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