  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 $C_1^*$, $C_2^*$ and $C_3^*$ 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.
