  We have found a remarkably large response of the transition temperature of
CaFe2As2 single crystals grown out of excess FeAs to annealing / quenching
temperature. Whereas crystals that are annealed at 400 C exhibit a first order
phase transition from a high temperature tetragonal to a low temperature
orthorhombic and antiferromagnetic state near 170 K, crystals that have been
quenched from 960 C exhibit a transition from a high temperature tetragonal
phase to a low temperature, non-magnetic, collapsed tetragonal phase below 100
K. By use of temperature dependent electrical resistivity, magnetic
susceptibility, X-ray diffraction, Mossbauer spectroscopy and nuclear magnetic
resonance measurements we have been able to demonstrate that the transition
temperature can be reduced in a monotonic fashion by varying the annealing /
quenching temperature from 400 to 850 C with the low temperature state
remaining antiferromagnetic for transition temperatures larger than 100 K and
becoming collapsed tetragonal / non-magnetic for transition temperatures below
90 K. This suppression of the orthorhombic / antiferromagnetic phase transition
and its ultimate replacement with the collapsed tetragonal / non-magnetic phase
is similar to what has been observed for CaFe2As2 under hydrostatic pressure.
Transmission electron microscopy studies indicate that there is a temperature
dependent, width of formation of CaFe2As2 with a decreasing amount of excess Fe
and As being soluble in the single crystal at lower annealing temperatures. For
samples quenched from 960 C there is a fine (of order 10 nm), semi-uniform
distribution of precipitate that can be associated with an average strain field
whereas for samples annealed at 400 C the excess Fe and As form mesoscopic
grains that induce little strain throughout the CaFe2As2 lattice.
