Revealing disorder parameter and deformation electron density using electron diffraction.

Local disorders of lattice, charge, orbital, and spin perturbate the electron density distribution in materials, profoundly influencing their properties. Consequently, experimental determination of local electron density offers a powerful, universal approach to probe such disorder. Although quantitative convergent beam electron diffraction (QCBED) is widely employed for electron density measurements in ordered crystals, its applicability to disordered structures, where the translational symmetry of the electrostatic potential is broken, remains uncertain. Here, a multi-beam off-zone axis CBED technique combined with a coherent potential approximation in Bloch wave formalism is used to simultaneously determine chemical disorder parameters, deformation electron density ∆ρ, and Debye-Waller factors (DWF) in both chemically-ordered L1 FePd and chemically-disordered γ-phase FePd solid solution. The CBED results reveal that chemical disordering significantly increases DWFs while having a negligible impact on ∆ρ. Density functional theory calculations on supercells with randomly distributed Fe and Pd atoms support these experimental findings. This work validates QCBED as a robust method for quantifying local disorder parameters in chemically disordered systems, bridging a critical gap in the characterisation of disordered materials.