Atomistic Modeling of the Negative Thermal Expansion in δ- Plutonium Based on the Two-State Description

Tongsik Lee; Michael I Baskes; A C Lawson; Shao Ping Chen; Steven M Valone

doi:10.3390/ma5061040

Atomistic Modeling of the Negative Thermal Expansion in δ- Plutonium Based on the Two-State Description

Materials (Basel). 2012 Jun 7;5(6):1040-1054. doi: 10.3390/ma5061040.

Authors

Tongsik Lee¹, Michael I Baskes^{2

3}, A C Lawson⁴, Shao Ping Chen⁵, Steven M Valone⁶

Affiliations

¹ Los Alamos National Laboratory, Los Alamos, NM 87545, USA. tongsik_lee@lanl.gov.
² Los Alamos National Laboratory, Los Alamos, NM 87545, USA. baskes@lanl.gov.
³ Jacobs School of Engineering, University of California, San Diego, La Jolla, CA 92093, USA. baskes@lanl.gov.
⁴ Los Alamos National Laboratory, Los Alamos, NM 87545, USA. aclawson@cybermesa.com.
⁵ Los Alamos National Laboratory, Los Alamos, NM 87545, USA. sc@lanl.gov.
⁶ Los Alamos National Laboratory, Los Alamos, NM 87545, USA. smv@lanl.gov.

Abstract

The δ phase of plutonium with the fcc structure exhibits an unusual negative thermal expansion (NTE) over its narrow temperature range of stability, 593-736 K. An accurate description of the anomalous high-temperature volume effect of plutonium goes beyond the current capability of electronic-structure calculations. We propose an atomistic scheme to model the thermodynamic properties of δ-Pu based on the two-state model of Weiss for the Invar alloys, inspired by the simple free-energy analysis previously conducted by Lawson et al. The two-state mechanism is incorporated into the atomistic description of a many-body interacting system. Two modified embedded atom method potentials are employed to represent the binding energies of two competing electronic states in δ-Pu. We demonstrate how the NTE takes place in δ-Pu by means of Monte Carlo simulations implemented with the two-state mechanism.

Keywords: Invar; Monte Carlo method; Weiss model; modified embedded atom method; negative thermal expansion; plutonium.