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Rare earth silicates are promising environmental/thermal barrier coating (E/TBC) materials facing severe CMAS (CaO‐MgO‐Al2O3‐SiO2) corrosion. Previous studies mainly focused on the intrinsic properties of precorrosion coatings, but there were few studies on their CMAS corrosion products that play a crucial role in the performance of coatings in postservice stage. In this work, the mechanical and thermal properties of nine corrosion products between lutetium silicates and CMAS are studied using first‐principles calculations. Their differences of elastic stiffness are attributed to the different crystal structures and bonding strength. The T:O ratio is identified as a factor of the crystal structure for silicate products, and it has a good correlation with their elastic stiffness. Moreover, the divergences of thermal conductivity are dominated by three essential factors, that is, atomic vibration intensity, lattice vibrational anharmonicity, and complexity of crystal structure. Compared with rare earth silicates, six products, that is, the α‐CaSiO3, β‐CaSiO3, Ca2MgSi2O7, Ca2Al2SiO7, CaAl2Si2O8, and Ca2Lu8(SiO4)6O2, showing good damage tolerance and low thermal conductivities, are predicted to be advantageous to E/TBCs. These discoveries reveal the mechanical/thermal properties of corrosion products between lutetium silicates and CMAS and are expected to support the future researches on the performance of E/TBC in the postservice stage. [ABSTRACT FROM AUTHOR]

Surface structure and relevant oxygen vacancy play an important role in the application of RE2Si2O7 for environmental barrier coatings, in which the oxygen vacancies in RE2Si2O7 may influence their thermal and optical properties. In this work, the structure and thermodynamics of (0 0 1) and (1 1 0) surfaces of RE2Si2O7 (RE = Yb, Lu) are studied via first‐principles calculations to reveal the underlying mechanism of the surface formation and the associated oxygen vacancy behavior. The (1 1 0) surface is preferred energetically, being in good agreement with the experiential results. It is uncovered that the weak chemical bond broken dominates the decrease of the surface energy, together with the contribution from the polyhedral distortion. Furthermore, the [O3Si–O–SiO3] site is found to be the preferred site for oxygen vacancies on (1 1 0) surface. The formation energies of oxygen vacancies on the (1 1 0) surfaces are lower than those in the bulk, suggesting their segregation on the surfaces. These findings provide essential insights into the surface excitation and oxygen vacancy behaviors of RE2Si2O7, which could shield light on the experimental improvement of the thermal, mechanical, and corrosion properties for RE2Si2O7‐based environmental barrier coating materials. [ABSTRACT FROM AUTHOR]

To explore the underlying mechanism of chemical disorder in high-entropy pyrochlores, ten rare earth zirconates (nRE1/n)2Zr2O7 (n = 1, 2, and 4, RE = La, Nb, Sm, Eu, and Gd) are studied by using first-principles calculations. The mechanical and thermal properties are carefully analyzed with a special focus on local structural evolution and interatomic interaction. It is found that all three kinds of bond lengths increase linearly with lattice parameters whether the pyrochlore involves chemical disorder or not. Compared with the single-component counterparts, the multi-component pyrochlores are recognized to exhibit higher elastic constants and moduli but lower elastic anisotropy. Meanwhile, (LaSmEuGd)2Zr2O7 shows the lowest thermal conductivity, which can be attributed to the larger La atoms and the weaker La–O bonding. Furthermore, the abnormal strengthening of phonon anharmonicity in (SmEu)2Zr2O7 emphasizes the significance of fluctuation in local distortion rather than enhancement in chemical disorder on decreasing thermal conductivity for high-entropy ceramics. This work uncovers the physical origins of the chemical disorder effect on mechanical and thermal properties for pyrochlores and further shed some lights on the design of high-performance high-entropy ceramics with great potential applications including thermal barrier coatings. [ABSTRACT FROM AUTHOR]

