  Single crystals of Ca1-xLaxFe2As2 for x ranging from 0 to 0.25 have been
grown and characterized by structural, transport and magnetic measurements.
Coexistence of two superconducting phases is observed, in which the low
superconducting transition temperature (Tc) phase has Tc ~ 20 K, and the high
Tc phase has Tc higher than 40 K. These data also delineate an x - T phase
diagram in which the single magnetic/structural phase transition in undoped
CaFe2As2 appears to split into two distinct phase transitions, both of which
are suppressed with increasing La substitution. Superconductivity emerges when
x is about 0.06 and coexists with the structural/magnetic transition until x is
~ 0.13. With increasing concentration of La, the structural/magnetic transition
is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15
< x < 0.19. A domelike superconducting region is not observed in the phase
diagram, however, because no obvious over-doping region can be found. Two
superconducting phases coexist in the x - T phase diagram of Ca1-xLaxFe2As2.
The formation of the two separate phases, as well as the origin of the high Tc
in Ca1-xLaxFe2As2 is studied and discussed in detail.
