Composition-dependent Phase Research Articles

Studies on the compositional dependent structural and electrical properties of CaTiO3-modified K0.5Na0.5NbO3 piezoelectric system

Lead-free piezoelectric ceramics of (1 − x)K0.5Na0.5NbO3-xCaTiO3 were fabricated, and their crystal structure, microstructure, and electrical properties were systematically studied. Rietveld refinement of the x-ray diffraction data and Raman spectroscopic analyses revealed a composition-dependent structural phase transition: three phase transitions, namely, from a pure orthorhombic phase for x ≤ 0.02 to a mixed phase of orthorhombic and tetragonal phases (0.03 ≤ x ≤ 0.08) and finally another mixed phase of tetragonal + cubic for x = 0.10 and 0.15 at room temperature (RT). The morphological study reveals a decrease in grain size along with a more uniform distribution of grains as the concentration of CaTiO3 (CT) increases; notably, a homogeneous distribution of grains is observed for x = 0.05. The temperature-dependent dielectric properties show two phase transitions, from orthorhombic to tetragonal (TO-T) and tetragonal to cubic (TC), for unmodified K0.5Na0.5NbO3 (KNN). However, both the phase transition temperatures (TO-T and TC) decrease, and the transition peaks broaden with an increase in CT substitution, and for x > 0.06, the TO-T shifted below RT. The broadening of the transition peak at TO-T may be due to the relaxation behavior. Among the prepared samples, the 5 mol. % CT-modified KNN shows the optimum electrical properties (d33 = 114 pC/N, ɛr = 412, and 2Pr = 15.25 μC/cm2) at RT. The enhanced electrical properties for x = 0.05 are due to the coexistence of orthorhombic and tetragonal phases, facilitating easy polarization rotation and flattening of the free energy profile. A phase diagram has been constructed based on the information gathered from the temperature-dependent dielectric measurements, RT x-ray diffraction, and Raman spectroscopy data and is discussed in detail.

Bridging scales with Machine Learning: From first principles statistical mechanics to continuum phase field computations to study order–disorder transitions in Li[formula omitted]CoO[formula omitted]

LixTMO2 (TM=Ni, Co, Mn) forms an important family of cathode materials for Li-ion batteries, whose performance is strongly governed by Li composition-dependent crystal structure and phase stability. Here, we use LixCoO2 (LCO) as a model system to benchmark a machine learning-enabled framework for bridging scales in materials physics. We focus on two scales: (a) assemblies of thousands of atoms described by density functional theory-informed statistical mechanics, and (b) continuum phase field models to study the dynamics of order–disorder transitions in LCO. Central to the scale bridging is the rigorous, quantitatively accurate, representation of the free energy density and chemical potentials of this material system by coarse-graining formation energies for specific atomic configurations. We develop active learning workflows to train recently developed integrable deep neural networks for such high-dimensional free energy density and chemical potential functions. The resulting, first principles-informed, machine learning-enabled, phase-field computations allow us to study LCO cathodes’ order–disorder transitions in terms of temperature, microstructure, and charge cycling. We highlight several insights gained to the dynamics of the phase transitions, and that have been made possible by the quantitatively rigorous scale bridging. To the best of our knowledge, such a scale bridging framework has not been previously demonstrated for LCO, or for materials systems of comparable technological interest. This approach can be expanded to other materials systems and can incorporate additional physics to that studied here.

Synthesis and characterization of 1T′ and 2H phase coexistence 2D Re1-xMoxS2 alloy films and their application for photodetectors

Alloying is considered an efficient strategy to engineer the crystal structure, modulate the energy band structure and charge carrier concentration by adjusting the elemental composition of transition metal dichalcogenides (TMDs). Here, the two-dimensional (2D) Re1-xMoxS2 alloy films with tunable composition and phase structure were prepared on mica substrates by chemical vapor deposition (CVD) method. The back-gated field effect transistors (FETs) based on 2D Re1-xMoxS2 alloy films were prepared. The Mo component x in 2D Re1-xMoxS2 alloy films was varied from 0, 0.2, 0.4, 0.5, 0.6, 0.8–1 by changing mole ratio of ReO3 to MoO3 while the total number of moles remains the same. The Mo composition-dependent morphology phase structure, electrical transport properties and photoelectric properties of Re1-xMoxS2 alloy films were studied in detail. The results indicate that the 1T′ phase Mo-doped ReS2 and 2H phase Re-doped MoS2 coexist, and the ratio of 2H phase Re-doped MoS2 to 1T′ phase Mo-doped ReS2 can be modulated by changing Mo component x in Re1-xMoxS2 alloy films. 1T′ phase Mo-doped ReS2 exhibits the p-type conduction feature while 2H phase Re-doped MoS2 shows the n-type conduction behavior. The Re1–xMoxS2 alloy devices show obvious photoelectric response to 405, 532, 635 and 808 nm lasers irradiation. The Re0.5Mo0.5S2 (x= 0.5) has the maximum responsivity R of 1024.1 mA/W and external quantum efficiency EQE of 239.3% under 532 nm laser irradiation.