Exploration and exploitation of efficient and nonprecious catalysts for hydrogen evolution reaction (HER) is one of the key solutions to energy problems. Two‐dimensional (2D) MXenes have attracted extensive attention as potential catalysts and supports due to their unique structure and customizable properties. In this work, we have systematically investigated the structural stability and electronic properties of chalcogen‐functionalized MXenes Ti2CT2 (T = O, S, Se, and Te) and Pt‐anchored Ti2CT2 MXenes (Pt‐Ti2CT2). Density functional theory calculations have discovered that all Ti2CT2 exhibit metallic electronic structures except Ti2CO2 with an indirect bandgap. Additionally, the electronic densities of states at Fermi level for Pt‐Ti2CT2 are greatly enhanced after anchoring Pt atoms. The reduction of chalcogen electronegativity is recognized to mainly contribute to weaker bond strength but greater electronic activity. Moreover, Pt‐Ti2CTe2 is predicted to exhibit near‐zero Gibbs free energy of hydrogen adsorption especially at high hydrogen coverage, indicating its significant potential as HER catalysts. It is also revealed that Ti2CT2 and Pt‐Ti2CT2 MXenes are not only ideal candidates of catalyst materials but also could serve as good catalyst substrates. This work provides insights into the structural and chemical complexity of MXenes for catalyst applications but also demonstrates a new strategy to modulate high‐efficiency and low‐cost catalyst and support materials. [ABSTRACT FROM AUTHOR]

The research of nanocrystalline pyrochlores highlights the importance of the surface structure, composition and segregated point defect in their thermal, electrical, optical, magnetic, and catalytic performances. In order to provide a basic view on the surface‐related phenomena, thermodynamic stabilities of three low‐index (100), (110), and (111) surfaces for A2Sn2O7 (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd), together with their configurations, electronic structures and related oxygen vacancies are investigated using first‐principles calculations. The (111) surfaces with A3SnO8 and ASn3O6 terminations are predicted to be stable due to their low surface energies. Meanwhile, the (110) surfaces with A2Sn2O8 and A2Sn2O6 terminations are found to may also form. For these surface structures, the amount of broken bonds play the main role in their structural stability, and the local coordination environment variation also has minor contribution to it. Moreover, oxygen vacancies are observed to segregate on the surface layer, owing to lower energy of breaking bonds accompanying with oxygen vacancy formation and the larger relaxation space comparing to the counterpart in bulk. These results are expected to provide guidance on optimizing the performances of these compounds through surface engineering. [ABSTRACT FROM AUTHOR]

RE2SiO5 (RE = Yb and Lu) are significant environmental barrier coating (EBC) materials, in which surface and oxygen vacancy play crucial roles in their structural stability and functionality. In this work, the structural configuration and thermodynamics of (1 0 0), (0 1 0), and (0 0 1) surfaces of RE2SiO5 are investigated by first‐principles calculations. The (0 0 1) surface is preferred energetically, which is attributed to the weak bond broken environment and large rare‐earth polyhedron distortion on this surface. Moreover, the formation energies of various oxygen vacancies on the stable (0 0 1) surface are estimated and the optimal location for oxygen vacancies is held by the [SiO4] tetrahedron. The oxygen vacancies are more likely to segregate on the surface because of the lower formation energies on the surfaces compared with those in the bulk. These findings are expected to enable the development of RE2SiO5‐based EBCs by tuning grain size and/or thin film growth orientation. [ABSTRACT FROM AUTHOR]

The surface structures, bond variations, and segregation of oxygen vacancies play crucial roles in the structural stability and functionality of nanocrystalline rare‐earth zirconate pyrochlores. In this work, the stabilities of (1 0 0), (1 1 0), and (1 1 1) surfaces of pyrochlore A2Zr2O7 (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd) are investigated by first‐principles calculations. Surface reconstruction occurs on (1 1 0) surface with a transition of ZrO6 octahedron to ZrO4 tetrahedron, leading to their large relaxation energies. In combination with the small amount of broken bonds during the surface formation process, the (1 1 0) surfaces are identified having the lowest surface formation energies than the (1 0 0) and (1 1 1) surfaces. Moreover, the reconstructed (1 1 0) surface has characteristics of the segregation of oxygen vacancies. The surface oxygen vacancies have the low migration barriers (<1.2 eV), which are comparable with those in bulk and ensure the long‐distance diffusion of oxygen vacancies in A2Zr2O7. These discoveries provide fundamental insight to the surface structure and related oxygen vacancy behavior, which are expected to guide the optimization of the surface related properties for nanocrystalline rare‐earth zirconates. [ABSTRACT FROM AUTHOR]

