  All phonons in a single crystal of NaBr were measured by inelastic neutron
scattering at temperatures of 10, 300 and 700 K. Even at 300 K the phonons,
especially the longitudinal-optical (LO) phonons, showed large shifts in
frequencies, and showed large broadenings in energy owing to anharmonicity. Ab
initio computations were first performed with the quasiharmonic approximation
(QHA), in which the phonon frequencies depend only on $V$, and on $T$ only
insofar as it alters $V$ by thermal expansion. This QHA was an unqualified
failure for predicting the temperature dependence of phonon frequencies, even
300 K, and the thermal expansion was in error by a factor of four. Ab initio
computations that included both anharmonicity and quasiharmonicity successfully
predicted both the temperature dependence of phonons and the large thermal
expansion of NaBr. The frequencies of LO phonon modes decrease significantly
with temperature owing to the real part of the phonon self-energy from explicit
anhamonicity, originating from the cubic anharmonicity of nearest-neighbor
Na-Br bonds. Anharmonicity is not a correction to the QHA predictions of
thermal expansion and thermal phonon shifts, but dominates the behavior.
