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First principle investigations of the physical properties of hydrogen-rich MgH2

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2013 Phys. Scr. 88 065704

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Phys. Scr. 88 (2013) 065704 (8pp) doi:10.1088/0031-8949/88/06/065704

First principle investigations ofthe physical properties ofhydrogen-rich MgH2

Mohammed Zarshenas1, R Ahmed1, Mohammed Benali Kanoun2,Bakhtiar ul Haq1, Ahmad Radzi Mat Isa1 and Souraya Goumri-Said3

1 Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, UTM Skudai,81310 Johor, Malaysia2 KAUST Catalysis Center, King Abdullah University of Science & Technology (KAUST),Thuwal 23955-6900, Saudi Arabia3 Phyical Sciences Engineering Division, King Abdullah University of Science & Technology (KAUST),Thuwal 23955-6900, Saudi Arabia

E-mail: rashidahmed@utm.my, rasofi@hotmail.com and Souraya.Goumri-Said@kaust.edu.sa

Received 7 January 2013Accepted for publication 3 October 2013Published 28 November 2013Online at stacks.iop.org/PhysScr/88/065704

AbstractHydrogen being a cleaner energy carrier has increased the importance of hydrogen-containinglight metal hydrides, in particular those with large gravimetric hydrogen density likemagnesium hydride (MgH2). In this study, density functional and density functionalperturbation theories are combined to investigate the structural, elastic, thermodynamic,electronic and optical properties of MgH2. Our structural parameters calculated with thoseproposed by Perdew, Burke and Ernzerof generalized gradient approximation (PBE-GGA) andWu–Cohen GGA (WC-GGA) are in agreement with experimental measurements, however theunderestimated band gap values calculated using PBE-GGA and WC-GGA were greatlyimproved with the GGA suggested by Engle and Vosko and the modified Becke–Johnsonexchange correlation potential by Trans and Blaha. As for the thermodynamic properties thespecific heat values at low temperatures were found to obey the T 3 rule and at highertemperatures Dulong and Petit’s law. Our analysis of the optical properties of MgH2 alsopoints to its potential application in optoelectronics.

PACS numbers: 71.15.Mb, 71.20.Be, 71.20.±b, 82.47.Cb, 88.30.rd

(Some figures may appear in color only in the online journal)

1. Introduction

The rapid exhaustion of conventional hydrocarbon energyresources and their well-known severe effects on globalwarming, due to ozone layer damage and environmentalpollution, strongly demand more efficient, cleaner andcheaper energy sources. In this regard, hydrogen (H2)is considered as an environmentally friendly and cleaneralternative to fossil fuels. However, for application as a fuelon a large scale, a highly compact storage material containinga high-mass content of H2 is necessitated [1]. For this purpose,hydrides of group-I [2] and group-II [3] of the periodic tableare considered promising candidates.

Magnesium hydride (MgH2) with its light weight and lowcost is always at the fore-front due to the fact that it containsan appreciable amount (7.66%) of H2. The major hurdle for itsapplication on a commercial scale is its high thermodynamicstability, i.e. high dehydrogenation temperature, and slowabsorption kinetic limits [3–7]. In several investigationsimprovements to dehydrogenation and kinetic properties isreported through synthesis techniques [8–13], by reducing thediffusion barrier of H2 atoms in MgH2 and using catalysts onthe metal surface [14–17], however a comprehensive solutionis keenly desired for its application.

Due to its H2-rich energy storage and, more recentlyshown, its relevance to optoelectronics and rechargeable

0031-8949/13/065704+08$33.00 Printed in the UK & the USA 1 © 2013 The Royal Swedish Academy of Sciences

Phys. Scr. 88 (2013) 065704 M Zarshenas et al

batteries, MgH2 has been actively investigated experimentallyand theoretically for a long time. At the theoreticallevel different schemes of calculation [16–26] have beenimplemented. To calculate the lattice energy of MgH2, aBorn–Mayer type scheme of calculation [16] is employedassuming the purely ionic nature of the compound, howeverobtained results also indicate the contribution of covalentbonding in MgH2. The existence of both types of (ionicand covalent) bonding another study has also been pointedout [17]. In addition some recent studies reveal its potentialfor other energy technologies. Ramzan et al [27] in adensity functional theory (DFT) study highlighted MgH2as a potential material for combining fuel cell and batterytechnologies. Although the literature on MgH2 coversmany aspects for the understanding of this importantmaterial, the dilemma of its practical application is yetunresolved. To unveil its potential for practical application,further experimental and theoretical studies are passionatelydemanded.

