Rhombohedral-cubic Phase Transition Research Articles

Orchestrating phase transition in GeTe thermoelectrics: An investigation into the role of electronegativity

In thermoelectrics, mastering solid-state phase transition is paramount for enhancing the figure of merit and potentially designing novel thermoelectric materials. Focusing on GeTe, its reversible rhombohedral-to-cubic phase transition poses challenge for practical application. The determinant governing this phase transition remains obscure. Here, we introduce a simple, intuitive, and broadly applicable strategy to facilitate this phase transition by exploiting the large electronegativity disparity between cations and anions. Leveraging this concept, we adeptly identify AgInTe2 as a potent alloying agent and incorporate Sb to enhance the dissolution of AgInTe2 within the matrix, hastening the formation of cubic GeTe thanks to the increased electron transfer. These leads to the enhanced band degeneracy, formed multiscale microstructures, enhanced lattice anharmonicity, and reduced sound velocity, substantially increasing the density-of-states effective mass and decreasing the lattice thermal conductivity. Notably, AgInTe2 alloying engenders impurity bands, significantly narrowing the bandgap and rendering it ideal for low-temperature power generation. Consequently, the cubic (Ge0.94Sb0.06Te)0.9(AgInTe2)0.1 achieves a peak zT ∼ 1.01 at 473 K, resulting in a maximum energy-conversion efficiency ∼ 5.3% (9.2%) in a single-leg device under a 300 K (500 K) temperature differential. These results underscore the critical role of electronegativity difference in managing the rhombohedral-cubic phase transition in chalcogenides.

Electrochemical capacitance and cubic-rhombohedral phase transition of sodium intercalated ferrimagnetic manganese hexacyanoferrate based open framework material

We report a synthesis of a highly crystalline, sodium intercalated manganese hexacyanoferrate open framework structure based on Prussian blue analogues with chemical stiochiometry of Na2xMn[Fe(CN)6]2(x+1)/3·zH2O (x = 0, 0.5, and 1) using controlled co-precipitation method. The effect of Na intercalation on structural properties, electrochemical performance along with magnetic ordering are prersnted. The formation of uniformly cube shaped sodium‑manganese hexacyanoferrate is evident from field-emission scanning electron microscopy. Infra-red spectroscopy shows presence of characteristic absorption peaks of MnII-C ≡ N-FeIII bonds, confirm the formation of manganese hexacyanoferrate. The Rietveld refinement of x-ray data reveals that the compounds are crystallized in Fm3m cubic symmetry up to x = 0.5, whereas, excess Na intercalation (x = 1) leads to a R-3 mH, rhombohedral symmetry. The dc magnetization measurements show ferrimagnetic nature of these compounds. The magnetic transition temperature increases with Na intercalation and was found to be ∼11, 12.5, and 13.4 K for x = 0, 0.5, and 1. The Mössbauer spectroscopy study confirms low and intermediate spin states of both Fe+2 and Fe+3 ions in the studied compounds, consistence with their magnetic properties. The electrochemical performance of the compounds with Na intercalation in terms of charging-discharging are studied using cyclic voltammetry. The compounds show capacitance values of 182.6 F/g at 10 A/g, 117.5 F/g at 6A/g, and 110.1 F/g at 2.8 A/g for x = 0, 0.5, and 1, respectively, suggesting their suitability as electrochemical capacitors.

Regulating the Configurational Entropy to Improve the Thermoelectric Properties of (GeTe)1-x(MnZnCdTe3)x Alloys.

