phase identification

Every crystalline phase has its own unique set of peak positions and relative intensities — a diffraction fingerprint as distinctive as a human fingerprint. Phase identification is the everyday task of taking an unknown powder, recording its pattern, and finding out which crystalline substances are present by matching that fingerprint against a database of known ones. It is the single most common use of X-ray diffraction.

The mechanism is a search-and-match. You measure the pattern, then a program compares its peak positions (the d-spacings) and relative heights against a reference library — the ICDD Powder Diffraction File holds hundreds of thousands of known phases. A convincing match needs both the d-spacings and the intensity ranking to line up, not just one. If the sample is a mixture, its pattern is simply the sum of each phase's pattern superimposed, and the software peels the components apart one fingerprint at a time.

This tells you not just the elements but the actual compounds and, importantly, which polymorph you have — calcite and aragonite are both calcium carbonate, CaCO3, with the same chemistry but different structures and therefore completely different patterns. Extended with peak areas, the method becomes quantitative phase analysis (how much of each). Two honest limits: X-ray diffraction only sees crystalline phases, so amorphous or glassy content shows up merely as a broad hump and is easily overlooked; and a trace phase below roughly one to five percent may produce peaks too weak to spot at all.