skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: 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.  more » « less
Award ID(s):
1740111 1434897
PAR ID:
10072413
Author(s) / Creator(s):
; ; ; ; ; ; ;
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
More Like this
  1. ; ; ; (, Dalton Transactions)
    null (Ed.)
    A series of new Ce( iv ) based fluorides with two different compositions, Cs 2 MCe 3 F 16 (M = Ni 2+ , Co 2+ , Mn 2+ , and Zn 2+ ) and Na 3 MCe 6 F 30 (M = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) were synthesized as high quality single crystals via a mild hydrothermal route. The compounds with the composition Cs 2 MCe 3 F 16 (M = Ni 2+ , Co 2+ , Mn 2+ , and Zn 2+ ) crystallize in the hexagonal crystal system with space group P 6 3 / mmc and are isotypic with the uranium analogs, whereas the Na 3 MCe 6 F 30 (M = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) compounds crystallize in the trigonal space group P 3̄ c 1 and are isotypic with the uranium and thorium analogs Na x MM′ 6 F 30 (M′ = Th, U). The Cs 2 MCe 3 F 16 compounds exhibit a complex 3D crystal structure constructed of edge-sharing cerium trimers, in which all three Ce atoms share a common μ 3 -F unit. The Na 3 MCe 6 F 30 compounds are constructed of edge- and vertex-sharing cerium polyhedra connected to each other to form Ce 6 F 30 6− framework, which can accommodate only relatively smaller trivalent cations (M 3+ = Al 3+ , Ga 3+ , Fe 3+ , and Cr 3+ ) as compared to uranium and thorium analogs. Magnetic susceptibility measurements were carried out on the samples of Cs 2 MCe 3 F 16 (M = Ni 2+ and Co 2+ ), which exhibit paramagnetic behavior. 
    more » « less
  2. ; ; ; (, Physical Chemistry Chemical Physics)
    The hydrogen-bonding environments at the COOH moiety in eight polycrystalline polymorphs of palmitic acid are explored using solid-state NMR. Although most phases have no previously reported crystal structure, measured 13 C chemical shift tensors for COOH moieties, combined with DFT modeling establish that all phases crystallize with a cyclic dimer ( R 22(8)) hydrogen bonding arrangement. Phases A 2 , B m and E m have localized OH hydrogens while phase C has a dynamically disordered OH hydrogen. The phase designated A s is a mix of five forms, including 27.4% of B m and four novel phases not fully characterized here due to insufficient sample mass. For phases A 2 , B m , E m , and C the anisotropic uncertainties in the COOH hydrogen atom positions are established using a Monte Carlo sampling scheme. Sampled points are retained or rejected at the ±1 σ level based upon agreement of DFT computed 13 COOH tensors with experimental values. The collection of retained hydrogen positions bear a remarkable resemblance to the anisotropic displacement parameters ( i.e. thermal ellipsoids) from diffraction studies. We posit that this similarity is no mere coincidence and that the two are fundamentally related. The volumes of NMR-derived anisotropic displacement ellipsoids for phases with localized OH hydrogens are 4.1 times smaller than those derived from single crystal X-ray diffraction and 1.8 times smaller than the volume of benchmark single crystal neutron diffraction values. 
    more » « less
  3. ; ; (, Scientific Reports)
    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). 
    more » « less
  4. ; (, Journal of Computational Chemistry)
    The optimized geometries, vibrational frequencies, and dissociation energies from MP2 and CCSD(T) computations with large correlation consistent basis sets are reported for (H2S)2and H2O/H2S. Anharmonic vibrational frequencies have also been computed with second‐order vibrational perturbation theory (VPT2). As such, the fundamental frequencies, overtones, and combination bands reported in this study should also provide a useful road map for future spectroscopic studies of the simple but important heterogeneous H2O/H2S dimer in which the hydrogen bond donor and acceptor can interchange, leading to two unique minima with very similar energies. Near the CCSD(T) complete basis set limit, the HOH⋯SH2configuration (H2O donor) lies only 0.2 kcal mol−1below the HSH⋯OH2structure (H2S donor). When the zero‐point vibrational energy is included, however, the latter configuration becomes slightly lower in energy than the former by <0.1 kcal mol−1. © 2018 Wiley Periodicals, Inc. 
    more » « less
  5. ; ; ; ; ; (, Journal of Materials Chemistry C)
    null (Ed.)
    In this paper, the photoluminescent properties of a lead-free double perovskite Cs 2 NaInCl 6 doped with Sb 3+ are explored. The host crystal structure is a cubic double perovskite with Fm 3̄ m symmetry, a = 10.53344(4) Å, and rock salt ordering of Na + and In 3+ . It is a wide bandgap compound ( E g ≈ 5.1 eV), and substitution with Sb 3+ leads to strong absorption in the UV due to localized 5s 2 → 5s 1 5p 1 transitions on Sb 3+ centers. Radiative relaxation back to the 5s 2 ground state, via a 3 P 1 → 1 S 0 transition, leads to intense blue luminescence, centered at 445 nm, with a photoluminescent quantum yield of 79%. The Stokes shift of 0.94 eV is roughly 33% smaller than it is in the related vacancy ordered double perovskite Cs 2 SnCl 6 . The reduction in Stokes shift is likely due to a change in coordination number of Sb 3+ from 6-coordinate in Cs 2 NaInCl 6 to 5-coordinate in Cs 2 SnCl 6 . In addition to the high quantum yield, Cs 2 NaInCl 6 :Sb 3+ exhibits excellent air/moisture stability and can be prepared from solution; these characteristics make it a promising blue phosphor for applications involving near-UV excitation. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email