EPR Study of Structural Phase Transition in Manganese-Doped [(CH₃)₂NH₂][Zn(HCOO)₃] Metal–Organic Framework

We present an electron paramagnetic resonance (EPR) study of [(CH₃)₂NH₂]­[Zn­(HCOO)₃] metal–organic framework (MOF) powder doped with a small amount of paramagnetic Mn²⁺ ions. Our EPR measurements indicate a successful incorporation of local Mn²⁺ probes into the structure allowing us to detect and investigate an order–disorder structural phase transition in the studied MOF. The temperature-dependent continuous wave (CW) X- and Q-band EPR measurements reveal a sudden change in the spectra at a phase transition temperature <i>T</i>₀ = 163 K. Simulations were performed to determine the spin Hamiltonian parameters of the spectra which reflect the local symmetry of the Mn²⁺ probes in the disordered and ordered phases. The temperature dependence of the axial zero-field splitting parameter <i>D</i> demonstrates that the observed phase transition at <i>T</i>₀ is discontinuous. Additionally, this dependence follows the prediction of the Landau theory. We also performed preliminary pulse EPR measurements which reveal a rather long phase memory time sufficient to detect a spin echo even in the high-temperature disordered phase. The modulation with the proton and nitrogen Larmor frequencies of the electron spin echo was observed as well