Title: Anharmonic contribution to the stabilization of Mg(OH)₂ from first principles

Abstract

Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P$$\bar{3}$$ (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P$$\bar{3}$$m1 (No. 164, point group -3m) 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 PP$$\bar{3}$$m1 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$$\bar{3}$$m1 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$$\bar{3}$$m1 structure towards P$$\bar{3}$$. Here, 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 C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.

Authors:

Treviño, P. ^[1]; Garcia-Castro, A. C. ^[2]; López-Moreno, S.  ^[3];

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• Search DOE PAGES for ORCID "0000-0001-6292-8275"

Bautista-Hernández, A.  ^[4];

• Search DOE PAGES for author "Bautista-Hernández, A."
• Search DOE PAGES for ORCID "0000-0003-1360-9483"

Bobocioiu, E. ^[5]; Reynard, B. ^[5]; Caracas, R. ^[5]; Romero, A. H.  ^[6]

• Search DOE PAGES for author "Romero, A. H."
• Search DOE PAGES for ORCID "0000-0001-5968-0571"

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+ Show Author Affiliations

1. Center for Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV-IPN), Mexico City (Mexico)
2. Univ. of Liege, (Belgium); Univ. Industrial de Santander, Bucaramanga (Colombia)
3. San Luis Potosí Institute of Scientific Research and Technology (IPICyT), San Luis Potosí (Mexico)
4. Benemerita Universidad Autónoma de Puebla (BUAP) (Mexico)
5. Lab. de Gélogie de Lyon UMR CNRS (France)
6. Benemerita Universidad Autónoma de Puebla (BUAP) (Mexico); West Virginia Univ., Morgantown, WV (United States)

Publication Date:

Tue Jun 05 00:00:00 EDT 2018

Research Org.:

West Virginia Univ., Morgantown, WV (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); CONACyT; National Science Foundation (NSF); eDARI

OSTI Identifier:

1594015

Alternate Identifier(s):

OSTI ID: 1455083

Grant/Contract Number:  

SC0016176; 152153; 1434897; 1740111; TKII-R2018-SLM1; ACI-1053575; A0011006368

Resource Type:

Accepted Manuscript

Journal Name:

Physical Chemistry Chemical Physics. PCCP

Additional Journal Information:

Journal Volume: 20; Journal Issue: 26; Journal ID: ISSN 1463-9076

Publisher:

Royal Society of Chemistry

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; equation-of-state; ABINIT software package; augmented-wave method; initio total-energy; high-pressure phase; magnesium-hydroxide; neutron-diffraction; thermal-expansion; Hartree-Fock; brucite