Abstract We present a systematic investigation of thermodynamic stability, phase-reaction, and chemical activity of Al containing disordered Ti 2 (Al-Ga)C MAX phases using machine-learning driven high-throughput framework to understand the oxidation resistance behavior with increasing temperature and exposure to static oxygen. The A-site (at Al) disordering in Ti 2 AlC MAX (M=Ti, A=Al, X=C) with Ga shows significant change in the chemical activity of Al with increasing temperature and exposure to static oxygen, which is expected to enable surface segregation of Al, thereby, the formation of Al 2 O 3 and improved oxidation resistance. We performed in-depth convex hull analysis of ternary Ti–Al–C, Ti–Ga–C, and Ti–Al–Ga–C based MAX phase, and provide detailed contribution arising from electronic, chemical and vibrational entropies. The thermodynamic analysis shows change in the Gibbs formation enthalpy (Δ G form ) at higher temperatures, which implies an interplay of temperature-dependent enthalpy and entropic contributions in oxidation resistance Ga doped Ti 2 AlC MAX phases. A detailed electronic structure and chemical bonding analysis using crystal orbital Hamilton population method reveal the origin of change in phases stability and in oxidation resistance in disorder Ti 2 (Al 1−x Ga x )C MAX phases. Our electronic structure analysis correlate well with the change in oxidation resistance of Ga doped MAX phases. We believe our study provides a useful guideline to understand to role of alloying on electronic, thermodynamic, and oxidation related mechanisms of bulk MAX phases, which can work as a precursor to understand oxidation behavior of two-dimensional MAX phases, i.e., MXenes (transition metal carbides, carbonitrides and nitrides).
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Anharmonic contribution to the stabilization of Mg(OH) 2 from first principles
Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P 3̄ (No. 147, point group −3), C 2/ m (No. 12, point group 2), P 3 m 1 (No. 156, point group 3 m ) and P 3̄ m 1 (No. 164, point group −3 m ) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH) 2 , in particular for the P 3̄ m 1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P 3̄ m 1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P 3̄ m 1 structure towards P 3̄. In this paper, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C 2/ m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.
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- NSF-PAR ID:
- 10072413
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 20
- Issue:
- 26
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 17799 to 17808
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Perovskite photovoltaic ABX3systems are being studied due to their high energy‐conversion efficiencies with current emphasis placed on pure inorganic systems. In this work, synchrotron single‐crystal diffraction measurements combined with second harmonic generation measurements reveal the absence of inversion symmetry below room temperature in CsPbBr3. Local structural analysis by pair distribution function and X‐ray absorption fine structure methods are performed to ascertain the local ordering, atomic pair correlations, and phase evolution in a broad range of temperatures. The currently accepted space group assignments for CsPbBr3are found to be incorrect in a manner that profoundly impacts physical properties. New assignments are obtained for the bulk structure: (above ≈410 K),
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Abstract Next‐generation electronics and energy technologies can now be developed as a result of the design, discovery, and development of novel, environmental friendly lead (Pb)‐free ferroelectric materials with improved characteristics and performance. However, there have only been a few reports of such complex materials’ design with multi‐phase interfacial chemistry, which can facilitate enhanced properties and performance. In this context, herein, novel lead‐free piezoelectric materials (1‐
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Abstract Using molten‐salt synthetic techniques, NaNbO3(Space group
Pbcm ; No. 57) was prepared in high purity at a reaction time of 12 hours and a temperature of 900°C. All NaNbO3products were prepared from stoichiometric ratios of Nb2O5and Na2CO3together with the addition of a salt flux introduced at a 10:1 molar ratio of salt to NaNbO3, that is, using the Na2SO4, NaF, NaCl, and NaBr salts. A solid‐state synthesis was performed in the absence of a molten salt to serve as a control. The reaction products were all found to be phase pure through powder X‐ray diffraction, for example, with refined lattice constants ofa = 5.512(5) Å,b = 5.567(3) Å, andc = 15.516(8) Å from the Na2SO4salt reaction. The products were characterized using UV‐Vis diffuse reflectance spectroscopy to have a bandgap size of ~3.5 eV. The particles sizes were analyzed by scanning electron microscopy (SEM) and found to be dependent upon the flux type used, from ~<1 μm to >10 μm in length, with overall surface areas that could be varied from 0.66 m2/g (for NaF) to 1.55 m2/g (for NaBr). Cubic‐shaped particle morphologies were observed for the metal halide salts with the set of exposed (100)/(010)/(001) crystal facets, while a truncated octahedral morphology formed in the sodium sulfate salt reaction with predominantly the set of (110)/(101)/(011) crystal facets. The products were found to be photocatalytically active for hydrogen production under UV‐Vis irradiation, with the aid of a 1 wt% Pt surface cocatalyst. The platinized NaNbO3particles were suspended in an aqueous 20% methanol solution and irradiated by UV‐Vis light (λ > 230 nm). After 6 hours of irradiation, the average total hydrogen production varied with the particle morphologies and sizes, with 753 µmol for Na2SO4, 334 µmol for NaF, 290 µmol for NaCl, 81 µmol for NaBr, and 249 µmol for the solid‐state synthesized NaNbO3. These trends show a clear relationship to particle sizes, with smaller particles showing higher photocatalytic activity in the order of NaF > NaCl > NaBr. Furthermore, the particle morphologies obtained from the Na2SO4flux showed even higher photocatalytic activity, though having a relatively similar overall surface area, owing to the higher activity of the (110) crystal facets. The apparent quantum yield (100 mW/cm2,λ = 230 to 350 nm, pH = 7) was measured to be 3.7% for NaNbO3prepared using the NaF flux, but this was doubled to 6.8% when prepared using the Na2SO4flux. Thus, these results demonstrate the powerful utility of flux synthetic techniques to control particle sizes and to expose higher‐activity crystal facets to boost their photocatalytic activities for molecular hydrogen production.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8CP02490A