In thermoelectrics, entropy engineering as an emerging paradigm-shifting strategy can simultaneously enhance the crystal symmetry, increase the solubility limit of specific elements, and reduce the lattice thermal conductivity. However, the severe lattice distortion in high-entropy materials blocks the carrier transport and hence results in an extremely low carrier mobility. Herein, the design principle for selecting alloying species is introduced as an effective strategy to compensate for the deterioration of carrier mobility in GeTe-based alloys. It demonstrates that high configurational entropy via progressive MnZnCdTe3 and Sb co-alloying can promote the rhombohedral-cubic phase transition temperature toward room temperature, which thus contributes to the enhanced density-of-states effective mass. Combined with the reduced carrier concentration via the suppressed Ge vacancies by high-entropy effect and Sb donor doping, a large Seebeck coefficient is attained. Meanwhile, the severe lattice distortions and micron-sized Zn0.6Cd0.4Te precipitations restrain the lattice thermal conductivity approaching to the theoretical minimum value. Finally, the maximum zT of Ge0.82Sb0.08Te0.90(MnZnCdTe3)0.10 reaches 1.24 at 723 K via the trade-off between the degraded carrier mobility and the improved Seebeck coefficient, as well as the depressed lattice thermal conductivity. These results provide a reference for the implementation of entropy engineering in GeTe and other thermoelectric materials.

Entropy engineering enhances the thermoelectric performance and microhardness of (GeTe)1−x(AgSb0.5Bi0.5Te2)x

As an endogenous material genetic property, entropy is an emerging fingerprint factor in the material genome. By designing multicomponent thermoelectric materials with high configuration entropy, the lattice thermal conductivity can be significantly reduced via severe lattice distortion, and the Seebeck coefficient can be improved by enhancing the crystal symmetry. However, high entropy also causes the deterioration of carrier mobility, restraining the improvement of the dimensionless figure of merit zT. Herein, we design the (GeTe)1−x(AgSb0.5Bi0.5Te2)x, aka TABGS alloys, via replacing half of the Sb by Bi in the well-known (GeTe)1−x-(AgSbTe2)x, aka TAGS alloys, to attest the efficacy of entropy engineering. In view of the low carrier mean free path of TAGS alloys approaching to the Mott-Ioffe-Regel limit, further Bi substitution and increased configuration entropy cannot impair the carrier mobility any longer. In addition, the suppressed rhombohedral-cubic phase transition via high configuration entropy and the reduced carrier concentration contribute to the substantially improved Seebeck coefficient. Furthermore, the elicited multiscale microstructures and the decreased sound velocity upon AgSb0.5Bi0.5Te2 alloying effectively restrain the lattice thermal conductivity. As a result, (GeTe)0.80(AgSb0.5Bi0.5Te2)0.20 achieves a high zT of 1.60 at 723 K and an average zTave of 1.23 from 300 to 773 K. Meantime, its room-temperature Vickers hardness reaches 2.21 GPa due to solid solution hardening. These results highlight the efficacy of entropy engineering in enhancing the thermoelectric performance, especially for numerous materials with intrinsically low carrier mobility.

High-Voltage and Super-Stable Aqueous Sodium-Zinc Hybrid Ion Batteries Enabled by Double Solvation Structures in Concentrated Electrolyte.

Aqueous sodium-zinc hybrid ion batteries are attracting extensive attention due to high energy density, low cost, and environmental friendliness. Unfortunately, there are still some drawbacks associated with the low voltage and cycle performance degradation that limit their practical application. Here, a concentrated aqueous electrolyte with solvation-modulated Zn2+ is reported that reduces the hydrogen evolution reaction on the surface of Zn metal, avoiding the generation of ZnO and uneven deposition. Accordingly, the Zn anode exhibits 1600 h Zn plating/stripping and ≈99.96% Coulombic efficiency after 700cycles. In addition, solvation-modulated Na+ promotes the excellent structural stability of zinc hexacyanoferrate (ZnHCF) due to the rhombohedral-rhombohedral rather than rhombohedral-cubic phase transition. A ZnHCF//Zn full cell delivers an average voltage of 1.76 V and 98% capacity retention after 2000 cycles at 5 C rates.

Medium Entropy-Enabled High Performance Cubic GeTe Thermoelectrics.