The role of Ca/Zr ratio on the local structure and phase transitions in lead-free (Ba,Ca)(Zr,Ti)O3

In this study, the structural phase transitions are investigated as a function of composition and temperature for polycrystalline x(Ba0.7Ca0.3)TiO3-(1 − x)Ba(Zr0.2Ti0.8)O3 (x = 0.40, 0.45, 0.50, 0.55, and 0.60) through a combination of Raman spectroscopy, synchrotron X-ray diffraction, and dielectric spectroscopy. The aim is to gain insight into the complex phase boundary region responsible for the excellent electromechanical properties. The results demonstrate the correlation between local site substitutions based on the stoichiometric variations to the microstructure and dielectric properties. The dielectric response has been correlated with the BCT/BZT content displaying a maximum depending on the phase content. Additionally, in situ temperature-dependent Raman, permittivity, and pair distribution function (PDF) studies were performed from −60 °C to 130 °C to highlight the structure and phase evolution. In particular, the in situ temperature-dependent Raman measurements reveal sudden discontinuities in the vibrational modes that correspond to the structural changes in the perovskite structure. Therefore, the results of the permittivity response, based on the average of a large volume, are consistent with the local structural changes obtained by other techniques. Indeed, correlations of Raman, X-ray diffraction, and PDFs obtained from synchrotron X-ray total scattering data, along with permittivity measurements allowed the identification of the discrete ferro- to paraelectric phase transitions and a more robust characterization. The rotation and distortion of the octahedral caused by oxygen displacement are driving forces behind symmetry changes and phase transitions, explaining the mechanism of polymorphic phase transition based on A- and B-site substitution. Therefore, this work provides a comprehensive understanding of temperature and composition-dependent phase transitions in BCZT.

Compositional induced structural phase transitions in (1 − x)(K0.5Na0.5)NbO3–x(Ba0.5Sr0.5)TiO3 ferroelectric solid solutions

Ferroelectric materials exhibiting switchable and spontaneous polarization have strong potential to be utilized in various novel electronic devices. Solid solutions of different perovskite structures induce the coexistence of various phases and enhance the physical functionalities around the phase coexistence region. The construction of phase diagrams is important as they describe the material properties, which are linked to the underpinning physics determining the system. Here we present the phase diagram of (K0.5Na0.5NbO3)–(Ba0.5Sr0.5TiO3) (KNN-BST) system as a function of composition and their associated physical properties. Lead-free (1 − x)KNN–xBST (0 ≤ x ≤ 0.3) solid solution ceramics were synthesized by conventional solid-state reaction technique. The X-ray diffraction and Raman spectroscopic studies indicate composition-dependent structural phase transitions from an orthorhombic phase for x = 0 to orthorhombic + tetragonal dual-phase (for 0.025 ≤ x ≤ 0.15), then a tetragonal + cubic dual-phase (x = 0.2) and finally a cubic single phase for x ≥ 0.25 at room temperature (RT). Among these, the orthorhombic + tetragonal dual-phase system shows an enhanced value of the dielectric constant at room temperature. The phase transition temperatures, orthorhombic to tetragonal (TO-T) and tetragonal to cubic (TC), decrease with the increase in BST concentrations. The ferroelectric studies show a decrease of both 2Pr and EC values with a rise in BST concentration and x = 0.025 showed a maximum piezoelectric coefficient.

