Abstract
Brillouin-scattering spectra of ${\mathrm{KHCO}}_{3}$ have been observed as a function of temperature. The two pure transverse acoustic modes propagating along the ${a}^{*}$ and $b$ axes show an anomalous softening near the phase transition temperature ${T}_{N}$=318 K. Their frequencies, however, are nonzero even at ${T}_{N}.$ This incomplete softening behavior indicates an antiferrodistortive phase transition, though a ferroelastic transition was suggested by a previous ultrasonic study. A phenomenological discussion is given for the antiferrodistortive phase transition.
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