[Paper Review] Vibrational and electronic properties of the B$_7$Al$_2$ cluster

Peter L. Rodríguez‐Kessler|arXiv (Cornell University)|Sep 26, 2024

Boron and Carbon Nanomaterials ResearchMaterials Science3 citations

TL;DR

This study employs density functional theory (DFT) to investigate the structural, vibrational, and electronic properties of B₇Al₂ clusters, revealing a bipyramidal inverse sandwich structure as the global minimum. The most stable configuration features a B₇ ring coordinated with two Al atoms, while higher-energy isomers exhibit peripheral Al₂ doping, with distinct IR spectra and density of states indicating significant differences in electronic localization and reactivity.

ABSTRACT

In this work, we employ density functional theory (DFT) to explore the structure of boron clusters doped with two aluminium atoms (B$_7$Al$_2$ or Al$_2$B$_7$). The results show that the most stable structure is a bipyramidal configuration formed by a B$_7$ ring coordinated with two Al atoms, while the higher energy isomers correspond to peripheral Al$_2$-doped B$_7$ structures. The IR spectra and density of states reveal remarkable differences between the global minimum and the higher energy isomers.

Motivation & Objective

• To determine the most stable geometric structure of B₇Al₂ clusters using DFT calculations.
• To investigate the vibrational characteristics of B₇Al₂ isomers via IR spectroscopy calculations.
• To analyze the electronic structure of B₇Al₂ using total and partial density of states (DOS).
• To compare the reactivity and electronic localization between the global minimum and higher-energy isomers.
• To contribute to the growing library of M₂-doped B₇ clusters for potential nanomaterial applications.

Proposed method

• Employed DFT with the PBE0 functional and Def2-TZVP basis set for electronic structure calculations.
• Used the modified basin-hopping (MBH) method to overcome limitations of standard basin-hopping in exploring the potential energy surface.
• Applied BFGS algorithm with tight SCF convergence criteria (10⁻⁸ au) for geometry optimization.
• Included dispersion corrections via Grimme DFT-D3(BJ) to account for van der Waals interactions.
• Calculated IR spectra using Hessian matrix diagonalization via the NumFreq keyword.
• Computed total and partial density of states (DOS) using the Multiwfn program for electronic structure analysis.

Experimental results

Research questions

• RQ1 What is the most stable geometric structure of the B₇Al₂ cluster?
• RQ2 How do the vibrational modes differ between the global minimum and higher-energy isomers of B₇Al₂?
• RQ3 What are the electronic differences, particularly in HOMO-LUMO gap and orbital localization, between the stable and metastable B₇Al₂ isomers?
• RQ4 How does the inclusion of dispersion corrections affect the predicted stability and geometry of B₇Al₂?
• RQ5 To what extent does the Al₂ doping alter the structural rigidity and electronic properties compared to the Al₂B₈ cluster?

Key findings

• The most stable structure of B₇Al₂ is a bipyramidal inverse sandwich configuration with a B₇ ring and two Al atoms coordinated above and below.
• The global minimum (7M2.1) has a relative energy of 0.00 eV, while the next lowest-energy isomer (7M2.2) is 0.42 eV higher in energy.
• The IR spectrum of the global minimum (7M2.1) shows a single dominant peak at 303.89 cm⁻¹, whereas 7M2.2 exhibits three main peaks at 536.22, 638.31, and 1279.19 cm⁻¹.
• The vibrational frequency range for 7M2.1 is 303.89–881.34 cm⁻¹, while for 7M2.2 it spans 100.65–1473.28 cm⁻¹, indicating greater vibrational dispersion due to lower symmetry.
• Partial density of states (PDOS) analysis shows more localized electronic states in the global minimum (7M2.1), suggesting higher reactivity compared to 7M2.2.
• The HOMO-LUMO gap is larger in the 7M2.2 isomer, indicating greater kinetic stability and reduced reactivity compared to the more localized 7M2.1 structure.