Materials properties of magnesium and calcium hydroxides from first-principles calculations

TLDR: In this article, a systematic study of ground state properties, electronic structure, dielectric and optical properties of the materials by first-principles calculations within PBE-GGA and range-separated hybrid functional schemes as well as by using GW approximation is presented.

About: This article is published in Computational Materials Science.The article was published on 2014-12-01 and is currently open access. It has received 31 citations till now. The article focuses on the topics: Band gap & GW approximation.

Figures

Table VIII: Charge characteristics related to the host ions in Mg(OH) 2 and Ca(OH) 2 (in units of |e|). The Pauling electronegativities are taken from93 δO describes the electronegativity difference with respect to O. The quantities QB(M) denote the Bader effective charges calculated from electronic densities. Z∗ij is the matrix of the Born effective charges. The quantity Z̄∗(M) = (1/3)(Z∗xx(M) + Z ∗ yy(M) + Z ∗ zz(M)) represents the average of the principal values over crystal axes.

Figure 1: Schematic presentation of unit cell of X(OH)2 (X=Mg and Ca). Mg and Ca atoms are sketched by the large grey color balls.

Table XI: The calculated values of WF (eV) for Mg(OH) 2 and Ca(OH) 2 as compared with relevant materials.

Table XII: Predictions of fundamental band gaps (in eV) for the bulk Mg(OH) 2 and Ca(OH) 2 . As an additional test, MetaGGA calculations with the TB-mBJ potential117,118 have been carried out (MBJ column).

Figure 9: The electron potential energy across X(OH)2 and vacuum. Fermi level EF is plotted by dotted line.

Figure 5: Band structure of X(OH)2 calculated within both PBE-GGA and HSE06m hybrid functional schemes. The Fermi level is set to zero.

Frequently asked questions

Q1. What are the contributions in this paper?

This work presents a study of ground state properties, electronic structure, dielectric and optical properties of magnesium and calcium hydroxides X ( OH ) 2 ( X= Mg and Ca ) within PBE-GGA and range-separated hybrid functional schemes as well as by using GW approximation. The origin of the conduction and valence band states near the band edges has been studied in terms of orbital and site projected density of states as well as by comparison with the X-ray photoelectron spectroscopy measurements. On the base of the obtained results the potential of applicability of the Mg ( OH ) 2 and Ca ( OH ) 2 crystalline hydroxides in semiconductor device engineering and optoelectronics has been analyzed. 

Q2. Why does the covalency have a partially polar character?

Due to a vertical (oriented) deformation of the localized electronic cloud from sphericity this covalency has partially polar character. 

Q3. What can be done to characterize the leading chemical interactions?

Characterization of activities and functional roles of the leading chemical interactions can be made in terms of electron partitioning schemes. 

Q4. What is the main issue related to the structural stability of the X(OH)2 hydro?

The principal issue related to the structural stability of the X(OH)2 hydroxides is connected with results of the recent computer simulations71 which have suggested that at least for the magnesium hydroxide the P3̄m1 symmetry unit cell may be less convenient as compared with the hypothetical low-symmetry P3̄ √ 3× √ 3×1 supercell. 

Q5. What is the key feature of the bonding picture in X(OH)2?

This feature is a principal component of the multifunctionality of X(OH)2 compounds because it provides the equal utilization of electronic characteristics such as large band gap and low refractive index (relevant to purely ionic systems) and the covalent contributions from the oxygen. 

Q6. What is the significance of the tensor of the Born effective charges?

In this context, analysis of the components of the tensor of the Born effective charges is of great importance because their enhanced values with respect to some reference (nominal) ionic values serve as confirmation for many-body electronic polarization effects that contribute to ionic dipole polarization. 

Q7. What is the key feature of the bonding in the X O H unit?

the key feature of bonding in the X− O −H unit is that the one-electron oxidation processes on each half-side lead to an inequivalency of chemical bonds on the left and right of the central oxygen. 

Q8. Why is the d-related anisotropy of the hydroxides mainly?

In other words, due to the lack of any noticeable d-related contributions near the fundamental absorption edge one can suggest that the anisotropy of optical properties of the hydroxides is mainly determined by electronic transitions from the three highest valence bands and governed in terms of dipolar selection rules associated with the (2px, 2py) and pz oxygen states, respectively. 

Q9. What is the rTF of the Thomas-Fermi screening?

Based on that, the authors could also add that due to strong inequality rTF >> (2/µ) universally common to ion-covalent insulators such parametrization is generally safe for tuning the optimal a; here rTF is the Thomas-Fermi screening length and µ characterizes the range separation in the HSE06 hybrid functional. 

Q10. What is the key feature of the bonding picture in the hydroxides?

In particular, as compared with the simple oxides the key feature of the bonding picture in the hydroxides, which is mainly essential in materials context, is the regulation of the charge transfer degree by the bridging oxygen anion. 

Q11. What is the reason for the difference in the valence band states?

Analysis based on Figs. 2, 3 and 4 has shown that the microscopic reason of this difference is that the valence electrons tend to be tightly bound within OH– anions what in turn results in almost flat structure of the valence band states. 

Q12. What is the main structural feature of the X(OH)2 hydroxides?

Apart from different details of the hexagonal symmetry of the X(OH)2 hydroxides, the major structural feature is strictly determined by 2D layers of XO6 octahedra that are attached together by an ordered set of perpendicular O−H connections. 

Q13. What is the main characteristic of the X(OH)2 hydroxides?

As a result, all the responses of XO6 octahedra appear to be direction dependent, what in turn gives rise to a planar anisotropy of physical properties of the X(OH)2 hydroxides. 

Q14. What is the effect of the HSE06 functional viatuning?

the comparison with results of previous studies performed within the Hartree-Fock formalism (HF) and some representative DFT based methods shows that their adaptation of the HSE06 functional viatuning the mixing coefficient has a profoundly positive effect on the structural optimization of the X(OH)2 hydroxides. 

Q15. How can the authors determine the chemical stability of Mg(OH)2?

In addition to examination of structural stability the chemical stability of Mg(OH)2 and Ca(OH)2 can be analyzed in terms of the heat of formation (∆H0f ).