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)(2)NH2][Zn(HCOO)(3)] metal organic framework (MOP) 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 T-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 D demonstrates that the observed phase transition at To is Additionally, this dependence follows the prediction of the Landau theory. We also performed preliminary pulse 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.