It is commonly thought that the successful applicationof a material as a base material in any kind of technologycan be well-understood by securing a clear picture of itselectronic structure. Although a number of efforts havealready been made to accomplish an obvious depiction ofthe electronic structure of MgH2, its electronic band structureand related properties are under debate. For example, itsmaterial nature as an insulating or wide band semiconductoris still under discussion. This is due to the reported valuesof fundamental band gap energy (an important parameterestablishing a material as a semiconductor or insulator),both theoretically and experimentally, being contradictoryincluding most recently the reported value in the study ofPaik et al [28], together with many other previously reportedinvestigations [29–35]. This inconsistency among differentcalculated and measured values of energy band gap might bethe main hurdle in the tailoring of electronic properties for itsapplication in different energy technologies.

In computational material science DFT-based approacheshave achieved almost the central role in understandingthe physical properties of materials at the atomic scaleand are playing a pivotal role in the design/discovery ofnew materials without any prior experimental knowledge.However the accuracy of the calculation of electronicstructure properties depends upon the suitable incorporationof exchange correlation (XC) energy functional/potential insolving Kohn Sham (KS) equations.

In fact, there is no such tractable tool available toevaluate XC exactly. Several approximations are in practicerelated to XC, for example, local spin density approximations(LSDA or LDA) [36], Perdew, Burke and Ernzerofgeneralized gradient approximations (PBE-GGA) [37], etc.Although these approximations describe the ground statephysical properties very resourcefully, excited state propertiessuch as the electronic band gap energy are significantlyunderestimated. In order to overcome this deficiency, theGGA suggested by Engle–Vosko (GGA-EV) [38] and themodified Becke–Johnson (mBJ) potential proposed by Tranand Blaha [39] are practiced on a large scale to deal withelectronic properties.

In this work we carry out our investigations forthe electronic, bonding and optical properties of MgH2

using DFT- and DFPT (density functional perturbationtheory)-based computational approaches as implemented inWIEN2k and CASTEP codes in addition to investigating itsstructural, elastic and thermodynamic properties. Structuralproperties have been calculated within the framework ofDFT by incorporating PBE-GGA [37] and Wu and Cohen(WC)-GGA [40] as an XC energy functional, whereas forthe calculation of electronic properties GGA-EV and mBJare used besides PBE-GGA and WC-GGA for the purposeof comparison and to realize their effect on the electronicproperties. For the calculations of elastic, mechanical andthermodynamic properties, the DFPT is employed whereasoptical properties are calculated using PBE-GGA by applyinga correction using the scissors approach.

2. Computational details

In order to study the structural and electronic propertiesof MgH2, we employ the DFT-based full potential (FP)linearized augmented plane wave + local orbital L(APW + lo)

approach as implemented in the WIEN2k package [41]. TheXC energy/potential functional is initially described withinthe GGA based on the PBE and WC schemes. Usuallyfundamental band gap values are underestimated by simpleLDA and GGA. To overcome this shortcoming, we employin this study another GGA proposed by EV, and the Tranand Blaha exchange potential, known as a mBJ potential [42],which have been predicted to result in band gaps that are moreaccurate than those computed with standard PBE-GGA. Allcalculations are performed self-consistently to an accuracyof better than one meV per unit cell, using 1000 k-points inthe full first Brillouin zone (BZ). Muffin-tin (MT) spheresradii (RMT) values are chosen as 2.36 au for Mg and 1.27 aufor H atoms; the RMT × KMAX = 8 cutoff for the plane-waveexpansion. These calculations are performed by optimizingthe atomic positions of the simulated crystal unit cell as wellas their internal parameters. The self-consistent calculationsare considered to be converged when the difference, betweentwo consecutive steps, in the total energy of the crystal is0.1 mRyd and in the total electronic charge is 0.001e.

