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) 2 ·3H 2 O ( 1 · 3H 2 O), 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 Mössbauer spectroscopies, magnetism at different pressures up to ca. 9 kbar, thermogravimetry, calorimetry, and X-ray powder diffraction. At room temperature, 1 · 3H 2 O 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 Fe 2+ sites now undergo a transition from low-spin (LS, S = 0) to high-spin (HS, S = 2) states. This transformation, denoted as 1 · 3H 2 O (LS) → 1 (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 T 1/2 around 100 K. This transition is cooperative around T 1/2, with a thermal hysteresis of about 10 K, but already begins immediately below room temperature. At 4 K, about 15% of the Fe 2+ sites have been found to remain in the HS state. The pressure dependence of the LS ⇔ 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.