Phase structure, dielectric and energy storage properties of Na0.5Bi0.5TiO3-BaTiO3 ceramics with Bi(Mg2/3Nb1/3)O3 modification

Na0.5Bi0.5TiO3 (NBT)-based ceramics are promising lead-free candidates for energy-storage applications owing to their individual crystal structure and phase transition information. However, the high coercive field (EC) and large remnant polarization (Pr) are detrimental for practical applications. In this work, the composition-dependent phase structure, micromorphology, dielectric and energy storage properties of (1-x)(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-xBi(Mg2/3Nb1/3)O3 (NBBT-100xBMN, x = 0.18, 0.20, 0.22 and 0.25) ternary ceramics were hereby investigated. The addition of BMN facilitates the phase transformation from rhombohedral (R3c) to tetragonal (P4bm), which is beneficial for the enhancement of relaxation behavior, and meanwhile impels the grain growth. Combined with the influence of grain size on breakdown field strength, an optimal recoverable energy storage density (Wrec) of 1.88 J/cm3 with a high energy storage efficiency (η) of 82.15% was achieved in the NBBT-20BMN ceramic composition under the dielectric breakdown strength (DBS) of 150 kV/cm, accompanied by the excellent temperature stability from room temperature to 200 °C. All these manifest that the NBBT-20BMN ceramics have an attractive application prospect in the energy storage field.

Uncovering the origin of physical properties associated with magneto-structural transition of Ni-Mn-In-X alloys by ab initio methods

Ni-Mn-In metamagnetic shape memory alloys exhibit magneto-structural coupling features during martensitic transformation. In the present work, the composition-dependent crystal structure, phase stability, elastic and magnetic properties of the off-stoichiometric Ni2Mn1.5In0.5 alloy doped with the fourth element X (X = Co, Fe, and Cu) are investigated by using the ab initio calculations. Results show that Co prefers to occupy the Ni sublattice, Fe or Cu takes the Mn site. Doping X constituent reduces the stability of the austenite (A) and non-modulated (NM) martensite. For doped Co, martensitic transformation can occur at 0 ≤ x ≤ 0.75, and there exists a magneto-structural coupling characteristic with the Co range in 0 ≤ x ≤ 0.25. For the case of Fe or Cu doping, the martensitic transformation is accompanied by the change of magnetism. The results of elastic properties indicate that the plasticity and toughness increase significantly with Fe or Cu doping and decrease with Co doping. Furthermore, the origin of the physical properties associated with the magneto-structural coupling transition is explained in terms of the electronic density of states.

Effect of Sm3+ doping on ferroelectric, energy storage and photoluminescence properties of BaTiO3 ceramics

Rare earth doped ferroelectric ceramics have attracted much attention due to their great potential application for novel multifunctional optical-electro devices. Herein, the x% mol Sm3+ doped BaTiO3 (BTO:xSm3+) ceramics were fabricated by the conventional solid-state reaction method. The Sm3+ ions composition dependent phase structure, ferroelectric, energy storage and photoluminescence properties were systematically investigated. With the increase of Sm3+ ions composition, the remanent polarization decreases dramatically from 15.705 μC/cm2 (BTO) to 7.132 μC/cm2 (BTO:3.0%Sm3+), but the energy storage density and efficiency increase greatly with a relative change of 79.76% and 31.13%, respectively. Furthermore, Sm3+ doping causes the transformation from the tetragonal to pseudo-cubic phase for BTO ceramics at room temperature, resulting in a broader temperature transition range from the ferroelectric to paraelectric phase and a lower Curie temperature. Particularly, the pure BTO and BTO:xSm3+ ceramics show great thermal stability for energy storage properties. In addition, under the excitation of 408 nm near-ultraviolet light, the BTO:xSm3+ ceramics exhibit the strongest orange-red emission peak around 596 nm with a large relative tunability of intensity by 88.97%. The results suggest that the BTO:xSm3+ ceramics are suitable for the design of optoelectronic devices.

Comparative study on mechanical and tribological properties of Cr–Al–N and Cr–Al–O coatings deposited by cathodic arc evaporation

A comparative study was performed for (Cr1−xAlx)2O3 and (Cr1−xAlx)N coatings with different Al contents (x = 0, 0.5, and 0.7) concerning structural, mechanical, and tribological properties. The composition-dependent phase structure was found for Cr–Al–O coatings, where higher Al content promotes a mixed structure of cubic and minor amorphous phases. Moreover, the mixed-phase structure results in lower indentation hardness, ∼27 GPa, and residual compressive stress, ∼ −4 GPa, as compared to Cr2O3 and Cr–Al–N coatings. Besides, (Cr0.5Al0.5)2O3 and (Cr0.3Al0.7)2O3 show worse wear resistance with a wear rate higher than 1.0 × 10−5 mm3/N·m at 200 − 600 °C due to reduced hardness and lower residual stress. A higher bias potential of −150 V induces higher compressive residual stress and hardness, which is beneficial to the high-temperature wear resistance of Cr–Al–O coatings.

