Investigation on Anharmonicity, Vibrational Anisotropy and Thermal Expansion of an Amorphous Ni${}_{46}$Ti${}_{54}$ Alloy Produced by Mechanical Alloying using Extended X-ray Absorption Fine Structure

A method to investigate anharmonicity, vibrational anisotropy and thermal expansion using correlated mean-square relative displacements (MSRD) parallel and perpendicular to the interatomic bonds obtained only from Extended X-ray Absorption Fine Structure (EXAFS) analysis based on cumulant expansion is suggested and applied to an amorphous Ni${}_{46}$Ti${}_{54}$ alloy produced by mechanical alloying. From EXAFS measurements taken on Ni and Ti K edges at several temperatures, the thermal behavior of ${\text{MSRD}}_{\parallel }$, ${\text{MSRD}}_{\perp }$ and of the cumulants $C_1^{\ast }$, $C_2^{\ast }$ and $C_3^{\ast }$ of the real distribution functions ${\varrho }_{ij}(r,T)$, and also the Einstein temperatures and frequencies associated with parallel and perpendicular motion were obtained, furnishing information about the anharmonicity of the interatomic potential, vibrational anisotropy and the contribution of the perpendicular motion to the total disorder and thermal expansion.