  Wurtzite-type ferroelectrics are highly promising for next-generation
microelectronic devices due to their ferroelectric properties and integration
with exiting semiconductors. However, their high coercive fields, which are
close to breakdown electric fields, need to be lowered. To deal with this issue
and secure device reliability, much effort has been devoted to exploring novel
wurtzite compounds with lower polarization switching barriers and implementing
doping strategies. Here, we report first-principles calculations on
polarization switching in cation-vacancy ordered wurtzite {\alpha}-Al2S3,
unveiling its uniaxial quadruple-well ferroelectricity and moderate switching
barrier, 51 meV/cation, which is much lower than that of conventional wurtzite
ferroelectrics. There are three important features relevant to the Al vacancies
leading to the uncommon quadruple-well ferroelectricity and the moderate
switching barrier: mitigation of cation-cation repulsion, structural
flexibility that alleviates an in-plane lattice expansion, and formation of
{\sigma}-like bonding states consisting of Al 3pz and S 3pz orbitals. Biaxial
compressive strain and Ga doping lower the switching barriers by up to 40%.
This study encourages experimental investigation of the ferroelectric
properties for defective wurtzite {\alpha}-Al2S3 as a new promising material
with unconventional and intriguing ferroelectricity and suggests a potential
strategy for reducing switching barriers in wurtzite ferroelectrics:
introducing cation vacancies.