Composition-dependent phase transformation path involving 4O martensite in Ni–Mn–Sn magnetic shape memory alloys

The experimental discovery of four-layer orthorhombic (4O) martensite has added new research motivation to the Ni–Mn–Sn magnetic shape memory alloy. Herein, the martensitic transformation, magnetic properties, and electronic structures of Ni2Mn1+xSn1−x alloys are investigated using the first-principles calculations. The results show that the increasing Mn content destabilizes the stability of austenite (A) compared to the non-modulated (NM) martensite. This composition adjustment promotes the occurrence of martensitic transformation in the range of 0.375 ≤ x ≤ 0.75, and the corresponding phase transition sequence is A → 4O → NM during cooling. An intense hybridization bond exists between excess Mn and its surrounding atoms. The increasing antiferromagnetic interaction between excess Mn and normal Mn weakens each atomic moment and, thus, the total magnetic moment. Furthermore, the physical essence of the phase stability and magnetic properties variation with composition was explained based on the electronic density of states.

Mechanical, electrical properties and microstructures of combinatorial Ni-Mo-W alloy films

Nickel-molybdenum-tungsten (Ni-Mo-W) alloys have been suggested for applications of advanced micro-electro-mechanical systems (MEMS) in extreme environments due to their high strength, good electrical conductivity, and excellent thermal stability. In this work, we have carried out combinatorial experiments to reveal composition-dependent phase formation, physical properties, and thermal stability of Ni-Mo-W alloy films. Combinatorial synthesis through a magnetron sputtering process fabricated thin films with a composition range of Ni 56–92 Mo 6–38 W 0.5–7.5 . Structural analysis for the as-deposited combinatorial film revealed that a nanotwinned and strongly textured nano-columnar fcc structure was formed in the Ni-rich compositions, while solute content greater than 30% resulted in the formation of metallic glasses. The specimen exhibited very high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ ∙ cm), and both the properties were found to increase with solute concentration. Annealing of the combinatorial film at 600 ℃ for 1 h revealed that nanotwinned structure is thermally stable for high Ni concentrations. Metallic glasses with high solute concentration also exhibited high thermal stability and were not crystallized after annealing. Alloys with intermediate solute content experienced the transformation from the amorphous to fcc nanocrystalline phase with nanotwins. The annealed combinatorial film maintained high hardness and moderate electrical resistivity after annealing. • Composition-dependent phase formation and physical properties of Ni 56–92 Mo 6–38 W 0.5–7.5 alloy films were investigated. • A nanotwinned (NT) and nano-columnar fcc structure was formed in the Ni-rich compositions. • The solute content greater than 30% resulted in the formation of metallic glasses. • The specimen exhibited high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ · cm). • The NT structure with high Ni content and metallic glasses with high solute concentration were thermally stable.

Full Composition Tuning of W1-xNbxSe2 Alloy Nanosheets to Promote the Electrocatalytic Hydrogen Evolution Reaction.

Composition modulation of transition metal dichalcogenides is an effective way to engineer their crystal/electronic structures for expanded applications. Here, fully composition-tuned W1-xNbxSe2 alloy nanosheets were produced via colloidal synthesis. These nanosheets ultimately exhibited a notable transition between WSe2 and NbSe2 hexagonal phases at x = 0.6. As x approaches 0.6, point doping is converted into cluster doping and eventually separated domains of WSe2 and NbSe2. Extensive density functional theory calculations predicted the composition-dependent crystal structures and phase transitions, consistently with the experiments. The electrocatalytic activity for the hydrogen evolution reaction (HER) in acidic electrolyte was significantly enhanced at x = 0.2, which was linked with the d-band center. The Gibbs free energy for the H adsorption at various basal and edge sites supported the enhanced HER performance of the metallic alloy nanosheets. We suggested that the dispersed doping structures of Nb atoms resulted in the best HER performance. Our findings highlight the significance of composition tuning in enhancing the catalytic activity of alloys.

