The temperature dependence of the magnetic susceptibility, optical reflectivity and electrical conductivity of [Fe(HB(pz)3)2] (pz = pyrazolyl) revealed irreversible changes in the material during the low-spin to high-spin transition when the “as-prepared” sample was heated above ∼400 K for the first time. During this first heating sequence, the initially fine powder sample became coarse, and its crystal structure changed from tetragonal to monoclinic. Single-crystals of the monoclinic form suitable for X-ray analysis could be isolated after the first thermal cycle, and their structure was resolved in the P21/n (Z = 4) space group. Successive cooling and heating cycles did not lead to further modification of the crystal structure, and the temperature dependence of the physical properties remained invariable. Remarkably, the electrical conductivity of the sample measured at 293 K dropped from 6.1 × 10−8 to 2.1 × 10−11 S m−1 following the first thermal cycle—suggesting possible applications of this material in read-only memory devices (ROM).
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