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

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 [NH 3 (CH 2 ) 4 NH 3 ][Zn(HCOO) 3 ] 2 dense metal–organic framework. Continuous-wave (CW) EPR measurements indicate successful incorporation of Mn 2+ and Cu 2+ ions at the Zn 2+ lattice sites. Pulse ENDOR spectrum reveals at least four different proton species in the vicinity of the Mn 2+ center showing excellent agreement with the X-ray diffraction experiments. Temperature-dependent CW EPR spectra demonstrate that Mn 2+ and Cu 2+ local paramagnetic probes are sensitive to the phase transition in the studied compounds. The analysis of the temperature dependence of the Cu 2+ 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 Mn 2+ EPR spectrum indicates an order–disorder-type phase transition.