Furthermore, the calculations of the thermodynamicand dynamical properties of MgH2 DFPT are employedusing CASTEP code [43]. Although a plane wave (PW)basis set can be very large, the use of optimizedpseudopotential significantly reduces the number of planewaves needed to accurately represent the electronic states.Interactions of the electrons with ions were represented bya Vanderbilt-type ultrasoft pseudopotential [44]. In FP-L(APW + lo) calculations, the electronic XC energy was treatedunder the GGA-WC [40]. In the process of setting up theCASTEP run [43], all the possible structures are optimizedand relaxed by the BFGS algorithm proposed by Broyden,Fletcher, Goldfarb and Shannon, which provides a fastway of binding the lowest energy structure and supportscell optimization. The optimization is performed until theforces on the atoms are less than 0.01 eV Å−1 and all thestress components are less than 0.02 GPa, the tolerance inthe self-consistent field calculation is 5 × 10−7 eV atom−1.Ultra-soft pseudo-potentials are expanded within a plane wavebasis set with a 520 eV cutoff energy in the process of

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Phys. Scr. 88 (2013) 065704 M Zarshenas et al

Figure 1. The calculated energy volume (E–V) plots obtained withPBE-GGA and WC-GGA. For these E–V plots the data points arefitted with the Mornaghan equation of states.

Table 1. The calculated structural parameters with PBE-GGA andWC-GGA.

a b c µ B0 B ′

0

PBE 4.77 4.77 3.11 1.034 51.68 3.33WC 4.70 4.70 3.14 0.996 53.19 3.36Exp [45] 4.516 4.516 3.02

optimization. The k-points sampling is 7 × 7 × 11 accordingto the Monkhorst–Pack method, leading to 270 irreduciblek-points. For all the optimized structures, the Mullikencharges and bond populations are investigated in detail witha method that projects plane wave states onto a linearcombination of atomic orbitals basis set which is widely usedto perform charge transfers and populations analysis.

3. Results and discussion

3.1. Structural, elastic, mechanical and thermal properties

At ambient conditions, MgH2 crystallizes in a rutile phasewith the space group no. 136(P42/mnm), atomic positionMg (0, 0, 0), H (0.304, 0.304, 0) and lattice constants a =

b = 4.516 Å, and c = 3.02 Å [45]. To calculate the structuralproperties each simulated structure (over the entire range ofvolume ±10% around the equilibrium position) is optimized.For comparison, the calculated values of the total energyand volume corresponding to each optimized structure usingPBE-GGA and WC-GGA are shown in figure 1. Data is thenfitted into the Murnaghan [46] equation of states to find outthe equilibrium structural parameters. Our calculated valuesof lattice constants along with bulk modulus and its pressurederivative at the level of PBE-GGA and WC-GGA are listedin table 1 together with experimental measurements. Fromtable 1 it is clear that our results with WC-GGA are morereasonable and closer to the experimentally measured valuesof lattice constants [45].

Elastic constants play a crucial role in providingfundamental knowledge about inter-atomic bonding forcesand elastic stability that subsequently reveal crystal stability.Our calculated elastic constants (C11, C12, C13, C33, C44

and C66), along with other related parameters for the rutilephase of MgH2 are presented in table 2. The elastic stabilityof MgH2 is evaluated using the Born–Huang criteria [47]where the bulk modulus B and shear modulus G are estimatedusing the Voigt–Reuss–Hill (VRH) approach. Furthermore,the Young’s modulus, E, and Poisson’s ratio, υ, are calculatedfrom the shear and bulk moduli [48]. All these calculatedparameters are summarized in table 2. The calculated zeropressure bulk modulus (B0) using the equation of states [46]within WC-GGA is found to be in close agreement tothat obtained from the VRH approximation. The value ofB0 = 53.19 GPa indicates that MgH2 is a mechanically hardmaterial. This may be due to its dense atomic packing in therutile phase.

In comparison with DFT, a more accurate approach hasbeen attained by DFPT as it enables precise calculations inevery point of the BZ. The calculations for phonon dispersionrelation were carried out within the framework of DFPTusing CASTEP code [43]. A schematic overview of thephonon dispersion curve in the BZ as depicted in figure 2endorses the dynamically stable nature of MgH2. This resultedin all the frequencies being positive throughout the BZ.Moreover, the phonon dispersion curves exhibit two separatebands indicating a large mismatching between the atomicmasses of Mg and H. The effect of temperature on boththe vibrational heat capacity at constant volume CV and thethermal expansion coefficient α are shown in figure 3. It isevident from the figure that in low temperature limits CV

is in agreement with the T 3 power law; however, at highertemperatures it attains a classic limit of 70 J K−1 mol−1 thatfollows Dulong and Petit’s law. The temperature dependencethermal expansion coefficient α plot shows that α increasesrapidly up to 550 K, however, for temperatures above 550 Kthe increase in α is very small. For the next 450 K the increasein α is also small, around 2.15 units. In figure 3, the variationof α with CV is shown. The value of α is zero for CV up to7.7011 J K−1 mol−1, on the other hand, for higher values ofCV, α starts increasing. This variation in α with CV is morerapid up to 58.3314 J K−1 mol−1, whereas for higher values ofCV variation in α is very low.

