Question

Which one of the following is NOT a diagnostic parameter in identifying a crystalline phase using powder X-ray diffraction data?

• A. Peak width at half-maximum
• B. Relative intensity
• C. Fluorescence
• D. Peak position

Hint:

Fluorescence is a background effect from the sample-source interaction, not a structural fingerprint of the crystal.

Answer 1

The Correct Option is C

Step 1: Understanding the Question.
Powder X-ray diffraction (XRD) identifies crystalline phases in a sample by matching a set of measured diffraction data against reference patterns. The question asks which of the four listed items is not actually used as a diagnostic parameter for phase identification.

Step 2: Key Concept.
A phase is identified from its powder XRD pattern mainly through peak position, which gives the d-spacing through Bragg's law, \(n\lambda = 2d\sin\theta\), and the relative intensity of the peaks, which reflects the arrangement and type of atoms in the unit cell. Peak width at half maximum (FWHM) is also used diagnostically, since it relates to crystallite size and lattice strain and helps distinguish well crystallised phases from poorly crystallised material. Fluorescence, however, is an unwanted background effect that occurs when the sample contains an element, commonly iron, that absorbs the incident X-ray wavelength, commonly Cu K-alpha, and re-emits it as a broad background signal, degrading the pattern rather than helping identify the phase.

Step 3: Detailed Explanation.

(A) Peak width at half-maximum: Used to judge crystallite size, strain, and crystallinity, so it does carry diagnostic information about the phase.

(B) Relative intensity: Along with peak position, this is one of the two core pieces of data used in standard phase matching databases such as the ICDD PDF files. It is diagnostic.

(C) Fluorescence: This is a background noise effect caused by the sample composition interacting with the X-ray source wavelength. It carries no structural information about the crystalline phase itself, so it is not a diagnostic parameter.

(D) Peak position: This directly gives the d-spacing values of the lattice planes through Bragg's law and is the single most important diagnostic parameter for phase identification.

Step 4: Final Answer.
Peak position, relative intensity, and peak width at half maximum all carry information about the crystal structure and are used to identify a phase, while fluorescence is only a background artefact. So the parameter that is NOT diagnostic is
\[ \boxed{\text{Fluorescence}} \]