Influences of temperature, pressure, and lattice solvents on the spin transition regime of the polymeric compound [Fe(hyetrz)₃]A₂•3H₂O (hyetrz = 4-(2´- hydroxyethyl)-1,2,4-triazole and A¯ = 3-nitrophenylsulfonate)

[Fe(hyetrz)3](3-nitrophenylsulfonate)2â3H2O (1â3H2O), with hyetrz ) 4-(2¢-hydroxyethyl)-1,2,4-triazole, has been synthesized, and its physical properties have been investigated with several techniques, including optical and Mo¨ssbauer spectroscopies, magnetism at different pressures up to ca. 9 kbar, thermogravimetry, calorimetry, and X-ray powder diffraction. At room temperature, 1â3H2O is in the low-spin (LS) state and the color is pink. As the compound is heated, the three noncoordinated water molecules are released, and the Fe2+ sites now undergo a transition from low-spin (LS, S ) 0) to high-spin (HS, S ) 2) states.
This transformation, denoted as 1â3H2O (LS)f1 (HS), is irreversible in a normal atmosphere and is accompanied by a change of color between pink and white. Depending on the rate of increasing temperature, it is possible to observe compound 1 in the metastable LS state. The thermal transformation from this metastable state to the stable HS state is then extremely abrupt, occurring within ca. 1 K. Compound 1 has been found to exhibit a spin transition in the low-temperature range, with T1/2 around 100 K. This transition is cooperative around T1/2, with a thermal hysteresis of about 10 K, but already begins immediately below room temperature. At 4 K, about 15% of the Fe2+ sites have been found to remain in the HS state. The pressure dependence of the LS S HS transition for 1 has been investigated. A pronounced effect has been observed; the transition temperature is shifted from 100 to 270 K as the pressure varies from 1 bar up to 8.9 kbar.