Powder XRD identifies a crystalline phase by comparing its diffraction pattern with reference data. The question asks which of the four choices plays no part in that comparison.
- Peak width at half-maximum: tells us about crystallite size and strain, and an unusually broad or sharp peak is a real clue about the sample, so it counts as diagnostic.
- Relative intensity: the height of each peak compared to the strongest peak is recorded in every reference pattern and matched during a search, so this is diagnostic.
- Fluorescence: this happens when the sample element absorbs the X-ray beam and glows back a broad, featureless background, mostly with iron rich samples under copper radiation. It adds noise, not structural information, so it plays no role in identifying the phase.
- Peak position: converted to d-spacing through Bragg's law, this is the backbone of phase identification, since every crystal structure gives its own unique set of d-spacings.
Peak position, relative intensity, and peak width all describe the crystal lattice in some way. Fluorescence describes only an unwanted interaction between the sample and the source wavelength.
Let's summarize:
- Phase identification in XRD rests on peak position and relative intensity, with peak width adding detail on crystallite size and strain.
- Fluorescence is a background problem caused by element-source interaction, not a structural fingerprint.
So the parameter that is NOT diagnostic for identifying a crystalline phase is fluorescence.