  Inorganic metal halide perovskites are promising materials for
next-generation technologies due to a plethora of unique physical properties,
many of which cannot be observed in the oxide perovskites. On the other hand,
the search for ferroelectricity and multiferroicity in lead-free inorganic
halide perovskites remains a challenging research topic. Here, we
experimentally show that cubic fluoroperovskites exhibit proximity to incipient
ferroelectrics, which manifested in the softening of the low-frequency polar
phonons in the Brillouin zone center at cooling. Furthermore, we reveal the
coupling between harmonic and anharmonic force constants of the softening
phonons and their correlation with the perovskite tolerance factor. Next, using
first-principles calculations, we examine the lattice dynamics of the cubic
fluoroperovskites and disclose the incipient lattice instability at which the
harmonic force constants of low-lying phonons tend to decrease with a reduction
of tolerance factor at all high-symmetry points of the Brillouin zone. The
correlations with the tolerance factor indicate the geometric origin of
observed incipient lattice instability in the cubic fluoroperovskites caused by
the steric effect due to the volume filling of the unit cell by different
radius ions. These results provide insights into the lattice dynamics and
potential ferroelectric properties of inorganic lead-free metal halide
perovskites, relevant to further design and synthesis of new multifunctional
materials.
