Elsevier

Chemical Physics Letters

Volume 365, Issues 5–6, 13 November 2002, Pages 487-493
Chemical Physics Letters

Path integral molecular dynamics simulation of solid para-hydrogen with an aluminum impurity

https://doi.org/10.1016/S0009-2614(02)01505-1Get rights and content

Abstract

The equilibrium properties of an aluminum impurity trapped in solid para-hydrogen have been studied. The results were compared to those of a previous study by Krumrine et al. [J. Chem. Phys. 113 (2000) 9079] with an atomic boron. In the presence of vacancy defect, when the orientation-dependent Al–pH2 potential is used, the Al atom is displaced to a position half way between its original substituted site and the vacancy site. Thermodynamic results also indicate that the presence of a neighboring vacancy helps to stabilize the Al impurity to a far greater extent than in the case of the B impurity.

Introduction

The embedding of atomic impurities in solid hydrogen has been the object of many recent experimental and theoretical studies. From a fundamental point of view solid hydrogen is the simplest molecular solid and is dominated by nuclear quantum effects [2]. More practically, impurity doped hydrogen is a possible candidate for future cryo-propellants [3]. Light impurities such as Li, B and Al have been successfully trapped in solid hydrogen and studied spectroscopically by Fajardo and coworkers [4], [5], [6]. Numerous simulation studies have appeared, which determined the structural and dynamical properties of pH2 with different impurities. Scharf et al. [7] used Path Integral Monte Carlo (PIMC) methods to investigate the structural properties of Li impurities in solid pH2 and oD2, and subsequently studied the Li excitation spectroscopy. Jang and Voth [8], [9] looked at the stability and reactivity of Li and B atoms in solid pH2 using path integral molecular dynamics (PIMD) and centroid molecular dynamics (CMD) methods. Krumrine et al. [1] used PIMD simulations to investigate the equilibrium properties of a B impurity in solid pH2. These authors looked specifically at the effect of the electronic anisotropy of the the B–pH2 interaction in the matrix and carried out a semi-classical simulation of the 3s  2p absorption of the embedded B atom.

Here we extend these simulation studies to the atomic Al impurity. Al is a particularly promising dopant for future high-energy density cryo-propellants, with a predicted 9% increase in the specific impulse (Isp) values over the traditional liquid-H2/liquid-O2 fuel at 5 mol% concentrations [3]. The goal of the present study is the understanding of the changes in the equilibrium properties of the solid pH2 system when doped with an Al impurity, as compared to the changes previously investigated with a B impurity [1]. We will focus in particular on the effect of the electronic anisotropy of the Al and pH2 interaction, as was done for the B doped system by Krumrine et al. [1].

Section snippets

Methods and calculations

The Silvera–Goldman [10] potential was used to represent the pairwise intermolecular interaction between H2 molecules. The incorporation of this potential into PIMD simulations of solid H2 has already been discussed elsewhere [8]. The modelling of the interaction of the Al atom with the matrix pH2 molecules was based on diatomic potential energy curves developed from ab initio calculations by Williams and coworkers [11]. Since Al has a singly filled 3p orbital in the ground state, its

Results and discussions

Insight into the structure of the equilibrated system is given by the Al–pH2 pair distribution functions, g(R). The g(R)s for all the cases studied here are presented in Fig. 2. For the site-substituted solid without a vacancy, shown in Fig. 2a, there is a striking difference between the g(R) obtained from the orientation dependent potential as compared with the spherically averaged potential. Most notably, the overall profile of the g(R) from the orientation dependent potential is shifted to

Acknowledgements

This research was supported by the Air Force Office of Scientific Research under Grants F49620-97-1-0023 (to GAV) and F49620-98-1-0187 (to MHA). DTM and GAV would like to thank Misha Ovchinnikov and Martin Čuma for helpful discussions and proofreading.

References (18)

  • M.E. Fajardo et al.

    Chem. Phys.

    (1994)
  • J.R. Krumrine et al.

    J. Chem. Phys.

    (2000)
  • I.F. Silvera

    Rev. Mod. Phys.

    (1980)
  • P. Carrick

    Specific Impulse Calculations of High Energy Density Solid Cryogenic Rocket Propellants 1: Atoms in Solid H2

    (1993)
  • M.E. Fajardo

    J. Chem. Phys.

    (1993)
  • S. Tam et al.

    J. Chem. Phys.

    (2000)
  • D. Scharf et al.

    J. Chem. Phys.

    (1993)
  • S. Jang et al.

    J. Phys. Chem.

    (1999)
  • S. Jang et al.

    J. Chem. Phys.

    (1998)
There are more references available in the full text version of this article.
View full text