  Using density functional tight-binding theory, we investigated the elastic
properties and deformation and failure behaviors of pristine and defective
carbon-phosphide (CP) monolayers subjected to uniform uniaxial tensile strain
along armchair (AC) and zigzag (ZZ) directions. Two variants of carbon
phosphide were studied and two types of carbon and phosphorous vacancies
(single and double) were considered. It was found that carbon monovacancies
have the lowest formation energy, while phosphorous divacancies have the
highest one in both CP allotropes. A strong mechanical anisotropy for carbon
phosphide was found with the Young\'s modulus and the failure stress along ZZ
direction being an order of magnitude larger than those along AC direction. In
both allotropes, the Youngs modulus, failure stress and strain are considerably
affected by vacancies, especially along AC direction. Fracture of pristine CP
monolayer occurred via the rupture of phosphorous-phosphorous bonds when CP
monolayer is stretched along AC direction, while via the rupture of
carbon-phosphorous bonds when stretched along ZZ direction. The failure strain
and stress along the AC direction are affected only by phosphorous vacancies,
while along the ZZ direction, they are almost equally affected by both
phosphorous and carbon vacancies. These understandings may provide useful
guidelines for potential applications of CP monolayers in nanoelectromechanical
systems.