Enhanced relaxor behavior and high energy storage efficiency in niobium substituted (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics

The microstructure, dielectric properties, relaxor behavior, and energy storage efficiency of un-substituted and niobium (Nb) substituted (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 (for x = 0, 0.02 and 0.05) samples prepared by the solid-state reaction method has been studied in detail. All the samples exhibited perovskite structure with no trace of impurity. Composition-dependent phase transition was also observed on the addition of Niobium. At room temperature, the co-existence of rhombohedral and tetragonal phases is observed in the unsubstituted samples. As the composition changes from x = 0.02 to x = 0.05, a structural change from tetragonal to cubic is observed. A remarkable reduction in grain size, from 90 μm (for x = 0) to 1.21 μm for (x = 0.05), is observed with the addition of Niobium. This result suggests that Niobium acts as a grain growth inhibiter in barium calcium zirconium titanate (BCZT) ceramics. The effect of Niobium on transition temperature is studied from the temperature-dependent dielectric permittivity graph. It was clear that the transition temperature shifted to a lower temperature region, and for x = 0.05, at a very low temperature (−23 °C/250 K) the tetragonal to cubic transition was observed. At x = 0.05, the temperature-dependent dielectric permittivity showed a broadened curve, indicating a diffuse phase transition. The diffuse phase transition in Nb substituted samples is explained by Uchino and Nomura modified Curie Weiss law.Moreover, the observations on temperature-dependent dielectric permittivity measurements at various frequencies suggest that the substitution of Nb5+ induces relaxor behavior. The energy storage efficiency of unsubstituted and Nb substituted samples was calculated from the polarization versus electric field graph. A high storage efficiency of 84% was obtained for the Nb substituted sample (x = 0.05) at 12 kV cm−1. Enhanced relaxor behavior and increased storage efficiency were observed in (Ba0.85 Ca0.15)(Zr0.1 Ti0.9) 1-x NbxO3 at x = 0.05. Thus we suggest that these are promising materials for energy storage applications.

Atomic reconfiguration among tri-state transition at ferroelectric/antiferroelectric phase boundaries in Pb(Zr,Ti)O3

Phase boundary provides a fertile ground for exploring emergent phenomena and understanding order parameters couplings in condensed-matter physics. In Pb(Zr1-xTix)O3, there are two types of composition-dependent phase boundary with both technological and scientific importance, i.e. morphotropic phase boundary (MPB) separating polar regimes into different symmetry and ferroelectric/antiferroelectric (FE/AFE) phase boundary dividing polar and antipolar dipole configurations. In contrast with extensive studies on MPB, FE/AFE phase boundary is far less explored. Here, we apply atomic-scale imaging and Rietveld refinement to directly demonstrate the intermediate phase at FE/AFE phase boundary exhibits a rare multipolar Pb-cations ordering, i.e. coexistence of antipolar or polar displacement, which manifests itself in both periodically gradient lattice spacing and anomalous initial hysteresis loop. In-situ electron/neutron diffraction reveals that the same parent intermediate phase can transform into either FE or AFE state depending on suppression of antipolar or polar displacement, coupling with the evolution of long-/short-range oxygen octahedra tilts. First-principle calculations further show that the transition between AFE and FE phase can occur in a low-energy pathway via the intermediate phase. These findings enrich the structural understanding of FE/AFE phase boundary in perovskite oxides.

Composition dependent phase structure, dielectric and electrostrain properties in (Sr0.7Bi0.2□0.1)TiO3–PbTiO3–Bi(Mg0.5Ti0.5)O3 systems

Composition dependent transitions from normal ferroelectrics to nonergodic and finally to ergodic relaxor phase are observed in 0.7(Sr0.7Bi0.2□0.1)TiO3–(0.3 − x)PbTiO3–xBi(Mg0.5Ti0.5)O3 system (SBT–PT–xBMT, □ represents A–site vacancy). Rietveld refinement results show that with increasing BMT content, the system experiences a gradual transition from coexistence of pseudocubic and tetragonal (P c + T) to P c phase. The ferroelectric–relaxor phase transition and freezing temperature gradually decreases with addition of BMT content accompanied by an enhanced relaxor degree, which produces local disorder and polar nanodomains. This is also verified by Raman spectra and piezoelectric force microscopic analysis. The P–E loops transform from square to slant and finally to slim shape with increasing BMT component and an electric field-induced strain of ∼0.21% with ultralow hysteresis of ∼3.7% is obtained for x = 0.04 composition. The underlying mechanism for the large strain with low hysteresis lies in the existence of nonergodic and ergodic relaxor phase boundary and polar nanodomains at room temperature. Additionally, the multiphase coexistence contributes to a flatten free energy profile and thus contributing to such superior performances, as explained by a modified phenomenological model. High electrostrain with ultralow hysteresis in SBT–PT–xBMT systems are promising candidates in high–precision actuator applications.

