  We study the magnetic susceptibility of mixed-valent La2-xSrxCoO4 single
crystals in the doping range of 0.5<= x <= 0.8 for temperatures up to 1000 K.
The magnetism below room temperature is described by paramagnetic Co2+ in the
high-spin state and by Co3+ in the non-magnetic low-spin state. Above room
temperature, an increase in susceptibility compared to the behavior expected
from Co2+ is seen, which we attribute to a spin-state transition of Co3+. The
susceptibility is analyzed by comparison to full-multiplet calculations for the
thermal population of the high- and intermediate-spin states of Co3+.
