Path Integral Effective Classical Potential Method Applied to Anharmonicity and Quantum Effects in Thermal Expansion of Invar Alloy

Abstract

Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is a powerful experimental technique to investigate anharmonic vibrational properties of solids especially when one combines experimental EXAFS with quantum mechanical theoretical evaluations. In this article, the path-integral effective-classical-potential (PIECP) theory is applied to temperature dependence of EXAFS in a real system. The anharmonicity and quantum effects in the Invar alloy Fe_64.6Ni_35.4 that shows anomalously small thermal expansion are investigated. Experimental Fe and Ni K-edge EXAFS measurements and the computational PIECP simulations have been performed. It is experimentally revealed that the first nearest-neighbor (NN) shells around Fe show almost no thermal expansion, while those around Ni exhibit meaningful but smaller expansion than that of fcc Ni. At low temperature (<100 K), the vibrational quantum effect is found to play an essentially important role, which is confirmed by comparing the quantum mechanical simulations to the classical ones, the latter of which exhibit large (normal) thermal expansion at low temperature. It is also clarified that thermal expansion for the Ni-Ni and Ni-Fe pairs is noticeably suppressed, even though the Ni electronic state may not vary depending on the temperature. On the other hand, the anharmonicity (asymmetric distribution) clearly exist for all the first-NN shells as in the case of the normal thermal expansion system, where thermal expansion originates almost exclusively from the anharmonic interatomic potential. [DOI: 10.1380/ejssnt.2012.486]