The configurational entropy is an emerging descriptor in the functional materials genome. In thermoelectric materials, the configurational entropy helps tune the delicate trade‐off between carrier mobility and lattice thermal conductivity, as well as the structural phase transition, if any. Taking GeTe as an example, low‐entropy GeTe generally have high carrier mobility and distinguished zT > 2, but the rhombohedral‐cubic phase transition restricts the applications. In contrast, despite cubic structure and ultralow lattice thermal conductivity, the degraded carrier mobility leads to a low zT in high‐entropy GeTe. Herein, medium‐entropy alloying is implemented to suppress the phase transition and achieve the cubic GeTe with ultralow lattice thermal conductivity yet decent carrier mobility. In addition, co‐alloying of (Mn, Pb, Sb, Cd) facilitates multivalence bands convergence and band flattening, thereby yielding good Seebeck coefficients and compensating for decreased carrier mobility. For the first time, a state‐of‐the‐art zT of 2.1 at 873 K and average zT ave of 1.3 between 300 and 873 K are attained in cubic phased Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te. Moreover, a record‐high Vickers hardness of 270 is attained. These results not only promote GeTe materials for practical applications, but also present a breakthrough in the burgeoning field of entropy engineering.

Achieving long-life Prussian blue analogue cathode for Na-ion batteries via triple-cation lattice substitution and coordinated water capture

Prussian blue and its analogues have attracted tremendous interests as cathode materials for sodium ion batteries because of their low cost and high capacity. However, their commercial applications are hindered by low capacity utilization and insufficient cycle life. Here, we report on a MnCoNi-co-doped Prussian blue analogue with high purity and crystallinity by a citrate-assisted controlled crystallization process. Moreover, a novel strategy of in situ capturing for residual coordinated water during sodiation/desodiation process is proposed to achieve a high cyclability by using the aluminium chloride Lewis acid as electrolyte additive. The reversible cubic-rhombohedral phase transition is revealed by in situ X-ray diffraction, ex situ X-ray absorption spectroscopy and transient electrochemical impedance spectroscopy. The MnCoNi-co-doped composite delivers a capacity of 111 mAh g−1 even at 1C and retains a capacity retention of 78.7% after 1500 cycles for sodium ion batteries. This work provides important insights on eliminating the parasitical coordinated water in lattice and modulating the crystallinity to develop high-performance Prussian blue and its analogues.

Investigation of rhombohedral-cubic phase transition of La0.58Sr0.4Co0.2Fe0.8O3-δ using high temperature XRD

The phase transition of the La0.58Sr0.4Co0.2Fe0.8O3-δ oxygen transport membrane material has been investigated by X-ray diffraction at elevated temperatures. It has a rhombohedral symmetry at room temperature that transforms to a cubic symmetry at operation relevant higher temperatures, which was investigated in detail in the current work on the basis of bar type and powder specimens and discussed in terms of correlations with mechanical behavior.

On the Mechanism of the Improved Operation Voltage of Rhombohedral Nickel Hexacyanoferrate as Cathodes for Sodium-Ion Batteries.

We reported a rhombohedral Na-rich nickel hexacyanoferrate (r-NiHCF) with high discharge voltage, which also possesses long cycle stability and excellent rate capability when serving as the cathode material of Na-ion batteries. First-principles calculations suggest that the high working voltage of r-NiHCF is correlated to the asymmetric residence of Na+ ions in the rhombohedral framework in parallel with the low charge density at the Fe2+ ions. In both aqueous and ether-based electrolytes, r-NiHCF exhibits higher voltage than that of cubic NiHCF. Rate and cycle experiments indicate that r-NiHCF delivers a specific capacity of 66.8 mAh g-1 at the current density of 80 mA g-1, which is approximate to the theoretical capacity of r-NiHCF. A capacity retention of 96% can be achieved after 200 cycles. The excellent stability of r-NiHCF can be assigned to the absence of rhombohedral-cubic phase transition and negligible volume variation during electrochemical redox, as proven by the ex situ XRD patterns at different depths of charge/discharge and the DFT calculations, respectively.

