurring building blocks) in order to find out more about their general building principles. Among complex intermetallics, two very distinct phenomena can be observed when inspecting the different structures. On one hand, chemically very diverse compounds can crystallize in rather similar structures, as was shown for a group of more than 40 complex face-centered cubic compounds [2]. On the other hand, small compositional changes within select intermetallic systems can yield a number of different structures, which can also belong to the class of complex intermetallics. A showcase system is found among Al-Cu-Ta, where a small range of the ternary phase diagram contains at least four different structures, all of them complex [3]. By gaining deeper knowledge of the factors influencing structure stabilization, we aim at a better understanding of the mechanisms that are responsible for the formation of intermetallic structures. Thus, we hope to contribute to the quest for explaining why certain intermetallic structures form, while countless other theoretically possible structures do not. Consequently, the prediction of intermetallic structures, especially in still largely unknown parts of multinary phase diagrams, will come into reach."

Experimental studies on NiSO4 by thermal analysis and calorimetry

This work includes very new experimental studies on pure NiSO4 with Differential Thermal Analysis (DTA), simultaneous Thermo Gravimetry (TG), and Dynamic Scanning Calorimetry (DSC). Literature only describes the decomposition of NiSO4, and all related multi-nary phase diagrams with nickel sulfate as a compound show a lack in data in the NiSO4-rich region. Indeed, our experiments demonstrate that NiSO4 is stable in equilibrium state and melts at 1483±3K with an enthalpy of melting of, ΔfusH1483K=44±2kJmol−1. Prior to this, it undergoes a never before observed structure change at 1243±3K (nominated as ‘dkostruct’) with the transformation energy ΔtrH1243K=3.9±0.2kJmol−1. The change from this high to low temperature structure (orthorhombic) is probably connected with a volume expansion. The standard heat capacity Cp(298K)0=93±7Jmol−1K−1, and from the polynomial for Cp(T)0, derived quantities for the enthalpy increment and the entropy could be deduced for the temperature range from 298 to 950K. From empirical analysis the heat of formation for NiSO4(c) is ΔfH298K=−880±10kJmol−1.

Ca1–xRExAg1–ySb (RE = La, Ce, Pr, Nd, Sm; 0 ≤ x ≤1; 0 ≤ y ≤1): Interesting Structural Transformation and Enhanced High-Temperature Thermoelectric Performance

For materials used in high-temperature thermoelectric power generation, the choices are still quite limited. Here we demonstrate the design and synthesis of a new class of complex Zintl compounds, Ca(1-x)RE(x)Ag(1-y)Sb (RE = La, Ce, Pr, Nd, Sm) (P63mc, No. 186, LiGaGe-type), which exhibit a high figure of merit in the high-temperature region. Compared with the parent structure that is based on CaAgSb (Pnma, No. 62, TiNiSi-type), an interesting structural relationship is established which suggests that important size and electronic effects govern the formation of these multinary phases. According to theoretical calculations, such a structural transformation from the orthorhombic TiNiSi-type to the hexagonal LiGaGe-type also corresponds to an obvious modification in the electronic band structure, which explains the observed significant enhancement of the related thermoelectric properties. For an optimized p-type material, Ca(0.84)Ce(0.16)Ag(0.87)Sb, a figure of merit of ~0.7 can be achieved at 1079 K, which is comparable to that of Yb14MnSb11 at the same temperature. In addition, due to the excellent thermal stability and high electrical conductivity, these materials are very promising candidates for high-temperature thermoelectric power generation.

Recent advances in the theory of hydrogen storage in complex metal hydrides

Abstract

Thermodynamic Aspects of the Synthesis of Thin‐Film Materials for Solar Cells

A simple and useful thermodynamic approach to the prediction of reactions taking place during thermal treatment of layers of multinary semiconductor compounds on different substrates has been developed. The method, which uses the extensive information for the possible binary compounds to assess the stability of multinary phases, is illustrated with the examples of Cu(In,Ga)Se(2) and Cu(2)ZnSnSe(4) as well as other less-studied ternary and quaternary semiconductors that have the potential for use as absorbers in photovoltaic devices.

CdSSe layer-sensitized TiO2 nanowire arrays as efficient photoelectrodes

Complete composition-tuned CdSxSe1−x alloy layers (avg. thickness = 50 nm) were deposited on pre-grown TiO2 nanowires by the thermal vapor transport of CdS/CdSe powders, producing core–shell nanocable arrays. CdSxSe1−x alloy nanowires were also synthesized with full composition tuning by the same method for comparison. The CdSSe nanowires consisted of Se-rich and S-rich pseudo binary phases, while the nanocable shell consisted of more complex multinary phases including CdSe and CdS. Remarkably, unique CdS–CdSSe–CdSe multishell structures were produced in the Se-rich composition range. The photoelectrochemical (PEC) cells fabricated using the as-grown nanocable arrays show higher solar photocurrents and hydrogen generation rates for the Se-rich shelled TiO2 nanocable arrays. This suggests that the CdS–CdSSe–CdSe multishell structures increase greatly the PEC performance by producing novel band alignment for efficient electron–hole separation following enhanced visible-range photon absorption.

Reactivity of multinary phases

Reactivity of multinary phases

Flux Growth and Characterization of Multinary Phases in the CuGaTe2-ZnTe System

Phase relationships in the CuGaTe2–ZnTe system have been investigated in order to establish the growth techniques for the relative quaternary phases. A number of single crystals of Cu2(1-x)Zn4xGa2(1-x)Te4 composition has been grown by using ZnCl2 as flux material. The best growth conditions are: molar ratio flux to solid 20: 1; cooling rate, above 700°C, 3°C hr-1. All crystals behave as p-type semiconductor. The analysis of absorption and reflection spectra give evidence for a heavy doping of crystals. The direct gaps evaluated for crystals of composition CuZn6GaTe8 and CuZn8GaTe10 are respectively Eg=1.49 eV and Eg=1.56 eV at room temperature.