M₂X (M= Li, Na; X= F, Cl): the smallest superalkali clusters with significant NLO responses and electride characteristics

Abstract

Hypervalent M₂X (M = Li, Na; X = F, Cl) clusters are prototype species possessing lower ionisation potentials than Li, therefore classified as superalkalis. This study reveals some interesting properties of these small clusters using ab initio MP2/aug-cc-pVTZ and QCISD/aug-cc-pVTZ methods. These clusters are shown as an ionic species, composed of positively charged cage of alkali metals (M₂⁺) and halogen anion (X⁻). Therefore, the stability of M₂X is governed by both ionic and covalent interactions. We show that the excess valence electron of (M₂⁺) is pushed out by anionic X⁻, which allows M₂X clusters to possess ‘electride’ characteristics. It is also due to this excess electron that M₂X clusters exhibit significant non-linear optical (NLO) properties. The dipole moment, mean polarisability and hyperpolarisability suggest their significant NLO responses, which are explained on the basis of electronic transitions in crucial excited states using TD-B3LYP/aug-cc-pVTZ method. The first static hyperpolarisabilities of Li₂F and Na₂F take the values of order of 10⁴ a.u. due to their lower transition energies. This study should provide new insights into the design of novel materials with significant NLO responses useful for electro-optical applications.

Keywords:

• Alkali halide clusters
• hypervalent
• superalkali
• NLO
• electrides

Acknowledgement

A.K. Srivastava thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India for a research fellowship [grant number 09/107(0359)/2012-EMR-I]. The computational help received from Mr Sarvesh Kumar Pandey, Department of Chemistry, Indian Institute of Technology Kanpur and technical support from Gaussian Inc. are also acknowledged.