Single Cubic Phase Scandia-Stabilized Zirconia: Stabilization and Aging at 600oC

Zirconia stabilized with 8-12 mol% scandia shows the highest value of oxide ion conductivity among zirconia-stabilized oxides. However, the cubic-rhombohedral phase transition at approximately 600°C prevents its use as high temperature solid electrolyte. To circumvent this problem, several additives have been proposed. In this work, ZrO2:10 mol% Sc2O3 was mixed to MnO2 to evaluate its effect on cubic phase stabilization, oxide ion conductivity and aging at 600oC. Pressed ceramic pellets of zirconia stabilized with 10 mol% scandia with addition of 1.0, 1.5, 2.0 and 2.5 mol% MnO2 were sintered at 1500 °C/2 h. X-ray fluorescence analysis was carried out to evaluate the manganese content. In situ high temperature X-ray diffraction data were collected to follow the rhombohedral-to-cubic transition. All specimens were analyzed by X-ray diffraction for structural phase evaluation and dilatometry for monitoring shrinkage. Polished and thermally etched surfaces of the sintered pellets were observed in scanning electron and scanning probe microscopes. Impedance spectroscopy measurements were carried out in the frequency range 5 Hz-13 MHz in the 300-500 °C temperature range. The single cubic structure is achieved for the specimen with 1 mol% MnO2 nominal addition, with no traces of the rhombohedral phase present in the ScSZ sintered pellets. The oxide ion conductivity of the YSZ + 1 mol%MnO2 specimen was monitored at 600 oC for 100 days by impedance spectroscopy measurements. After an initial decay, the ionic conductivity value of the solid electrolyte remained constant, suggesting its use as solid electrolyte in SOFCs operating at that temperature.

The heat capacities of thermomiotic ScF3 and ScF3–YF3 solid solutions

Scandium trifluoride (ScF3) exists in a cubic ReO3 structure that exhibits negative thermal expansion from 10 to 1100 K, while substituted Sc1−xYxF3 materials display the same behavior at room temperature but transition into positive thermal expansion rhombohedral phases upon cooling. We have measured the heat capacity of ScF3 from 0.4 to 390 K and found no evidence of a phase transition, but do find that its low-temperature heat capacity is anomalously high. The heat capacities of substituted Sc1−xYxF3 materials are also reported and show evidence of the cubic-rhombohedral phase transition for x ≥ 0.1 and smaller anomalies in the low-temperature heat capacity of the positive thermal expansion rhombohedral phases. To aid in interpretation of these results, the heat capacity of ScF3 was calculated from its phononic structure using density functional theory.

Effect of Manganese Dioxide Addition on the Cubic Phase Stability, Densification and Electrical Conductivity of Scandia-Stabilized Zirconia

Zirconia stabilized with 8-12 mol% scandia shows the highest value of oxide ion conductivity among zirconia-based oxides. However, the cubic-rhombohedral phase transition at approximately 600 °C prevents its use as high temperature solid electrolyte. To circumvent this problem, additives have been proposed. In this work, ZrO2:10 mol% Sc2O3 was mixed to MnO2 and sintered at 1200 oC/2 h + 1450 °C/2 h with addition of 1, 1.5, 2 and 2.5 mol% MnO2. All specimens were analyzed by X-ray diffraction for structural phase evaluation and dilatometry for densification. Polished and thermally etched surfaces were observed in scanning electron microscope and scanning probe microscope. Impedance spectroscopy measurements were carried out in the frequency range 5 Hz-13 MHz in the 300-500 oC temperature range. The main results show that the manganese dioxide addition prior to sintering leads to the formation of single cubic phase, to the reduction of the maximum shrinkage temperature, and to an increase in the value of the total oxide ion conductivity.

Structural, electronic, elastic and vibrational properties of BiAlO3: A first principles study

We present a study of the structural, electronic, elastic and vibrational properties of the rhombohedral BiAlO3 structure within the local density approximation of density functional theory using norm-conserving pseudopotentials. The calculated equilibrium lattice constant, angle and atomic position are in reasonable agreement with the available experimental and theoretical dates. Based on the elastic constants and their related parameters, the crystal mechanical stability have been discussed. The elastic constants for BiAlO3 are also needed to completely determine its elastic properties including polycrystalline bulk, shear and Young’s moduli, Poisson’s ratio and the elastic anisotropy. Energy band structure shows that the rhombohedral BiAlO3 has an indirect band gap between D and Г-D symmetry points. We compute Born effective charge tensor, which is found to be quite anisotropic of Bi and O atoms. BiAlO3 have been studied by applying the direct method and deriving the phonon dispersion relations which include the longitudinal/transverse optical phonon mode splitting. In the rhombohedral phase the phonon dispersion curves show a soft mode between X and Г-point. This soft mode leads to the observed rhombohedral-cubic phase transition. The results are compared with the previous calculations and available experimental data.

