Density functional study of electronic structure, elastic and optical properties of MNH$_2$ (M=Li, Na, K, Rb)

TL;DR

This study uses density functional theory to analyze the electronic, elastic, and optical properties of nitrogen-based solid hydrogen storage materials LiNH₂, NaNH₂, KNH₂, and RbNH₂, revealing their stability, electronic structure, and optical anisotropy.

Contribution

First principles calculations of structural, elastic, electronic, and optical properties of alkali metal amides, highlighting differences in van der Waals interactions and stability among them.

Findings

LiNH₂ is the stiffest material among the studied amides.

All compounds are insulators with significant band gaps.

Materials exhibit notable optical anisotropy.

Abstract

We report systematic first principles density functional study on the electronic structure, elastic and optical properties of nitrogen based solid hydrogen storage materials LiNH$_2$, NaNH$_2$, KNH$_2$, and RbNH$_2$. The ground state structural properties are calculated by using standard density functional theory and also dispersion corrected density functional theory. We find that van der Waals interactions are dominant in LiNH$_2$ whereas they are relatively weak in other alkali metal amides. The calculated elastic constants show that all the compounds are mechanically stable and LiNH$_2$ is found to be stiffer material among the alkali metal amides. The melting temperatures are calculated and which follows the order RbNH$_2$ $<$ KNH$_2$ $<$ NaNH$_2$ $<$ LiNH$_2$. The electronic band structure is calculated by using the Tran-Blaha modified Becke-Johnson potential and found that all the compounds are insulators with a considerable band gap. The [NH$_2$]$^-$ derived states are completely dominating in the entire valence band region while the metal atom states occupy the conduction band. The calculated band structure is used to analyze the different interband optical transitions occur between valence and conduction bands. Our calculations show that these materials have considerable optical anisotropy.