= Extended-zone representation of a phonon dispersion
The <crystal momentum> of a <phonon> is defined modulo a <reciprocal lattice vector>. Extending its <dispersion relation> periodically beyond the <first Brillouin zone> labels equivalent excitations by different extended <wavevectors>. A scattering transfer $Q=G+q$ can be outside that zone while its reduced <phonon> <wavevector> $q$ is inside. The measured <neutron> recoil depends on the actual transfer $Q$, not on the reduced representative alone.
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