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 [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 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.