  Negative piezoelectrics contract in the direction of applied electric field,
which are opposite to normal piezoelectrics and rare in dielectric materials.
The raising of low dimensional ferroelectrics, with unconventional mechanisms
of polarity, opens a fertile branch for candidates with prominent negative
piezoelectricity. Here, the distorted $\alpha$-Bi monolayer, a newly-identified
elementary ferroelectric with puckered black phosphorous-like structure [J.
Guo, {\it et al}. Nature \textbf{617}, 67 (2023)], is computationally studied,
which manifests a large negative in-plane piezoelectricity (with
$d_{33}\sim-26$ pC/N). Its negative piezoelectricity originates from its unique
buckling ferroelectric mechanism, namely the inter-column sliding.
Consequently, a moderate tensile strain can significantly reduce its
ferroelectric switching energy barrier, while the compressive strain can
significantly enhance its prominent nonlinear optical response. The physical
mechanism of in-plane negative piezoelectricity also applies to other
elementary ferroeletric monolayers.
