A beta-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (beta Alatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(beta Alatrz)(3)](CF(3)SO(3))(2)center dot 0.5H(2)O (1.0.5H(2)O). The temperature dependence of the high-spin molar fraction derived from (57)Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (T(c)(up arrow) = 232 K and T(c)(down arrow) = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mossbauer spectroscopy. The freshly prepared sample of 1.0.5H(2)O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(beta Alatrz)(3)](CF(3)SO(3))(2) (1) was found to exhibit a similar spin transition curve however much temperature was increased by (T(c)(up arrow) = 252 K and Tc(down arrow) = 248 K). The removal of lattice water molecules from 1.0.5H(2)O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced.