A student reasons: "Gallium arsenide's conventional cubic cell holds four gallium…
PHYSICS · Subjective · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Crystal structure, bonding and diffraction · Lattice dynamics and thermal properties of solids
Condensed Matter Physics, Ch Two atoms in the cell: acoustic and optical branches
Problem
A student reasons: "Gallium arsenide's conventional cubic cell holds four gallium atoms and four arsenic atoms, eight in total, so it has 3×8=24 branches, twenty-one of them optical." Find the fault in this reasoning, state the correct branch count for gallium arsenide, and say whether its optical branches are seen in an infrared absorption experiment.