  Single-layer pentagonal materials are an emerging family of two-dimensional
(2D) materials that could exhibit novel properties due to the building blocks
being pentagons instead of hexagons as in numerous 2D materials. Based on our
recently predicted single-layer pentagonal CoS$_2$ that is an antiferromagnetic
(AFM) semiconductor, we replace two S atoms by As atoms in a unit cell to form
single-layer pentagonal CoAsS. The resulting single-layer material is
dynamically stable ac-cording to the phonon calculations. We find two drastic
changes in the properties of single-layer pentagonal CoAsS in comparison with
those of CoS$_2$. First, we find a magnetic transition from the AFM to FM
ordering. We understand that the transition is caused by the lower
electronegativity of As atoms, leading to the weakened bridging roles on the
superexchange interactions be-tween Co ions. Single-layer pentagonal CoAsS also
shows significantly stronger magnetocrystalline anisotropy energy due to
stronger spin-orbit coupling. We additionally perform Monte Carlo simulations
to calculate the Curie temperature of single-layer pentagonal CoAsS and the
predicted Curie temperature is 103 K. Second, we find that single-layer
pentagonal CoAsS exhibits piezoelectricity, which is absent in single-layer
pentagonal CoS$_2$ due to its center of symmetry. The computed piezoelectric
coefficients are also sizable. The rare coexistence of FM ordering and
piezoelectric properties makes single-layer pentagonal CoAsS a promising
multifunctional 2D material.
