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

Electron paramagnetic resonance (EPA) 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)(4)NH3][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 T-c = 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.