American Chemical Society
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EPR of Structural Phase Transition in Manganese- and Copper-Doped Formate Framework of [NH3(CH2)4NH3][Zn(HCOO)3]2

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journal contribution
posted on 2016-08-19, 00:00 authored by Mantas Šimėnas, Aneta Ciupa, Mirosław Mączka, Georg Völkel, Andreas Pöppl, Ju̅ras Banys
Electron paramagnetic resonance (EPR) and pulse electron–nuclear double resonance (ENDOR) spectroscopies are applied to investigate the structural phase transition and the low temperature phase in manganese- and copper-doped [NH3(CH2)4NH3]­[Zn­(HCOO)3]2 dense metal–organic framework. Continuous-wave (CW) EPR measurements indicate successful incorporation of Mn2+ and Cu2+ ions at the Zn2+ lattice sites. Pulse ENDOR spectrum reveals at least four different proton species in the vicinity of the Mn2+ center showing excellent agreement with the X-ray diffraction experiments. Temperature-dependent CW EPR spectra demonstrate that Mn2+ and Cu2+ local paramagnetic probes are sensitive to the phase transition in the studied compounds. The analysis of the temperature dependence of the Cu2+ hyperfine coupling parameter reveals a first-order phase transition at Tc = 235 K into an antiferroelectric phase that is close to the tricritical point. The obtained logarithmic divergence of the line width of the Mn2+ EPR spectrum indicates an order–disorder-type phase transition.