3.2. Electronic structure and bonding

The calculated band structure of MgH2 with differentexchange and correlation potentials is illustrated in figure 4and the corresponding energy band gap values are listedin table 3. The calculated values of energy band gapwith PBE-GGA and WC-GGA (3.72 and 3.60 respectively)are considerably underestimated to experimental values5.16 eV [49] or 5.6 eV [50] but correspond with previousreported theoretical results with the same GGA. Thesediscrepancies in the band gap values are due to the fact thatthese XC functionals are designed for better reproductionof XC energy, and are not flexible enough to reproducecorresponding XC potential that subsequently leads tounderestimation in band gap values [51–53]. This deficiency

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Table 2. The calculated six elastic constants (GPa), and related (B0), shear (G), and Young’s (E) modulus and Poisson ratio (υ) of MgH2with Voigt (a), Reuss (b) and Hill (c) approximations for MgH2 in the rutile phase within the framework of DFPT.

C11 C12 C13 C33 C44 C66 B G E Ratio υ

Constants 72.01 38.19 31.99 140.47 41.10 54.44 54.31a 39.48a 95.35a 0.20a

51.04b 33.24b 81.93b 0.23b

52.68c 36.36c 88.64c 0.21c

a Voigt.b Reuss.c Hill.

Figure 2. Phonon dispersion curve as calculated with DFPT.

Figure 3. Temperature dependence behavior of the specific heat,CV, and the thermal expansion coefficient, α. The variation of αwith CV is shown in the inset of the figure.

of XC potentials is treated well within GGA-EV and mBJapproximations which is also confirmed (figure 4) fromour present results given in table 3 and some previousstudies [54, 55]. Our results endorse the insulating nature ofMgH2. Figure 4, also shows the conduction band minimumon the R point and the valence band gap maximum in betweenthe 0 and X points in BZ. This shows the indirect band natureof MgH2. In addition it also shows the narrower width of thevalence band with EV and mBJ than that of PBE and WC. Thecalculated valence band width is also listed in table 3.

The total and partial densities of states of MgH2 areplotted in figure 5. From the figure it is clear that the valenceband is mainly due to Mg-3p and H-s states including a weak

effect of Mg-s states with maximum intensity at about −3.02and −0.38 eV, respectively. The Mg-3p and H-s states arestrongly hybridized with H-s states. DOS plots also indicatethe transfer of Mg-3s electrons to the adjacent H atoms withlower energies near the valence band [19]. Moreover, thebottom of the valence band is mainly dominated by Mg-p ands states.

The electron charge density maps of MgH2,corresponding to the crystallographic plane parallel tothe (110) plane which are shown in figure 6, indicate thebonding between Mg and H atoms. Figure 6 shows that eachMg atom is bonded with four H-atoms. The Mg atom has aspherical charge density distribution around it, whereas the Hatom has a non-spherical distribution with low charge densityas compared to Mg. This non-spherical distribution is causedby the slight shift in the direction of the nearest neighbor Mgand H. The charge density map shows the typical ionic bondnature of MgH2 because the Mg atoms seem to be stronglycharge depleted whereas H-atoms appear to be electronrich. Moreover figure 6 shows covalent bonding betweennearest neighbors H–H atoms and the sharing of a borrowedcharge by overlapping states. The overlapping between the Hatoms is due to the short distance that causes the complexitythat is observed for the band of MgH2 [56]. The remainingdelocalized charge from the Mg atom has been extendedspatially to the other next nearest neighbor; however, some ofit has been squeezed between the four Mg and four H atomsas it can be seen in the plot.

