In this paper we present detailed M\"ossbauer spectroscopy study of
structural and magnetic properties of the undoped parent compound
CaFe2​As2​ single crystal. By fitting the temperature dependence of the
hyperfine magnetic field we show that the magneto-structural phase transition
is clearly first-order in nature and we also deduced the compressibility of our
sample to be 1.67×10−2GPa−1. Within the Landau's theory of phase
transition, we further argue that the observed phase transition may stem from
the strong magneto-structural coupling effect. Temperature dependence of the
Lamb-M\"ossbauer factor show that the paramagnetic phase and the
antiferromagnetic phase exhibit similar lattice dynamics in high frequency
modes with very close Debye temperatures, ΘD​∼270\,K.Comment: 6 pages,5 figures Accepted by J. Phys.: Condens. Matte