Crystal Structure and Concentration-Driven Phase Transitions in Lu(1-x)ScxFeO3 (0 ≤ x ≤ 1) Prepared by the Sol-Gel Method.

The structural state and crystal structure of Lu(1−x)ScxFeO3 (0 ≤ x ≤ 1) compounds prepared by a chemical route based on a modified sol–gel method were investigated using X-ray diffraction, Raman spectroscopy, as well as scanning electron microscopy. It was observed that chemical doping with Sc ions led to a structural phase transition from the orthorhombic structure to the hexagonal structure via a wide two-phase concentration region of 0.1 < x < 0.45. An increase in scandium content above 80 mole% led to the stabilization of the non-perovskite bixbyite phase specific for the compound ScFeO3. The concentration stability of the different structural phases, as well as grain morphology, were studied depending on the chemical composition and synthesis conditions. Based on the data obtained for the analyzed samples, a composition-dependent phase diagram was constructed.

Colloidal synthesis of AgGa(S1−xSex)2solid solution nanocrystals with composition-dependent crystal phase for efficient photocatalytic degradation of methyl violet

A series of AgGa(S1−xSex)2nanocrystals whose crystal phase features the dependence on their composition have been synthesized, and quasi-tetragonal AgGa(S0.7Se0.3)2presents the best photocatalytic efficiency of 88.48% for methyl violet.

Microstructural Evaluation of Phase Instability in Large Bandgap Metal Halide Perovskites

The optoelectronic performance of organic-inorganic halide perovskite (OIHP)-based devices has been improved in recent years. Particularly, solar cells fabricated using mixed-cations and mixed-halides have outperformed their single-cation and single-halide counterparts. Yet, a systematic evaluation of the microstructural behavior of mixed perovskites is missing despite their known composition-dependent photoinstability. Here, we explore microstructural inhomogeneity in (FAPbI3)x(MAPbBr3)1-x using advanced scanning probe microscopy techniques. Contact potential difference (CPD) maps measured by Kelvin probe force microscopy show an increased fraction of grains exhibiting a low CPD with flat topography as MAPbBr3 concentration is increased. The higher portion of low CPD contributes to asymmetric CPD distribution curves. Chemical analysis reveals these grains being rich in MA, Pb, and I. The composition-dependent phase segregation upon illumination, reflected on the emergence of a low-energy peak emission in the original photoluminescence spectra, arises from the formation of such grains with flat topology. Bias-dependent piezo-response force microscopy measurements, in these grains, further confirm vigorous ion migration and cause a hysteretic piezo-response. Our results, therefore, provide insights into the microstructural evaluation of phase segregation and ion migration in OIHPs pointing toward process optimization as a mean to further enhance their optoelectronic performance.

Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

Moisture-stimulated reversible thermochromic CsPbI3-xBrx films: In-situ spectroscopic-resolved structure and optical properties

We report a composition-dependent, moisture-stimulated reversible thermochromic phenomenon in CsPbI3-xBrx (0 ≤ x ≤ 2) films. The mechanism for the thermochromism is attributed to a reversible phase transition between perovskite (H-T phase) and nonperovksite (L-T phase) structure with the transition temperature points dependent on the I/Br ratio. Temperature and composition dependent thermochromic behavior of the films was recorded via in-situ x-ray diffraction, spectroscopic ellipsometry and digital photography in heating process. The color of the film changes from black to dark red with increasing Br content of 0 ≤ x ≤ 2 for the H-T phases, while chromically changed into yellowish-gray (semitransparent) to reddish-gray (transparent) for that of L-T phase. The three representative compositions of x = 0, 1, and 2 were selected as modelling compositions to illustrate the temperature-dependent opitcal constants evolution. We found two linear relationships: (i) TL-T→H-T=-80x+340(°C) for the phase transition point, and (ii) ΔEgCsPbI3-xBrx=0.17x+1.76(eV) for the bandgap evolution with composition. The composition dependent phase evolution and temperature-dependent optical parameters in CsPbI3-xBrx family not only provide the fundamental structure and thermal stability data for further understanding the relationship between composition and physical properties, but also important for the design and fabrication of effective devices for the promising optical, optoelectronic and thermochromic applications.