Materialinthis study were drawnfromthe experimental results conducted by Chu et.al [16], and use ABINIT Code [17] to implemented the modeling and GnuPlot for analysis and plotting data.
3.2 Method
3.2.1. Determination of structure
Study of convergeenergybeginsby testing of basis setto obtain thebeststructure. Samplingbasis set which is testedusing a standardof kineticenergycutoffrangingfrom10 to46Ha, toobtainthe energy differenceof 0.001Ha. If thedifference inenergy of the systemhas been reached,then the structureobtained hasconvergedand obtained the best structural model of Ca(BH4)2∙2NH3 crystal.
3.2.2. Hydrogen storage properties
This stageisrelated tothe previousresults, where it isfollowed bythe analysis ofthe crystal structureandproperties ofdehydrogenationCa(BH4)2∙2NH3.
4. Results and Discussion
Energyconvergencestudyby testing of electron wave function aims to get good full basis set in modelingcrystalstructure and get energyof systemswith increasingof kinetic energyin thesystem ofCa(BH4)2∙2NH3. The energy of Ca(BH4)2∙2NH3compound show in Fig. 1 is about -285,00 Ha or 178,838.355 kcal/mol (theoretically).
Fig. 1. Convergence energy study of Ca(BH4)2∙2NH3 crystal structure
Convergence criteria are taken in the form of the difference intwo energy of the system inside iteration processand reaches0.001Ha. The result of this study is get the energy system has not changed much to the energy difference with 25.7 Ha of cutoff energy or700eVis quite acceptable as a convergence of basis set parameters.
Optimization geometry using density functional theory method inside ABINIT code with generalized gradient approximation (GGA) as exchange and correlation energy [18] produce the geometry structure of Ca(BH4)2∙2NH3compound as lattice crystal which contains 76 atoms show in Fig. 2. This crystal lattice has orthorhombic space group (Pbcn:60), lattice parameter compare between experimental and calculation is show in Table 1.
-285,000 -284,500 -284,000 -283,500 -283,000 -282,500
5 10 15 20 25 30 35 40 45 50
Kinetics energy (Ha) ΔE = 0,001 Ha
Energy of system (Ha)
1 5 4 Kurniawan., “The Utilization Density Functional Theory inStructure Determination And Hydrogen
StorageProperties of Ca(BH4)2∙2NH3 Compounds”
ISBN:978-602-99849-2-7
Copyright © 2015by Advances on Science and Technology
Fig. 2. Structure of Ca(BH4)2∙2NH3 crystal lattice
Table 1. Lattice Parameter between calculation and experimental Lattice parameter Calculation Experimental*
a (Å) 06,492 06,416
b (Å) 08,317 08,390
c (Å) 12,683 12,702
Volume (Å3) 684,912679 683,751708
α (deg) 90 90
β (deg) 90 90
γ (deg) 90 90
*Source [16]
Fig. 3 Bond and interaction length inside of Ca(BH4)2∙2NH3 compounds
Ca B N H
Ca B N H 2,524
2,831
H1(B) 3,855
H6(N) 2,044
H3(B)
H1(B) H2(B) 1,239
2,524 3,087
1,027 2,524
1,239
1,027 H3(B)
H7(N)
2,377
H5(N)
H𝜹−+H𝜹+→H2
154
Kurniawan., “The Utilization Density Functional Theory inStructure Determination And Hydrogen StorageProperties of Ca(BH4)2∙2NH3 Compounds”
ISBN:978-602-99849-2-7
Lattice structure in Table 1. shown that both calculation and experimental result does not havemuch of a difference, it indicatesthe results ofoptimization hassucceeded in obtainingthe beststructural model of Ca(BH4)2∙2NH3 crystal.
In order
to better provide some insights into the understanding of hydrogen storage properties of Ca(BH4)2∙2NH3 crystal, the further study is about the complex structure of this compound as octahedral shaped with Ca2+ ionic as center and surrounds by four of BH4− molecules in side position and two NH3 molecule in vertical position. The detail visualization is show in Fig. 3.Bond length of N-H has longer than N-H inside NH3 molecule in gas phase (1,027:1,0170), because electron density of N has increased for interaction with Ca, this indicates interaction between nitrogen (N) atom and calcium (Ca) as acid-base complex interaction, bond density of N-H has decreased and bond length between N-H increased. The structure of BH4− molecule is tetrahedral with B-H bond length is 1,239 Å, if compared with borane as BH3 with bond length of B-H 1,190Å, it indicates that B-H bond length in BH4− has increased. The atoms H(B) are nearby calcium (Ca) in octahedral structure, it showed atom H(B) has negative charge that used for interaction with positive charge in Ca.
Inside Fig. 3 showed the interaction between two adjacent hydrogen atom, that is H6(N) and H1(B), closest interaction distance is about 2,044 Å, this interaction is called dihydrogen interaction between HN𝛿+ from NH3 molecule with HB𝛿− from BH4− molecule, which potentially as source of hydrogen molecule (H2). Interaction between two adjacent hydrogen atom as dihydrogen interaction is similar with hydrogen bond but differ in assumption, dihydrogen interaction in which thehydrogen atomsnormally would not becontiguouswiththe otherhydrogenatomwith a distance oflessthan 2.4Å, the characteristic of dihydrogenbondingis the distance betweenthe hydrogen atomsapproaching1.8Å[19].
5. Conclusion
Density functional theoryis one of theoretical chemistry that can be usedto explain the phenomenathat occur inchemical systemsin micro scale, which can not beexplained inthe experimental section. In this case, modelinga good geometry structureof Ca(BH4)2∙2NH3compound is a necessary condition to obtain the correct understanding of Ca(BH4)2∙2NH3crystal structure, lattice parameters and its properties as hydrogen storage material. This method very useful for research in most chemical system and phase, or simulating the development of new research in physics, chemist, biologist, and much more.
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