Structural phase transition study of the morphotropic phase boundary compositions ofNa0.5Bi0.5TiO3–PbTiO3

Neutron, synchrotron x-ray powder diffraction and dielectric studies havebeen performed for morphotropic phase boundary (MPB) compositions of the(1−x)Na1/2Bi1/2TiO3–xPbTiO3 system. At room temperature, the MPB compositions(0.10<x≤0.15) consist of a mixture of rhombohedral (space groupR3c) and tetragonal(space group P4mm) structures with the fraction of tetragonal phase increasing with increasingPbTiO3 content. On heating, while the rhombohedral phase just outside the MPB region, i.e. x = 0.10, transforms directly to a cubic phase, the rhombohedral phase of the MPB compositionstransforms gradually to a tetragonal phase, until interrupted by a rhombohedral–cubicphase transition. The correspondence of the dielectric anomalies with the structuraltransitions of the different compositions has been examined and compared with earlierreports.

Phonons and heat capacity of LaAlO 3

The ab initio calculations of the phonon dispersion relations for cubic and rhombohedral phases of LaAlO 3 were performed using the density functional perturbation theory together with plane-wave expansion and norm conserving pseudopotentials. In the cubic phase the phonon dispersion curves show a soft mode at the R-point. This soft mode leads to the observed cubic-rhombohedral phase transition. The phonon dispersion relations of rhombohedral LaAlO 3 confirm the dynamical stability of this phase. The calculated heat capacity of rhombohedral LaAlO 3 is in very good agreement with available experimental result.

Proton-Containing Yttrium-Doped Barium Cerate: A Simultaneous Structural and Dynamic Study by Neutron Scattering

The structural response to proton incorporation in a yttrium-doped barium cerate (BCY), a perovskite exhibiting high protonic and low oxide ion conductivity, and the proton motion therein are investigated in situ as a function of temperature and surrounding atmosphere (flow of moist or dry air). Undoped barium cerate (BC) is taken as a nonproton-conducting reference. Using simultaneous neutron diffraction and quasielastic neutron scattering measurements, we determine and correlate the phase transitions in the BCY lattice, the hydrogen content, and the relative mean square displacement (MSD) of hydrogen atoms over the temperature range of 150-900 C. Presence of OH groups in the BCY system affects the temperature of the high-temperature rhombohedral-cubic phase transition. Uptake of hydrogen occurs from room temperature up to 500-600 C, beyond which the system dehydrates. Onset of dehydration is observed before the rhombohedral-cubic phase transition at 700 C. The relative MSD follows the trend of hydrogen content, peaking also at 500-600 C. Decrease in MSD over a large range of temperatures (500-800 C) suggests a corresponding significant variation of the bonding strengths between the hydrogen atoms and the lattice.

On the thermodynamics of the rhombohedral-cubic phase transition of thulium(III)oxide fluoride

