  The origin of transition metal vacancies and their effects on the properties
of ThCr2Si2-type compounds have been less studied and poorly understood. Here
we carefully investigate the structure, physical properties, and electronic
structure for a series of lanthanum cobalt arsenides with nominal composition
of LaCoxAs2 (1.6 < = x < = 2.1). It is revealed that the occupancy of Co can be
tuned between 1.98(1) and 1.61(1). The structural analyses based on X-ray and
neutron diffractions show the existence of Co vacancies results from charge
balance due to the formation of bond between As-As. These Co vacancies affect
the magnetic and electrical properties greatly, adjusting the Curie temperature
from 205 to 47 K and increasing the resistivity by more than 100%. First
principles calculations indicate that the Co vacancies weaken the spin
polarization and reduce the density of states at the Fermi level, resulting in
decreased Curie temperature and increased resistivity, respectively. Our
results address the importance of transition metal vacancies in ThCr2Si2-type
materials and offer a reliable route to tune the magnetism of ThCr2Si2-type
structure.
