X-ray diffraction (XRD) is a workhorse characterization tool. A good text explains why the structure factor zeros out certain reflections (e.g., why FCC has no (100) peak) – not just the math, but the experimental implication for phase identification.
A phonon is a quantum of lattice vibration energy. Just as light is composed of photons, heat/sound in solids is composed of phonons. X-ray diffraction (XRD) is a workhorse characterization tool
Since we cannot see atoms with optical microscopes, we use X-rays. we use X-rays.