To further explore the bonding nature in MgH2 we presentcharge analysis/bond population analysis by calculating theelectronic structure using the CASTEP package [57]. Sincethe CASTEP code is based on a technique proposed bySanchez-Portal et al [58] in which atomic charge, chargetransfer, bond population, etc are treated in a natural wayusing the projected plane wave approach regarding a localizedelectron system to overcome the drawback of the planewave basis set (it does not provide any knowledge aboutlocalized electrons because of their delocalizing nature),and subsequently population analysis is performed using theMulliken formulism [59] and Hirshfeld charge [60]. Thoughthe Mulliken formulism provides some very useful qualitativeinformation to the Chemists’ interest [61], Mulliken chargeanalysis is also exaggerated and depends strongly onthe atomic basis. Moreover the Hirshfeld charge analysisapproach [60–62] has also been applied for the betterunderstanding of Mg–H and H–H bonding character. Averagevalues of the calculated bond overlap population, usingMulliken analysis for Mg–H and H–H, are −0.21 and −0.16respectively: our estimated average value of the bond overlap

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Figure 4. The electronic band structure of MgH2 calculated with different XC potentials: a significant improvement in the energy gap isobtained by applying GGA-EV and mBJ potentials.

Table 3. The calculated energy gap, total energy of unit cell and thevalence band width 1Ev calculated within different XC potentials.

Eg E0 Valence band width (1Ev)

PBE 3.723 −806.082 6.4503WC 3.604 −805.284 6.5046EV 5.104 −806.468 5.7484mBJ 5.770 −802.489 5.5255

population −0.21 indicates a dominancy of ionic bondingbetween Mg–H atoms. Similarly, the calculated bond overlappopulation between H–H is pointing to an ionic nature too.Whereas our calculated average values, within the Hirshfeldcharge approach, for Mg–H and H–H are −0.31|e| and−0.15|e| respectively. Furthermore, the amount of the backcharge donation indicates the presence of the higher degreeof ionicity in the Mg–H bond than that of the H–H bond.Thus our bonding population analysis also confirms the strongionic and weak covalent bonding nature of H2 in MgH2similar to our results of the charge density contour plots.It should be noted that for the improvement of the MgH2dehydrogenation performance, its ionic bonding must beweaker.

3.3. Optical properties

Although MgH2 has been intensively investigated to exploitits potential as a H2 energy carrier, it has also gainedimportance due to its application in optical switching. Mostrecently, Paik et al [28] have reported their experimentalmeasurements regarding optical properties and optical bandgap energy using electron energy loss spectroscopy in theplasmon energy range in order to understand specificallythe controversial optical band gap by finding its dielectricfunction. We also evaluate the optical properties of MgH2,using the DFT approach, such as dielectric function,refractive index, absorption and reflection coefficient. Forthe calculation of optical properties, we have used the bandgap correction with scissor operation. Underestimation ofthe band gap makes it difficult to calculate the opticalband gap; so the scissor operation of 1.88 eV has beenimplemented to fit the absorption edge to the experimentalvalue. The 1.88 eV scissor energy value corresponds tothe difference between the experimental [50] (5.6 eV) andPBE-GGA calculated (3.72 eV) band gap values. The scissoroperation has been taken into consideration to make theresults more reliable. Complex dielectric function is evaluated

in order to calculate optical properties and other relatedparameters. The dielectric constant is displayed in figure 7(a).It is apparent from figure 7(a) that the real part of thedielectric constant (ε1), in the low energy range first increaseswith photon energy and attains its maximum value at about6.46 eV, then decreases with the increase of photon energyand approaches its minimum value of −2.21 at a photonenergy of 9.71 eV. The dielectric constant then increases againand stabilizes to an average value of 0.9 from 30 eV. Ourcalculated value of the static dielectric constant calculatedε1(0) = 3.33 is in good agreement with the most recentexperimental measurements [28]. The calculated energy for ε1

when passing through zero with the positive point is 15.31 eV,which show a discrepancy with the measured value of Paiket al [28] of 10.34 eV. The fundamental absorption edge ofε2 (the energy corresponding to direct transition) is found tobe at 3.98 eV. There are three peaks in the imaginary partof dielectric functions which show the electron excitation.These peaks are situated at 7.28, 7.83 and 9.03 eV. The firstpeak is attributed to the transition from Mg-p states to Mg-sunoccupied states. The second peak relating to the interbandtransition from Mg-s to Mg-p shows the highest transitionenergy, and the last peak is ascribed to the H-s electronexcitation. By comparing the calculated transition energies(7.28, 7.83 and 9.03 eV) with the calculated band gap of5.16 eV [49] or 5.60 eV [54], one may come to the conclusionthat, as the optical assessment overates the band gap energyit could not be considered as a proper criterion for band gapcalculation.

