Ab initio electronic structure of Alkali Borohydrides: An Ab Initio Study

W. Gempel and N. Kioussis

California State University Northridge

D. Papaconstantopoulos

Naval Research Laboratory

• Introduction

• Method

• Results and discussion

• ConclusionSupported by NSF

Low-T Structure

Simple Tetragonal

P42/nmc

High-T Structure

Disordered fcc

Fm-3m

Calculational Details

• VASP

• PAW / GGA

• Converged Primitive Cells to 1 meV

• 700 eV Plane Wave Cutoff

• 5x5x5 Monkhorst-Pack Mesh

NaBH4 Lattice

KBH4 Lattice

Comparison with Experiment Low-T Structures

NaBH4 Tetra VASP 10K

a (Å) 4.3389 4.3320

c (Å) 5.8649 5.8690

H-B (Å) 1.2252 1.2200

Angle (°) 109.144 109.223

  110.127 109.969

Experimental Data from Fischer, Zuettel Materials Science Forum 443-444 pg. 287

KBH4 Tetra  VASP 1.5K

a (Å) 4.7810 4.7004

c (Å) 6.6934 6.5979

H-B (Å) 1.22840 1.1960

Angle (°) 109.440 109.392

  109.533 109.511

Experimental Data from Renaudin, Gomes, Hagemann, et al J Alloys & Compounds, 375 pg. 96

FCC 295K

a (Å) 6.1144 6.1737

H-B (Å) 1.2241 1.1782

FCC 295K

a 6.7500 6.7306

H-B 1.2287 1.1963

Low Temperature Density of States

Low TemperatureCharge Density Accumulation

NaBH4

High Temperature Charge Density Accumulation

Heat of FormationKJ per mol H2: MBH4->MH + B + 3/2 H2

NaBH4 Low Temperature -79.980

NaBH4 High Temperature -76.945

KBH4 Low Temperature -99.467

KBH4 High Temperature -98.479

KJ per mol H2: MBH4->M + B + 2H2

NaBH4 Low Temperature -100.286

NaBH4 High Temperature -97.251

KBH4 Low Temperature -119.388

KBH4 High Temperature -118.401

Brief Comparison with NaAlH4

• Borohydrides are sp-Hybridized

• Sodium Alanate is Ionic

• Larger Heat of Formation for Borohydrides

Conclusions

Acknowledgements

• W.M. Keck Foundation

• NASA/JPL Grant

• Naval Research Laboratory