The rhombohedral(β)→cubic(α) phase transition of stoichiometric TmOF and the phase behaviour of samples of the analytical composition Tm(O,F) y with 1.85≤ y≤2.10 were studied by quantitative differential thermal analysis (DTA) and by X-ray powder diffraction analysis. It was found that the β→α transition occurs at 808±3 K with an increment of the formula volume of 0.5±0.05 Å 3, and that α-TmOF decomposes in a peritectoid reaction at ca. 915 K into a fluorite-related, presumably orthorhombic phase of composition Tm(O,F) 1.93 ± 0.02 and an orthorhombic vernier-type phase of composition Tm 9O 8F 11 with a=5.3765(5) Å, b=49.402(5) Å and c=5.4607(4) Å. Tm(O,F) 1.93 is formed by a eutectoid reaction of C-Tm 2O 3 with α-TmOF at ca. 900 K. An accurate determination of the enthalpy of the β→α transition of TmOF was found to be impossible because of the proximity of the temperatures of transition and decomposition. Therefore the enthalpy of transition was determined indirectly by measuring the temperatures and enthalpies of transition of solid solutions Tm 1- x Ca x O 1- x F 1+ x with 0.017≤ x≤0.062 by differential scanning calorimetry (DSC). It was found that both, T tr and Δ tr H T(tr) 0, depend linearily on the CaF 2 content of the solid solutions, and that the decomposition of the cubic α-modification is suppressed by the incorporation of CaF 2 into the TmOF lattice. The following thermodynamic data relating to the β→α phase transition of stoichiometric TmOF could be derived by extrapolation to the formula TmOF: Δ tr H 808 0=5343±267 J mol −1 and Δ tr S 808 0=6.84±0.34 J mol −1 K −1. By correlating the entropies of the β→α transition with the temperatures of transition of ROF with R=Y, La–Tm it was found that the entropies for R=La–Eu and Tm are clustering around 6.6±0.4 J mol −1 K −1 and those for R=Gd–Er and Y around 8.5±0.5 J mol −1 K −1, independently of the cationic radii, and that TmOF occupies a special position within the ROF series.

On the rhombohedral-cubic phase transition of solid solutions ROFCaF2 with RGd, Tb and Dy

The rhombohedral-cubic phase transition typical of stoichiometric rare earth oxide fluorides of the formula type ROF was investigated for the ROF-based solid solution systems R1−xCaxO1+x F1+x, with RGd, Tb, Dy, by differential scanning calorimetry and by X-ray powder diffraction. It was found that the solubility limit of CaF2 in these ROF is almost identical with x⪷ 0.09 and that the transition temperatures decrease with increasing CaF2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb temperatures decrease with increasing CaF2 content. At x ≈ 0.9 this decrease is approximately 290 K, 250 K and 240 K for RGd, Tb and Dy, respectively, as compared to the transition temperatures of pure ROF. The formula volumes of the rhombohedral as well as the cubic solid solutions were determined as a function of x, and the structural implications of the observed trend is discussed for the rhombohedral modification. A partial tentative phase diagram is exemplarily proposed for Gd1 −xCaxO1 −x F1 −x.

The investigations of the R3c R3m phase transition in Pzt-Type ceramics. correlation with ordering effects

It has been measured temperature dependencies of dielectric permittivity and electric conductivity for several samples of solid solution Pb(ZrxTi1−x)O3 for concentration range 0.095 < x < 0.145. The concentration dependencies of dielectric permittivity, piezoelectric parameters and the parameters of diffusion of rhombohedral-cubic phase transition also have been measured. It has been done for explanation of the role of short-range ordering in B-type sublattice. It was stated that many of the measured parameters are nonlinear near x = 0.125. So, we can say that for Zr/Ti proportion equal 7:1 short-range ordering takes place. It was confirmed by observation of diffuse scattering.

High-temperature X-ray diffraction study of the rhombohedral-cubic phase transition of ROF with R=Y, La, Pr, Nd, SmEr

The rhombohedral-cubic phase transition of ROF with R=Y, La, Nd, SmEr and the expansion coefficients of both modifications were studied by the high-temperature Guinier X-ray powder diffraction method. It was found that a constant formula volume increment of 0.37±0.04 Å 3, which is independent of the cationic size, is characteristic of this transition. The formula volumes depend practically linearly on temperature, and the volume expansion coefficients, which are generally larger for the cubic than for the rhombohedral modification, vary irregularly across the R 3+ series. For the case of YOF, LaOF, NdOF, SmOF, GdOF and ErOF it could be shown that experimental data for the transitional volume changes, entropies and pressure dependence of the transition temperatures obey the Clausius-Clapeyron equation within the accuracy limits of the data.