In order to explore the temperature‐dependent structural and mechanical/thermal property evolution of pyrochlores, multicomponent rare‐earth zirconates (4RE1/4)2Zr2O7 (RE = La, Nd, Sm, Eu, and Gd) and corresponding single‐component compounds are investigated by molecular dynamics simulations. The structural parameters and mechanical/thermal properties reported in experiments are well reproduced. With temperature enhancement, the bond lengths become large and polyhedrons tend to deform more, whereas the second‐order elastic constants and polycrystalline mechanical moduli gradually reduce. It can be explained by the obvious change of (ZrO6) polyhedrons originating from the Zr–O bond stretching. Furthermore, thermal conductivities show decreasing tendency owing to a sharp decline of the phonon mean free path related to enhanced phonon scattering. As a big influence of temperature on (ZrO6) polyhedrons occurs, the multicomponent design at the Zr‐site is suggested to be paid more attention. This work is expected to shield light on the design for multicomponent pyrochlores as thermal barrier coatings. [ABSTRACT FROM AUTHOR]

Understanding the mechanism of point defects and the oxygen vacancy diffusion in rare earth (RE) silicates is important to evaluate their performance as environmental barrier coatings. In this work, the defect formation energies are calculated and the similar predominant native point defect type, that is, O Frenkel defect, in RE2SiO5 and RE2Si2O7 (RE = Yb and Lu) is uncovered. Furthermore, the nonstoichiometry related defects and phase composition in RE2SiO5 and RE2Si2O7 is also studied. For RE2SiO5, SiRE·+Oi″ or SiRE·+VRE″′ is the predominate defect complex accompanied with the RE2Si2O7 separation under SiO2‐rich condition, while RESi′+VO·· or RESi′+REi··· is favorable under RE2O3‐rich condition. In the case of RE2Si2O7, SiRE·+VRE″′ or SiRE·+Oi″ is the favorable defect complex under SiO2‐rich condition while RESi′+VO·· or RESi′+REi··· dominates the defects, accompanied with the formation of RE2SiO5, under RE2O3‐rich case. Finally, the high oxygen migration barriers are revealed for both RE2SiO5 and RE2Si2O7, indicating the low oxygen permeability in these silicates. These results will not only guide the future performance optimization of RE2SiO5 and RE2Si2O7 through tailoring their composition, but also suggest their potential application as oxidation protective coatings. [ABSTRACT FROM AUTHOR]

The effect of AlxGa1−xN electron blocking layer (EBL) on suppressing electron leakage from the multiple quantum wells (MQWs) active region has been systematically investigated by conducting the first-principles calculations. Our results revealed that the Al concentration in EBL plays a dominant role in modulating the band offsets of AlxGa1−xN/GaN(0001) (0 < x ≤ 1) heterointerfaces. The existence of charge accumulation and band bending at these heterointerfaces can be attributed to the strong polarization electric field with the order of MV/cm. We further demonstrated that the EBL can suppress electron leakage from the active region as well as prevent the hole injection from the p-GaN region. Lastly, to boost the quantum efficiency of light-emitting diodes (LEDs), we proposed a "synergistic effect" of Al concentration in EBL and In-content in MQWs. These results provided a fundamental insight into the physical mechanism of the AlxGa1−xN EBL to reduce the efficiency droop in GaN-based LEDs. [ABSTRACT FROM AUTHOR]

To explore the underlying mechanism of chemical disorder in high-entropy pyrochlores, ten rare earth zirconates (nRE1/n)2Zr2O7 (n = 1, 2, and 4, RE = La, Nb, Sm, Eu, and Gd) are studied by using first-principles calculations. The mechanical and thermal properties are carefully analyzed with a special focus on local structural evolution and interatomic interaction. It is found that all three kinds of bond lengths increase linearly with lattice parameters whether the pyrochlore involves chemical disorder or not. Compared with the single-component counterparts, the multi-component pyrochlores are recognized to exhibit higher elastic constants and moduli but lower elastic anisotropy. Meanwhile, (LaSmEuGd)2Zr2O7 shows the lowest thermal conductivity, which can be attributed to the larger La atoms and the weaker La–O bonding. Furthermore, the abnormal strengthening of phonon anharmonicity in (SmEu)2Zr2O7 emphasizes the significance of fluctuation in local distortion rather than enhancement in chemical disorder on decreasing thermal conductivity for high-entropy ceramics. This work uncovers the physical origins of the chemical disorder effect on mechanical and thermal properties for pyrochlores and further shed some lights on the design of high-performance high-entropy ceramics with great potential applications including thermal barrier coatings. [ABSTRACT FROM AUTHOR]