ISO 25498:2025

Overview

ISO 25498:2025 - "Microbeam analysis - Analytical electron microscopy - Selected area electron diffraction analysis using a transmission electron microscope" specifies a standardized method for performing selected area electron diffraction (SAED) with a transmission electron microscope (TEM) on thin crystalline specimens. The standard covers acquisition of SAED patterns from micrometre and sub‑micrometre test areas, indexing of diffraction patterns, and calibration of the camera constant (Lλ). It also describes limitations related to spherical aberration (minimum practical selected area diameter approaches hundreds of nanometres for modern TEMs) and when alternative techniques such as microdiffraction (nano‑beam diffraction) or convergent beam diffraction may be preferred.

Key topics and technical requirements

• Scope and applicability: SAED for thin crystalline specimens; test areas from micrometres down to sub‑micrometres; notes on limits when selected area is smaller than spherical aberration restriction.
• Fundamental principles: Ewald sphere construction, Bragg angle, reciprocal lattice vectors and R vectors that link direct beam to diffraction spots.
• Apparatus and recording: Requirements for TEM configuration, camera/screen recording of diffraction patterns and image acquisition in diffraction mode.
• Specimen preparation: Guidance for preparing thin crystalline specimens suitable for SAED.
• Measurement procedure: Instrument setup, acquiring SAED from single crystals, determination of the diffraction constant (Lλ), and practical steps for capturing reliable patterns.
• Pattern analysis: Selection of basic parallelogram in spot patterns, indexing diffraction spots using Miller/Miller‑Bravais notation, and addressing the 180° ambiguity in indexing.
• Calibration and uncertainty: Procedures for camera constant calibration, use of reference materials, and estimation of measurement uncertainty (including sections on uncertainty in camera constant and d‑spacing values).
• Informative annexes: Interplanar spacings of reference materials and example spot patterns for common crystal structures (BCC, FCC, HCP).

Practical applications

ISO 25498:2025 is intended to standardize SAED practice across research, industrial, and inspection contexts. Typical applications include:

• Phase identification and determination of lattice type and parameters
• Crystal orientation and orientation relationships between phases
• Analysis of defects, twins, interfaces, and habit planes
• Studies of phase transformations and preferred orientations (texture)
• Supporting R&D for new materials and quality control in manufacturing

Who should use this standard

• Electron microscopists and TEM operators performing SAED
• Materials scientists and metallurgists conducting microstructural analysis
• Quality control and failure analysis laboratories using SAED for inspection
• Calibration and accreditation bodies referencing standardized microbeam methods

Related Standards

• ISO 15932:2013 (related TEM terminology and imaging concepts)
• ISO/IEC Guide 98‑3 (GUM:1995) - included in the bibliography for uncertainty evaluation
• ISO/IEC 17025 - referenced in the history of revisions (moved to bibliography)

Keywords: ISO 25498:2025, SAED, selected area electron diffraction, TEM, camera constant, Lλ, microbeam analysis, spherical aberration, microdiffraction, nano‑beam diffraction.