Issue
EPL
Volume 88, Number 3, November 2009
Article Number 36005
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
DOI http://dx.doi.org/10.1209/0295-5075/88/36005
Published online 17 November 2009
EPL, 88 (2009) 36005
DOI: 10.1209/0295-5075/88/36005

Elastic constants of NaBH4 and LiBH4: Instability of $\beta$-LiBH4

Xiao-Dong Zhang1, Zhu-Feng Hou2, Zhen-Yi Jiang1 and Yu-Qing Hou3

1   Institute of Modern Physics, Northwest University - Xian 710069, PRC
2   Department of Physics, Fudan University - Shanghai 200433, PRC
3   Department of Electronic Science, Northwest University - Xian 710069, PRC

jiangzy@nwu.edu.cn

received 16 September 2009; accepted in final form 19 October 2009; published November 2009
published online 17 November 2009

Abstract
The structural properties and mechanical stabilities of $\alpha$-NaBH4 and $\beta$-LiBH4 have been investigated using first-principles calculations. Elastic constants of $\alpha$-NaBH4 and $\beta$-LiBH4 are calculated from the second derivative of total energy as a function of strain. The calculated structural parameters and elastic constants of $\alpha$-NaBH4 are in good agreement with the available experimental data. Our calculations show that $\beta$-LiBH4 with P63mc space group is unstable under the distortions (0, 0, 0, $\delta $, $\delta $, 0) and (0, 0, 0, 0, 0, $\delta $) and these distortions lead to a Martensitic transformation into Cc and Cmc21 structures, respectively. Thermodynamically, $\beta$-LiBH4 with P63mc space group is more likely changed into a Cmc21 structure, which is consistent with experimental results.

PACS
62.20.D- - Elasticity.
81.30.Kf - Martensitic transformations.

© EPLA 2009