To obtain the refractive index of MgH2, a mathematicalrelation between the complex refractive index and thecomplex dielectric function, as given in [63], is used, and theobtained corresponding results are displayed in figure 7(b).The refractive index n(0) = 1.9765 eV is in good agreementwith the experimental value, 1.95 eV, measured by Ellingeret al [64] and data of the ellipsometry and spectrophotometryexperiments [63], where n = 1.94.

The absorption coefficient, which indicates the fractionof energy lost by the wave when it passes through thematerial, is displayed in figure 7(c) along with the reflectioncoefficient. Our results show that there is no absorption ofenergy in the range of photon energy less than 3.85 eV andgreater than 38.37 eV. The value of the absorption coefficientstarted to increase at the photon energy larger than 3.85 eV,and approached a maximum value at 183 340.27 cm−1 whenenergy is at 8.15 eV. This exposes the MgH2 potential for

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Phys. Scr. 88 (2013) 065704 M Zarshenas et al

Figure 5. A 3D picture of total and partial density of states calculated with (a) GGA-PBE, (b) GGA-WC, (c) GGA-EV and (d) mBJ.

Figure 6. The calculated electronic charge densities as calculated with mBJ potential.

optoelectronics in the ultraviolet region because of its verysharp cut-off response in this region. From the reflectivespectrum displayed in the same figure, it is also observed thatthe transition between bands occurs mainly in the 7–25 eVenergy range.

The real and complex part of the optical conductivityσ(ω) is shown in figure 7(d). It is clear from the figure thatthere is no optical conductivity in the energy range less than2.10 eV and greater than 37 eV, however, its maximum valueis at 7.83 eV. These results are in accordance with the bandstructures and the DOS calculations.

To obtain a more comprehensive understanding of theoptical properties of MgH2, the electron energy-loss factorthat illustrates the energy loss of a fast electron traversingin a material has also been evaluated. The energy lost by theelectron while it is traversing through a medium is describedby the energy loss function L(ω) and mathematically canbe written as 1/ε(ω). The structures appearing in L(ω)spectrum are associated with the plasma frequency indicatingthe collective behaviour of the loosely bound electronsvalence to conduction bands. Figure 7(e) shows the MgH2electron energy loss function (EELS); it can be seen that

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Phys. Scr. 88 (2013) 065704 M Zarshenas et al

Figure 7. The optical properties of MgH2: (a) dielectric function ε(ω), (b) optical constants (n,k), (c) absorption and reflectivity, (d) opticalconductivity and (e) electron energy-loss function.

the largest energy loss peak occurs at about 14.88 eV,which correspond to the edge energy of MgH2 plasma. Ourvalue is in good agreement with the recent experimentalmeasurements performed by Paik et al [28], where they foundit equal to 14.8 eV. But this experimental value was still anoverestimation compared to the theoretical value of 13.7 eVobtained by the same author from a free electron calculation.The electronic energy loss of MgH2 is zero when the energyis more than 34.59 eV. The peak position in the EELS,corresponding with the plasma frequency, represents thetransition point from semiconducting to dielectric behavior ofa material.

4. Summary

In conclusion, the structural, elastic, mechanical,thermodynamic, electronic, bonding and optical properties ofMgH2 have been investigated in the frame work of DFT andDFPT calculations. Examining the structural and electronicproperties have been done within the DFT framework, and forthe thermodynamic properties, DFPT has been adopted. Ourcalculations were found to be in agreement with experimentalvalues. The calculated lattice constants with WC-GGA werefound to be more reliable than with PBE-GGA and closerto the experimental values. Moreover, the band gap valuesthat have been severely underestimated with PBE-GGA and

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WC-GGA show significant improvement with GGA-EVand mBJ XC potentials and are closer to experimentalmeasurements. As an important result, we have theoreticallyrevealed that the bonding nature of MgH2 is a mixture ofstrong ionic and weak covalent bonding. The results showthe existence of strong ionic bonding as compared to theweak covalent bond between Mg and H. The CV curve wasfound to obey the T 3 power rule for low temperature limits,however for higher temperature it was in agreement withDulong and Petit’s law. The optical properties were reportedand compared to early and recent literature and experimentalmeasurements.

Acknowledgments

The authors would like to thank the financial support of theMinistry of Higher Education (MOHE) Malaysia/UniversitiTeknologi Malaysia (UTM) of this research work throughgrant numbers Q.J13000.7126.00J33; R.J130000.7726.4D034; Q.J130000.2526.02H89; R.J130000.7826.4F113.Moreover SGS wishes to thank the research computingservice (KAUST-IT) for access to CASTEP code.

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