The low-frequency dynamics of BaZnF4 has been investigated in the temp
erature range T=300-15 K. The dispersion surfaces at room temperature
display a deep valley located along the (0.5, zeta, 0.5) zone-boundary
line. This valley becomes deeper when T decreases, showing a soft-mod
e behavior. In addition a new depression appears at low-temperature al
ong the (xi, 0.5, 0.5) line, corresponding to an incipient instability
toward an incommensurate phase. A symmetry analysis of the vibration
spectrum including the compatibility relations between the different i
rreducible representations, is presented. The soft-mode symmetry is de
termined as belonging to the I-1 representation. The present results a
re compared to previous Raman and x-ray investigations performed on th
is compound.